Geoscience for Petroleum Engineers 9811976392, 9789811976391

Citation preview

Sandeep Narayan Kundu

Geoscience for Petroleum Engineers

Geoscience for Petroleum Engineers

Sandeep Narayan Kundu

Geoscience for Petroleum Engineers

Sandeep Narayan Kundu Department of Geology Ravenshaw University Cuttack, India

ISBN 978-981-19-7639-1 ISBN 978-981-19-7640-7 (eBook) https://doi.org/10.1007/978-981-19-7640-7 © Springer Nature Singapore Pte Ltd. 2023 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

I dedicate this book to all Geoscientists and Engineers who are continuously endeavouring to secure energy for our future to sustain and improve our wellbeing.

Preface

Geoscience is the study of the planet Earth and its sub-systems. Its holistic understanding is essential for its application in petroleum exploration. As most engineers and geologists do not specifically train in the subject, but eventually end up working for the petroleum industry, it is essential that they must be exposed to the fundamental aspects of the subject at the undergraduate level. This is why several engineering and geoscience undergraduate programs have started including petroleum geoscience as an elective subject. Petroleum Geoscience is an important component of geoscience which essentially deals with a major component of the carbon cycle. As petroleum engineers are tasked with drilling and production of hydrocarbons, having an understanding of geoscience is critical to their effective collaboration with geoscientists in any organization. As a petroleum engineers’ job starts with inputs from a geoscientist, it is imperative that they develop the perspective of a geoscientist to be able to comprehend their inputs. Most engineers possess a strong understanding of rocks from a chemical and/or mechanical perspective, but they seldom have a geological perspective. Having this perspective is highly beneficial for effective and informed decision-making. This book is aimed at providing engineers and budding geologists, a foundational perspective of geoscience that is required for hydrocarbon exploration. Geoscience is a vast subject and has several sub-disciplines, which revolve around all rock types (igneous, metamorphic, and sedimentary). However, as hydrocarbon accumulations are mostly confined to sedimentary basins, the focus of this book shall be on sedimentary rocks. Geological processes within a sedimentary basin, since the formation of the basin, to the present-day, are critical to identification of petroleum systems. Sedimentary environments are complex as they are shaped by several concurrent processes, which run at different periodicities. Weathering, erosion, transportation, and deposition happen in tandem with other geological processes like tectonics, eustasy, and isostasy. All these processes, despite being very different from each other, are intricately related in time and space. As we increasingly look at Earth from a systems perspective, geologists and engineers must gain

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a sound understanding of petroleum systems, in different geological settings. Sedimentary geology and basin analysis are therefore of paramount importance to the understanding of basin evolution and identification of petroleum systems. Identification of petroleum systems require thorough scientific investigation, which involves collection of rock specimens, their laboratory analysis, and their direct and indirect interpretation. From early field-based investigations, the exploration process progresses through laboratory based studies to establish the physical and mechanical properties of the rocks in the subsurface. These are then integrated to build a subsurface geological model. Non-destructive indirect methods, like geophysical methods (gravity and magnetics), are deployed at the basin scale, which is later followed by seismic methods for smaller focus areas. Geophysical methods are the backbone of modern petroleum exploration, and both engineers and geologists must be aware of how geophysical data is acquired, processed and interpreted. Geophysics also plays an important role in post-exploration stages like development and production. Most universities and institutions develop engineers and geoscientists, but only a few, who opt for specializing in petroleum exploration and production, are exposed to geoscience. In reality, many who did not opt for this specialization still end up being employed with oil companies. This book, “Petroleum Geoscience for Engineers”, aims to fill the gap in the education of such professionals. The book is expected to be an easy self-read for gaining insights on the geoscientific activities practiced in the oil and gas industry. The book, in addition, shall also serve as a reference for petroleum related elective subjects, at the undergraduate level. The book has 12 Chapters in total. The first six chapters deal with the fundamentals of geoscience. The next six chapters deal with the application of these fundamentals for hydrocarbon exploration. Each chapter is written in a way that the concepts are lucidly explained. The educator can plan his lectures by following the sequence in which the chapters are written in this book with the liberty to add further information from the advanced suggested readings provided at the end of each chapter. The preceding chapter scaffolds the next one, transitioning information from the former to the latter. Learners can always expand their horizons by exploring the suggested further readings. Throughout this book, the term ‘petroleum’, ‘oil and gas’, and ‘hydrocarbons’ are interchangeably used and are therefore deemed as synonyms. The author hopes that this book serves as a primer on geoscience and petroleum exploration for students, who aim for a career with the petroleum and related industry. Cuttack, India

Prof. Dr. Sandeep Narayan Kundu

Acknowledgements

Firstly, I would thank the National University of Singapore for providing me the opportunity to introduce Geoscience and Petroleum Exploration to the students pursuing undergraduate programs in Science and Engineering. I would like to thank the South East Asian Petroleum Exploration Society (SEAPEX) and the Economic Development Board (EDB) of Singapore for funding my position at NUS for over 6 years. This provided the opportunity to focus on academic research and to conduct numerous field trips in the region, which have both supported the sourcing of rich inputs into this book. A special thanks to Dr Grahame Oliver, former Senior Lecturer at NUS, for constantly inspiring and supporting me in the endeavors to author this book. Secondly, I would like to thank all my students, whose inquisitiveness for the subject was a constant inspiration, and this provided the needed drive to complete this book. My ex-colleagues at NUS too, were also very supportive of the idea of this book. Finally, I would like to thank my parents Dr. Manmatha Kundu and Dr. Binodini Patra who founded the academic fervour in me. Their illustrative academic career was an inspiration for me and helped me switch from the corporate world to the noble profession of teaching and research. My wife, Priyanka, daughter Shreya, and son Shakti, need a special mention, as without their support this book would never have seen daylight. Prof. Dr. Sandeep Narayan Kundu

ix

Contents

1

Planet Earth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Origin of Earth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.1 Nebular Hypothesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.2 Planetesimal Hypothesis . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.3 Tidal or Gaseous Hypothesis . . . . . . . . . . . . . . . . . . . . . . . 1.2.4 Protoplanet Hypothesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.5 Big-Bang Theory and the Space–Time Singularity . . . . . 1.3 Age of Earth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.1 Early Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.2 Radioactive Dating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.3 Geological Time Scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Earth’s Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4.1 The Crust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4.2 The Mantle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4.3 The Core . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 Earth System & Interactive Processes . . . . . . . . . . . . . . . . . . . . . . . 1.5.1 The Earth System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5.2 Interactive Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Further Readings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 1 2 2 2 3 3 4 5 5 6 8 9 10 12 12 13 13 13 15 15

2

Crustal Processes and Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1 Crustal Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2 Isostasy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Crustal Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1 Minerals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2 Rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3 The Rock Cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Further Readings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

17 17 17 23 25 25 28 30 30 31 xi

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3

Sedimentary Basins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Basin Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Geodynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2 Climatic Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Basin Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Continental Rift Basins . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2 Flexure Basins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.3 Strike-Slip and Wrench Basins . . . . . . . . . . . . . . . . . . . . . . 3.3.4 Intra-Cratonic Basins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Basin Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.1 Sedimentological Analysis . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.2 Stratigraphic Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.3 Structural Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.4 Geohistory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Further Readings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

33 33 33 33 34 35 35 36 38 38 39 40 40 40 41 42 42

4

Sediments and Sedimentary Rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Weathering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1 Physical Weathering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2 Chemical Weathering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.3 Biological Weathering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.4 Climatic Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Erosion, Transportation and Deposition . . . . . . . . . . . . . . . . . . . . . . 4.3.1 Gravity Erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2 Fluvial Erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.3 Aeolian Erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.4 Glacial Erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Lithification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.1 Compaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.2 Cementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.3 Diagenesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5 Sedimentary Rock and Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.1 Clastic Sedimentary Rocks . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.2 Non-clastic Sedimentary Rocks . . . . . . . . . . . . . . . . . . . . . 4.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Further Readings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

43 43 44 45 45 46 46 47 47 48 49 50 51 51 52 52 52 52 57 59 59

5

Stratigraphy and Sedimentary Structures . . . . . . . . . . . . . . . . . . . . . . . 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Stratigraphy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1 Lithostratigraphy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.2 Biostratigraphy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.3 Chrono-Stratigraphy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

61 61 63 63 64 65

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5.3

Relative Geologic Dating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.1 Uniformitarianism and Catastrophism . . . . . . . . . . . . . . . 5.3.2 Principles of Stratigraphy . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4 Sedimentary Structures and Contacts . . . . . . . . . . . . . . . . . . . . . . . . 5.4.1 Sedimentary Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.2 Contacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5 Stratigraphic Correlation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Further Readings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

67 67 67 70 70 72 74 76 77

6

Depositional Environments and Facies . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Depositional Environments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Identifying Depositional Environments . . . . . . . . . . . . . . . . . . . . . . 6.3.1 Lithology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.2 Sedimentary Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 Sedimentary Facies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5 Typical Sediments Tied to Depositional Environments . . . . . . . . . 6.5.1 Turbidites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.2 Varves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.3 Marine Limestone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.4 Tsunami Deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.5 Deltas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.6 Fluvial Deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Further Readings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

79 79 79 80 80 81 84 85 85 86 86 86 87 88 89 89

7

The Petroleum System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1 The Carbon Cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1.1 Slow and Fast Carbon Cycle . . . . . . . . . . . . . . . . . . . . . . . . 7.2 The Petroleum System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3 Source Rock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.1 Kerogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4 Reservoir Rock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.1 Porosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.2 Permeability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5 Seal Rock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.6 Overburden . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.7 Petroleum System Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.7.1 Trap Creation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.7.2 Maturation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.7.3 Migration and Accumulation . . . . . . . . . . . . . . . . . . . . . . . 7.8 Extents of a Petroleum System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.9 Petroleum System Event Chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.10 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Further Readings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

91 91 91 92 93 94 95 96 96 97 98 98 99 100 102 102 103 104 104

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Exploration Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1 Seeps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 Geological Mapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.1 Mapping from Air and Space . . . . . . . . . . . . . . . . . . . . . . . 8.2.2 Drainage Pattern and Subsurface Geology . . . . . . . . . . . . 8.3 Geophysical Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.1 Gravity Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.2 Magnetic Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4 Sedimentology and Geochemical Methods . . . . . . . . . . . . . . . . . . . 8.4.1 Micro-seeps and Soil Analysis . . . . . . . . . . . . . . . . . . . . . . 8.4.2 Result Visualization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4.3 Other Geochemical Methods . . . . . . . . . . . . . . . . . . . . . . . 8.5 Seismic Exploration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5.1 Fundamentals of Seismic Reflection . . . . . . . . . . . . . . . . . 8.5.2 Seismic Surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5.3 Seismic Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5.4 Seismic Resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5.5 Seismic Interpretation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5.6 Modelling Geology from Seismic Data . . . . . . . . . . . . . . . 8.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Further Readings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

105 105 107 108 109 110 110 112 113 115 115 115 116 117 117 120 121 122 123 123 124

9

Drilling and Logging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1 Drilling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1.1 Drilling Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1.2 Vertical and Directional Wells . . . . . . . . . . . . . . . . . . . . . . 9.1.3 The Drilling Rig . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1.4 Geology Influenced Problems . . . . . . . . . . . . . . . . . . . . . . 9.1.5 Well Completions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2 Well Logging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.1 The Borehole Environment . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.2 Equipment, Types and Tools . . . . . . . . . . . . . . . . . . . . . . . . 9.2.3 Spontaneous Potential (SP) Log . . . . . . . . . . . . . . . . . . . . . 9.2.4 Natural Gamma Ray (GR) Logs . . . . . . . . . . . . . . . . . . . . . 9.2.5 Resistivity Logs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.6 Neutron Porosity Log . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.7 Gamma Density Log . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.8 Acoustic (Sonic) Porosity Log . . . . . . . . . . . . . . . . . . . . . . 9.2.9 Caliper Log . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Further Readings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

125 125 126 127 128 131 131 133 133 134 136 137 138 139 139 140 140 145 145

Contents

xv

10 Hydrocarbon Reserves and Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.1 Resources and Reserves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2 Volumetric Estimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.1 Gross Reservoir Volume (GRV) and Net Pay Volume (NPV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.2 Porosity (.) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.3 Fluid Saturation (Sw , Sg , So ) . . . . . . . . . . . . . . . . . . . . . . . . 10.2.4 Formation Volume Factor (FVF) . . . . . . . . . . . . . . . . . . . . 10.2.5 Recovery Efficiency (RE) . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.6 Hydrocarbons Initially in Place (HCIIP) . . . . . . . . . . . . . . 10.3 Uncertainty and Probabilistic Methods . . . . . . . . . . . . . . . . . . . . . . 10.3.1 Probability Density Function (PDF) . . . . . . . . . . . . . . . . . 10.3.2 Monte Carlo Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.4 Hydrocarbon Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.4.1 API Gravity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.4.2 Sulphur Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.4.3 Dry Gas and Wet Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.4.4 Processing of Crude . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Further Readings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

147 147 148

11 Production Geology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Reservoir Fluid and Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Reservoir Drive Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2.1 Solution Gas Drive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2.2 Gas Cap Drive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2.3 Water Drive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3 Production Well Planning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3.1 Rate of Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3.2 Drive Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3.3 Reservoir Geology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3.4 Fluid Saturation, Wettability and Relative Permeability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.4 Decline Curve Analysis (DCA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.4.1 Forecasting Using Decline Curves . . . . . . . . . . . . . . . . . . . 11.4.2 Decline Curves for Different Drive Mechanisms . . . . . . . 11.4.3 Reserve Estimation from Production History . . . . . . . . . . 11.5 Enhanced Oil Recovery (EOR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.5.1 Secondary Recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.5.2 Tertiary Recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

159 159 159 160 161 161 162 162 162 162

149 150 150 150 150 151 151 152 153 154 154 155 155 155 157 157

162 163 164 166 167 167 168 169 171 171

xvi

Contents

12 Unconventional Hydrocarbons Resources . . . . . . . . . . . . . . . . . . . . . . . . 12.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.1.1 Energy Returned on Energy Invested (EROEI) . . . . . . . . 12.1.2 Types of Unconventional Hydrocarbon Resources . . . . . 12.2 Shale Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2.1 Exploration and Production . . . . . . . . . . . . . . . . . . . . . . . . 12.3 Oil Shale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.3.1 Exploration and Production . . . . . . . . . . . . . . . . . . . . . . . . 12.4 Tight Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.4.1 Exploration and Production . . . . . . . . . . . . . . . . . . . . . . . . 12.5 Coal Seam Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.5.1 Exploration and Production . . . . . . . . . . . . . . . . . . . . . . . . 12.6 Oil (or Tar) Sands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.6.1 Extraction and Production . . . . . . . . . . . . . . . . . . . . . . . . . . 12.7 Natural Gas Hydrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.7.1 Composition, Structure and Stability . . . . . . . . . . . . . . . . . 12.7.2 Development and Production . . . . . . . . . . . . . . . . . . . . . . . 12.8 Global Unconventional Resources and Distribution . . . . . . . . . . . . 12.8.1 Shale Gas Resource Distribution . . . . . . . . . . . . . . . . . . . . 12.8.2 Tight Gas and Coal Seam Gas Resource Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.8.3 Oil Shale Resource Distribution . . . . . . . . . . . . . . . . . . . . . 12.8.4 Gas Hydrates Resource Distribution . . . . . . . . . . . . . . . . . 12.9 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Further Readings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

173 173 173 174 175 176 177 177 179 180 181 181 182 182 183 184 184 186 187 188 189 191 191 192

About the Author

Prof. Dr. Sandeep Narayan Kundu is a British Council Fellow and has over 20 years of experience in Industry, which includes the likes of Fugro, Reliance Exploration and Production, and BHP Billiton. He has taught Exploration Geosciences and Spatial Data Science at NUS Singapore, at the Department of Civil and Environmental Engineering and also at the Department of Geography. Professor Kundu’s prime interests lie in Energy Geosciences and Geospatial applications. He has edited and authored three books, published several scientific articles, and also has presented at several international forums. Name: Sandeep Narayan Kundu Designation: Professor Affiliation: Department of Geology, Ravenshaw University, College Road, Cuttack-753003 Address: Flat 1101, Tower-3, Tata Ariana, Kalinganagar (near Ghatikia), Bhubaneswar-751029, India Email(O): [email protected] OrcidID: 0000-0002-8567-253X

xvii

Abbreviations

0

C 2D 3D 4D AAS Al API Ba BTU C Ca Cl cm CMP Co CSS Cu DCA DMO EOR EROEI Fe FVF GC GIP GOC GPS GR GRV GWC H

Degree Centigrade Two Dimension Three Dimension Four Dimension Atomic Absorption Spectroscopy Aluminium American Petroleum Institute Barium British Thermal Unit Carbon Calcium Chlorine Centimetre Common Mid-Point Cobalt Cyclic Steam Stimulation Copper Decline Curve Analysis Dip Move-out Enhanced Oil Recovery Energy Returned on Energy Invested Iron Formation Volume Factor Gas Chromatography Gas in place Gas Oil Contact Global Positioning System Gamma Ray Gross Reservoir Volume Gas water contact Hydrogen xix

xx

HCIIP Hz K Km LLD LLM LLS LWD Ma Mg mGAL Moho mV MWD Na Ni NMO NMR NPV nT O OIP OWC Pb PDF pH RE RF Ro ROP S SAGD Si SP T TAI TCF Th TOC U UR USA V

Abbreviations

Hydrocarbon Initially in place Hertz Potassium Kilometre Laterolog Deep Laterolog Medium Laterolog Shallow Logging While Drilling Megannum Magnesium Milli Gal Mohorovicic Discontinuity MiliVolt Measuring while Drilling Sodium Nickel Normal Move-out Nuclear Magnetic Resonance Net Pay Volume Nano Tesla Oxygen Oil in place Oil water contact Lead Probability Density Function Potential of Hydrogen Recovery Efficiency Recovery Factor Vitrinite Reflectance Rate of Penetration Sulphur Steam-assisted Gravity Drainage Silicon Spotaneous Potential Titanium Thermal Alteration Index Trillon Cubic Feet Thorium Total Organic Carbon Uranium Ultimate Recovery United States of America Vanadium

List of Figures

Fig. 1.1 Fig. 1.2 Fig. 1.3 Fig. 1.4 Fig. 1.5 Fig. 1.6 Fig. 2.1 Fig. 2.2 Fig. 2.3 Fig. 2.4 Fig. 2.5 Fig. 2.6 Fig. 2.7 Fig. 2.8 Fig. 2.9 Fig. 3.1 Fig. 3.2 Fig. 3.3 Fig. 3.4 Fig. 3.5 Fig. 3.6 Fig. 3.7 Fig. 4.1 Fig. 4.2 Fig. 4.3 Fig. 4.4 Fig. 4.5 Fig. 4.6 Fig. 4.7

Big Bang Theory of Space & Time (adapted from NASA JPL) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Radioactive Dating and Half life . . . . . . . . . . . . . . . . . . . . . . . . . The geological time scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Earth’s internal structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Elemental Composition of Earth’s crust . . . . . . . . . . . . . . . . . . . The Earth System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mantle Convection and Plate Tectonics . . . . . . . . . . . . . . . . . . . . The Wilson Cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Continental plates through geologic time . . . . . . . . . . . . . . . . . . Models for Isostasy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Comparison of mineral hardness scales . . . . . . . . . . . . . . . . . . . . Classification of Silicates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bowen’s reaction series . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Classification of Rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Rock Cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sedimentary Basins of the World . . . . . . . . . . . . . . . . . . . . . . . . . Stages of Continental Rifting . . . . . . . . . . . . . . . . . . . . . . . . . . . . Geological Setting of Flexure Basins . . . . . . . . . . . . . . . . . . . . . Plan view of Strike-Slip Basins . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanisms of Intra-cratonic Basins . . . . . . . . . . . . . . . . . . . . . Sedimentation and Tectonics . . . . . . . . . . . . . . . . . . . . . . . . . . . . Deconstructing Deposition, Burial and Compaction . . . . . . . . . Types of Weathering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Climate Zones on Earth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hjulstrom’s Curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Erosion and transportation of sediments by water, wind and ice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Lithification Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sphericity and Roundness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Flowing Current and Pebble Orientation . . . . . . . . . . . . . . . . . . .

4 7 9 10 11 14 19 22 23 24 26 27 28 28 31 34 36 37 38 39 41 42 44 47 48 50 51 54 55 xxi

xxii

Fig. 4.8 Fig. 4.9 Fig. 5.1 Fig. 5.2 Fig. 5.3 Fig. 5.4 Fig. 5.5

Fig. 5.6 Fig. 5.7 Fig. 5.8 Fig. 5.9 Fig. 5.10

Fig. 6.1 Fig. 6.2 Fig. 6.3 Fig. 6.4 Fig. 6.5 Fig. 6.6 Fig. 7.1 Fig. 7.2 Fig. 7.3 Fig. 7.4 Fig. 7.5 Fig. 7.6 Fig. 7.7 Fig. 7.8 Fig. 7.9 Fig. 7.10 Fig. 7.11 Fig. 8.1

List of Figures

Common Clastic Sedimentary Rocks . . . . . . . . . . . . . . . . . . . . . Non-clastic Sedimentary Rocks . . . . . . . . . . . . . . . . . . . . . . . . . . A stratigraphic column of Sandstone and Shale formations in Labuan, East Malaysia . . . . . . . . . . . . . . . . . . . . . . Major Fossils and their Geological Time Ranges . . . . . . . . . . . . Principles of Relative Dating . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tilted beds and the concept of Dip and Strike . . . . . . . . . . . . . . . Sedimentary structures (a), Graded bedding (b), Convolute bedding (c), Ripple marks (d), Mud cracks (e) and Burrows (f) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Depositional and Intrusive contacts . . . . . . . . . . . . . . . . . . . . . . . Angular Uniformity (in blue) along the Miri-Bintulu coastal road, East Malaysia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Normal and Reverse faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stratigraphic Correlation using strata columns at three different locations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biostratigraphic correlation and the reconstruction of Gondwanaland. NB: Gondwana split into the continents we have today due to continental drift . . . . . . . . . . . . . . . . . . . . . Primary Depositional Environments . . . . . . . . . . . . . . . . . . . . . . Textural maturity in sedimentary environment . . . . . . . . . . . . . . Schematic of Facies Association and lateral shift during Transgression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Field Photographs of Sediments tied to Depositional Environments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Mahanadi Delta in Odisha, with its distributaries . . . . . . . . Avulsion and Subsidence in Fluvial Environment . . . . . . . . . . . Slow and Fast Carbon Cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . Elements of a Petroleum System . . . . . . . . . . . . . . . . . . . . . . . . . The van Krevelin diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Porosity and Permeability in rocks . . . . . . . . . . . . . . . . . . . . . . . Membrane and Hydraulic seals . . . . . . . . . . . . . . . . . . . . . . . . . . Structural traps (a Fold and c Fault), Stratigraphic trap (b) and Mixed trap (d) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Stages of Maturation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermal Maturity: a based on TAI and Ro. b Fission Tracks . . Maturation and Migration of Oil and Gas . . . . . . . . . . . . . . . . . . Geographic and Stratigraphic Extents of a Petroleum System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Petroleum System Event Chart (* critical moment) . . . . . . . . . . Petroleum seeps. a Schematics of a petroleum seep. b A seep in sandstone reservoir in East Malaysia. c SAR imagery with repeat slicks. d Multibeam backscatter for seabed seep detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

56 57 62 65 68 69

71 73 74 75 76

77 81 83 84 85 87 89 92 93 94 96 97 99 100 101 102 103 104

106

List of Figures

Fig. 8.2 Fig. 8.3 Fig. 8.4

Fig. 8.5 Fig. 8.6 Fig. 8.7 Fig. 8.8 Fig. 8.9 Fig. 8.10

Fig. 8.11 Fig. 9.1 Fig. 9.2 Fig. 9.3

Fig. 9.4 Fig. 9.5 Fig. 9.6 Fig. 9.7 Fig. 9.8 Fig. 9.9 Fig. 9.10 Fig. 9.11 Fig. 9.12 Fig. 9.13 Fig. 9.14 Fig. 10.1

xxiii

An anticlinal fold below an unconformity (a) and a normal fault (b) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Satellite images of (left) Jashak salt dome in Iran and (right) San Andreas Fault in USA . . . . . . . . . . . . . . . . . . . . . Interpretation of subsurface geology from drainage patterns. a Dendritic drainage (horizontal strata). b Parallel drainage (gently dipping strata). c Trellis drainage (folded strata). d Rectangular drainage (faulted or jointed non-sedimentary rocks). e Deranged drainage (glacial till). f Radial drainage (dome structures) . . . . . . . . . . . . Gravity anomaly over salt domes (a) and Salt domes on the seabed of Gulf of Mexico (b) . . . . . . . . . . . . . . . . . . . . . . Magnetic anomaly map of Mumbai oil and gas fields, India . . . Microseeps in sandstone (a), sample gas chromatograph (b) and gas cluster map (c) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Schematic of a seismic survey on sea . . . . . . . . . . . . . . . . . . . . . The Synthetic Seismic Trace . . . . . . . . . . . . . . . . . . . . . . . . . . . . Seismic processing: a static correction, b CMP gather, c velocity (offset) analysis d NMO correction, e stacked trace and f migration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Seismic amplitude and section interpretation . . . . . . . . . . . . . . . The First oil well in Miri, East Malaysia (left) and different types of bit (right) . . . . . . . . . . . . . . . . . . . . . . . . . . Wells by geometry (MD = measured depth, VD = vertical depth) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Components of Drilling Rig. a The Rig Mast, b Drill pipes, c Mud pump, d The Engine to provide rotary motion to the drill pipes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Types of Rigs and their operational environment . . . . . . . . . . . . Components of Mud logging operations . . . . . . . . . . . . . . . . . . . Types of Well Completion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Borehole Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Well logging equipment and tool-types with their resolution and penetration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SP and Gamma ray logs over a Sand-Shale alternating section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Resistivity Logging Tools and log data . . . . . . . . . . . . . . . . . . . . Porosity logging tools. Neutron and gamma density (left) and acoustic (right) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Caliper log tool (left) and deviations from planned hole diameter in a well (right) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sequence of Well log interpretation . . . . . . . . . . . . . . . . . . . . . . . Basic Well log interpretation (NR: non-reservoir) . . . . . . . . . . . The concept of Resource and Reserve for Hydrocarbons . . . . . .

107 109

109 111 114 116 118 119

121 122 126 127

128 129 130 132 134 135 137 138 140 141 143 143 148

xxiv

Fig. 10.2 Fig. 10.3 Fig. 10.4 Fig. 10.5 Fig. 10.6 Fig. 11.1 Fig. 11.2 Fig. 11.3 Fig. 11.4 Fig. 11.5 Fig. 11.6 Fig. 11.7 Fig. 11.8 Fig. 11.9 Fig. 12.1 Fig. 12.2 Fig. 12.3 Fig. 12.4 Fig. 12.5 Fig. 12.6 Fig. 12.7 Fig. 12.8 Fig. 12.9 Fig. 12.10 Fig. 12.11 Fig. 12.12 Fig. 12.13 Fig. 12.14 Fig. 12.15 Fig. 12.16

List of Figures

Gross Reservoir volume (GRV), Net Pay volume (NPV), Porosity (F) and Saturation (Sw, So) . . . . . . . . . . . . . . . . . . . . . Different probability density functions . . . . . . . . . . . . . . . . . . . . The Monte Carlo simulation workflow . . . . . . . . . . . . . . . . . . . . Crude oil classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fractional Distillation and Petroleum Products . . . . . . . . . . . . . Reservoir Drive Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wettability of oil (top) relative to different reservoir types (bottom) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Relative Permeability and Fluid Saturation in Oil-wet Reservoir . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Decline Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Decline curves for various drive mechanisms . . . . . . . . . . . . . . . Oil Recovery Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Injection Well Patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Secondary Recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemical Flooding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Unconventional Hydrocarbons and Geological Settings . . . . . . Trapped gas in shale and desorbed gas post hydraulic fracturing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Horizontal Drilling and Hydraulic Fracturing in Shale Formations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Oil Shale Extraction Workflow . . . . . . . . . . . . . . . . . . . . . . . . . . Hydrocarbon Reservoir Rocks and Permeability . . . . . . . . . . . . Well Stimulation methods for Tight Gas Reservoirs . . . . . . . . . Coal Seam Gas Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Organization of Oil Sand Constituents . . . . . . . . . . . . . . . . . . . . Cyclic Steam Stimulation (CSS) and Steam-Assisted Gravity Drainage (SAGD) methods . . . . . . . . . . . . . . . . . . . . . . . Structure of a Methane Clathrate (left) and its stability range (right) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gas hydrate (white) in clay (black) found in deep-water drilling (inset: a burning clathrate) . . . . . . . . . . . . . . . . . . . . . . . . Primary Energy Share from Fossil Fuels by Country (BP Statistical Review 2022) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Global distribution of Shale Gas (in trillion cubic meters) . . . . . Global coal seam gas reserve estimates . . . . . . . . . . . . . . . . . . . . Oil Shale Mined annually since 1950 . . . . . . . . . . . . . . . . . . . . . Inferred and Recovered Gas Hydrate locations . . . . . . . . . . . . . .

149 152 153 154 156 160 163 164 165 166 168 168 169 170 175 176 177 178 179 180 182 183 184 185 186 187 188 189 190 191

List of Tables

Table 1.1 Table 1.2 Table 1.3 Table 2.1 Table 4.1 Table 5.1 Table 6.1 Table 6.2 Table 6.3 Table 7.1 Table 10.1 Table 12.1 Table 12.2

Common Radionuclides and their half-life periods . . . . . . . . . . Ages of Meteorites from dating methods . . . . . . . . . . . . . . . . . . Earth System Components and Interactive Processes . . . . . . . . Oceanic and Continental plates . . . . . . . . . . . . . . . . . . . . . . . . . . Udden-Wentworth Scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stratigraphic Units and their Rank equivalence . . . . . . . . . . . . . Continental Depositional Environments . . . . . . . . . . . . . . . . . . . Transitional Environments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Marine Depositional Environments . . . . . . . . . . . . . . . . . . . . . . . Kerogen types and characteristics . . . . . . . . . . . . . . . . . . . . . . . . Composition of crude oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ERORI averages of various energy resources . . . . . . . . . . . . . . . Tight Gas and Coal Seam Gas resource estimates by Region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

8 8 14 19 54 66 82 82 83 95 154 174 190

xxv

Chapter 1

Planet Earth

1.1 Introduction Earth is the third planet from the Sun in the solar system and the only one known in the Universe to host living beings. Earth appears like a blue marble from space because of its vast water cover which scatters blue wavelengths. Earth is divided into two halves at the equator and the upper half is called the Northern Hemisphere and the lower one is called the Southern Hemisphere. Earth orbits the sun once in every 365 days along an elliptical orbit. It is closest (91 million miles) to the sun at “perihelion,” in the month of January and farthest from it (95 million miles) at “aphelion” in July. Earth spins at the equator at a rate of 1,000 miles per hour with a single full spin taking 24 hours. The spin axis, which is an imaginary line through the centre of the planet from the North Pole to the South Pole, is tilted at an angle of 23.5° and imparts a precession while it rotates. The inclined axis, the precession & rotation and the elliptical orbit ensures that the solar irradiation received at any location is not the same and this differential cyclic phenomenon causes day and night, different seasons and climate zones on Earth. Earth is also the densest planet of the solar system. Earth’s gravitational field interacts with other planets and moon to stabilize its orientation along its axis, and its rotation causes ocean tides in the oceans. Lithosphere is the solid outer layer of Earth, where most of the life forms dwell and interact with water and atmosphere. The lithosphere is rigid and brittle, underneath which the elastic layer of upper mantle is situated. There is a gradual change in chemistry of Earth’s material with depth. The Earth’s crust is divided into several rigid tectonic plates which float over this viscous mantle and these plates laterally move at 2-3 cm per year. Plates collide with each other forming mountains and when two plates move further away from each other, oceans are formed. Earth’s surface consists of oceans (71%) and land (29%) and part of each is covered with ice. Earth’s interior is hot and dense and consists of a solid inner core of iron and nickel under the lower mantle.

© Springer Nature Singapore Pte Ltd. 2023 S. N. Kundu, Geoscience for Petroleum Engineers, https://doi.org/10.1007/978-981-19-7640-7_1

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1 Planet Earth

1.2 Origin of Earth Being a part of the solar system, the origin and age is understood to be related to the origin of the solar system. Several theories have been proposed on the origin of the solar system, which postulates different mechanisms. Despite the fact that each theory is inadequate in explaining all the aspects of the planetary system, it is important to know the basis on which the early hypothesis on Earth’s origin were made. The concept of time-space continuum and the Big-Bang, are the ones which strike the chord with most scientists, and are therefore the most acceptable theory for the origin of the solar system and Earth.

1.2.1 Nebular Hypothesis As proposed in 1755 by the German philosopher Immanuel Kant, the nebular hypothesis suggests that the solar system formed out of a disc shaped cloud of interstellar gas called ‘Nebula’. In 1796, French mathematician Pierre Laplace, laid the scientific foundations to this theory, which elaborate that at some period in the distant past, a great nebula existed, which extended by a distance as large as the distance between Sun and Pluto. This nebula, which slowly rotated along an axis, gradually lost energy into space. The cooling and shrinking of this nebula increased its speed of rotation centrifuging gaseous masses. As the nebula continued to contract, its speed of rotation increased. This led to several rings of gaseous material to be thrown off, which coalesced to form the planets with the sun at the centre. This hypothesis, though popular in the nineteenth century, was untenable from later research and was therefore abandoned. The prime objection was the fact that the sun actually rotates at a much slower rate compared to the outer planets, which contradicts the hypothesis.

1.2.2 Planetesimal Hypothesis In 1905, a geologist named Thomas C. Chamberlin and an astronomer named Forest R. Moulton, both from the University of Chicago, proposed the planetesimal hypothesis. This hypothesis assumes the existence of sun without planets. Another star, at some point in the past, travelled so close to the sun that the resulting gravitational pull tore off great masses from the sun. These separated masses later cooled and condensed to form ‘planetesimals’. The gravitational pull between these planetesimals led to accretion, cooling and growth into present day planets. Major planets were produced from material from the sun and the minor planets and the asteroids were formed from the parts of the passing star. Small clusters of planetesimals which were located near the planets, formed their natural satellites.

1.2 Origin of Earth

3

Despite being widely accepted for several decades, this hypothesis met several geological and astronomical objections. The current understanding on the structure of Earth indicates that it was initially in a molten state and this is contrary to the postulation by Chamberlin and Moulton that Earth was initially solid. Moreover, some argue that the gravitational pull between planetesimals could have created collision leading to their further size reduction and therefore, the accretion of matter to form larger bodies was not possible.

1.2.3 Tidal or Gaseous Hypothesis In 1918, two British scientists, Sir James Jeans and Sir Harold Jeffreys, proposed the gaseous hypothesis to counteract some of the objections that was raised to the planetesimal hypothesis. Assuming the near-collision interaction between the sun and the passing star actually happened, the material from the sun could have come out in the shape of a long spindle-shaped filament, just as gravitational pull causes tides in oceans (hence the name ‘tidal hypothesis’). This gaseous filament later broke up into initially molten units, which gradually condensed into the solid state forming planets. But later, modern astronomers proved that such a gaseous filament would rather disappear in space, than form solid bodies like planets. Hence, this hypothesis was no longer acceptable.

1.2.4 Protoplanet Hypothesis Several years later, scientists took a closer look at the nebular origin. In light of modern findings, a revised theory, known as the protoplanet hypothesis was first proposed in 1944 by C. F. von Weizsacker. This was later modified by Gerald P. Kuiper. The protoplanet hypothesis suggests that, about 5 billion years ago a great cloud of gas and dust, which was 10 billion kilometres in diameter, rotated slowly in space. With time, this cloud shrank or collapsed, either under the pull of its own gravitation or by the explosion of a passing star. It was found that rapidly rotating nebulas will develop large whirlpools or vortexes at the disk of nebular material while smaller whirlpools are developed inside some of the larger vortexes. These gave rise to spinning discs that became the satellites, or moons, of the planets. The understanding that some of these massive swirl of gas and dust in space are condensing to form new stars, supports this hypothesis. Various astronomers have revealed numerous nebulas between the stars from space observations using large telescopes, which are in resounding support of this hypothesis. The proptoplanet hypothesis, however, fails to explain the origin of the nebula.

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1.2.5 Big-Bang Theory and the Space–Time Singularity Until the turn of the 20th century, space and time were considered as separate entities. Space was perceived to have three-dimensional geometry, described in terms of position, shape, distance and direction. Time on the other hand was one-dimensional. In 1905, Albert Einstein’s special theory of relativity postulated that the speed of light through void space has one definitive constant value, that is independent of the motion of the light source. The important consequences of this were that the distances and times between pairs of events vary, when measured in different inertial frames of reference. This concept of space-time fuses the three dimensions of space and the one dimension of time into a single 4-dimensional continuum. This led to the theory that, when there was no space there existed no time. Scientists believe the entire vastness of the observable universe, including all its matter and radiation, was compressed into a hot, dense mass which just a few millimetres in size. This state is referred to as the space-time singularity. A massive blast allowed all the universe’s known matter and energy to spring out of this spacetime singularity, and the universe continuously expanded with incomprehensible speed from its pebble-size origin to the present day astronomical scope (Fig. 1.1). Expansion was associated with cooling, the rate of which has eventually slowed over billions of years. Post Big Bang, scientists believe that, as time passed and matter cooled, more diverse kinds of atoms began to form, and they eventually condensed to form the stars and galaxies of the universe. Cosmologists agree that this theory matches with what they have observed so far. The theory is based on many different observations, one of which being the redshift of very far away galaxies. Redshift is the Doppler’s Effect from an object, which moves farther away from Earth. It gives a red colour as the away movement stretches the wavelength of light emitted by the object. As red is the longest wavelength in

Fig. 1.1 Big Bang Theory of Space & Time (adapted from NASA JPL)

1.3 Age of Earth

5

the visible spectrum, more the redshift, faster is the object moving away. Measurement of the redshift has led to the understanding that the universe is expanding, and its back calculations have yielded what scientists believe is the space-time singularity, which happenned approximately 13.8 billion years ago. The Big Bang theory doesn’t explain the cause of the big bang itself, to which several hypothesis have been proposed, but none has been equivocally accepted.

1.3 Age of Earth Like the initial hypotheses of its origin, the age of Earth was anybody’s guess. Age of Earth has been mentioned religious scriptures. Surface and internal processes on Earth makes it a constantly changing planet, and its crust is continually and cyclically being created, modified, and destroyed. Rocks, which were formed when Earth was formed from interstellar material, are most likely completely recycled. The doctrine of uniformitarianism, which states that processes which operated in the past are still in operation today, was formalized by Scottish geologist James Hutton. This doctrine, popularly stated as ‘present is the key to the past’, has been a basis for all geological investigations including the dating of Earth.

1.3.1 Early Methods Charles Darwin, in the first edition of his book ‘Origin of Species’, estimated that it took 300 million years to erode the Weald (a chalk deposit in southern England). Keeping in mind that Darwin only addressed the erosion of the chalk deposit and not its deposition, Earth could be much older than 300 million years. Geologic dating was challenged by physicist Lord Kelvin, who used the principles of physics to solve the riddle. Assuming that Earth was a molten ball of fire at time of its origin, and observing that the present Earth is cooler at the surface and hot in its interior, the rate of heat loss to arrive at this stage could indicate its age. Kelvin used estimates of the temperature of the Earth’s core, temperature gradient and thermal conductivity of rocks to put the age of Earth between 20 and 400 million years. T.C. Chamberlain, a distinguished glacial geologist, believed that Earth had formed from gradual accumulation of solid material like asteroids. He argued that Earth had never been a molten sphere and therefore Kelvin’s estimates could not be believed. All these early approaches, which used Earth’s cooling rate, rate of salinity increase in oceans, snow and sediment records and species evolution, all yielded different values for Earth’s age. It was only when radiometric methods of dating came into existance, definitive age calculation of Earth was possible.

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1.3.2 Radioactive Dating Radioactivity was coined by Marie Curie in 1903, but it was physicist Ernest Rutherford who demonstrated its use as a clock for dating old rocks. In 1923, Arthur Holmes was the first to publish the age of Earth as 1.6 billion years in Nature, where he used the Uranium-Lead decay method. Since then, of the various dating methods which have been applied on different rock types, radiometric methods have been the most accurate of all. All the previous age estimates of Earth were discarded after the rock fragments from the Canyon Diablo iron meteorite were radiometrically dated to be 4.56 billion years old.

1.3.2.1

Principles and Methods

All ordinary matter is made up of combinations of chemical elements, each with its own atomic number, indicating the number of protons in the atomic nucleus. Many elements may exist in different isotopic states, with each isotope having different number of neutrons in the nucleus. Nucleus of some isotopes are inherently unstable and undergo radioactive decay eventually transform into a daughter nuclide. This transformation is accomplished either through emission of alpha particles or beta particles or both. When rocks are formed from molten matter, trace radioactive impurities were selectively incorporated at the time. Once the rock is formed, the radioactive impurities start decaying to form daughter elements at a constant rate of decay. In the radiometric method, the proportion of the parent radioactive isotope to the daughter isotope is calculated, and this ratio helps us back-calculate the time, when no daughter isotopes would be present in the rock. Radiometric dating is now the prime method used to date the age of Earth’s materials, which include rocks and fossilized life forms. Its application on dead organic matter has extended its use in forensic science. While the stage of decay of a nucleus is unpredictable, the principle of half-life is used to determine the age. At one half-life, half of the atoms of the radioactive nuclide in question will have decayed to form the “daughter” nuclide. In many cases, the daughter nuclide itself could be radioactive, resulting in a decay chain, eventually culminating with the formation of a stable (non-radioactive) daughter nuclide. Uranium (U234 ) is a radioactive nuclide which decays to Thorium (Th230 ). Half-life of U234 is 245500 years, which means if 100 units of U234 was initially present at the time of mineral formation, then after 245500 years only 50 units shall be remaining, and the ratio of parent Uranium to daughter Thorium shall be 50:50 (Fig. 1.2). Halflife depends solely on nuclear properties and is a constant for a radioactive nuclide. The radioactive decay processes are not affected by external factors, such as temperature, pressure, chemical environment, or presence of a magnetic or electric fields. For most nuclides, the proportion of the original nuclide to its decay products, changes in a predictable way as the original nuclide decays over time. This allows the relative

1.3 Age of Earth

7

Fig. 1.2 Radioactive Dating and Half life

abundances of related nuclides to be used as a clock to measure the time from the incorporation of the original nuclides into a material at present.

1.3.2.2

Dating Rocks, Minerals and Meteorites

Materials from the time of Earth’s formation are highly unlikely to exist today. Of all the rocks dated, the Acasta Gneiss in Canada was found to be 4 billion years old. The toughest mineral found in ancient rocks, which contains radioactive uranium (U) is zircon. Zircon is normally dated using its thorium (Th) and lead (Pb) ratios along with radioactive nuclides of Uranium. The oldest zircon (dated 4.4 billion years) was found in Jack Hills of Western Australia. There could be other rocks and minerals, which could be even older but are yet to be discovered and dated. Most geological samples are unable to give a direct estimate of its time of its formation. This is because Earth has undergone differentiation into the core, mantle, and crust. The on-going processes of plate tectonics may also have adversely impacted the isotopic compositions through partial or even complete removal of various radionuclides. Common Radionuclides used for dating rocks are listed in Table 1.1. Meteorites from space enter Earth’s gravitational field and their remnants are found on the crust. These remnants have been radiometrically dated. Meteorites

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Table 1.1 Common Radionuclides and their half-life periods Parent

Daughter

Half life period (billion years)

Uranium-235

Lead-207

0.7

Uranium-238

Lead-206

4.47

Potassium-40

Argon-40

1.25

Rubidium-87

Strontium-87

48.8

Samarium-147

Neodymium-143

106

Thorium-232

Lead-208

14

Table 1.2 Ages of Meteorites from dating methods Dating methods

St. Severin (Chondrite) (billion years)

Juvinas (Basaltic achrondrite) (billion years)

Allende (Carbonaceous chrondrite) (billion years)

Pb-Pb isochron

4.543 ± 0.019

4.556 ± 0.012

4.553 ± 0.004

Sm-Nd isochron

4.55 ± 0.33

4.56 ± 0.08

–

Rb-Sr isochron

4.51 ± 0.15

4.50 ± 0.07

–

are primitive material and are conceived to have formed at the same time as Earth. Since they have not undergone any internal differentiation through processes of aggregation, melting and recrystallization, their age is likely to be close to the age of Earth. Several meteorites have been dated (Table 1.2) yielding ages around to 4.5 billion years. The oldest one, dated so far, is the Canyon Diablo meteorite (4.65 billion years) found in Canada. Together with indirect geological approaches, radiometric dating methods are used for geochronological dating and establishing the geological time scale. The geological time scale provides a significant source of information on the ages of fossils and the rates of evolutionary change supported by the extension of radiometric dating to date archaeological materials and ancient artefacts.

1.3.3 Geological Time Scale Chronological dating of rocks has led to the development of a timescale that is used by geoscientists to describe the timing and relationships of events that have occurred during its history. The International Commission on Stratigraphy has agreed on a nomenclature of timespans based on which a Geological Time Scale for use (Fig. 1.3). The primary groups are eons, era, period and epoch. Eons are divided into eras, which are, in turn, divided into periods, epochs and ages.

1.4 Earth’s Structure

9

Fig. 1.3 The geological time scale

1.4 Earth’s Structure Scientific understanding of Earth’s internal structure is based on observations. Topography (and bathymetry), rock outcrops, materials from volcanic eruptions, analysis of seismic waves through the Earth, gravitational and magnetic field experiments and experiments with crystalline solids at high pressures and temperatures etc. were some of the methodologies adopted to understand the Earth’s internal structure. These have established that the Earth is layered in spherical shells with an inner solid core at the centre, an outer viscous core, a mantle in the middle and a solid crust at the surface (Fig. 1.4). These spherical shells vary in their chemical and rheological properties.

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Fig. 1.4 Earth’s internal structure

1.4.1 The Crust The crust is bounded by our atmosphere on the outside and an upper mantle underneath it. In 1909, Andrija Mohorovicic, a Croatian researcher working in Zagreb, discovered that the velocity of Earthquake waves abruptly increased at a depth of tens of kilometres beneath the Earth’s surface. This pointed to the existence of a boundary at this depth. Later studies showed that such velocity changes existed most everywhere around the planet, although the depth from where sound was reflected varied from location to location. This depth was more beneath continents and less under oceans. This reflector was considered as the base of the Earth’s crust and was commonly referred to as Mohorovicic discontinuity or simply ‘Moho’. The Moho occurs at depths between 7 to 70 km from the Earth’s surface which makes the crust a thin veneer (given that the Earth’s radius is 6731 km). The average elemental composition of Earth’s crust is provided in Fig. 1.5. The crust exhibits compositional variations at continents and at oceans, and therefore it is differentiated into continental and oceanic crust.

1.4 Earth’s Structure

11

Fig. 1.5 Elemental Composition of Earth’s crust

1.4.1.1

Oceanic Crust

The oceanic crust covers the ocean basins and consists of dark-coloured rocks called basalt. Basalts are rich in magnesium silicate minerals and have a density of 3 g/cm3 which is much higher than the rocks which belong to continents. Differential heating underneath the oceanic crust generates lateral motion leading to its collision with other oceanic or continental plates at a location and separation with another. At points of separation, molten matter from the mantle below moves upward and solidifies, adding material to the solid crust, whereas at points of collision the denser crust is subducted deep into the mantle, resulting in its melting and eventual destruction. This process, in geological time, recycles the oceanic crust constantly, which is why no oceanic crust older than 200 Ma is ever found.

1.4.1.2

Continental Crust

Accounting for 40% of the surface of the Earth, the continental crust is made up of light-coloured granitic rocks, which are rich in aluminosilicates minerals. Rocks of the crust have a density of 2.6 g/cm3 . The continental crust is much thicker as compared to the oceanic crust. The continental crust runs up to 70 km in thickness as compared to mere 5-10 km in case of oceanic crust. Being lighter, the continental crust rests above the oceanic crust and in equilibrium over the viscous mantle.

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1.4.2 The Mantle The mantle starts below the Moho and forms a 2,885 km thick layer, encapsulating the Earth’s core. Researchers have found that Earthquake-wave velocity changes at a depth of 400 km and again at a depth of 660 km in the mantle. Based on this observation, the mantle is subdivided into the upper mantle (down to a depth of 660 km), and the lower mantle (from 660 km down to 2,900 km). The lower and the upper mantle, together form the largest part of Earth by volume. Even though the mantle is solid, its temperature is so high that the rocks become ductile and fluid.

1.4.2.1

Upper Mantle

The upper mantle extends up to 660 km from below the crust with its temperature near the crust ranging from 300 to 870 °C, which increases further at greater depths. The upper mantle, together with the crust is referred to as the ‘lithosphere’ and its thickness is not uniform throughout. Its topmost, thin layer is very similar to the Earth’s crust and its composition grades into peridotite, which is an ultramafic (dark and dense) rock. This makes peridotite, which is rarely ever exposed on the Earth’s surface, the most abundant rock.

1.4.2.2

Lower Mantle

The lower mantle is dense and viscous and ranges from 60 km below the surface, to a depth of 2900 km. Being so deep inside the Earth, the temperature and pressure of the lower mantle are extremely high and can soar to over 3800° centigrade near the core. It experiences pressures which are 1.3 million times of that at the surface. Such high pressures triggers the formation of minerals which are hardly seen in the Earth’s crust.

1.4.3 The Core Early estimations of the density of the Earth’s central part were compared with the density of gold. This lead to the belief that Earth’s core is mostly composed of the precious metal. Eventual studies of the Earth’s magnetic field led to the conclusion that the core consisted of iron and nickel along with other minor elements. Like the mantle, the core can also be divided into two rheological parts, the outer core (between 2,900 km and 5,155 km deep) and the inner core (between 5,155 km down to the Earth’s centre at 6,371 km). The outer core consists of iron alloy, is fluid like the lower mantle and it generates Earth’s magnetic field. The inner core, with a radius

1.5 Earth System & Interactive Processes

13

of about 1,220 km, reaches temperatures above 4,700 °C. Though molten, the inner core is still in a solid state because of high confining pressure. The solid crust and upper mantle are referred to as the lithosphere, which is an important sub-system of Earth as it interacts with the hydrosphere and atmosphere and is home to crustal processes and landforms. Underlying the lithosphere is the viscous asthenosphere, which consists of the lower mantle and outer core. Both material and heat is constantly under circulation in the asthenosphere and this generates convection currents which drive plate tectonics.

1.5 Earth System & Interactive Processes Earth is a complex system of interacting physical, chemical and biological processes, which have been running since its origin. Earth has been divided into various components, which collectively form the Earth system, among which, energy and matter is exchanged through interactive processes.

1.5.1 The Earth System The holistic Earth system comprises of components, which are commonly referred to as “spheres” or “subsystems”. These are the atmosphere, the hydrosphere, the biosphere and the geosphere (Fig. 1.6). The study of the interactive processes embraces chemistry, physics, biology, mathematics and applied sciences, which treat Earth as an integrated system. Such integrated studies evolve a deeper understanding on the past, current and future state of the Earth. Earth system science provides a physical basis for understanding the world we live in, which helps sustain life and related activities.

1.5.2 Interactive Processes Energy and material exchanges between the atmosphere, the hydrosphere, the biosphere and the geosphere, take place through coupled processes, which are commonly known as Earth’s biogeochemical cycles (Table 1.3). The prime ones are the Water cycle, the Carbon cycle and the Rock cycle. The Rock cycle is important for geoscientists as it governs the formation of energy and mineral resources in the Earth’s crust. Petroleum and other fossil fuels are governed by the Carbon cycle, and the Carbon cycle together with Water cycle defines climate and weather systems on the planet.

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Fig. 1.6 The Earth System Table 1.3 Earth System Components and Interactive Processes Atmosphere

Hydrosphere

Biosphere

Atmosphere

Air circulation and wind systems driving weather

Hydrosphere

Heating, Evaporation and water transfer. Surface currents and waves

Ocean upwelling and circulation between cold arctic and hot equatorial waters

Biosphere

Respiration and transpiration. Dispersal of life (spores and pollen by wind)

Removal of Food cycle and dissolved ecosystems materials by shells and diatoms. water for life support

Lithosphere

Stored solar heat and topography impacted air movements

Precipitation, erosion and transportation of sediments to basins

Biophysical weathering and soil formation. Sourcing of mineral nutrients

Lithosphere

Plate tectonics (seafloor spreading, mountain building)

Further Readings

15

1.6 Summary Since its origin, planet Earth has evolved through time, transforming itself to its present form. Earth processes are interactions between its subsystems. Being driven by multiple forces (both external and internal which operate at different periodicity and scales), these processes are difficult to quantify and predict. These processes have, not only shaped our Earth over millions of years, but also have influenced the way natural resources (like minerals and petroleum) are distributed across the globe. Geoscience, being an applied and integrative science, plays an important role in understanding them through evidences preserved in our rock records. Having a thorough understanding of geoscience is useful in exploration of minerals and petroleum resources, which are both critical to sustaining modern ways of life.

Further Readings 1. Skinner, B. J., & Murck, B. W. (2011). The blue planet: An introduction to Earth system science (3rd ed.). Wiley. 2. Monroe, J. S., & Wicander, R. (1997). Physical geology: Exploring the Earth (3rd ed.). West/Wadsworth.

Chapter 2

Crustal Processes and Materials

2.1 Introduction In the previous chapter, we discussed Earth’s age, structure and its interacting sub-systems. Spectroscopic studies of materials from outer space indicate that, when Earth was formed, almost all the elements we know as of today, existed in varying relative abundance. As these stellar materials aggregated to form Earth, the elements coalesced and later combined under pressure and temperature to form various compounds. Although it is difficult to reconstruct all those reactions and processes which triggered them, fundamental principles of thermodynamics can explain several chemical combinations and decompositions behind their formation. The densest compounds sank and concentrated at Earth’s core. The moderately dense materials remained in the mantle and lighter ones formed the solid crust (oceanic and continental). Relatively denser compounds like olivine and pyroxenes (which are rich in oxygen (O), silica (SiO4 ), magnesium (Mg), iron (Fe), aluminium (Al), calcium (Ca) and other trace elements) settled at the mantle. Lighter materials, which included feldspars and quartz, formed the crust. These solid compounds, which are formed by natural processes, are called minerals. Minerals are the building blocks of rocks, which are essentially inorganic solids having a specific internal structure and a definitive chemical composition. Earth’s surface and crustal processes are responsible for the formation of minerals from mantle material and for their recycling through volcanism, evaporation, and tectonism.

2.1.1 Crustal Processes The rocks found on Earth’s crust were dated to be of different ages (from over 4 billion years to a few years old). As material can neither be created nor destroyed, this wide age range can be explained by the fact that older rocks undergo some © Springer Nature Singapore Pte Ltd. 2023 S. N. Kundu, Geoscience for Petroleum Engineers, https://doi.org/10.1007/978-981-19-7640-7_2

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transformation to form newer rocks. The processes on Earth, like tectonism and precipitation, constantly recycle materials in Earth’s crust. Precipitation generates runoff, which weathers and erodes existing rocks. Erosion removes and transports the weathered material to depositional environments like ocean basins, where these sediments accumulate and lithify forming sedimentary rocks. Addition of subsequent layers of later sediments bury the older sediments deeper into the Earth’s crust. As they go deeper, the temperature and pressure increases, and some minerals recrystallize giving rise to metamorphic rocks. When metamorphic rocks are subducted further down to the top of asthenosphere, it forms molten rock or Magma. Magma, being mobile and buoyant, travels upward to the surface through cracks and fissures in the solid crust forming volcanoes. On its way up the volcanic pipe, magma gradually cools, and several minerals crystallise forming intrusive igneous rocks. The residual magma that reaches the surface, forms extrusive igneous rocks. This essentially is a summary of the rock cycle, which is an on-going process ever since the Earth was formed. Crustal processes are driven by mantle convections and this continuously churns the crustal materials transforming the landscape and composition of the lithosphere. Heat flow in the mantle exerts differential pressure beneath the crust, impacting the shape and volume of crustal matter due to ductile and brittle deformation.

2.1.1.1

Plate Tectonics

Plate tectonics theory states that the Earth’s lithosphere is divided into several rigid fragments or plates which float over the dense and viscous mantle. Plate tectonics evolved from the theory of ‘Continental Drift’, which was first proposed by Alfred Wegener in 1912. Plate tectonics theory explains the mechanism of movement of plates on Earth, which the Continental Drift theory failed to explain. It explains the origin of features, which range from the deepest of ocean trenches, to the tallest of mountains.

2.1.1.2

Mantle Convection

The driving force behind plate tectonics is mantle convection (Fig. 2.1). Hot matter near the Earth’s lower mantle rises, and colder molten rock under the lithosphere sinks. This circulates the dense and viscous materials of the mantle in eddies, which then drives the movement of plates. Lithospheric plates spread both towards and away from each other. For example, at mid-ocean ridges, two plates move further away from each other. The away movement of plates creates a gap allowing mantle material to move up and solidify forming crustal rocks. On the other hand, when an oceanic plate and continental plate collide, the heavier one subducts at oceanic trenches. The deeper portions of a sinking plate eventually melts and part of it is accommodated by the molten mantle and the other part finds its way upwards to form volcano chains like the “Ring of Fire” that surrounds the Pacific Ocean.

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Fig. 2.1 Mantle Convection and Plate Tectonics

2.1.1.3

Plate Types

Lithospheric plates can be of two primary types: oceanic plates or continental plates. The prime differentiators of these plate types are in their formative process, composition, density, age, extent and thickness (Table 2.1). Oceanic plates are formed at divergent plate boundaries, where mid-ocean ridges represent areas of upwelling magma. Continental plates, on the other hand, collide at Table 2.1 Oceanic and Continental plates Oceanic plates Formative process Formed by plate divergence at mid oceanic ridges

Continental plates Formed by plate convergence and subduction processes

Composition

Composed of coarse-grained mafic rocks like Gabbro rich in iron, magnesium and calcium

Composed of felsic rocks like granite abundant in silica, aluminium, sodium and potassium

Density

Being rich in Iron and Magnesium, oceanic plates are denser

Abundant aluminium, sodium and potassium, make continental plates less dense

Age

Oceanic plates are relatively younger as being heavier, they tend to originate at mid oceanic ridges and merge with magma at subduction zones. Oldest oceanic crust is about 200million years old

Being less dense continental plates do not subduct and therefore are rarely destroyed. Most continental places are more than a billion years old in age with oldest being little over 4 billion years, closer to the time of Earth’s origin

Extent, thickness

Oceanic plates cover about 70% of Earth’s crust and average under 10 kms in thickness

Continental plates consist of 30% of the Earth’s crust and have an average thickness of 40 km or more

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convergent plate boundaries, giving rise to mountain ranges. Where an oceanic plate collides with a continental plate, the oceanic plate being denser plunges underneath to form an oceanic trench. As a plate subducts deep, a part of it melts to form magma. This magma finds it way through the cracks and cools over time to produce intrusive igneous rock.

2.1.1.4

Plates Boundaries

The margins of plates (which are mid-oceanic ridges and oceanic trenches) are mostly associated with active earthquakes or detectable seismicity. Modern global positioning system (GPS) and global network of seismograms have led to the identification of most plate boundaries. Based on the types of motion of plates in relation to each other, three types of plate margins are possible, namely convergent, divergent, and transform margins. At convergent margins, the higher density of a cooler lithospheric plate above a hot, plastic asthenosphere, leads to the subduction of the plate into deeper regions of the mantle. As the plate sinks into the asthenosphere, it gradually warms, losing rigidity and eventually becoming a part of the asthenosphere. This down-draft part of the convection cycle at the ocean basins is marked by deep trenches in the sea floor. As plates can be composed of oceanic crust or continental crust, three types of tectonic settings are possible for convergent margins. 1. Oceanic-oceanic convergence: Collision of two oceanic crusts with the heavier one subducted underneath the other at an oceanic trench. Examples are the oceanic trenches at the east of Japan where oceanic lithosphere and crust of the Western Pacific region is being subducted beneath the plate that carries the Asian continent. Oceanic trenches are the deepest part of the oceans where depths can be more than 10 km. 2. Oceanic-continental convergence: There are convergent plate margins where oceanic lithosphere, being heavier, is subducted beneath continental lithosphere. The trench along the western coast of South America where oceanic lithosphere of the Pacific is being subducted eastward beneath the South American continental plate. The subducted parts of the oceanic plate generate magma which rises up into the continental crust producing a chain of volcanoes. A greater portion of the molten magma solidifies within the continental crust forming plutons which isostatically compensate forming mountain chains like the Andes. 3. Continent-continent convergence: Collision of continental plates result in formation of mountain chains due to crustal thickening. Because of its relatively low density, continental crust does not generally subduct to form magma or vent up as volcanic chains. The Himalayas are formed as a result of the collision of the Indian plate and the Chinese plate some 40–50 Ma. Divergent margins occur along spreading centres where plates are moving apart due to mantle convection and new crust is created by magma pushing up from the mantle. The best example of a divergent margin is the Mid-Atlantic Ridge which is a

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submerged mountain range, extending from the Arctic Ocean to beyond the southern tip of Africa. The rate of spreading margins is about 2.5 cm per year, which is slow by human standards, but at geological time scales such small rates have resulted in plate movement involving thousands of kilometres. Over millions of years, the newly added crust at divergent margins, spreads away and arrives at a convergent margin and sinks beneath lighter crust to eventually melt into the mantle. Transform margins are where two plates slide horizontally past one another. Most transform margins are associated with spreading centres and originate because of differential rates of spreading at mid oceanic ridges. Transform boundaries commonly offset the active spreading ridges and generate shallow earthquakes and produce zigzag plate margins.

2.1.1.5

Wilson’s Cycle

Abundant paleomagnetic data from rocks of the oceanic crust, from spreading centres to subduction zones, have aided the reconstruction of past plate movements. Such reconstructions reveal that all the continental masses were once together as a supercontinent of Earth. Plate tectonics resulted in the disintegration of this supercontinent, dispersing parts to their present-day configuration. Canadian scientist Tuzo Wilson, in 1965, proposed that an ocean basin starts at the divergent spreading centre and undergoes several stages before its subduction at a convergent margin. These stages, later, came to be known as the Wilson Cycle (Fig. 2.2). Because of Wilson Cycle, we do not find any oceanic crust older than 200 million years. In the Wilson cycle, an undivided plate rifts due to mantle convection currents. The rifted parts diverge giving birth to an ocean, which expands due to continued divergence of the rifting plates. Earth being spherical, the oceanic plates eventually meet another oceanic or continental plate at a convergent margin. At this convergent margin, the denser plate subducts under the lighter one. With continued convergence, the balance shifts and the diverging margin begin to converge and the ocean gradually starts to shrink. When one lithospheric plate subducts underneath another, the crust thickens and forms mountains. Over time, the mountains are eroded and peneplained to form a single landmass. The Red Sea represents an early ocean that has resulted from the rifting of the African plate and the Arabian plate. Oceanic-continental collision and subduction is best represented by the Peru-Chile trench, where the Nazca oceanic plate is sinking underneath the South American continental plate. This convergence has resulted in thickening of the land mass, creating the Andes volcanic range. Closing of an ocean and subsequent convergence two continental plates leads to mountain building just like the Himalayas. The ancient sea, called Tethys, once existed where currently Himalayas stand tall.

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Fig. 2.2 The Wilson Cycle

2.1.1.6

Plate Reconstruction

Historic location of tectonic plates can be derived from remnant magnetism exhibited by rocks of oceanic crust. Rocks of the oceanic crust exhibit geomagnetic reversals which can be linked with the geochronology of the Earth’s magnetic field. Other evidences, like genetic similarity of animals and their evolutionary graphs, indicate that the continents were once one across which these animals thrived and migrated freely. The striking similarity of the sequences of rocks found at opposite shores of present-day continents indicative that they were once together, which allowed continuous deposition of rocks under one climatic zone and depositional environments. The position of continents through geological periods is presented in Fig. 2.3 where the geometry of eastern coastline of South America strikingly matches with the coastline of West Africa like a jigsaw puzzle. This, in itself, is a visible evidence that these continents were once conjoined, and this is validated by the striking similarity of rocks records at each shore. Based on studies of ancient magmas and minerals preserved in rocks, most researchers perceive that plate tectonics began about 3 billion years ago. As per the Wilson cycle, continents keep drifting apart and at times come together to form a single landmass or supercontinent. About 200 Ma ago a supercontinent called

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Fig. 2.3 Continental plates through geologic time

Pangaea existed, which was a single landmass on Earth, from which the presentday continents have originated. Reconstruction of its shape, where Africa, South America, North America and Europe are seen fitting together, is based on characteristic pattern of fossils and rocks found at different parts of present-day continents. Some 200 million years ago, Pangea broke into Laurasia to the north and Gondwanaland to the south. About 135 million years ago, Laurasia broke into North America, Alaska & Canada to the west and Eurasia in the east. Gondwanaland later broke to form South America, Africa, India, Antarctica and Australia-New Zealand.

2.1.2 Isostasy Isostasy is the understanding that the lighter lithosphere must be floating on the denser asthenosphere which is used to explain the varying topographies on Earth’s surface. It is conceived that the lithosphere and the asthenosphere are in isostatic equilibrium, which reacts to perturbations caused by plate tectonics, weathering and erosion and climate change. Understanding the dynamics of isostasy helps explain

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Fig. 2.4 Models for Isostasy

the complex tectonic events which result in formation of mountains, ocean basins and continental drift.

2.1.2.1

Airy’s and Pratt’s Hypothesis

In the mid-19th century, two prime theories were proposed on the existence of high mountainous terrains and low-lying plains (Fig. 2.4). Pratt hypothesized that the densities of rocks in the lithosphere vary laterally but terminate at a common depth above the uniformly dense asthenosphere. To maintain equilibrium, it is needed that rocks under higher topography, i.e. mountains should be less dense and the converse applied to lithosphere at lower heights. A striking example is the density of rocks in the Himalayas, which is much lower that the rocks of the low plains south of it. Airy’s theory proposes uniform density of the lithosphere but postulates that crustal blocks must have different thickness to be under isostatic equilibrium. This essentially indicates that higher mountains have deeper roots into the asthenosphere. Both the theories are supported by evidence, albeit at different parts of the world. Airy’s theory provides a better explanation of continental mountains, whereas Pratt’s theory essentially explains the difference between continents and oceans, as compositionally continental crust is granitic and of less density than the basaltic oceanic rocks.

2.1.2.2

Implications of Isostasy

Isostasy is based on the laws of buoyancy and explains the geodynamic processes observed today and the present day landforms. Ice ages led to the formation of ice sheets over land, which added mass to the column thereby causing it to sink

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isostatically. The opposite happened in between glacial phases, when this ice melted and caused the land to isostatically rebound upward. These events are evidenced by geological features, which include former sea cliffs associated with wave cut platforms, that are found hundreds of meters above present day sea level. Melting of ice sheets and erosion of hills both cause uplift and rebound of the lithosphere. Likewise, temperature changes in the upper mantle can induce density changes in the lower crust which can drive partial detachments due to melting. This could result in the rise of the asthenosphere and cause uplift forming mountains asin the case of the late Cenozoic uplift of the Sierra Nevada in California. Such is evidenced by seismic data, which defines and finds the Moho to be asymmetric. Isostasy can drive continental drift and plate tectonics can result in isostatic compensation. Both these processes contribute to the geodynamic processes on Earth.

2.2 Crustal Materials Materials of the Earth’s crust are rocks. Rocks are composed of minerals which are defined as a naturally occurring, crystalline solid of definite chemical composition which have a characteristic form. This makes rock a naturally occurring coherent aggregate of one or more minerals. Rock forming minerals are chemical compounds resulting from crustal processes and their distribution depends on the different temperature, pressure and climatic zones on Earth’s surface and within the lithosphere.

2.2.1 Minerals Minerals are compounds of elements that occur naturally. Although our Earth is made up of thousands of different minerals, but only a few of them are rock forming and the rest are rarely present in quantities large enough to form rocks.

2.2.1.1

Mineral Properties

Some of the most obvious physical properties of a mineral are hardness (resistance to scratching), tenacity (resistance to impact), specific gravity (relative density), cleavage (the tendency of crystals to split along weak planes of atomic bonding), magnetism, electrical conductivity, reactivity with acids etc. These physical properties, in combination to their optical properties like luster (shine), refractive index and selective absorption of light rays, enable their identification using scientific instruments like microscopes. Hardness is typical of minerals and is used to identify them in hand specimens, as a harder mineral can always scratch a softer mineral. Hardness can be tested using

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Fig. 2.5 Comparison of mineral hardness scales

Vickers method, which consists of indenting the mineral sample with a diamond indenter (a right pyramid with a square base) with a known load for 10–15 s. A more practical approach was developed by Frederich Mohs, where 10 minerals of known hardness were identified with a hardness index ranging from 1 to 10. Mohs hardness is comparable to Vicker’s scale and the relationship is presented in Figure 2.5.

2.2.1.2

Mineral Classification

Minerals are classified based on their composition. Silicates are those minerals that have silicon as a component, while non-silicates do not have silicon. Silicates form more than 90% of the Earth’s crust and each silicate consists of oxygen and silicon tetrahedral anion along with few metal cations. Silicates are further classified based on the arrangement of the silicate tetrahedron (Fig. 2.6). Non-silicates include the likes of native elements (like gold & silver), oxides (like hematite), carbonates (like calcite), halides (like common salt) and others.

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Fig. 2.6 Classification of Silicates

2.2.1.3

Bowen’s Reaction Series

Experimental studies, into the order of crystallization of the common silicate minerals from a magma in early 20th century, led to the understanding that specific minerals form at specific temperatures as the magma cools. Silicates of relatively heavier elements start crystallizing at temperature of 1400 °C and the crystallization progresses into two branches with decrease in temperature (Fig. 2.7). The first branch explains the formation of mafic (consisting of heavier elements) minerals like olivine, pyroxene, amphibole, and biotite. Olivine is first formed and as the temperature gradually drops, the crystallized olivine would react with the residual magma to form pyroxene, which is why it is called the discontinuous series. Olivine shall be preserved if there is no magma left after its crystallization. The continuous branch, on the other hand, crystallizes plagioclase (dark colored) feldspars, composition of which ranges from calcium-rich at higher temperatures to sodium-rich at intermediate temperatures. At lower temperatures, felsic (consisting of less heavy elements) minerals like orthoclase and muscovite crystallize, followed by quartz. The order of crystallization is known as Bowen’s reaction series which supports the compositional segregation of Earth’s crust. The deeper crust is rich in mafic minerals and the shallower crust is more felsic. This series also defines the susceptibility to chemical weathering, which makes quartz the most resistant mineral and olivine and plagioclase to be the least resistant ones.

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Fig. 2.7 Bowen’s reaction series

2.2.2 Rocks The Earth’s solid outer layer, which is the crust, is composed of rocks. Rocks are natural substances which are essentially solid aggregates of one or more minerals. Based on the mode of formation and other properties, rocks can be grouped into three major types, namely igneous, sedimentary, and metamorphic (Fig. 2.8).

Fig. 2.8 Classification of Rocks

2.2 Crustal Materials

2.2.2.1

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Igneous Rocks

The cooling of magma results in crystallization of minerals which constitute igneous rocks. The minerals and rocks formed are governed by Bowen’s reaction series, the initial composition of the magma and its rate of cooling. Igneous rocks are either extrusive or intrusive. The former is formed by volcanism, where the molten magma rapidly crystallizes in contact with atmosphere (and at times with water). Intrusive rocks are formed when magma gradually cools and the constituent minerals crystallize in the subsurface without being exposed to the atmosphere. The rate of cooling governs the size of the mineral crystals in igneous rocks. Rapid cooling forms glassy minerals like obsidian, whereas slow rates of cooling allow mineral crystals to grow larger like feldspars and biotite in granites. Based on chemistry, igneous rocks can be either acidic or basic. Acidic rocks are normally light colored (leucocratic) like granites. Basic rocks are mostly dark colored (e.g. basalt). There are rocks like andesite, which are of intermediate composition.

2.2.2.2

Sedimentary Rocks

Sedimentary rocks from out of sediments, which are products of weathering and erosion. Sediment particles are transported far away from where they are produced and are deposited under water in favorable environmental conditions where they eventually are lithified into rocks. Sedimentation is the collective name for processes that cause detritus to settle and is broadly physical and chemical in nature. Sedimentary rocks are deposited in large depressions called sedimentary basins. 75% of Earth’s surface is covered with sedimentary rocks. Examples of common sedimentary rocks are sandstones, shale and limestone. The texture, sedimentary structures and composition of the sediments in the sedimentary rocks are used to classify sedimentary rocks, details of which are covered in Chap. 3.

2.2.2.3

Metamorphic Rocks

An existing rock, igneous, sedimentary or even metamorphic, transforms itself under the influence of elevated temperature and pressure. As rocks are buried deeper into the subsurface, the increase in temperature and pressure trigger chemical reactions which form new minerals and also cause recrystallization of existing minerals. An example of the former is the metamorphosis of mudstone and shale to form garnet schist. Recrystallization of existing minerals transform one mineral form into another e.g. limestone into marble. Metamorphism can be of two types, regional and contact metamorphism. Regional metamorphism occurs over large areas during periods of mountain building which is primarily driven by pressure. Contact metamorphism occurs when pre-existing rocks (often called country rock) meets molten magma (magma) and is primarily driven by high temperatures.

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Metamorphic rocks are classified based on whether they are foliated or nonfoliated. Foliated metamorphic rocks such as gneiss, phyllite, schist, and slate have a layered or banded appearance that is produced by exposure to heat and directed pressure. Non-foliated metamorphic rocks, like marble and do not exhibit any layered or banded appearance.

2.2.3 The Rock Cycle Igneous rock forms when molten matter from the mantle cools and crystallizes. Depending on the original composition of this melt, and the location where it solidifies, the rock assumes different mineralogy and physical properties. Igneous rock formed in the confines of the crust is called plutonic igneous rocks, and those which form at the surface by volcanic processes are called volcanic rock. Once igneous rocks are exposed weathering agents, they are broken down and transported as sediments, which eventually settle and lithify to form sedimentary rocks. Sedimentary rocks are laid one above the other in a stratified manner, with the layer laid first at the bottom. With time, sedimentary rocks are buried deeper exposing them to high temperature and pressure. This is because Earth has a natural geothermal gradient that averages about 35 0 C/km of depth. Buried sedimentary rocks transform into metamorphic rock as high pressure and temperature form new minerals and impart their preferential orientation in the metamorphosed rock. Igneous rock, too, when buried deeper transform into metamorphic rocks. Deeper burial towards the mantle, melts the rock completely, and it forms a part of the mantle matter. Thus, rocks transform from one type to another and processes are mapped in the Rock Cycle (Fig. 2.9). Rock cycle is one of the oldest processes, which still continues unabated, ever since the Earth was differentiated into crust, mantle and core. The Rock cycle is driven by the Wilson Cycle.

2.3 Summary The diversity of minerals and rocks in the Earth’s crust is because of crustal processes. These processes are slow and are driven by mantle convections and by weathering and erosional processes. Enrichment of minerals take place at tectonic boundaries of crustal plates. The topography of Earth is shaped by crustal processes, as the crustal matter of varying densities and thickness hang in balance isostatically over the mantle. Subduction zones are destructive boundaries, where crustal matter in recycled. Collision of continental plates results in crustal thickening and mountain building (or Orogeny), which has resulted in the formation of present-day mountain ranges like the Himalayas. The shape and extent of sedimentary basins, which are low lying areas, which provide accommodation space for sediments to accumulate, is also governed by crustal tectonic processes. Crustal processes, therefore,

Further Readings

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Fig. 2.9 The Rock Cycle

are of high significance as they also influence the formation of hydrocarbon and mineral resources.

Further Readings 1. Lutgens, F. K., & Tarbuck, E. J. (2011). Essentials of geology (11th ed.). Pearson Education Inc. 2. Skinner, B. J., & Murck, B. W. (2011). The blue planet: An introduction to Earth system science (3rd ed.). Wiley. 3. Monroe, J. S., & Wicander, R. (1997). Physical geology: Exploring the Earth (3rd ed.). West/Wadsworth.

Chapter 3

Sedimentary Basins

3.1 Introduction A sedimentary basin is a depression on the Earth’s crust that provides an accommodation space for sediments. A sedimentary basin may not be a depression at present, but in its geological past, it was a low depression, that accommodated sediments from the surrounding regions. Therefore, a sedimentary basin refers to an area where sedimentary rocks are found (Fig. 3.1). Sedimentary basins range in size, from as small as hundreds of meters, to as large as oceans. Each sedimentary basin has a distinctive tectonic and sedimentation history. The rock successions in a sedimentary basin provide an insight into its evolution, specifically with respect to the variations in depositional environments, sediment supply in space and time, paleoclimates and relative changes in sea level.

3.2 Basin Controls Formation of a sedimentary basin and its evolution over time is controlled by major Earth processes. A set of processes, collectively known as geodynamics, includes plate tectonics and isostatic compensation and is a prime driver of sedimentation and evolution of sedimentary basins. The other driver is climatic which is mostly linked to relative sea level changes.

3.2.1 Geodynamics Plate tectonics is the predominant factor responsible for the plate motions and transformations on Earth’s crust. Plate margins, therefore, are always associated with

© Springer Nature Singapore Pte Ltd. 2023 S. N. Kundu, Geoscience for Petroleum Engineers, https://doi.org/10.1007/978-981-19-7640-7_3

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Fig. 3.1 Sedimentary Basins of the World

different types of sedimentary basins. Geodynamics is a collective term for continental drift and plate tectonics models, which can be defined mathematically though scaled experiments. The solid-state convention of the mantle is normally explained by rheological models.

3.2.2 Climatic Factors Classification of strata into units is based on their physico-chemical characteristics, that is mappable from field observation. Such classification has resulted in definition of several lithostratigraphic units, which were all formed in varying climates and depositional environments. Climate change is a cyclic process, and our Earth has witnessed several ice ages and hot periods. Climate change, through ice ages, load the Earth’s crust causing depressions in stable continental mass generating differential isostatic compensation. When the ice forms, it load the crust enforcing isostatic compensation. When it melts, the underlying landmass is left with a gentle depression, and this forms a continental sag basin. Climate change too impacts surface processes causing different depositional environments in the same region which influences the physico-chemical characteristics of the sediments in the basin.

3.3 Basin Classification

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3.3 Basin Classification Sedimentary basins form by several mechanisms in different parts of Earth. Based on their relationship with plate margins, they can be classified as . . . . .

Continental rift basins Flexure basins Strike-slip & wrench basins Intra-cratonic basins Hybrid basins.

3.3.1 Continental Rift Basins Rifting is a process where a continental landmass breaks into two, giving birth to a lake or a nascent ocean basin, in between. The process is entirely driven by mantle convection currents, which impart extensional stress underneath the crust. Rifting has several stages (Fig. 3.2). Rifts exhibit a central linear faulted depression, forming down-thrown blocks with uplifted flanks at sides. The axis of rifting is perpendicular to the direction of extensional stress. The down-faulted blocks are called grabens and the up-faulted ones are called horsts. A flank of a graben separated far apart by rifting is referred to as a half-graben. The flank of a rift, which is relatively uplifted, undergoes weathering and the sediments are eroded, transported and later deposited in the basin. Rift basins, when located above the sea level, accommodate a water body forming a lake. When they are large and situated below the sea level, it assumes the form of a sea or an ocean.

3.3.1.1

Crustal Thinning & Brittle Fracture

Stretching of the crust due to extensional forces causes the crust to thin. This results in necking of the viscous mantle underneath the crust and forms a depression above it. At this stage, the rocks are still ductile, as the stretching is within elastic limits.

3.3.1.2

Faulting and Formation of Half Grabens

Stretching beyond the elastic limit leads to brittle fracturing and the formation of near vertical cracks in the rock. Crustal blocks across each crack or fracture plane get displaced relatively to form a series of normal faults. The faults form nested, conjugate pairs and at each bank the terraced structures are referred to as half-grabens. The depression encloses a water body which is a lake or a sea. This lake or sea grows larger and deeper as the extensional forces continue to increase the separation between the two rifted crustal blocks.

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Fig. 3.2 Stages of Continental Rifting

3.3.1.3

Ocean Basin

Mantle matters condense and consolidate to occupy the space created by the separating crustal blocks. The solidified mantle matter is formed the base of the rift basin, on which the sediments from the uplifted surroundings start depositing in layers. Rifting is the most common mechanism of basin formation. Rifting produces extensive basins (~70 km wide) and can go much deeper than 10 km. Some examples of present-day rift basins are the Red Sea Rift and the East Africa Rift.

3.3.2 Flexure Basins At convergent plate margins, where two plates collide, the denser plate subducts under the lighter plate (Fig. 3.3). In case of the collision between an oceanic plate and continental plate, crustal matter accretes at the point of collision thickening the landmass. The subducting plate melts as it gets deeper, and the resulting magma rises upward forming a volcanic arc.

3.3 Basin Classification

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Fig. 3.3 Geological Setting of Flexure Basins

3.3.2.1

Fore-Arc Basins

The depression between the accretion and the volcanic arc is the fore-arc basin. The sediments deposited in the fore-arc basin are sourced from the highlands of the volcanic arc. Therefore, they are likely to be composed of weathered material and inter-bedded with volcanic tuff. Subsidence in fore-arc basins is primarily because of stratigraphic loading rather than from tectonic forces.

3.3.2.2

Trenches

The depression between the accretion and the ocean where both plates meet is a trench basin. Trenches are likely to be filled with sediments from adjacent highlands and are less likely to contain volcanic material. Being deep with steep slopes, trenches mostly consist of sediments transported by slope processes. Subsidence in trenches is primarily due to tectonic factors.

3.3.2.3

Back-Arc Basins

Behind the volcanic arc is a depression, which is the back-arc basin. The stress setting at this part may not necessarily be compressional. The principal sediments in backarc basins are sourced from the volcanic arc and its underlying rocks. Therefore, such basins are usually underfilled and huge successions of rocks are unlikely to form.

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Fig. 3.4 Plan view of Strike-Slip Basins

3.3.3 Strike-Slip and Wrench Basins Horizontal shear stresses result in the formation of strike-slip faults. If the fault trace is bent (non-linear), then it results in a deep basin (Fig. 3.4). True strike-slip motion is rare and in most cases, it involves rotational motion between the faulted blocks. When rotational motion is involved, the strike-slip basin is referred to as a ‘wrench basin’. Strike-slip basins can be of many different shapes, but in general they have smaller aerial extent although they tend to be very deep. Strike-slip basins range in size from small depressions to large ones as wide as 50 km. They are normally elongated in shape. The process of basin formation results in high subsidence rates which is also evident from steep thermal history of the sediments in such basins. Sediments are mostly sourced from the crustal blocks surrounding the basin. A good example of a strike-slip basin is the San Andreas transform fault of California, USA. Qaidam basin of China is a great example of a wrench basin.

3.3.4 Intra-Cratonic Basins Basins, which are formed within cratons or stable lithosphere, and are situated far from plate boundaries, are called intra-cratonic basins. Formation of such basins can be explained by several mechanisms. During glaciation, large ice sheets are formed which isostatically load the lithosphere. Isostatic compensation causes the loaded part of the lithosphere to sag giving rise to a shallow depression (Fig. 3.5). As ice melts at the end of glacial cycle, a lake is formed at the depression, which accommodates sediments from the surrounding highlands. Mantle convection currents, under a stable craton, cause thickening and downwelling of the ductile lithosphere (Fig. 3.5). Thermal changes in the lithosphere,

3.4 Basin Analysis

39

Fig. 3.5 Mechanisms of Intra-cratonic Basins

i.e. heating and subsequent cooling, can cause sagging of the lithosphere to form intra-cratonic basins. Uneven heating under the lithosphere can develop a shallow but extensive intracratonic basin. The lithosphere, at normal phase, bulges due to temperature rise from the heat transferred from the underlying mantle (Fig. 3.5). The bulge exposes part of the lithosphere to erosion and consequently the convex top is peneplained. In the ensuing cooling stage, the lithosphere contracts leaving behind a depression, resulting in an intra-cratonic basin. Intra-cratonic basins accumulate deposits in a shallow marine or lacustrine environment and often have significant sedimentation gaps (or hiatus) between consecutive rock sequences. As tectonic margins are not involved, thermal changes in the basin are gradual due to low subsidence rates. Examples of intra-cratonic basins are the Congo basin in Africa and the Amazon basin in Brazil.

3.4 Basin Analysis Succession of sedimentary rocks in a sedimentary basin can be studied to analyse the changes in depositional environments. Such changes can further be correlated at the local, regional or global context. Depositional facies and their spatial extents are impacted by sediment supply and sea level change, which in turn depend on

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3 Sedimentary Basins

tectonic settings of the sedimentary basin. Basin analysis is an aspect of geology, which looks into the sedimentological, stratigraphic and, structural evolution from the geologic records in a sedimentary basin. Basin analysis can be extended further into basin modeling, which is a critical tool for prospecting petroleum and other minerals. Basin analysis can be done using very basic information gathered from field observations or it can be very sophisticated involving advanced geophysical surveys. Basin modeling essentially contributes to the complete understanding of a sedimentary basin and provides insights into all the past geological processes through paleo-environmental reconstructions.

3.4.1 Sedimentological Analysis Sedimentological analysis primarily involves in identifying the sediment source (or provenance), the paleo-environment of deposition and their changes through geologic time in a sedimentary basin. Provenance is indicative of the setting of the sedimentary basin. For example, rift basins are more likely to contain sediments arising from the uplifted half-graben margins and emplaced plutons at the rift. Clasts in sedimentary rocks, their shape, size and mineralogy, serve as indicators of their provenance.

3.4.2 Stratigraphic Analysis Relative dating of sequences in between unconformities can establish a time framework of the successions. Once they are established, the rock successions in a sedimentary basin can then be grouped into different facies as per their sedimentological characteristics. Thickness of strata, which is indicative of the rate of sedimentation, is usually more in rift basins than in intra-cratonic basins. A high resolution stratigraphic analysis, especially with the support of sequence stratigraphic principles, can provide insights into detail scale sea level changes.

3.4.3 Structural Analysis Tectonics forces continue unabated after the formation of sedimentary basins impacting the sediment supply, burial rates and inversion of strata. Patterns of rock deformation and tilting can provide information on the changing stress regimes within the basin. Faults, folds and cross-cutting intrusive bodies can help correlate the timing of structural events in relation to sedimentation. Sedimentary successions can be classified as syn-tectonic, pre-tectonic and post-tectonic in a sedimentary basin based on tectonic relationships (Fig. 3.6). Post-tectonic sediments are undisturbed

3.4 Basin Analysis

41

Fig. 3.6 Sedimentation and Tectonics

whereas syn-tectonic sediments are wedge shaped owing to tilting of the basin base. Pre-tectonic sediments are tectonically disturbed from faulting or rifting episodes.

3.4.4 Geohistory Deposition of sediments results in loading and subsidence in the basin. Subsidence buries the sediments deeper to higher temperatures and pressures. Increase in temperature and pressure triggers diagenetic processes in the sediments. Later deposits provide the much needed lithostratigraphic pressure on earlier sediments for their compaction and lithification. It is therefore important to understand the geohistory of the sedimentary basin and this is done by quantitatively reconstructing the burial and by decompacting the sedimentary layers (Fig. 3.7). In doing so, insights on the thermal history of the sedimentary basin are gained, and this has implications on maturity of source rocks for petroleum generation, migration and accumulation. Geohistory analysis is complex as it requires accounting of several aspects like the palaeo-bathymetry of the basin and compaction history of the sediments. Palaeo-bathymetry is estimated from the facies of the rock, whereas compaction rates are based on lithology of the sediments. Burial history of strata in a succession can be determined from indirect evidences like vitrinite-reflectance, sporepollen coloration, and fission-track studies. Vitrinite, a maceral found in organic rich rocks, change their reflectivity based on the temperature to which it was buried to. Spore and pollens are fossilized in sediments and these change their coloration with increase in temperature. Radioactive elements in minerals emit sub-atomic particles which leave scars in the mineral containing the element. These scars, called fission tracks, vanish when the mineral approaches its recrystallization temperatures. Hence, multiple generations of fission tracks, depicting multiple episodes of temperature rise and fall in the sedimentary basin, can be recorded. Geohistory analysis is fundamental

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3 Sedimentary Basins

Fig. 3.7 Deconstructing Deposition, Burial and Compaction

to petroleum exploration as it provides critical information for establishing a suitable petroleum system. Source rock maturation and reservoir rock properties can also be estimated through geohistory and burial analysis.

3.5 Summary As petroleum is found in sedimentary basins, the rock records are key to the identification of petroleum systems. The controls of sedimentation in the basin, its relation to tectonic settings and its evolution through geologic time is crucial in determining the exact stage of maturation of hydrocarbon in the source rock. Basin analysis is therefore, a prime component of geological investigations, which helps in narrowing down petroleum exploration to specific regions in a basin, where potential hydrocarbon accumulation are likely.

Further Readings 1. Nichols, G. (2009). Sedimentology and stratigraphy (2nd edn). Wiley-Blackwell. 2. Monroe, J. S., & Wicander, R. (1997). Physical geology: Exploring the Earth (3rd ed.). West/Wadsworth.

Chapter 4

Sediments and Sedimentary Rocks

4.1 Introduction Earth surface processes include weathering of rocks, its erosion and transportation, and deposition in sedimentary basins under the influence of agents like gravity, flowing water, air, and ice. Other surface processes like volcanism and seismicity have influences from forces underneath the crust. The morphology and landforms of Earth’s surface are mostly from the results of sedimentary processes. Depositional landforms like deltas, beaches and alluvial fans constitute sediments resulting from disintegration of rocks that are exposed to the atmosphere above the water table. These are eroded and transported to the location where they are deposited to form sedimentary rocks. Weathering is the prime process which breaks down existing rocks and produces soils and sediments. Of the various interactive processes on Earth, the hydrologic cycle is a key driver of weathering and erosion. Water, in all its forms (liquid, vapour and snow), in the various spheres of Earth (hydrosphere, atmosphere and cryosphere), is an important weathering agent. Weathering involves physical, chemical, and biochemical breakdown of rocks at the interface between lithosphere and atmosphere, and the resulting sediments are transported under the influence of gravity, runoff, wind and glaciers to sedimentary basins where they deposit and lithify. Soil is terrigenous (land derived) sediment and is a primary product of weathering. Soil contains all the elements needed to support the biosphere. It contains sediment grains, called clasts, which are eroded and transported to far off places by erosional agents. In the process, grains abrade and become smaller in size and some unstable minerals chemically disintegrate. Shape and size of these clasts are results of the process of transportation. The mineralogy and chemistry of these sediments helps decipher the source of its origin (or provenance). Weathered material, which chemically disintegrates, are easily taken into solution and are transported by surface and ground water. They reach locations, where the conditions lead to precipitation of the dissolved chemicals as solid minerals. It is now well established that roots of plants and trees aid weathering,

© Springer Nature Singapore Pte Ltd. 2023 S. N. Kundu, Geoscience for Petroleum Engineers, https://doi.org/10.1007/978-981-19-7640-7_4

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4 Sediments and Sedimentary Rocks

although they prevent its erosion. Dead organic matter alters the pH of the environment, triggering chemical disintegration of rock and minerals. In this chapter, we shall learn about Earth’s surface processes, which effect weathering and the transportation of sediments by various agents of nature. Details of the sediments and the mechanisms of their transport to sedimentary basins shall be entailed. Characteristics of the clasts produced under different surface processes and the transformation of sediments into rocks shall be discussed.

4.2 Weathering Weathering is the breakdown of rocks by physical, chemical, and biochemical means. It is a process which occurs at the interface of our Earth’s geosphere and its atmosphere. Rocks, at the surface of Earth, come in contact with the atmosphere and are subjected to alteration. Alteration is fundamentally a chemical process involving silicate minerals at low temperatures. Solid rocks are also broken into fragments with little or no chemical change. Breaking of a rock into smaller parts by physical means increases the surface area of the rock and exposes it to erosion. Weathering, therefore, results in mineralogical, chemical, and grain-size changes in rock materials. Observable aspects of weathering result from partial or complete decomposition of selective minerals, based on their stability, in prevailing environmental conditions. Such environmental conditions, which influence weathering, are mostly climatic and involve processes like freezing and thawing or rusting (oxidation). Saturation of water in rock pores, reacts with some of the constituent minerals which are removed by solution. Resistance of minerals to weathering is in the reverse order to their formation from molten magma (as reflected in the Bowen’s reaction series). Quartz and muscovite are the most resistant minerals to any form of chemical weathering. There are three prime types of weathering processes (Fig. 4.1).

Fig. 4.1 Types of Weathering

4.2 Weathering

45

4.2.1 Physical Weathering A prime mechanism of physical weathering is the freeze–thaw cycle of water in rocks pores. Water expands, when its freezes to form ice. This exerts pressure up to one kilobar, which is enough to break most rocks. Freeze–thaw is uncommon in the tropics and permafrost areas and therefore, is primarily restricted to temperate climates. Such weathering results in angular rock fragments, that are eroded by gravity (or slope) processes which form scree or talus deposits. Salt weathering is another form of physical weathering. Salt crystals are deposited in rock pores by flowing groundwater. These grow and exert pressure on the rock and breaks it when its tensile strength is exceeded. Salt growth weathering is predominant in arid climatic conditions in both warm and cold regions. Another mechanism of physical weathering is by thermal expansion and contraction of minerals in rocks. During diurnal temperature changes, individual minerals in rocks tend to differentially expand and contract, resulting the physical breakdown of the rock. Extreme climatic events produced by the likes of forest fires also can produce significant breaking of rock slabs. Jointed rocks at coastal regions have been found to have mechanically weathered by storm events.

4.2.2 Chemical Weathering The exterior surface of a rock, which is exposed to the atmosphere and precipitation, appears much different to its interior. This is because the surface of the rock has been subjected to chemical weathering. Chemical weathering is selective to particular minerals in the rock and some minerals are disintegrated by oxidation, hydrolysis and carbonation. The water that causes chemical weathering is sourced from precipitation. The pH of rainwater varies seasonally and differs from region to region. Rainwater is acidic (pH of around 5.7) because it takes carbon dioxide into solution along its way from the atmosphere. It is more acidic where gaseous industrial effluents like nitrogen and sulfur are present. Therefore, chemical weathering depends on the constituent elements of the minerals and the chemistry of water.

4.2.2.1

Hydrolysis

The chemical breakdown of a substance when combined with water is called hydrolysis. Feldspars, a constituent mineral of granite, undergo hydrolysis and form clay minerals. This selective alteration of minerals weakens the rock, thereby exposing it to other forms of weathering. Feldspars react with weak carbonic acid in water to form clay (Kaolinite) minerals and release calcium and carbon di-oxide to the environment, which are prime constituents of the mineral calcite.

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4.2.2.2

4 Sediments and Sedimentary Rocks

Oxidation

Another form of chemical weathering is oxidation, which essentially is the reaction of a mineral with oxygen. An example is when iron-rich minerals react with oxygen and form rust (iron oxide). Rusting gives a brownish appearance to the exposed surface of rocks and at the same time it weakens it for further weathering by physical agents.

4.2.2.3

Carbonation

Carbonation is another form of chemical weathering where carbonic acid rich water reacts with carbonate minerals to form carbon dioxide and lime. Carbonation results in the formation of caverns in limestone where dissolved carbon dioxide in rainwater (carbonic acid) reacts with limestone (CaCO3 ) and partially dissolves it. This imparts an elephant skin like texture on the surface of limestones (Fig. 4.2).

4.2.3 Biological Weathering In urban environments, we have seen many buckled pavement slabs underneath which tree roots are exposed. This is a form of biological weathering which is purely mechanical in nature. Microbes in groundwater can also alter the rock’s chemical composition, enhancing its susceptibility to weathering. An example is that of lichen, a fungus, which releases chemicals that breakdown rocks to release minerals that serve as its food. Continued microbial activity develops holes and gaps exposing the rock to other forms of weathering.

4.2.4 Climatic Factors Solution, hydrolysis and oxidation processes are aided by water and therefore availability of water and its chemistry is important. High temperature accelerates chemical reaction and wide variation of diurnal temperature results in thermal expansion and contraction of rocks breaking them mechanically. Climate governs the temperatures, its variation and precipitation. Certain climatic zones favour dense growth of vegetation and support biological weathering. Based on precipitation and temperatures, our Earth can be divided into several climatic zones (Fig. 4.2). Tropical climates experience the highest rates of weathering and polar climates the least. In temperate climate, weathering is found to be moderate.

4.3 Erosion, Transportation and Deposition

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Fig. 4.2 Climate Zones on Earth

4.3 Erosion, Transportation and Deposition Weathering produces broken material, which seldom stay put at the place where they are produced. Agents like wind, following water, moving ice keep disturbing the broken-down material, transporting them to places where they settle to forms sedimentary rocks. Detrital material may be transported downslope a hill under gravity. It may also be blown further away by wind or washed by flowing water. Erosion, transportation and deposition happen in tandem and the relationship between the three depends on the size of the sediment and the energy of the agent (Fig. 4.3). For example, a river flows through three stages: the first stage is where it originates and flows at high velocity aiding erosion only, the second stage is where it flows through moderate slope mostly transporting the sediments, and in the final stage where topography is flat and it is already laden with sediments, it slowly meanders towards the sea shedding all the sediments on its way to the sea.

4.3.1 Gravity Erosion Downslope movement under gravity depends on the slope of topography and on the type of material being eroded. If the detrital material is a coherent mass of rock, which has moved without disintegrating further, then it is called a landslide. Else

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it is referred to as a rock fall. Rock falls and landslides are rapid movements which are often triggered by seismic tremors. A mass of soil on a gentler slope creeps downslope at a very slow rate. This creep can be coherent or plastic depending on its water saturation. Thus, between a landslide and a creep, there are several types of gravity based erosion like slumping, debris flow and mud flow. Landforms resulting from gravity erosion are accumulationed at the base of the slope or foothill. These accumulations are called scree, which are mostly reworked by flowing water and blowing wind. When preserved, these form conical landforms called talus.

4.3.2 Fluvial Erosion Flowing water in river erodes the riverbed and the banks, generating sediments that are carried along with the flow. Surface water flow or runoff erodes the loose material causing sheet erosion or gully erosion. Hill slopes are prone to both these types of erosion and the amount of eroded material depends on the vegetation on the slopes. When a thin layer of topsoil is removed over a larger area, we call it sheet erosion. Gully or rill erosion occurs when runoff cuts channels up to a meter deep into the surface. Hjulstrom’s curve (Fig. 4.3) defines the relationship between fluvial flow velocity and sediment size with reference to erosion, transportation, and deposition.

Fig. 4.3 Hjulstrom’s Curve

4.3 Erosion, Transportation and Deposition

4.3.2.1

49

Erosion

Rivers erode by means of four prime processes. These are hydraulic action, abrasion, attrition, and corrosion. The hydraulic action is because of the force of water which hits riverbanks pressurizing into the rock cracks, prying into the rocks, and removing the broken matter. Hydraulic action is at its peak at waterfalls and rapids. Abrasion action is by the sediment carried by rivers which scour the river bed and river banks. Attrition is due to the collision of the rock fragments with each other during turbulent flow due to which they break into smaller particles. Corrosion is chemical reaction triggered by acidic rainwater.

4.3.2.2

Transportation

Weathered and eroded matter is transported by rivers through several means (Fig. 4.4a). Corroded material is transported in solution as chemical (or solution) load. Very fine particles like silt and clay are transported in suspension in turbulent waters. Sediments like pebbles, gravel and sand are transported as bed load by rolling and saltation.

4.3.3 Aeolian Erosion Wind may not be as energetic and prevalent as rivers but it is the predominant agent for erosion and transportation in arid environments. Wind transports only loose, unconsolidated fragments of sand, as larger particles are too heavy for wind to move except in storm conditions. Being more intermittent than flowing water, sediments are mostly transported by saltation with only the dust (fine) material being transported in suspension. Sand dunes are the most common landforms in desserts which result from sand piled by wind action. Shape and size of dunes vary from one area to another, and this variation depends on the availability of sediments and the speed (and direction) of wind. Sand climbs to the crest of the dune, along the gently sloping windward face, and cascades over it to the slip-faced side forming arcuate layers. These arcuate layers form a pattern called ‘cross-bedding’ which dips towards the current direction of the wind(Fig. 4.4b). Such action leads to migration of the dunes over long periods of time. Over a period, the sand dunes advance several kilometers, forming hundreds of cross-bedded layers of sand.

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Fig. 4.4 Erosion and transportation of sediments by water, wind and ice

4.3.4 Glacial Erosion Glaciers are slow moving huge blocks of ice which do not melt seasonally. Glacial erosion, and resulting sediments, are characteristic and distinct from wind and river sediments. Glacial erosion occurs by the processes of plucking and abrasion. When the glacial bottom melts, the melt water seeps into the cracks of the bedrock and freezes. As the glacier advances, the rocks encapsulated by this frozen melt water is plucked and carried along embedded with the glacier (Fig. 4.4c). Abrasion follows when the plucked rock at the base, scours the bedrock producing striations, which indicate the direction of movement of the glacier. Glacial sediments are transported embedded with ice till the point where the it melts completely. This is where the sediments are released and dumped. Finer sediments flow away with the melt waters, but the heavier ones stay behind as till. If the glacier terminates at a lake, as in the case of fjords, icebergs form. These

4.4 Lithification

51

icebergs contain embedded rock fragments that were plucked by the glacier from the bedrock. When these icebergs melt, the rocks fragments are dropped onto the muddy lake sediments, where it is lithified as dropstones. Dropstones are helpful in establishing the direction of younging in sedimentary successions.

4.4 Lithification Sediments are deposited in specific regions called depositional environments. Depositional environments can be terrestrial, marine or transitional. In each depositional setting, the sediments settle and lithify to form sedimentary rocks. The process of deposition and lithification is evident from the texture and fabric of the rock, from which the depositional environment is deduced. Thus, a rock, which lithified several million years ago, bears distinct lithological and sedimentary signatures of the the ancient depositional environment. The two prime processes involved in lithification of rocks are compaction and cementation (Fig. 4.5).

4.4.1 Compaction Compaction is the process in which lithostatic pressure causes grains of sediment to come closer, thereby increasing the density of the collective material. The pressure is generated by later sediments, which are deposited above older sediments. During compaction, porosity reduces and the fluids in the pores are expelled. Compaction also increases the temperature and the concentration of dissolved material in the pore

Fig. 4.5 The Lithification Process

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4 Sediments and Sedimentary Rocks

fluids. This triggers the chemical precipitation of matter that cements the sediment particles together.

4.4.2 Cementation Chemical precipitation of new material from dissolved solids in the pore fluids in sediment grain interstices is called cementation. As percolating water brings in material in solution, with time its concentration increases and the matter is gradually deposited in the pores. Cement binds the sediment grains together. The most common cementing material are quartz and calcite, which are products of chemical weathering in rock carried by running water and groundwater.

4.4.3 Diagenesis Post lithification, several chemical reactions occur which result in replacement and alteration of minerals. These reactions are collectively known as diagenesis. At low temperatures and pressures, the rock’s original mineralogy and texture is impacted by chemical reactions aided by hydrothermal solutions and meteoric groundwater. Diagenesis impacts porosity and permeability of rocks. Diagenesis, in sand, witness a reduction of porosity, whereas in limestone, it creates secondary porosity.

4.5 Sedimentary Rock and Types Sediments are classified as clastic or non-clastic based on its constituents. Clastic sediments consist of clasts or grains that result from physical weathering. Examples of clastic sedimentary rocks are conglomerates, sandstone and shale. When dissolved material is carried in solution which chemically precipitate, they form non-clastic rocks. Calcite (calcium carbonate) and evaporites like gypsum and halite are good examples of non-clastic sedimentary rocks.

4.5.1 Clastic Sedimentary Rocks Clastic rocks (also known as siliciclastic) are composed of various silicate mineral/rock grains (or clasts), which even after being transported for long distances, do not chemically disintegrate. Clastic minerals are relatively chemically stable and

4.5 Sedimentary Rock and Types

53

their interstices preserve pores in which water or hydrocarbons are stored under suitable conditions. The process of clastic rock formation imparts distinct texture and fabric to the resulting sedimentary rock.

4.5.1.1

Clastic Textures

Texture refers to the physical makeup of rock, which are primarily characterized by shape and size and organization the sediments in space. Shape and size depend on the composition of sediment particles and on the distance of transportation. Textures very from one sedimentary depositional environment to another. Sediment Size Particles are generally classified by their grain diameter. The most common classification in use, is the Udden-Wentworth scale, where sediments are classified as gravel, sand, silt and clay with well-defined size limits (Table 4.1). Size classes and their proportions in a sediment are analysed statistically to characterize the type of depositional environment. A common practice in studying grain size distribution is to use ‘phi’ scale as defined by Udden and Wentworth. ‘Phi’ is the negative logarithm of grain size in mm to base 2. Phi (.) = −log2D where, D is particle size in mm. Sediment Shape Shape has multiple dimensions to its definition. Minerals tend to assume a shape based upon their chemistry and crystal structure. If all the three axes of the crystal are of equal length, the mineral tends to be equant. Shortening of one axis makes it platy, whereas the opposite imparts a elongated shape. Shape, in many case, reflects the erosional processes and is indicative of the parent rock. Shale, for example, shall produce platy sediments, whereas marble or quartzite shall produce equant particles. Sphericity and roundness are the fundamental shape descriptors. Sphericity is the degree to which a particle approaches a sphere of the same volume, whereas roundness is a measure of the sharpness of edges. A well eroded particle tends to be more spherical and rounded. Sphericity and roundness are measured with reference to a visual scale (Fig. 4.6). More quantitative treatise on shape and size are based on Wadell’s methods, which is implemented using computer based granulometric analysis using automated algorithms. Mature Sediments Maturity of sediments is defined based on size, shape and composition. Textural maturity refers to the grain shape and grain size distribution in the rock. A texturally

54 Table 4.1 Udden-Wentworth Scale

4 Sediments and Sedimentary Rocks Grain size (mm)

Phi (F)

Greater than 256 Less than −8

Description

Size class

Boulder

GRAVEL

Between 256 and 64

Between −8 and −6

Cobble

Between 64 and 4

Between −6 and −2

Pebble

Between 4 and 2 Between −2 and −1 Between 2 and 1 Between −1 and 0 Between 1 and 0.5

Granule SAND

Between 0 and 1

Between 0.5 and Between 1 and 2 0.25 Between 0.5 and Between 2 and 3 0.125 Between 0.125 and 0.0625

Between 3 and 4

Between 0.0625 and 0.031

Between 4 and 5

Between 0.031 and 0.0156

Between 5 and 6

Between 0.0156 and 0.0078

Between 6 and 7

Between 0.0078 and 0.0039

Between 7 and 8

Less than 0.00006

Greater than 8

Fig. 4.6 Sphericity and Roundness

SILT

CLAY

4.5 Sedimentary Rock and Types

55

mature clastic rock has well rounded, spherical grains where all grain size tend to be of similar size with little variation (i.e., well sorted). Compositionally, mature sediments consist of more resistant minerals like quartz and mica. Beach sands, which consist of mainly quartz, are compositionally more mature than glacial sediments. This is because, the latter has several unstable minerals like feldspars. Maturity is also an indicator of the erosional and depositional processes. For example, waves have a winnowing effect on beach deposits, where unstable and small sized materials like clay are removed and sand sized minerals are enriched. Hence, coastal processes make the sediments both compositionally and texturally more mature. Mature sediments tend to be more porous and permeable as grain size range is small. Such sediments form excellent hydrocarbon reservoirs and freshwater aquifers.

4.5.1.2

Clastic Fabric

Sediment fabric stresses the three-dimensional make-up of the sediment grains in the rock. Preferred orientation of grains in a sedimentary rock defines its fabric. Like texture, fabric is useful in determining the processes operating at the time of deposition. If clasts are aligned in one direction, then they are likely to be influenced by current direction (Fig. 4.7). A common descriptor of fabric is imbrication of gravels in conglomerates. In imbricated gravels, the long axes of the clasts lie sub parallel to the base and dip upstream. This is used to identify the paleo-current direction in sedimentary rocks. Conglomerates can be grain supported or matrix supported. Matrix is the fine sediments that fills up the space between the gravels. In grain supported sediments, the proportion of matrix is low as compared to matrix supported sediments. The long axis of pebbles and gravels is not always aligned to the direction of flow. In some

Fig. 4.7 Flowing Current and Pebble Orientation

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4 Sediments and Sedimentary Rocks

Fig. 4.8 Common Clastic Sedimentary Rocks

cases, it has been observed that the elongated pebbles roll along the long axis, which result in the alignment of the pebbles perpendicular to the flow direction.

4.5.1.3

Common Clastic Rocks

Conglomerate and Breccia Conglomerate and breccia (Fig. 4.8) are rocks composed of granules or larger clasts. The clasts are angular in breccia, whereas they are rounded in conglomerate. This is because, the clasts in conglomerate are eroded and transported, whereas in the case of breccia they are preserved in-situ. As roundness is a function of sediment transportation, breccia is formed in vicinity of the source rock, whereas in case of conglomerates, they are transported over some distance. Conglomerates are found atop unconformities. Breccia, on the other hand, are found in fault zones where the rock is crushed, pulverized, and trapped in between the hanging wall and the foot wall. Sandstone and Siltstone Sandstone is a sedimentary rock composed mostly of sand-sized grains. It is usually rich in quartz but also may contain mica, feldspar, and some proportion of silt and clay size particles in the matrix. Sandstone containing more than 90% of quartz is called quartz sandstone. It is called arkose, if the feldspar proportion exceeds 25%. Sandstone with significant amount of clay or silt, it is referred to as argillaceous sandstone. Sandstone and siltstone (Fig. 4.8) are variable in colour which depends on the constituent minerals. Siltstone is a sedimentary rock which has a grain size in the silt range and is finer than sand but coarser than clay.

4.5 Sedimentary Rock and Types

57

Shale and Mudstone Both shale and mudstone (Fig. 4.8) are formed from the compaction of silt and clay-size mineral particles. What differentiates shale from mud is that it is fissile (breaks in plane of weakness resulting from compaction) and laminated (thin layers). Compositionally, they both have clay minerals with little silt sized quartz grains and some organic matter. Organic rich shales produce hydrocarbons on thermal maturation.

4.5.2 Non-clastic Sedimentary Rocks Non-clastic sedimentary rocks (Fig. 4.9) form from chemical precipitation of solution load. Since they are chemically precipitated, they tend to be crystalline. Non-clastic sedimentary rocks are classified based on their mineral composition and proportion of mud. Most are often composed of a single mineral with a few impurities. Common examples of non-clastic rocks are limestones and evaporites. Limestone and Dolostone Collectively referred to as carbonates, limestone (Fig. 4.9) and dolostone predominantly consist of CaCO3 (calcite) and (Ca,Mg)(CO3 )2 (dolomite) respectively. Dolostones are products of recrystallization of pre-existing limestone where magnesium in seawater partially replaces calcium in calcite. Carbonates are the most extensive non-clastic sedimentary rock which form in the marine environment. Calcite is sourced from shells of marine animals, which get dissolved in sea water after their death and is later deposited under favourable conditions as limestone. In present day

Fig. 4.9 Non-clastic Sedimentary Rocks

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environment, carbonate deposition is favoured by warm, shallow waters in tropical regions where calcite bearing organisms like shells and reefs proliferate. Carbonates are significant for the petroleum industry as they form prolific reservoir rocks from which hydrocarbons are produced. Evaporites Evaporites are water soluble minerals which crystallize out of an aqueous solution. Evaporites form both in the marine environments and in terrestrial playa lakes. Gypsum, anhydrite and halite are common examples of evaporites. Gypsum (CaSO4 .2H2 O) is a common mineral and occurs as thick and extensive evaporite beds (Fig. 4.9) in association with other sedimentary rocks like halite and mudstone. Hydrothermal anhydrite (CaSO4 ) form in veins of intrusive rocks hydrates as gypsum when it comes in contact with water. Gypsum is the most commonly occurring sulfate mineral which is normally white or transparent. Halite (NaCl) form vast beds of sedimentary evaporites. Halite results from the drying up of an enclosed saline water body, where salt occurs in solution. Salt beds, hundreds of meters thick, have been found in the Gulf of Mexico. Being ductile and fusible, it assumes different shapes under duress and balloons upward to form salt domes in sedimentary basins. Salt domes are good traps for hydrocarbons as they have little or no porosity and can give rise to both stratigraphic and structural traps. Bioclastic Limestone Bioclasts are skeletal fossil fragments of once living marine or land organisms. They are preserved in sedimentary rocks, which are primarily deposited in marine environments. Most bioclasts are found in limestone and are essentially carbonate shells of marine organisms which get lithified with the sedimentary rock before they get chemically dissolved. An example is Coquina (Fig. 4.9), which entirely consists of physically broken and sorted fragments of the shelled marine organisms like molluscs, trilobites, brachiopods, or other invertebrates. Biochemical Chert Composed of microcrystalline or cryptocrystalline quartz, chert (Fig. 4.9) is chemically precipitated silica. Chert is formed from the accumulation of siliceous skeletons belonging to microscopic organisms like Radiolaria and Diatoms. It deposits in deep seas, where its precipitation in form of nodules, concretionary masses, and layered deposits, is favoured. Coal Coal forms from the accumulation of plant debris, in anaerobic and swampy environments. To form coal, the rate of plant debris accumulation must be higher than the rate of decay and the organic debris should be quickly overlaid and preserved by sediments like mud or sand. Chemically, coal is mostly carbon with variable amounts of other mineral impurities. Coal (Fig. 4.9) is black in colour and can grade from lignite (sedimentary) to anthracite (metamorphic). Methane trapped in coal is a good source of unconventional hydrocarbons.

Further Readings

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4.6 Summary Sedimentary processes produce sedimentary rocks, which in specific environments, are deposited and lithified into sedimentary rocks. The process of deposition governs the characteristics of the rock and its composition. Characteristics like texture, fabric and composition are important in predicting the depositional environment. In later chapters, we shall learn how these processes bear significance in understanding the depositional and environmental evolution of a sedimentary basin and this has a strong link to prospecting hydrocarbon resources.

Further Readings 1. Nichols, G. (2009). Sedimentology and stratigraphy (2nd ed.). Wiley-Blackwell. 2. Monroe, J. S., & Wicander, R. (1997). Physical geology: Exploring the Earth (3rd ed.). West/Wadsworth.

Chapter 5

Stratigraphy and Sedimentary Structures

5.1 Introduction The organization of sedimentary rocks in the subsurface plays an important role in the storage and migration of petroleum and the configuration of hydrocarbon traps. It is therefore important to understand how rocks are organised in space and time. Sedimentary rocks are deposited in layers, with the most recent one overlying the previous. As rock layers occur stratified, each is referred to as a stratum. In some geological settings, non-sedimentary (igneous or metamorphic) rocks occur in a stratified manner too. Stratigraphy is the study of the strata, which primarily involves establishing the time-related architecture of sediments in a basin. Stratigraphy details the sedimentary successions and helps in the reconstruction of the paleo-environments in a sedimentary basin. The analysis of sedimentary processes in the geological past, supports the correlation of regional and global geological events. Figure 5.1 represents a succession of strata exposed at a coast. Sedimentary strata are investigated for several reasons. The primary ones are to identify the rock composition, the sedimentary structures, the fossil assemblages and the heavy mineral content (for radioactive dating and age determination). There are three subdivisions of stratigraphy, which are based on the different methods of investigation. These are: . Lithostratigraphy—Classifying strata based on rock composition, texture and structures . Biostratigraphy—Classifying strata based on fossil content . Chrono-stratigraphy—Classifying strata based on the geologic time scale. Relative geologic dating is based on lithostratigraphy and biostratigraphy, which lays the foundation for the principles of stratigraphy. Sedimentary structures are the physical features of sedimentary formations that can be observed in a rock outcrop or seen in a hand-specimen of rock. Common sedimentary structures are bedding planes, ripple marks, trace fossils, and mud cracks. Sedimentary structures can be primary or secondary, depending on it’s relationship © Springer Nature Singapore Pte Ltd. 2023 S. N. Kundu, Geoscience for Petroleum Engineers, https://doi.org/10.1007/978-981-19-7640-7_5

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Fig. 5.1 A stratigraphic column of Sandstone and Shale formations in Labuan, East Malaysia

with the time of deposition. Primary sedimentary structures result from the energy and environment of sediment deposition. E.g., cross-bedding and ripple marks in sedimentary rocks are formed by currents and waves. Some of these structures are “stratigraphic up” indicators as they help establish the correct order of deposition. This is helpful in regions where the rocks are tectonically disturbed, and are overturned. Secondary structures, like tilted beds, folds and faults, are a result of tectonic events which disturb the original horizontal orientation of the strata in a sedimentary basin. Both stratigraphy and sedimentary structures help us understand the evolution of a sedimentary basin and decipher the geologic sequence of events. Such understanding is important for a geologist to pick up clues which indicate the existance of a petroleum system in the sedimentary basin.

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5.2 Stratigraphy 5.2.1 Lithostratigraphy Lithostratigraphy essentially classifies strata into units based on physico-chemical characteristics of the rocks mappable from field observations. Such classification has resulted in definition of several lithostratigraphic units like bed, formation and member.

5.2.1.1

Bed

A bed is the smallest unit which is assumed to be deposited by a single episode of sediment deposition. It is distinguishable from overlying and underlying beds by a distinctive plane, which is referred to as the bedding plane. A bed is described in terms of its type (e.g., sandstone) and thickness (e.g., 5 cm). When a bed is less than a centimetre, it is referred to as a lamination.

5.2.1.2

Formation and Member

Formation is a fundamental lithostratigraphic unit in a sedimentary succession. It is the unit that can be represented in a geological map of 50,000 scales (or higher). Formations usually vary in thickness from about 10 m to several hundred meters. They are unlikely to be of homogenous composition and usually contain smaller units of varying lithologies. Each such smaller unit, which are a set of beds with consistent lithology, is called a member. In stratigraphic nomenclature, the name of the locality where the formation is found or drilled, is usually used to name it. E.g., Jurong Formation and Kallang Formation of Singapore. Members on the other hand have consistent lithology and are named after the rock type. E.g., conglomerate member of Jurong Formation.

5.2.1.3

Group, Subgroup and Supergroup

While studying long successions of strata in a sedimentary basin, it is more practical to club formations into larger units called groups. A group usually has 3–6 formations and is bounded by significant unconformities or hiatuses. A group can be subdivided into two or more subgroups. Conversely, two or more groups can constitute a supergroup. Lithostratigraphic units carry regional connotation and therefore, the same succession deposited at the same geological time but at different places in the same sedimentary basin, can have different names. To standardize the nomenclature of lithostratigraphy units, it is essential that a type section with adequate outcrop be present in the

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locality. To avoid conflicting names in stratigraphic nomenclature, a name assigned earlier takes precedence and the same cannot be reused for another formation.

5.2.2 Biostratigraphy Biological remains of pre-existing flora and fauna are often preserved in sedimentary rocks. There are several known fossil forms that can be identified from both hand-held specimens and thin sections of rock studied under the microscope. As fossils belong to prehistoric organisms who came into existence at a particular age and became extinct at a later geologic time, different assemblages of fossils are usually found in similar formations in different parts of the same sedimentary basin. Biostratigraphy is fundamentally based on the fossil assemblages in sedimentary rock formations. Correlation of sedimentary rocks, at the global scale, was primarily supported by biostratigraphy, and this was later refined after the advent of radiometric dating methods. Biostratigraphy can be perceived as the application of evolution of life forms as evidenced from fossil records in strata of sedimentary basins (Fig. 5.2). Fossils of a particular species are representative of its form at its evolutionary stage. A sedimentary stratum, that is deposited after extinction of a species, is unlikely to contain any fossil form of the same species. Not all life forms are fossilized in rocks and not all the fossils have biostratigraphic applications. Life forms, with hard shells (resistant to weathering), are most likely to be preserved as fossils. Fossils of life forms, which had a very fast evolutionary rates, display strong morphological differentiation. Such fossils are very helpful in characterising close sedimentary successions. Fossils of life forms, which were globally widespread, are very useful in correlating rock types across continents. Fossils of life forms that habitated in specific climate and environment are helpful in determining paleo-climate and paleo-environment.

5.2.2.1

Biozones

A body of strata defined by its fossil content is called a biozone. Categorization of a biozone into an interval zone or an assemblage zone helps in biostratigraphic correlation at multiple (global, regional or local) scales. Interval zones are defined by strata which contain the fossilized form of a species between its first appearance and extinction. Zones of continuous succession, where three or more common fossils are found, form an assemblage zone.

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Fig. 5.2 Major Fossils and their Geological Time Ranges

5.2.3 Chrono-Stratigraphy Geological events are tied to time and can be identified from rock records. Correlation of rocks with these geological events, constitute a branch of stratigraphy called chrono-stratigraphy. Impacts of geologic events, signatures of which are found in rocks can be biological (e.g. mass extinction), depositional (e.g. a veneer of volcanic ash in a formation depicting a volcanic event) or related to remanence magnetism influenced by the reversal of Earth’s magnetic field. Such correlations with past geologic events are mostly relative in a succession of rocks. Establishing the absolute time scale, by the use of radiometric dating, constitutes a branch of geoscience called ‘Geochronology’.

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Chronozones

A chronozone includes all the deposits formed during a short geologic time interval bounded by two major geologic events. Succession between the appearance and extinction of a species, which essentially is a biozone, can also be a referred to as a chronozone. The difference is that a biozone is defined based on fossil content but a chronozone represents all the rocks that were formed during this interval irrespective of their fossil content. A lithostratigraphic unit, that has a wide occurrence and is of chronostratigraphic significance, acts as a marker bed. Marker beds are important for correlation of rock sequences in sedimentary basins and beyond. These are characterized by specific well-log signatures from exploration wells.

5.2.3.2

Stage, Series and Period

One or more chronozones constitute a stage. A stage covers a limited period, which is usually between one and ten million years. A Stage is the smallest chronostratigraphic unit that is used for stratigraphic correlation. It is defined by a geographical name linked to a type section. An age is the geochronological unit that relates to a stage. A series is a chronostratigraphic unit larger than a stage. Its geochronological equivalent is the epoch. A period as a chronological unit is larger a series. The older the period, the larger is the time. E.g. Cretaceous period us about 60–70 Ma, whereas Quaternary is about 2.5 Ma. The rocks formed during a period constitute a system. An era comprises of two or more periods. Equivalence of stratigraphic units based on lithostratigraphy, biostratigraphy and chronostratigraphy is provided in Table 5.1. Table 5.1 Stratigraphic Units and their Rank equivalence Lithostratigraphic

Biostratigraphy

Chronostratigraphic/Geochronological

Supergroup

Assemblage zone

Eonothem/Eon

Group

Range zone

Erathem/Era

Subgroup

Acme zone

System/Period

Formation

Interval zone

Series/Epoch

Member

Biozone

Stage/Age

Bed

Chronozone/Chron

5.3 Relative Geologic Dating

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5.3 Relative Geologic Dating 5.3.1 Uniformitarianism and Catastrophism The shape of Earth we see today is a result of the interplay of processes, which are operational ever since Earth was differentiated into its present subsystems (i.e., lithosphere, hydrosphere and atmosphere). Two schools of thoughts exist on how the present-day Earth was shaped. One is ‘Catastrophism’ and the other is ‘Uniformitarianism’. The doctrine of uniformity or Uniformitarianism is attributed to James Hutton (1726–1797). It assumes that the natural processes that operate today, has been operating in the geologic past too. This doctrine assumes constancy of cause-effect throughout space and time and is the fundamental principle which is used to explain the landscapes we find on Earth today. Many scientists consider natural processes to be non-uniform in time and space, although most of them accept that such processes do have a broad sense of regularity. Catastrophism, on the other hand, assumes that the shape and features of Earth are caused by global events which are sudden, violent and short-lived. This theory contradicts uniformitarianism which supports gradual, normal and long-term processes, like weathering and erosion, to be responsible for Earth’s geological features. The doctrine of uniformitarianism literally translates into the statement ‘Present is the key to the past’ and this forms the basis of most geological interpretations and predications. Present day scientists adopt a more inclusive view of geologic events and accept the fact that there have been catastrophic events in the past, which are essentially extreme events of the same natural processes that run in continuum.

5.3.2 Principles of Stratigraphy Geologists can determine the relative order of past events though observation of rock records, without having to determine their absolute ages. Relative dating methods, referred to as Principles of Stratigraphy, place strata in a chronological order. Based on the doctrine of Uniformitarianism, these principles have been developed from the works of Nicolaus Steno (alias Niels Steensen 1638–1686), James Hutton (1726– 1797), and William Smith (1769–1839). Six fundamental principles are popular for relative dating of geologic events (Fig. 5.3), of which the first four are based on Steno’s postulates.

5.3.2.1

Principle of Original Horizontality

Postulated by Steno in 1669, this principle states that all sedimentary rocks are deposited in a horizontal fashion in most sedimentary basin. Sedimentary rocks,

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Fig. 5.3 Principles of Relative Dating

which are not horizontal, must have been tilted from their original position from a process during or after its deposition and lithification. Strata which is not horizontal is said to be dipping. Dip of a rock is described by its direction and the amount of tilt. Strike is defined as the intersection of the dipping plane (formed by the dipping bed) with the imaginary horizontal at a given elevation (Fig. 5.4). Dip direction and strike are mutually perpendicular to each other. Dip is ‘zero’ for horizontal beds and in tilted beds, it can reach a maximum of ‘90’ degrees.

5.3.2.2

Principle of Lateral Continuity

Steno, in 1669, also said that material forming any strata were spread continuously over the surface unless something stood in their way preventing it to extend further.

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Fig. 5.4 Tilted beds and the concept of Dip and Strike

This forms the basis of the principle of lateral continuity, which says that sedimentary rocks spread laterally in a sedimentary basin. An analogy is snowfall, which spreads laterally to cover as far as it can. Sediments usually stop depositing at the edge of a sedimentary basin which acts as a barrier to lateral continuity.

5.3.2.3

Principle of Superposition

Steno postulated that when any stratum was formed in a body of water, no strata on top of this could have existed. This emulates into the principle of superposition which states that layers of sedimentary rocks superpose on each other with the bottom being oldest and the top being the youngest. The lowest lying stratum is always the oldest unless the sequence of rocks is overturned by a post-depositional tectonic process.

5.3.2.4

Principle of Cross-Cutting Relationship

Steno postulated that the geologic feature which cuts another is the younger of the two. If a rock intrudes into another, the the former is younger than the latter. This principle is applied to determine the sequence of events in a sedimentary basin, where the rock successions are impacted by magmatic intrusions and faulting.

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Principle of Inclusion

Inclusions are rock fragments embedded in a body of rock. Sedimentary rocks often contain inclusions of igneous, metamorphic or older sedimentary rocks. Inclusions, being parts of a pre-existing rock, are always older than the rock which contains them. While analysing evolution of sedimentary basins, the study of inclusions helps us understand the provenance (or source) of the sedimentary rock.

5.3.2.6

Principle of Faunal Succession

Different species have different evolutionary trends and extinction periods. Biozones (assemblage zones) have fossils of multiple species which establish the chronology of the sedimentary rocks. Index fossils (which have geologically short appearanceextinction timelines) are very useful in determining the chronology of sediment succession in biozones containing them. The very understanding, that a fossil that is extinct earlier than the time of deposition of a sediment can never be found in the sediment, forms the foundation of the principle of faunal succession. There are exceptions to the six principles in certain areas and circumstances. Especially in transition zones, where sea-level changes impact sedimentation processes, a new principle called the Walther’s law of facies is applicable.

5.4 Sedimentary Structures and Contacts Sedimentary structures are physical features of sedimentary rocks which are visible from a rock outcrop or in a large hand specimen. Common sedimentary structures include bedding, ripple marks, trace fossils and mud cracks (Fig. 5.5). Structural features that are associated with depositional and lithification process are considered primary structures. Sedimentary structures like concretions and vein fillings, which develop post deposition and lithification of the sedimentary rock, are secondary structures. Contacts are the relationship between adjacent strata in three dimension. The simplest contact is bedding which is a depositional structure. Tectonic forces and erosion form several different post-depositional contacts.

5.4.1 Sedimentary Structures The interface between consecutive beds is referred to as the bedding plane. The orientation of bedding depends on the depositional and stratification process.

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Fig. 5.5 Sedimentary structures (a), Graded bedding (b), Convolute bedding (c), Ripple marks (d), Mud cracks (e) and Burrows (f)

5.4.1.1

Crossbedding

Crossbedding (Fig. 5.5a) forms during deposition on the inclined surfaces of bed forms such as ripples and dunes; it indicates that the depositional environment contained a flowing medium (typically water or wind). Crossbedding helps in establishing the paleocurrent and also helps in determining the direction of younging in rock sequences in sedimentary basins.

5.4.1.2

Graded Bedding

Graded bedding (Fig. 5.5b) is characterized by a systematic change in grain or clast size from one end of the bed to the other. Most commonly this takes the form of normal grading where coarser sediments occur at the base and fines upward. The change in energy of the depositional environment can be inferred from graded bedding.

5.4.1.3

Convolute Bedding

Sediment layers get convolved due to deposition of dense sediments on soft sediments and this results in the formation of convolute beds (Fig. 5.5c). Convolute beds are a common form of soft sediment deformation. As the load from the top dense sediment exerts an imprint due to gravity on the underlying soft sediment, the soft sediments are convolved and this helps us identify the direction of younging in a sedimentary succession.

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Ripple Marks

Currents and waves in shallow water create ripples on the sand which form ripple marks (Fig. 5.5d) in the resulting sandstone. Current ripples are asymmetrical whereas wave (or oscillation) ripples are symmetrical and concave upwards. Ripple marks help identify the direction of younging in a sedimentary succession.

5.4.1.5

Mud Cracks

Mud cracks (Fig. 5.5e) form in mud as it dries under the sun. The cracks are preserved when later sediments with different grain size or composition fill those cracks. Mud cracks are wider at the top and taper to its base and this helps is identifying the ‘stratigraphic up’ in overturned successions.

5.4.1.6

Burrows

In specific environments, life forms dwell in burrows in soft sediments. The burrows are filled by later sediments, which are different in size and composition. When lithified, these burrows are preserved in the rock. Burrows (Fig. 5.5f) preserved in sedimentary rocks are referred to as ‘trace fossils’ as they depict existence of a life form although they are are not skeletal remains of the organism. Burrows by different animals have specific patterns and these patterns help us identify the organism and also the ‘stratigraphic up’ in a sedimentary sequence.

5.4.2 Contacts A geological contact is a boundary which separates one rock body from another. Contact can result from depositional processes, intrusive events or tectonic forces.

5.4.2.1

Depositional Contacts

Depositional contacts can be conformable, i.e. there is no time gap in deposition or non-conformation involving a hiatus in deposition. An example of a conformable contacts is bedding. Non-conformable contacts involve a time gap in deposition, during which the depositional environment changes drastically. A non-conformable contact is called an unconformity. An unconformity essentially represents a non-depositional and erosional period. The surface of an unconformity is undulating owing to the effects of erosion.

5.4 Sedimentary Structures and Contacts

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Fig. 5.6 Depositional and Intrusive contacts

Unconformities can be of different types based on the type and nature of the rock sequences above and under (Fig. 5.6). A ‘disconformity’ is an unconformity where the rocks overlying and underlying the unconformity surface are both sedimentary and have the same dip and strike (Fig 5.7). Where the dips and strike of the overlying succession and underlying succession are different, it is referred to as ‘angular unconformity’. ‘Nonconformity’ is another type of unconformity where non-sedimentary rocks underly the unconformity surface above which sedimentary rocks are laid.

5.4.2.2

Intrusive Contacts

Intrusive contacts are the surfaces formed by body of rock which intrudes into a pre-existing (or host) rock. Such intrusion usually occurs due to invasion of magma along the joints and cracks of the host rock. Depending on the relationship with the host rock, they can be defined as batholith, dykes or sills (Fig. 5.6). Magmatic intrusions alter the host rocks inducing a type of metamorphism called ‘contact’ metamorphism. This creates zones of rocks around the intrusion which are baked to different degrees. Contact metamorphism results in non-foliated metamorphic rocks like marble and quartzite.

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Fig. 5.7 Angular Uniformity (in blue) along the Miri-Bintulu coastal road, East Malaysia

5.4.2.3

Tectonic Contacts

Sedimentary basins are subjected to compressive and extensive stresses due to plate tectonics. Rocks are not completely ductile and hence they tend to break when its elastic limit is breached. Rocks break in planes across which there is observable displacement of one block relative to the other. These planes are called ‘faults’ and depending on their angle with the horizontal and the direction of relative displacement, they are termed as normal, reverse or thrust faults (Fig. 5.8). Normal faults are formed by extensive (pull apart) tectonism, where the hanging wall (the block resting over the fault plane) is displaced downwards with respect to the foot wall (the block below the fault plane). Reverse faults are formed by compressive forces where the direction of displacement is exactly the opposite. A low angle reverse fault is referred to as a thrust fault. Another type of faults are strike-slip faults (or transform faults), where the blocks are displaced horizontally. Hence, in strike-slip faults, the faulted blocks cannot be differentiated into a hanging wall and a foot wall.

5.5 Stratigraphic Correlation Stratigraphic correlation is the process of establishing the equivalence of sedimentary sequences with and across sedimentary basins. Stratigraphic correlation helps geologists reconstruct the depositional history and decipher the paleoenvironments. An example is the stratigraphic relationships and correlation between Canyonlands

5.5 Stratigraphic Correlation

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Fig. 5.8 Normal and Reverse faults

National Park and Zion National Park in Utah. At Canyonlands, the Navajo Sandstone overlies the Kayenta Formation which in turn overlies the cliff-forming Wingate Formation. In Zion, the Navajo Sandstone overlies the Kayenta formation which overlies the cliff-forming Moenave Formation. Based on the stratigraphic relationship, the Wingate and Moenave Formations correlate well. These two formations have unique names because their composition and outcrop pattern is slightly different. Other strata in the Colorado Plateau and their sequence can be recognized and correlated over thousands of square miles. Figure 5.9 is the depiction of stratigraphic correlation of rock sequences at three different localities. Correlation is the process of establishing similarities in stratal sequence across different sedimentary basins. At smaller scales, correlation is also done at different locations in the same basin to establish various depocentres within the basin through continuous facies associations. It is well recognized that there are distinct spatial and temporal similarities in lithological, faunal and floral distribution in different Gondwana Basins of southern continents. This includes India, which was once a part of Gondwanaland (Fig. 5.10). Correlations can make use pf magnetic polarity reversals, rock types and sequences, and biomarkers (using zone or index fossils). There are four main types of stratigraphic correlation: lithostratigraphic, chronostratigraphic, and biostratigraphic.

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Fig. 5.9 Stratigraphic Correlation using strata columns at three different locations

5.6 Summary Stratigraphy is the foundation of geological investigation in a sedimentary basin and is the very basis for global correlation of rocks. It has huge implication on petroleum exploration, as discovery in a basin triggers the search for analogous basins elsewhere in the world. An example is the discovery of petroleum in eastern margins of South America, which had triggered exploration activities in the western margin of Africa, as stratigraphic correlations indicated that they were a part of the same landmass in the geological past. In the next chapter, we shall learn about how sedimentary basins are formed and about the various sedimentary depositional environments.

Further Readings

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Fig. 5.10 Biostratigraphic correlation and the reconstruction of Gondwanaland. NB: Gondwana split into the continents we have today due to continental drift

Further Readings 1. Nichols, G. (2009). Sedimentology and stratigraphy (2nd edn.). Wiley-Blackwell. 2. Monroe, J. S., & Wicander, R. (1997). Physical geology: Exploring the Earth (3rd ed.). West/Wadsworth.

Chapter 6

Depositional Environments and Facies

6.1 Introduction Based on the doctrine of uniformitarianism, our understanding of present-day depositional processes can be applied on sedimentary sequences to reconstruct the geologic history of the basin. By observing the properties of sedimentary rocks, a geologist is able to deduce the sedimentary environment at the time of deposition. Such understanding is then used to correlate the events at analogous basins, which provides the geological picture at the regional or global scale. As sedimentary rocks are stratified, the depositional environment of each lithological unit in a sequence helps in understanding past climate. The principles of relative geologic dating can be applied to understand the stages where the depositional settings change and such changes could relate to major orogenic and tectonic events. Such correlations have established that there were several ice ages on Earth, which had influenced several cycles of sea level changes. It has been established that the continents, which are now far apart, were once a single large landmass, and that the present day continents, South America and Africa, were once conjoined. Sedimentary facies are the sum total of all physical, chemical and biological characteristics of rocks which are indicative of the rock’s depositional environment. Thus, an understanding of depositional environment and facies is critical in reconstructing the evolution of a sedimentary basin through geological time.

6.2 Depositional Environments A depositional environment, which is also known as a sedimentary environment, is a physiographic setting, where sediments are deposited. A depositional environment can be presented by a river, a lake, a delta, a lagoon or an ocean. Each depositional environment imparts distinctive signatures to the sediments, which is typical of the processes that were active during sedimentation. These signatures, © Springer Nature Singapore Pte Ltd. 2023 S. N. Kundu, Geoscience for Petroleum Engineers, https://doi.org/10.1007/978-981-19-7640-7_6

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which include the fossil assemblages, sedimentary structures, and lithological properties, are referred to as sedimentary facies. With geologic time, the physiographic setting of a location changes, which influences the formation of rock deposits that vary in their facies. Hence, analysis of the sedimentary column at a location helps us in deciphering the past depositional environments and in reconstructing the paleoenvironment and geologic history. The characteristics that are commonly observed and analysed include lithological properties, sedimentary structures and texture, and fossil assemblages. Sedimentary processes have been operational ever since weathering and erosional forces came into existence. Since its origin, our Earth has undergone several cycles of chemical and physical changes, evidence of which is recorded in sedimentary rock sequences. Some depositional environments of the past may not exist today, but the sedimentary formations of the time are testament to their existence. Understanding past depositional environments and processes has several applications. It helps us characterise regions in terms of its proximity and exposure to natural disasters, like volcanicity and seismicity. These understandings can be used today to evaluate the risks associated with anthropogenic activities, and these influence insurance policies for properties that exist or are being constructed. Geologists analyse depositional environments to establish sedimentary process elements, which then leads to discovery of petroleum, coal, gas and other minerals of economic importance. Depositional environments also help in finding aquifers, which are a major source for groundwater. Based on present-day analogues, depositional environments are classified into three major settings; continental, transitional and marine (Fig. 6.1). The details of each setting, which includes the facies elements are provided in Tables 6.1, 6.2 and 6.3.

6.3 Identifying Depositional Environments As each depositional environment imparts unique signatures on to the sedimentary rocks, the study of lithology, texture, sedimentary structures, and fossil content of the rocks helps us delineate the prevalent depositional environment.

6.3.1 Lithology Lithology is a combination of the composition and texture of the rock, which is largely summarized in the naming of the rock. E.g. Sandstone is a rock comprising of sand sized grains. However, a textural study will determine whether the sandstone is coarse grained or fine grained. Coarse grained sandstones with angular and poorly sorted grains are deposited at foothills whereas fine gained sandstones with well-rounded and well

6.3 Identifying Depositional Environments

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Fig. 6.1 Primary Depositional Environments

sorted grains are found in beaches and coasts. Hence, grain size distribution, which is an important textural element, is indicative of the depositional environment. Textural maturity and distribution of grainsize in sedimentary rocks for some depositional environments is provided in Fig. 6.2. Similarly, mineral content of a rock can be typical of a depositional environment. Mineralogy of shale and siltstone has been used to evaluate the nature of the parent rock (or provenance) and the intensity of weathering. These also point us to the regional tectonic setting of depositional environment. Mature sedimentary rocks mostly contain very stable minerals (like quartz), whereas immature sedimentary rocks have a wide variety of minerals which include several unstable minerals like feldspars. E.g. tillite is an immature sedimentary rock that contains grains of varying size and its mineralogy includes unstable minerals like olivine and plagioclase feldspars.

6.3.2 Sedimentary Structures Sedimentary structures like crossbeds, ripple marks, load structures (Fig. 6.2) help us in precisely determining the depositional environment. They also support in establishing the direction of younging where the sedimentary sequence is overturned. Sedimentary strata and their thickness provide an idea of the energy of the environment and rate of sediment supply. Sedimentation happens in periodic pulses. In

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Table 6.1 Continental Depositional Environments Depositional environment

Dominant rock types

Dominant sedimentary structures

Fossil habitats

Stream Channel deposits

Conglomerate, sandstone

Cross-beds, ripple marks High energy, oxidizing environment with few fossils

Stream Floodplain deposits

Shale

Mud cracks

Alluvial fans

Conglomerate, arkose

Poorly sorted, cross-beds High energy, oxidizing environment with few fossils

Aeolian dunes

Sandstone

Well sorted, large scale cross-beds

Terrestrial reptile traces

Glacial till

Tillite

Angular to rounded grains, poorly sorted, un-stratified (massive)

High energy environment with few fossils

Glacial outwash plains

Sandstone, conglomerate

Ripple marks, cross-beds, similar to stream channel

High energy, oxidizing environment with few fossils

Swamps

Coal

Cross-beds, ripple marks, Plant fossils mud cracks

Lakes

Silt, shale, freshwater limestone

Graded beds, thin beds, varves, ripple marks, mud cracks

Lake dwelling organisms

Tidal flats

Mudstone, siltstone, sandstone, possible evaporites

Fine-grained, ripple marks, cross-beds, mud cracks

Mollusc shells, bioturbation

Terrestrial plants and animals

Table 6.2 Transitional Environments Depositional environment

Dominant rock types

Dominant sedimentary structures

Fossil habitats

Delta

Marine and non-marine mudstone, siltstone, sandstone, coal

Possible cross-beds, ripple marks

Terrestrial plants, mollusc shells, bioturbation

Beach and coastal

Sandstone

Fine to medium-grained, well-sorted, cross-beds

Mollusc shells, bioturbation

Ephemeral lagoons

Gypsum, anhydrite, halite Mud cracks, thin beds, salt casts

Extreme chemical environment with few fossils

6.3 Identifying Depositional Environments

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Table 6.3 Marine Depositional Environments Depositional environment

Dominant rock types

Dominant sedimentary structures

Fossil habitats

Continental shelf

Limestone, shale, sandstone

Cross-beds, ripple marks

Fish, coral, mollusc shells, sponges, echinoderms

Reefs

Limestone

Massive

Coral

Continental slope or rise

Mudstone, graywacke

Graded beds, turbidites

Microscopic plankton

Deep marine

Chert, chalk, limestone, Thin beds mudstone

Microscopic plankton

Fig. 6.2 Textural maturity in sedimentary environment

dry seasons, there shall be little or no flow of water and therefore significant sediment transport and deposition is unlikely. In wet rainy season, when the river has more water and energy, both sediment transportation and deposition occur. In marine settings, wave, currents and tide influence the sediments and sedimentary structures like ripples and cross beds are observed. Because of these periodic variations, thickness of sedimentary beds range from few millimetres to several meters. The influences of depositional environments are inferred from the sedimentary structures. Fossils, like burrows, help us understand the energy of the environment (see Fig. 5.5 in Sect. 5.4.1) as burrowing organisms have specific habitats comprising of characteristic sediments and water depth.

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6.4 Sedimentary Facies Sedimentary facies are properties of sedimentary rock that are used to distinguish one depositional environment from another. For example, a sedimentary rock deposited in a beach can be distinguished from the sandstones deposited in tidal flats from their grain size distribution, fossils and mineralogy. As different sedimentary environments lie adjacent to each other, one facies does grade into another. As compared to the beach facies, the tidal flat facies will have smaller average sediment grain size, more bioturbation, and shall contain Herringbone cross-beds. A sharp boundary between the two lateral facies would be difficult to determine as the lithologies gradually blend into each other. Figure 6.3 presents the schematics of lateral facies association along a coastal profile during transgression. Several sedimentary facies are found adjacent to each other: a beach and tidal facies grade into a marine or near-shore portion of a continental shelf, which in turn grades into an offshore carbonate platform or reef. Sediments of beach and tide flat facies are mostly sand, whereas the tidal facies is mostly mud. Reef facies consists of carbonate shells and corals. When these sediments are buried and lithified into sedimentary rocks, beach sands turn into sandstone, the tidal flat into shale or mud, and the reef sediments into limestone. During level rise (Fig. 6.3b), the shoreline moves inwards shifting the facies landward. The rise of sea level makes the sedimentary environment more submerged and this allows corals reefs to build and deposit limestone on top of shoreface and fluvial facies. The sequence of sediments, therefore, records a gradual lateral shift of its facies during a marine transgression. During a regression, the converse is observed, where fluvial facies are deposited on top of shoreface facies, which in turn overlies on limestone resulting in a sequence opposite to a transgressive one. Regressive sequences are less likely to be preserved in the rock record as compared to transgressive sequences. This is because as sea level falls, the parts of the continent, which was previously below sea

Fig. 6.3 Schematic of Facies Association and lateral shift during Transgression

6.5 Typical Sediments Tied to Depositional Environments

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level, are exposed to weathering and erosional agents. Therefore, these sediments are likely to be eroded. Such facies shifts relative to sea level changes has been observed along all continental shores over geologic time. This caused facies to shift both laterally and vertically. Discovery of such variations in a sedimentary basin led to the formalization of the law of correlation of facies, which is otherwise known as “Walther’s law”.

6.5 Typical Sediments Tied to Depositional Environments 6.5.1 Turbidites Clastic sediments, that erode from the continents, build up at the edge of continental shelves. When these accumulations reach a critical mass, they slide down the continental slope forming turbidity currents, which transport sediments to the ocean floor where they are eventually deposited. In the process of deep-water deposition, the coarse-grained sediments settle to the bottom first, followed gradually by finer sediments. This creates a graded sequence of sediments, where at the base lie coarse grained sediments and it gradually gets silty and eventually muddy. Such graded sequence of sedimentary rocks are called turbidites. Over several years, one turbidite layer is likely to be deposited on top of another, and this cyclic process creates repeated sequences of graded beds (Fig. 6.4a).

Fig. 6.4 Field Photographs of Sediments tied to Depositional Environments

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6.5.2 Varves Varves are annual layers of sediment that are deposited rhythmically forming a cyclic sequence of beds. Typically, varves are deposited in lakes in cold climates. The springsummer thaw results in high stream discharge, which brings in and deposits silt. And during winter, wind driven clay sized particles are deposited. Silt is usually rich in quartz and feldspar and is light-coloured, whereas the clay-size particles are dark coloured as they contain organic matter derived from planktonic (floating, mostly microscopic) organisms which flourished during the summer and perished in winter. Cyclic deposits of silt and clay, result in a couplet of sediment layers each year. This annual couplet, consisting of a light-coloured stratum of silt and a darker stratum of clay, serve as a geologic clock (Fig. 6.4b). Sequences of varves are found in lacustrine basins near glaciers during ice ages. Ice ages are times when continental glaciers proliferated outside of polar regions and during which the glaciers dammed many streams and created temporary lakes where these varves accumulated.

6.5.3 Marine Limestone Limestone, primarily consists of the mineral calcite and, forms in multiple depositional environments. Typical environments include hot springs, lakes and coral reefs in the tropical oceans. However, the most dominant depositional environment of limestone is shallow waters of tropical seas, where carbonate-shelled plants and animals proliferate. Limestone is made from lime mud which originates from dead and disintegrated organisms that have a calcium carbonate exoskeleton. This lime mud forms massive limestone deposits which lack bedding planes (Fig. 6.4d). Layering in limestone is found in lithified coral reefs as their growth is impacted by transgression and regression. Regression exposes the corals to weathering and the weathered limestone clasts are deposited at the foothills of the reef. Transgression helps the coral to grow until the next regression and this cycle repeats with geologic time. Such layering, when present, helps in identifying transgression and regression episodes of the past.

6.5.4 Tsunami Deposits Quakes in offshore subduction zones trigger extremely large water waves called ‘tsunamis’. A tsunami deposit is a sedimentary unit deposited by a tsunami. Such deposits may be left onshore during the ‘inundation’ phase or offshore during the ‘backwash’ phase. There remain considerable problems, however, in distinguishing between deposits caused by tsunamis and those caused by storms or other extreme sedimentary processes. When there is a sudden drop in land level and

6.5 Typical Sediments Tied to Depositional Environments

87

then a tsunami washes over it, it creates distinctive markers in the sediment layers (Fig. 6.4e), where coarse sand deposited by the Tsunami is overlaid on relatively finer sand deposited during normal times. Such deposits are used to identify past tsunami events, which are indicative of past events of marine earthquakes.

6.5.5 Deltas At the point, where a river approach a lake or an ocean, its velocity significantly reduces. As a result, the sediments, which were in before transport, are deposited to form sand bodies. The sand bodies force the stream to meander its way to the ocean and in the process it splits into several rivulets (or distributaries) forming a triangular fan with a gentle slope. This triangular fan is called a delta, which is also impacted by waves and tides. Deltas, therefore present a transitional environment, where currents of fresh water carrying sediment meet waves and tides. It is an area, where there is huge nutrient supply from land that support the proliferation of a variety of flora and fauna. Deltas, where large rivers meet the ocean, are huge and extend well into the present day oceans. E.g., the Brahmaputra River delta in Bangladesh has a submarine component, known as the Bengal fan. Another example is the Mahanadi delta of Odisha, India, (Fig. 6.5) which has a coastal stretch of over 100 km.

Fig. 6.5 The Mahanadi Delta in Odisha, with its distributaries

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At places, where the river meanders along low slopes and at delta plains, dense vegetation produce loads of organic matter. In this humid habitat, the decomposition of organic matter (primarily from plant origin) is inhibited. This leads to the preservation of the organic matter, which is later converted to coal on lithification. The process of coalification starts when dead plant matter does not biodegrade, and is first converted into peat. The peat bogs are eventually buried deep by later sediments and in course of several millions years, the heat and pressure of the overburden compacts the peat. This compaction reduces water content and enhances the proportion of carbon, thereby transforming peat into coal. Coal deposits are therefore characteristic of humid, tropical deltaic settings. It is also common for oil deposits to be found in deltas. The organic matter sourced from marine flora is deposited with the clay sized sediments. This forms shale which is rich in organic carbon. When such shales are subsequently buried deeper to higher temperatures, the organic matter thermally matures to produce petroleum. As deltas have the depositional environment for both shale and sandstone, the hydrocarbons which matures from shale can conveniently migrate and accumulate in the adjacent sandstones, which then becomes the hydrocarbon reservoir. Drilling into delta deposits by oil companies has led to detailed knowledge of the composition, structure, texture, facies, and fossils that are typically deposited in different parts of a delta.

6.5.6 Fluvial Deposits A sedimentary sequence consisting of sandstones, conglomerates, siltstones, shales, and plant fossils indicate terresterial sedimentation by river systems. Thick woods and densely vegetated swampy areas around rivers would produce coal. The sedimentary structures and characteristic signatures of a fluvial depositional environment include meandering river channels, sandbars, stream bank erosion, and muddy flood plains. A river undergoes avulsion and subsidence (Fig. 6.6), and both influence the size and geometry of the sand bodies formed. Avulsion is the process by which flow of water is diverted out of an established river channel into a new course on the adjacent floodplain. This leaves behind a sand body as the river changes course. Over time, the river avulses laterally depositing sand on top and beside each other. Deposition of sand bodies over older sandstones and mud result in lithostratigraphic loading which leads to subsidence. Hence avulsion and subsidence happen in tandem in a fluvial system, where the deposits are primarily sandstone and mud. The rate of avulsion and the rate of subsidence impact the distribution of sand bodies in space. Low avulsion and subsidence will result in connected sand bodies whereas high subsidence and avulsion shall result in isolates sand bodies. Understanding avulsion and subsidence is critical to identification of connected or disconnected sand bodies in a fluvial system. This has implications on petroleum production from sandstone reservoirs in fluvial settings.

Further Readings

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Fig. 6.6 Avulsion and Subsidence in Fluvial Environment

6.6 Summary Depositional environments and facies distribution has huge implications on petroleum systems and on the geometry of reservoir rocks and their properties. As different environments tend to produce sediments of varying thickness and spatial extent, identification of the environment through their facies associations is important to estimate reservoir properties. As we explore for hydrocarbons and coal, depositional environments in a sedimentary basin play an important role in identification of petroleum systems. It also helps us in drawing a holistic picture of the sedimentary basin, which, not only helps identify petroleum system elements. But also increases the chances of petroleum discovery in analogous sedimentary basins.

Further Readings 1. Nichols, G. (2009). Sedimentology and stratigraphy (2nd ed.). Wiley-Blackwell. 2. Hans-Erich, R., & Singh, I. B. (1980). Depositional sedimentary environments. Springer.

Chapter 7

The Petroleum System

7.1 The Carbon Cycle The essential components of hydrocarbons are hydrogen and carbon, which are derived from the chemical breakdown of carbon dioxide and water through photosynthesis. Therefore, understanding hydrocarbons or petroleum requires an understanding of both biology and geology. The carbon cycle (Fig. 7.1) refers to the natural, biological pathways through which carbon is transferred from inorganic sources to living organisms and back. Plants are photoautotrophs, who make their own food from inorganic (non-living) sources like atmospheric carbon dioxide and water using sunlight. The process is called photosynthesis, which converts basic elements into more complex molecules. The basic building blocks of all hydrocarbon molecules come from carbon dioxide (CO2 ) and water (H2 O), which are combined during photosynthesis to form carbohydrates. Carbohydrates are then used by plant to grow. Living organisms die and the organic compounds (which are essentially hydrocarbons) in their bodies decompose, disintegrate and assimilate into the surrounding environment. However, in cases, where the organic matter is buried along with sediments before disintegrating, it is preserved in sediments under anaerobic conditions. Burial and compaction over geological time subjects the preserved organic matter to suitable temperature and pressure, after which its converts into fossil fuels like coal, oil and natural gas.

7.1.1 Slow and Fast Carbon Cycle As atmospheric carbon is assimilated into the geosphere through photosynthesis, it is also released back in to the atmosphere by natural processes. The assimilated carbon in organic matter is released back to the atmosphere by the process of decomposition. The carbon in fossilised organic matter in the sedimentary rocks take a longer time to be exposed to weathering and mass wasting processes, which release them back to © Springer Nature Singapore Pte Ltd. 2023 S. N. Kundu, Geoscience for Petroleum Engineers, https://doi.org/10.1007/978-981-19-7640-7_7

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Fig. 7.1 Slow and Fast Carbon Cycle

the atmosphere. The former, being a faster process, is referred to as the “fast carbon cycle” and the latter is known as the “slow carbon cycle” (Fig. 7.1). Another way where carbon re-enters the carbon cycle is through volcanoes. Volcanic activities are sporadic and occur at specific locations along plate margins. On an average, volcanoes emit somewhere between 130 and 380 million metric tons of carbon dioxide each year, whereas anthropogenic activities account for 30 billion metric tons.

7.2 The Petroleum System The petroleum system is a part of the “slow carbon cycle”. The concept was formulated by Magoon and Dow in 1994, where all of the controlling factors, which determine the presence of petroleum in an accumulation were identified. To establish a petroleum system in a sedimentary basin, the source rock, from where petroleum would have matured needs to be identified. The condition of maturation and timing of generation of hydrocarbons from the source rock must be estimated. In addition, the means of migration of these hydrocarbons to the reservoir rock and within the reservoir rock to the trap must also be identified. The elements of a petroleum system, which are illustrated in Fig. 7.2, are the source rock, the reservoir rock, the trap and the processes of hydrocarbon generation, migration, accumulation and preservation.

7.3 Source Rock

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Fig. 7.2 Elements of a Petroleum System

Most of the elements of a petroleum system are dependent on sedimentary depositional environments. The elements are influenced by the geography, tectonics and the flora and fauna that was prevalent in those sedimentary environments. The properties of these elements are intricately linked to the process of sedimentation, rate of burial and diagenesis. The physical entities of a petroleum system, therefore, are the source rock, reservoir rock and the seal rock. Whereas, the fundamental processes in petroleum system are maturation, migration and accumulation of hydrocarbons. When all the components of a petroleum system are found to exist in a sedimentary basin, the likelihood of discovering petroleum in economic proportions is high. Petroleum systems are more probable in sedimentary basins which have a long history of sedimentary deposition, and is evidenced by thick sedimentary sequences deposited in a variety of depositional environments. This increases the probability of occurrence of a source rock rich in organic matter and the occurrence of porous and permeable reservoir rocks in vicinity, and also the occurrence of a non-permeable seal or cap rock protected by thick overburden.

7.3 Source Rock One fundamental element of a petroleum system is the source rock. It is the rock which contains organic matter that thermally matures into petroleum. The source rock is deposited in favourable depositional environment that support the proliferation of organic activity. Source rocks are primarily organic rich shale. Shale varies in colour from black to grey to reddish to brownish to greenish grey. The green and grey shale are the ones with high organic content and are potential source rocks. Shale

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is generally porous but not permeable because of its small pore size. Based on the thermal history of the sedimentary basin, the source rock can be active or inactive. An active source rock is that part of the source rock which is buried deep enough to produce hydrocarbons. Whereas, the part which has not achieved that critical temperature to produce hydrocarbons, is the inactive source rock. Inactive shale is mined as oil shale and hydrocarbons are extracted by heating it in a furnace.

7.3.1 Kerogen Upon burial and heating, the organic matter in the source rock converts into “Kerogen”. Kerogen is the solid and insoluble organic matter which yields hydrocarbons upon heating. The assemblage of organic compounds in kerogen govern the type of hydrocarbons it shall generate upon maturing. Kerogen does not have a fixed chemical composition as it is essentially an assemblage of dead organic matter. Therefore, it can only be differentiated based on the atomic ratio of hydrogen (H), oxygen (O) and carbon (C). The relationship between the atomic ratios is inherited from the original organic material that was preserved. Dirk Willem van Krevelin (a chemist and professor of fuel technology at the TU Delft) explored this relationship and developed a cross plot diagram of the hydrogen:carbon atomic ratio as a function of the oxygen:carbon atomic ratio. This is popularly known as the Van Krevelin Diagram (Fig. 7.3).

Fig. 7.3 The van Krevelin diagram

7.4 Reservoir Rock Table 7.1 Kerogen types and characteristics

95 Kerogen type

Hydrocarbon potential

Hydrogen proportion

Depositional environment

I

Oil prone

Abundant

Lacustrine

II

Oil and gas prone

Moderate

Marine

III

Gas and coal prone

Small

Terrestrial

There are three types of kerogen which an be differentiated using Van Krevelin diagram. The characteristics of the types are provided in Table 7.1. Type I kerogen has a high H:C ratio and low O:C ratio. It is Sapropelic (meaning “putrid mud”) which is rich in proteins, waxes and fatty acids and is derived from algal matter. Kerogen of type II is characterized by the relatively hydrogen-rich spores and pollen of land plants, marine phytoplankton cysts and some land plant components such as leaf and stem cuticles. Type II kerogen is deposited in low-oxygen marine settings and can produce oil, gas or a combination of the two. Kerogen of type II can also be formed from partial degradation of type I kerogen or from varying mixtures of type I and type III kerogen. Type III kerogen contains compounds like cellulose which is derived from land plants. The relatively high concentration of oxygen is a distinguishing feature of this type of kerogen. Natural gas is the only significant hydrocarbon produced by the thermal maturation of this type of kerogen. When the source rock transitions from diagenesis to catagenesis, the kerogen matures and generates hydrocarbons. The thermal boundaries for catagenesis are typically between 70 and 200 °C, which is referred to as the oil window. Metagenesis starts around 200 °C, after which most liquid hydrocarbons disintegrate to form smaller gaseous hydrocarbons and eventually methane. Temperatures between 200 °C and 400 °C is referred to as the gas window and above 400 °C the source rock enters the metagenetic stage which chars the kerogen completely to form carbon residue.

7.4 Reservoir Rock Reservoir rocks are the porous and permeable formations which accommodate and store hydrocarbons expelled from the source rock. Therefore, the prime properties of a reservoir rock are it’s the ability to accommodate, permeate and store petroleum in economic proportions. Porosity and permeability of sedimentary rocks is illustrated in Fig. 7.4.

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Fig. 7.4 Porosity and Permeability in rocks

7.4.1 Porosity Rocks have voids inside them which can be microscopically small or megascopically as big as a cavern. The proportion of total volume of void spaces inside a rock to its bulk volume is used to quantify porosity. Por osit y = (V olume o f V oids/T otal V olume) × 100% Porosity in rocks can be of different types. The porosity resulting from the spaces between the sedimentary grains during the process of its deposition is known as primary porosity. In clastic reservoir rocks like sandstones, the porosity type is mostly primary. Much of the porosity found in sedimentary rocks, such as sandstone and siltstone, occurs in the spaces between the sediment grains. Secondary porosity, on the other hand, is the amount of space created after the rock was formed, especially from later processes like hydro fracturing and dissolution etc. However, porosity may diminish because soluble minerals tend to deposit as cement in the primary pore spaces. Interconnected pores allow the flow of fluids through rocks. Therefore, the proportion of connected pores, which facilitates fluid flow, is important for production of water or petroleum. This volume of interconnected pores is known as effective porosity.

7.4.2 Permeability Permeability is the capacity (or resistance) of a rock to transmit fluids through its pore spaces. The pores in the reservoir rock must be well connected to make it permeable. Most source rocks are not permeable as their pores are smaller in size and are not interconnected. When not charged with petroleum, the pores of a reservoir rock is normally filled with water or brine. Rocks are heterogeneous in nature. This is why porosity and permeability are not uniform and vary within the rock and exhibit anisotropy (different in different directions within the rock body). Characterising this heterogeneity is often challenging

7.5 Seal Rock

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and a geologist needs to understand the complex relationship between the pore size, shape, orientation, interconnectivity and their spatial distribution in the reservoir rock. A poor reservoir rock will have low permeability arising from smaller pore or from poorly connected pores. In such reservoirs, more effort is needed to extract the trapped petroleum. A good reservoir rock contains well-connected, large pores and is more permeable and therefore can produce hydrocarbons at ease.

7.5 Seal Rock The seal rock is a layer which is placed in a topographically higher location above the reservoir rock. Its function is to stop the hydrocarbons trapped in the reservoir rock from migrating further, thereby facilitating a localised accumulation called a trap. As petroleum is less dense than water, it migrates to higher grounds displacing the water above. Such buoyancy-driven accumulations are clearly marked by a water-oil interface in the petroleum trap. Changes in permeability along migration pathways within the reservoir rock can act as a seal too. In such cases, the reservoir and seal rock are one and the same. As seal rocks are as impermeable as source rocks, the source rocks which lie atop reservoir rocks act as the seal. The sealing mechanism differentiates seals to be a hydraulic seal or a membrane seal (Fig. 7.5).

Fig. 7.5 Membrane and Hydraulic seals

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Membrane seals are the ones which create a pressure differential below and above the seal. When the pressure differential exceeds the threshold displacement pressure, the sealing effect will cease and the seal rock will allow hydrocarbons to escape through it, until the pressure differential drops below the threshold again. In membrane seals, the pore size of the seal rock is a key factor as it influences the capillary pressure within the seal. Faults, which contain crushed material along the fault plane, act as membrane seals. Hydraulic seal occurs in rocks which have significantly higher displacement pressure. This requires the rock to fracture to allow the hydrocarbons to escape. The rock will fracture only when the pore pressure is greater than its tensile strength. After a fracturing episode, a hydraulic seal can reseal only when the fracture is healed. Salt domes normally act as hydraulic seals. Salt has low tensile strength and it fractures under higher displacement pressure. Being ductile, it fuses again, healing the fracture without leaving a trace.

7.6 Overburden The overburden is sum total of all the rocks which lie atop a petroleum accumulation. Overburden is necessary for several reasons; firstly, to contribute to the lithostratigraphic pressure to heat the underlying source rock to mature petroleum, and secondly, to protect the seal rock from being eroded so that the accumulation is preserved. To produce lithostatic pressure, the overburden must be pretty thick, and to contain the heat produced by the pressure in the source rock, it must be a poor conductor of heat. Usually, silica-rich sedimentary rocks such as sandstone, siltstone and shale are poorer conductors of heat as compared to limestone, and salt. Apart from the thermal characteristics of the overburden, the source rock also depends on heat flow within the section. The combination of thickness, thermal characteristics and the heat flow within the overburden helps the source rock attain the critical oil-forming temperatures.

7.7 Petroleum System Processes Petroleum system processes include trap creation, maturation, migration and accumulation. These processes are typically timed in a petroleum system to make the trap likely to have a hydrocarbon accumulation.

7.7 Petroleum System Processes

99

7.7.1 Trap Creation A trap is a mechanism which stops the migrating oil in a reservoir rock and allows it to accumulate in economically significant proportions. It typically occurs when a reservoir rock is positioned against a seal in such a way that obstructs migration and supports accumulation. This makes the reservoir, the seal and the geometric closure, the three essential components of a petroleum trap. In Fig. 7.5, the closure is provided by the dipping beds that abut against the faults and the salt dome. Trap creation can be tectonic, which result from folding and faulting or it can be stratigraphic (e.g. lateral facies variation). Another type of trap is caused by combination of both processes, as in the case of salt domes. Hence traps can be structural, stratigraphic or mixed (Fig. 7.6). The geometry of the closure formed by traps stops the migrating hydrocarbons, which then accumulate hydrocarbons as it displaces the water in the pores. As oil, gas and water are immiscible, they occur stratified in the trap. This stratification is planar and is limited to the confines of the reservoir rock. Gas being the lightest is always at the top and above oil. Water being the heaviest is always at the bottom.

Fig. 7.6 Structural traps (a Fold and c Fault), Stratigraphic trap (b) and Mixed trap (d)

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7.7.2 Maturation In addition to the proportion and type of kerogen, the thermal regime of the source rock also influences the process of maturation. With rise in temperature, the complex organic compounds in kerogen break down to form simpler hydrocarbons. Maturation is a non-linear process and the production of hydrocarbons increase exponentially in relation to temperature rise. With deeper burial, the temperature increases more and the rate of hydrocarbon production accelerates until all of the reactive kerogen gets converted into hydrocarbons (Fig. 7.7). The average geothermal gradient of Earth’s crust being 35 °C/km, kerogen maturation in initiated when the source rock is buried to a depth of 3 km or more. As a thumb rule, maturation process starts when the source rock is subjected to about 70 °C and continues exponentially with temperature rise. Higher temperatures facilitate breaking down of long chained hydrocarbons to short ones. When the temperatures reaches 200 °C, the shortest hydrocarbons namely ethane and methane, which are gases under normal conditions, are produced. Shallower depths can only maintain temperatures which are too low for intense oil generation, whereas higher temperatures are sustained at deeper depths which converts almost all of the kerogen into hydrocarbons. At the later stages of the oil window, temperatures are

Fig. 7.7 The Stages of Maturation

7.7 Petroleum System Processes

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very high and this decomposes oil into condensates or “wet gas.” When the source rock enters the gas window, all the oil and condensates turn into methane or “dry gas”.

7.7.2.1

Thermal Exposure

Source rocks can be examined to determine its thermal history. To ascertain the temperature to which a source rock was exposed to, methods like Vitrinite reflectance (Ro) and Thermal Alteration Index (TAI) are used. Vitrinite is a maceral and an essential constituent of coal which is found in most source rocks alongside with other organic matter. As vitrinite is heated during burial, it systematically changes its reflective properties. Reflectance is the proportion of incident light reflected from a polished surface observed under a microscope. Vitrinite reflectance can be correlated with the temperature of exposure of the source rock that contains it. TAI, on the other hand, is based on the change in colouration of spores and pollens. Spores and pollens are fossilised along with organic matter in the source rock. Both TAI and Ro, are used to determine the thermal maturity of the source rock (Fig. 7.8a). The radiometric technique, where fission tracks in uranium bearing minerals like zircon are studied, is also a way to determine source rock maturity. Fission tracks are linear marks left in the parent crystal which result from radioactive decay and escape of subatomic particles. These tracks are sensitive to temperature as they partially dissolve when the parent crystal recrystallizes when re-exposed to high temperatures. The generations of tracks can be studied to ascertain both the temperature of exposure and also the temperature variability over time in the sedimentary basin (Fig. 7.8b).

Fig. 7.8 Thermal Maturity: a based on TAI and Ro. b Fission Tracks

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Fig. 7.9 Maturation and Migration of Oil and Gas

7.7.3 Migration and Accumulation Upon maturation, the voids in the source rock which contained kerogen, now have oil and gas. Gas and oil occupy more volume than the solid kerogen. Therefore, maturation increases the pore pressure in the source rock. As pore pressure increases, the source rock is fractured, releasing the oil and gas to the formations above and below. This is primary migration of oil and gas. When the neighbouring rock is porous, the water in the pores is displaced by oil and gas. Being hydrodynamically lighter, oil and gas travels upward due to buoyancy within the reservoir rock until they reach the trap, where they accumulate. The migration of oil and gas within the reservoir rock to the trap is called secondary migration. A schematic of maturation and migration is provided in Fig. 7.9. Accumulation process involves displacing the original water in the reservoir by oil and gas. In petroleum accumulations, oil, gas and water lay stratified with respect to their densities. Gas overlies oil and oil overlies water in the trap (Fig. 7.9).

7.8 Extents of a Petroleum System Just like a sedimentary basin, a petroleum system has its extents. The extents of a petroleum system is defined by a stratigraphic column and a geographic boundary and these are illustrated in Fig. 7.10.

7.9 Petroleum System Event Chart

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Fig. 7.10 Geographic and Stratigraphic Extents of a Petroleum System

The extents of a petroleum system is from the base of the spread of the active source rock to the topmost point in the reservoir rock where further migration of hydrocarbons is curtailed by the seal rock and overburden. Its geographical extents constitute the aerial extent of the petroleum system, that can be mapped, and within which all the exploration activities must be confined to. The stratigraphic extent is the vertical rock column which runs from the lower limit the gas window to the highest point of the overburden. Knowing the precise extents of a petroleum system is challenging but a broad understanding of these limits is essential to focus the exploration targets in a sedimentary basin.

7.9 Petroleum System Event Chart A petroleum system events chart details the sequence and timing of the events in a petroleum system. The geological ages of the petroleum system elements (i.e. source, reservoir, seal and overburden) and the timing of the petroleum system processes (i.e. trap formation, maturation-migration-accumulation and preservation) are plotted on the event chart. An example of a petroleum system event chart is provided in Fig. 7.11. The components, processes and the critical moment of a petroleum system are depicted in the events chart, in which the geologic time lies along the horizontal axis and the petroleum system elements on the vertical axis. The chances of finding oil typically exist in basins where the a petroleum system elements and processes are in the correct sequence with respect to geologic time. Hence, establishing the sequence and timing of each of the petroleum system element and process is critical. The critical

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Fig. 7.11 Petroleum System Event Chart (* critical moment)

moment, which is the time of highest probability of entrapment and preservation of hydrocarbons in a petroleum system, is an event which must happen after the traps are formed. This is because, hydrocarbons migrate into a reservoir after the critical moment and should there be no trap at that point in time, petroleum is unlikely to accumulate in economic proportions. This makes the probability of finding a petroleum accumulation very low.

7.10 Summary Ever since the concept of petroleum system was introduced, there has been an increase in the rigour of establishing a petroleum system before a trap (or prospect) is drilled. Earlier, anything suggestive of a structural trap was indiscriminately drilled, which made discovery of petroleum accumulation more by chance than from any scientific rigour. As all the low hanging accumulations are discovered and their production is nearing exhaustion, newer petroleum accumulations, which are located at difficult terrains, must be explored. As such exploration is expensive, establishing a petroleum system is crucial for increasing chances of discovery. Petroleum systems have added a leaf to sedimentary basin analysis studies, which is now considered very essential for successful hydrocarbon exploration projects.

Further Readings 1. Magoon, L. B., & Dow, W. G. (1994). Petroleum system—From source to trap. AAPG Memoire, 60, 3–24. 2. Selley, R. C., Sonnenberg, S. A. (2014). Elements of petroleum geology.

Chapter 8

Exploration Methods

Geologists use a wide range of methods to explore petroleum. To build confidence in geological interpretations, multiple approaches are always adopted from different perspectives to validate a geological hypothesis. All these approaches help in understanding the distribution of the petroleum system elements and processes at different scales (from the basin scale to the pore space scale). Initially, the exploration methods were not based on geoscience. But now, methods like the petroleum systems approach are adopted for successful exploration. The current chapter is a compilation of major geoscientific approaches, that are employed for discovery of hydrocarbons.

8.1 Seeps Exploration for hydrocarbons is understood to have begun in 1959 with the discovery at Oil Creek, Pennsylvania, by “Colonel” Edwin Drake. Since then the methods of exploration have continually evolved. The most primitive method of exploration was mainly digging pits where oil seeps were encountered on land. During those times, crude oil was primarily used for medicinal purposes, waterproofing, and as a disinfectant or insect repellent. Shallow pits or horizontal tunnels were dug where seeps were detected. Soon rudimentary bamboo poles were used for drilling oil wells up to 20 m when these shallow pits dried up. Seeps were found and exploited across the world, including in China, Baku in Azerbaijan, Iraq, Caucasus in Chechnya, Ploesti in Romania, Assam in India, Sanga Sanga in Borneo and Talara in Peru. After the energy potential of oil was known, it soon replaced whale oil for lighting lamps. This was when there was no electricity. Crude oil was then used to produce kerosene. Much later, after the invention of automobiles, oil was used as engine fuel and increased use of automobiles drove the demand of oil. Exploration of petroleum was based on various theories which evolved with time. Exploration now includes a comprehensive study utilizing geology and geophysics to understand the sedimentology of basins and its petroleum system elements. © Springer Nature Singapore Pte Ltd. 2023 S. N. Kundu, Geoscience for Petroleum Engineers, https://doi.org/10.1007/978-981-19-7640-7_8

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Petroleum seeps serve as a direct evidence of a petroleum system. Petroleum seeps are locations where natural liquid or gaseous hydrocarbons escape to the surface of earth from a reservoir rock or trap (Fig. 8.1a). The trap is breached when the overpressure and buoyancy combined, overcome the capillary resistance of the seal rock. The seeping frequency of a seal depends on the pressure differential across the seal and its sealing mechanism. A seeping sandstone reservoir in East Malaysia discovered during a site visit by the author is presented in Fig. 8.1b. Seeps generally occur above petroleum accumulations, both on land and sea. Seeps, on land, are accidentally discovered by people living around. However, seeps in marine settings are difficult to detect. The currents in oceans disperse the seeping oil far from its initial location and this makes it difficult to pinpoint the accumulation. Oil from seeps form oil slicks on the surface of oceans. Oil slicks can also result from an oil spill involving the capsize of a vessel carrying oil. Repeated occurrence of oil slicks in an area without any incident linked to an oil tanker are indications of a marine oil seep. Satellite imageries are helpful in detecting such oil seeps through repeat imaging of the same location over several years. Temporal analysis of Synthetic Aperture Radar (SAR) satellite imagery is used to detect oil seeps (Fig. 8.1c). Once such

Fig. 8.1 Petroleum seeps. a Schematics of a petroleum seep. b A seep in sandstone reservoir in East Malaysia. c SAR imagery with repeat slicks. d Multibeam backscatter for seabed seep detection

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a slick is detected, a multibeam backscatter survey and analysis helps to decipher the exact location of the seep at the ocean floor (Fig. 8.1d).

8.2 Geological Mapping In the nineteenth century, the then director of Geological Survey of Canada, William Logan, associated seeps to crests of topographic highs. He found that these locations were essentially the convex tip of folds, based on which he formulated the anticlinal trap theory. The concept of types of traps was non-existent then. Hence, the understanding of the folded nature of rock sequences was deemed important for exploration success. The anticlinal theory dominated for the next couple of decades until all such anticlinal structures found were drilled. The understanding of structural configuration of rock sequences introduced geology as a driver for oil exploration. Intensive and detailed geological mapping was the prime exploration method and this was always a challenge in remote and inaccessible terrains. Geologists made their way past the forests of Burmah, India and Borneo leading to discoveries and thus, companies like Burmah Oil (now a part of British Petroleum) and Borneo Shell were formed. Geological exploration, at the time, involved detailed field mapping of rock exposures. The mapping included lithological characterization and distribution of potential reservoirs, seals and source rocks. The concept of petroleum system was not integrated into exploration at this stage. The investigation methods were lengthy and cumbersome as they were based on mapping of stratigraphy and structure in the field. Examples of some structures from field mapping are provided in Fig. 8.2. It was only around 1920, when geologists realised that hydrocarbons could also occur in regions, where petroleum traps were not based on structural features. With discoveries of accumulations at unconformities and at strata where lateral facies varied, played an important role in adopting the discipline of geoscience. Geoscience was taken up further to characterise such stratigraphic traps, although

Fig. 8.2 An anticlinal fold below an unconformity (a) and a normal fault (b)

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the initial discovery of non-structural stratigraphic traps was more by chance, than by design.

8.2.1 Mapping from Air and Space It was only after World War II that aerial photographs from planes could be used for interpreting and mapping geological features. This was a huge step as such reconnaissance mapping could be done without having to set foot on the ground. Later, in 1972, when the first Landsat satellite was launched, imageries from space fed repeated information on Earth’s changing landscape. Thereafter, the advent of hyper spectral satellites and airborne platforms provided multi resolution and multi spectral imageries that helped interpretation of lithology. Remote Sensing and Aerial photographs supported the efficient scanning of wide regions for structural and lithological indicators of petroleum traps. Such activity reduced the manual effort and enhanced the speed of exploration. Today, we have several space missions, where satellite imageries are being acquired at high temporal resolution, which cover almost all parts of the globe. Most rocks are buried under soil and vegetation, and therefore, they can only be studied from satelite images, when the rock outcrops. Field mapping of these outcrops was cumbersome and time consuming. With the need to efficiently map as much of the Earth’s land surface, air and space based mapping technologies were used extensively to map geological structures, which helped in identifying petroleum prospects. Images taken from space and air can be either multispectral and photo-geological. Multispectral data can cover wide range of the visible and near-visible spectrum (with wavelengths ranging from 0.1 μm to 50 cm). The wavelengths which are absorbed by atmosphere are excluded during the design of the spectral bands of space imaging sensors. Each narrow band of wavelength records the reflectance of the Earth’s surface and these bands can be used to quantitatively classify the image pixels into land cover categories. Textural classifiers can objectively classify complex objects with their spectral signature ranges and are important in determining lineaments which are manifestations of the underlying geology. Photo-geological images, mostly acquired by airborne platforms, have the advantage of seeing objects in the visible spectrum and in stereoscopic mode (using image pairs with a tilt angle). Both these methods have their advantages and disadvantages and are selectively used based on their scope of application in exploration projects. Both forms are used to identify large-scale geological structures (Fig. 8.3) which could not be identified by other means.

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Fig. 8.3 Satellite images of (left) Jashak salt dome in Iran and (right) San Andreas Fault in USA

8.2.2 Drainage Pattern and Subsurface Geology Where the surface is covered with soil and vegetation, interpretation of stream drainage patterns provide an understanding of the underlying geology. Drainage patterns develop based on the topography and underlying geology. Common drainage patterns and the likely underlying geology are provided in Fig. 8.4. Dendritic drainage is a random pattern which commonly develops on homogenous bedrock, such as flat and horizontal rock strata. Parallel drainage patterns develop mostly on gently dipping beds of rock. On folded terrains, the trellis pattern commonly forms where the streams are on the synclines that are separated by the ridges located at the hinge of the anticlines. Rectangular drainage patterns form on fractured and faulted rocks. Fractures are either annihilation cracks in igneous rocks or are formed due to decompression of deeply buried metamorphic rocks due to

Fig. 8.4 Interpretation of subsurface geology from drainage patterns. a Dendritic drainage (horizontal strata). b Parallel drainage (gently dipping strata). c Trellis drainage (folded strata). d Rectangular drainage (faulted or jointed non-sedimentary rocks). e Deranged drainage (glacial till). f Radial drainage (dome structures)

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removal of overburden rocks. Deranged drainage patterns signify geological events like glaciated landscape or landscapes impacted by active volcanoes. Radial drainage patterns are associated in terrains where multiple generation of folds which form domes and basins. Drainage patterns are easy to pick from satellite imageries and aerial photographs. They present a very useful means to identify and rule out nonsedimentary provinces during the search for petroleum systems. Identification of structural elements from drainage patterns helps in identifying potential traps which could be part of a petroleum system.

8.3 Geophysical Methods Post 1920, with the advent of geophysical methods like gravity, magnetic and seismic refraction, surface geological mapping could be extended into the subsurface. Gravity and magnetic methods, in particular, helped discover oil accumulations in salt domes on the shore of Gulf of Mexico. Around this time in France, the Schlumberger brothers developed methods to measure properties of rocks and fluids which were encountered during drilling for hydrocarbons. Resistivity of the rocks was the first to be measured which was soon followed by electrical, sonic and radioactive methods. Today, we have sophisticated methods to log porosity, permeability, mineralogy, and fluids in wells. These advanced methods are nothing but enhancements of the fundamental concepts devised by the Schlumberger brothers.

8.3.1 Gravity Method Gravitational prospecting uses Newton’s Law, which relates the force of mutual attraction between masses as a function of distance between them. The law essentially states that two masses separated by a distance shall be attracted to one another by a force F as follows: F =G

m1 m2 r2

where, G is the universal gravitational constant equals 6.67 × 108 . ‘m1 ’ and ‘m2 ’ are the masses of the individual bodies and ‘r’ is the separation between them. The unit of gravity is Gal and at the Earth’s surface the gravitational attraction is about 980 gals. In exploration geophysics, gravity measurements are at much sensitive in scale, which is why a unit of mGal (milliGal) is commonly used. Deviation from the expected gravity at a location is identified as an anomaly. These anomalies are caused by the heterogeneous densities of the rocks and their uneven distribution in space. As the measured gravity is a composite for all rocks underneath a

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Fig. 8.5 Gravity anomaly over salt domes (a) and Salt domes on the seabed of Gulf of Mexico (b)

location, any subtle change can be attributed to variation in density. Significant anomalies will reflect a significant deviation from normal density. An example is in the case of salt domes where large bodies of salt having low density is surrounded by denser sedimentary rocks (Fig. 8.5a). Gravity methods led to the discovery of salt domes in the Gulf of Mexico (Fig. 8.5b). On land, gravimeters are carried to the field for data acquisition, but nowadays a gravity surveys can be done efficiently from airborne platforms. Airborne platforms work equally well for both over land and over water. Land-based instruments do measure gravity anomalies with greater accuracy, precision, and resolution but airborne survey covers a larger area, is faster and is suitable for basin scale exploration. The raw gravity data is subject to several of corrections before they can be geologically interpreted. The location, time, elevation and water depth are necessary parameters which are used to process gravity data. The common corrections which are applied are as below. . Drift Correction: Instrumental drift is based on repeated readings at the same location at different time during the survey to establish the rate of dilation of the spring in the instrument. The correction is done by adjusting the dilation of the spring periodically during the survey. . Latitude Correction: Gravity readings vary with latitude because of the nonspherical shape of Earth and because of variations in angular velocity. Gravity at the poles exceeds gravity at the equator. The distance, from the Earth’s centre of mass to the point where gravity data is collected, is used for latitude correction. . Elevation Correction: Correction for the differing elevations of gravity stations is made in three stages. The first stage is the “Free Air” correction, which accounts for the decrease of gravity, because of free air in between the instrument and the geodetic datum. The second is “Bouguer” correction which accounts for the gravity effect of the rock present between the observation point and the geodetic datum. The last one is the “terrain” correction which takes topographic

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relief into account, which induces a lateral pull to the gravimeter readings. High terrain effect is experienced due to high relief. . Tidal Correction: Periodic variation in Earth’s gravity field due to changing position of the Sun and Moon causes tide. The same variation, also impacts gravity measurement during long surveys, require a correction for tide. . Eotv ¨ os ¨ Correction: This correction is applied to gravity measurements taken on a moving vehicle such as a ship or an aircraft. Depending on the direction of travel, vehicular motion will generate a centripetal acceleration which could both reinforce or oppose measured gravity values. The corrected data collected over an area is plotted as gravity contours. These contour maps display the horizontal variations of rock density, where anomalies can be detected at both regional and local scales. The size, shape, and orientation of gravity anomalies are expected to be similar to the shape, orientation, and areal extent of any underlying petroleum traps (specifically the structural traps). Gravity data can be used to define the regional tectonic regime, to prioritize areas for seismic work, and to identify the extent of sedimentary basins. Sedimentary rocks being less dense exhibit a negative gravity anomaly as compared to the dense igneous and metamorphic provinces. The ease of acquisition and low cost of gravity data works well where expensive seismic data is difficult to obtain. However, as interpretation of gravity data is very generic and therefore it of often studied in conjunction with magnetic data for more specific interpretation.

8.3.2 Magnetic Method Our Earth exhibits magnetic polarity as the electrical currents and its convection within the Earth’s liquid core generates a powerful magnetic field. Although most rock-forming minerals are non-magnetic, certain rock types contain sufficient magnetic minerals which are susceptible to the magnetic field of Earth. Presence of such magnetic minerals in the rock crystallizing from the magma at mid oceanic ridges acquire the remnant magnetism of Earth. Over time, when Earth’s magnetic field changes, the rocks bearing magnetic minerals exhibit significant magnetic anomalies. Magnetic surveys are widely used in the early stages of exploration. Important techniques have been developed to estimate the depth to the basement in sedimentcovered areas from aeromagnetic data. All magnetic anomalies, caused by rocks, are superimposed on the geomagnetic field, just in the same way as gravity anomalies are superimposed on the Earth’s gravitational field. However, unlike gravity data, magnetic data is a vector and therefore needs to be reduced to the equator or the poles. The magnetic anomalies are magnetic susceptibility deviations from the Earth’s magnetic field. Anomalies are measured in nanotesla (nT) and 1 nT is equivalent to 10–9 T.

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Rock magnetism is either induced or is remanent. The induced component is proportional to the Earth’s magnetic field and is referred to as “magnetic susceptibility.” A vector sum for the remanent and induced components is used the measure the degree of magnetization. Rocks, when exposed to a magnetic field, can exhibit Diamagnetism (repulsive to a magnetic field), Paramagnetism (attracted to magnetic fields) or Ferromagnetism (pronounced magnetic properties). Remanent magnetism exists where there is no applied magnetic field and is entirely independent of the induction effects. Airborne magnetic survey is the most cost effective methods where geolocation is aided by GPS. Ship borne magnetic surveys require a fish, carrying the instrument, to be towed behind a ship. Marine surveying is slower than aeromagnetic surveying, but is frequently carried out in conjunction with several other geophysical methods, such as multibeam bathymetry, gravity surveying and seismic profiling, which cannot be employed in the air. Just like gravity data, raw magnetic data is subjected to several corrections as below; . Diurnal Correction: Magnetic field changes over time during the day need to be corrected. Diurnal variation during an aeromagnetic survey may alternatively be assessed by arranging numerous crossover points in the survey plan. . Geomagnetic Correction: This is the equivalent of the latitude correction in gravity surveying. This correction removes the effect of a geomagnetic reference field from survey data. . Elevation and Terrain Corrections: The vertical gradient of the geomagnetic field is only some 0.03 nT/m at the poles and −0.015 nT/m at the equator. The influence of topography can be significant in ground magnetic surveys but is not completely predictable at most places. The interpretation of magnetic data is more complex than gravity data. Anomaly maps are interpreted in terms of the effects of basement (igneous and metamorphic) and basin fill (sedimentary) rock bodies. Magnetic data alone is ambiguous as far as petroleum exploration is concerned. In Fig. 8.6, where the producing wells of Mumbai oil and gas fields are plotted on the magnetic anomaly map, a clear correlation with the reservoir was absent. However, when magnetic data is studied along with gravity data things were more definitive. E.g. Salt domes show negative anomalies in both gravity and magnetic data. Gravity and Magnetic methods are apt for studies at the basin scale studies during petroleum exploration but are more important in identifying metallogenic provinces for exploration of ore minerals.

8.4 Sedimentology and Geochemical Methods Around 1950, after Walther’s law of facies change was postulated, the understanding of lateral and vertical variation of facies was widely accepted. This gave birth to the discipline of sedimentology, which is presently an essential means for reconstructing

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Fig. 8.6 Magnetic anomaly map of Mumbai oil and gas fields, India

modern analogues to past depositional sequences and helps in understanding the controls of depositional processes in sedimentary basins. Application of sedimentological techniques led to the discovery of several huge petroleum accumulations in several countries like USA (Yates Field), Mexico (Posa Rica Field), Middle East (Kirkuk field) and the Siberian oil fields of Russia. The discovery of carbonate reservoirs in West Texas and Canada created deep interest in sedimentology of those basins where inter-tidal carbonate-evaporite sequences were predominant. Sedimentological methods combine the use of stratigraphic studies and facies analysis to delineate the geohistory of the basin. Geohistory, reconstructs the different stages of evolution in a sedimentary basin through geological time, and this involves backstripping analysis which accounts for sediment loading, compaction, eustatic changes, isostatic effects and tectonic subsidence. Geochemistry is the application of chemistry to the study of the Earth matter. Developments in geochemistry, supported the understanding of petroleum systems through quantification of the source rock maturity, its kerogen content and the distribution of source rock potential at a regional scale. Biomarker analysis was done using paleontological studies and Isotope analysis was done to understand hydrocarbon origin. Geochemistry, along with biostratigraphy (which includes palynology: the study of spores and pollens) thus became de-facto exploration tools.

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Geochemical methods include the analysis of soil and water for their absorbed hydrocarbon content, salinity, micro seeps, vitrinite reflectance and spore (and pollen) coloration indices.

8.4.1 Micro-seeps and Soil Analysis Microseeps are seeps which are not visible to naked eye. Some traps leak to release small traces of hydrocarbons to the surface (Fig. 8.7a). These hydrocarbons are absorbed in the water and pores in the soil profile. Core samples from shallow depths (~2 m) are collected for surface geochemical studies. This includes gas chromatography (GC) analysis, atomic absorption spectroscopy (AAS), and microbial analysis. The soil samples are analysed for their light hydrocarbon gases like C1 , C2 , and C3 using GC (Fig. 8.7b). Powdered soil samples are chemically treated to remove silica and then made into a solution from which the concentrations of trace metals Ba, Co, Cr, Cu, Ni, Pb, T, U, and V are determined using AAS. For microbial analysis, soil samples are normally collected and the hydrocarbon oxidising bacteria are isolated and enumerated. These bacteria feed on crude oil to release carbon dioxide.

8.4.2 Result Visualization The analysed data is then visualized for identification of anomalous clusters (Fig. 8.7c). Once clusters are identified, they are investigated further to establish the underlying reservoir or trap. Microseeps in the seabed have been instrumental in several recent petroleum discoveries. Microseeps motivated researchers toward drilling a well in the Khourian Desert in Iran which led to the discovery of a large hydrocarbon accumulation. Conventional and indirect surface geochemical methods provide an insight into the relationship between surface and subsurface hydrocarbons (Fig. 8.7).

8.4.3 Other Geochemical Methods There are other geochemical methods, that are used for quantification of organic carbon content in the source rock, and its potential to generate hydrocarbons. Kerogen type is identified by determining oxygen-carbon and hydrogen-carbon ratios, and the maturity stage is determined by vitrine reflectance values. Determination of the Total Organic Carbon (TOC) is a fundamental part of source rocks evaluation, and

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Fig. 8.7 Microseeps in sandstone (a), sample gas chromatograph (b) and gas cluster map (c)

it is a very critical determinant of the potential productivity of discovered oil reservoirs. There are a number of techniques for the determination of the TOC which is included in the due diligence for most source rock evaluations.

8.5 Seismic Exploration With the help of seismic reflection, geoscientists were capable of detecting subsurface features at different scales. The seismic method also helped in estimating the physical properties and geometry of rock bodies and structures in the subsurface. The seismic reflection technology is based on reflection of sound waves from rock and structural interfaces. The time of arrival and intensity of the reflected waves at the receiver was helpful in detecting the depth of the rock interface where the acoustic contrast influences the reflection sound wave. With all the low hanging shallow prospects identified, discovered and exploited, geologists had no other option but to use seismic surveys to find oil in deeper and offshore locations. Seismic data were first used in the discovery of a Texan oil field. Reflection seismic provides an image of the subsurface in two or three dimensions (2D or 3D). It is the most widely used technique in hydrocarbon exploration today. The technique exploits the density contrast and the changes in acoustic velocity

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between rock layers. Seismic reflection is an active geophysical method where seismic pulses are generated by employing an air-gun, vibrator or dynamite. The reflected pulses are recorded by an array of detectors which are at a distance from the seismic source. Air-guns are used for marine acquisition whereas vibrator and dynamites are used for land seismic surveys. The receivers employed in land and sea are based on different technologies. The strength of pulses generated governs the depth of penetration into the subsurface and is decided based on the depth of the target in the exploration program.

8.5.1 Fundamentals of Seismic Reflection A pulse from a seismic source is initiated at or near the Earth’s surface. The amplitudes and travel times of the reflected waves are recorded by an array of receivers. Two kind of waves are generated by seismic source that can travel through solid rock. These are ‘P’ or compressional waves and ‘S’ or shear waves. P-waves travel twice as fast as S-waves. The sound waves travel between the layers with different velocities and at each interface shall refract, as per Snell’s law: Sinθ1 /Sinθ2 = V2 / V1 where V1 and V2 are the velocities of sound in the first and second rock layer, Sinθ1 and Sinθ2 are the sines of the incident and refracted ray angles. The velocity of sound in the two layers will be different as their densities are different. In such a case, a part of the sound wave shall be reflected back and the other part shall be refracted. The proportion of the sound energy reflected shall depend on the acoustic impedance of the rocks. Acoustic impedance is the product of density and acoustic velocity for a medium. The reflection coefficient (R) of a normally incident P-wave on a boundary is given by: R = (ρ 2 V2 − ρ1 V1 )/(ρ 2 V2 + ρ1 V1 ) where ρ1 and ρ2 are the densities of the upper and lower layers, V1 and V2 are their respective P-wave velocities. Any event and contact between subsurface rock formations which causes a large contrast in impedance shall generate a strong reflection.

8.5.2 Seismic Surveys The energy from the acoustic source propagates in many directions in form of a spherical wave front. The subsurface directed energy reflects and refracts as it encounters an interface between two rocks layers with varying acoustic impedance. An array of

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Fig. 8.8 Schematic of a seismic survey on sea

receivers record the reflected signal (typically in the frequency range of 5–100 Hz). P-waves are the ones which are recorded and analysed as they travel faster than S-waves. P-waves also react differently to different fluids in the rock pores. The schematic of a seismic survey on sea is provided in Fig. 8.8. The end result from a seismic survey is a set of seismic traces with the locations of each trace separated by a fixed interval. A seismic trace is the processed ray path from the location on the surface to the deepest location of the subsurface at the same location, that was calculated based on data recorded during the survey. The ray path has wiggles, which are proportional to the acoustic impedances at each rock interface. The best way to explain a seismic trace is through a synthetic seismic trace (Fig. 8.9), where known values for densities and seismic velocities in a rock column are used to calculate the acoustic impedance and reflection coefficients. A synthetic seismic trace, therefore, can be obtained for any stratigraphic column for which the rock properties are known. However, as the seismic survey is conducted before the area is drilled, the rock properties can only be estimated at this stage. Thus, the resulting seismic image from the survey is likely to contain artefacts of processing algorithms. Post drilling, when the properties of the rock column in the area could be more accurately assessed, synthetic seismograms are constructed based on which the processed seismic data is validated. Should there be mismatches, the seismic data must be reprocessed using the more accurate values for velocities and densities and these are usually obtained from well logs.

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Fig. 8.9 The Synthetic Seismic Trace

Seismic surveys, data acquisition and processing are very different for land and marine regions. In marine seismic acquisition, large ships are used where streamers consisting arrays of receivers are towed. Multiple streamers are used for 3D surveys. The seismic impulse source used is either an airgun or electrical sparks. The marine receivers are hydrophones. In land surveys, the seismic source is either a vibroseis or a dynamite. The receivers are geophones. Hydrophones work on the principle of piezoelectricity where water pressure from the reflected sound waves generates an electric pulse which is recorded. Geophones work on the same principle as that of a seismometer where ground vibrations move the instrument in relation to a hanging weight and an electromagnet produces electric pulses from this relative motion. Unlike a marine survey, a straight array of receivers are not possible on land due to various topographic constraints like lakes, buildings etc. A single array of data acquisition constitutes a 2D survey whereas multiple arrays of receivers from a single impulse are used for a 3D survey. 4D seismic surveys are multiple 3D surveys conducted over the same area with the same configuration at different times. The differences in 3D data acquired at different times but with identical acquisition configuration helps detect changes in pore fluid distribution in the reservoir. This is used for designing production wells to enhance recovery of petroleum in the reservoir. This is why 4D surveys are normally used at a production stage to periodically monitor the movement of fluid hydrocarbons in the reservoir rock.

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8.5.3 Seismic Processing Raw data from a seismic acquisition consists of coherent or incoherent noise. Thus the raw seismic data must be processed using signal processing methods to remove them and to enhance the signals, to obtain seismic traces that make geological sense. Coherent noise is unwanted seismic energy from the same impulse source which is generated during the seismic survey. It consists of direct waves which travel through air from the shot point to the receivers, ground roll that travels through the top of the surface (called the weathering layer), multiples which are internal reflections within the rock layers and water bottom reflections (in marine surveys). Non-coherent noise is background noise from sources not related to the seismic impulse used for the survey. They can originate from wind, moving vehicles, ship engine or overhead power lines. There is no standard sequence in seismic data processing but it can always be performed in steps to achieve the objectives of each stage. The sequence used for processing, therefore, varies from project to project depending on the purpose of investigation, type of acquisition, base assumptions and any other trade-off between cost and quality. 2D seismic processing steps typically include the below. . Static Corrections: Corrections that are applied in order to compensate for the effects of variations in elevation of source and receivers, weathering thickness, weathering velocity, or referencing to a datum plane. The goals are to determine the reflection arrival times which would have been observed if all measurements had been made on a level plane with no weathering or low-velocity material present in between the source and receivers (Fig. 8.10a). . Deconvolution: This is aimed at improving temporal resolution by compressing the effective source wavelet contained in the seismic trace to a spike. Deconvolution can be achieved by use of optimum Wiener filters. Often a single deconvolution operator is applied to all the traces on a shot record. . Velocity Analysis: The offset (distance between the impulse source and the receiver) and travel times are analysed for different shot point-receiver configurations for a single common mid-point (CMP) to derive the velocity information of the medium (Fig. 8.10c). CMP is a location in the subsurface where the impulse is reflected for which multiple offset configurations are available. Velocity analysis is performed on selected CMP gathers (Fig. 8.10b). A table of numbers as a function of velocity versus two-way zero-offset time is obtained which represents a measure of signal coherency and semblance along the hyperbolic trajectories governed by velocity, offset, and travel time. . Normal and Dip move-out corrections: Based on the assumption that, in a CMP gather, reflection travel times as a function of offset follow hyperbolic trajectories, the process of normal move-out (NMO) correction removes the move-out effect on travel times (Fig. 8.10d). Dip-move-out correction (DMO) is needed to correct for the dip effect on stacking velocities and thus preserve events with conflicting dips during CMP stacking.

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Fig. 8.10 Seismic processing: a static correction, b CMP gather, c velocity (offset) analysis d NMO correction, e stacked trace and f migration

. Stacking: The velocity field is derived from the velocity functions during NMO/DMO correction and is used to apply NMO correction to the CMP gathers. After this, a CMP stack is obtained by summing them over the offset axis. Stacking removes multiples and enhances the signal (Fig. 8.10e). . Migration: Migration improves the positional accuracy of geologic features especially where dipping events are moved to their supposedly true subsurface positions (Fig. 8.10f). Thus the processing artefacts, which make no geological sense, are removed.

8.5.4 Seismic Resolution Seismic resolution helps to distinguish between two features in a seismic section or image. Seismic data has a vertical and a horizontal resolution. The former determines the thickness of formation from the distance between two reflectors. Horizontal resolution helps in identifying the termination of lateral extensions of the strata. In seismic data, horizontal resolution is usually much poorer when compared to vertical resolution.

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8.5.5 Seismic Interpretation Interpretation of seismic data is done on the basis of seismic sections, time slices and horizon attributes. When all the sections and slices are interpreted, a geological model of the subsurface from summation of the horizons and faults is generated. The seismic sections represent slices through the geological model from which seismic sequences, horizons and faults can be digitised based on amplitude variations and trends. Seismic resolution and scale defines the purpose of the interpretation. Regional (basin) scale mapping, prospect mapping and reservoir delineation are some of the purposes of seismic interpretation. Seismic data also helps in direct hydrocarbon detection and 4D seismic data can be used for monitoring of producing reservoirs. From a hierarchical perspective, four stages of seismic interpretation is usually done. These interpretations are seismic facies, seismic structure, seismic attribute and seismic sequence stratigraphy. An example interpretation of a seismic section to identify seismic sequences and geological structure is provided in Fig. 8.11. The basic properties of seismic data which are used for interpretation are . Reflection Amplitudes: Reflection amplitude is proportional to the difference in acoustic impedance across the contact of two formations. Reflection amplitude and its polarity help us identify the different rock layers in the subsurface. . Reflector Spacing: The thickness of each bed is determined by the spacing or separation between adjacent reflectors. At seismic scale, beds with a minimum thickness are detected and thinner layers of rocks cannot be differentiated. . Internal Velocity: The internal velocity of acoustic waves in the bed, computed from velocity analysis of the seismic data, can provide an idea on the lithology and porosity of the rock layer.

Fig. 8.11 Seismic amplitude and section interpretation

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. Reflector Continuity: The continuity of reflectors determines the continuity of the rock layer and the trend of the continuity or a break provides information on the structural elements (folds and faults). This helps in reconstructing the tectonic environment in each seismic sequence in the basin. . Instantaneous phase: From the seismic trace (which is essentially an analytical signal), the instantaneous attributes can be mathematically computed. Instantaneous phase is a measure of the continuity of the events on a seismic section and the temporal change of which is referred to as the instantaneous frequency.

8.5.6 Modelling Geology from Seismic Data . Structural and Geological Modelling: Seismic interpreters draw horizons and faults on the seismic sections. The interpretations on individual sections can be spatially related and computationally merged to build a continuous geological and structural model. . Seismic Attribute Analysis: The study of amplitude, polarity, continuity and wave shapes of gathered seismic data is called seismic attribute analysis. The aim of seismic attribute analysis is to automate interpretation of morphological features. Seismic attributes like amplitude envelope, dominant frequency, and apparent polarity among others are used to prepare a 3D model of the subsurface geology. Over 100 seismic attributes can be extracted from seismic data which can be used for automatic delineation of geological structures, stratigraphy, and pore fluids. . Seismic Sequence Stratigraphy: Terminations of lateral continuity of amplitude in seismic sections can reveal sequences. Parasequences conforming to different stages in a cycle of transgression and regression (e.g. high stand system tract, low stand system tract etc.) can be interpreted from seismic sections. Doing so helps in reconstruction of the eustatic changes in the basin. Seismic sequence stratigraphy has huge implication on seismic facies analysis and in delineating reservoir bodies in transitional depositional environments.

8.6 Summary Petroleum exploration has evolved with time. Ancient methods are still practiced where applicable. However, modern techniques, which revolve around establishing a petroleum system before a prospect is drilled, are predominant. Most geological methods are field based and are complemented by geochemical and geophysical methods. Geochemistry plays a vital role in establishing the nature of the source rock, the reservoir rock and their provenance. On a spatial context, visualization of geochemical anomalies, help in pinpointing the location of microseeps for further investigation and subsequent drilling. Surface geophysical techniques like gravity,

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magnetic and seismic data determine the density, magnetic, and acoustic properties of a geologic medium. Of the three geophysical methods, reflection seismics is a game changer, especially for frontier exploration in marine environments. Remote sensing methods, especially the ones from airborne and space platforms are used at a reconnaissance stage alongside gravity and magnetic data. Prospect scale exploration is based on seismic data (2D or 3D). Exploration techniques still continue to evolve as they have to meet the challenges of both conventional and unconventional hydrocarbon exploration.

Further Readings 1. Yilmaz, O. (2001). Seismic data analysis: Processing (p. 2027). SEG Press. 2. Selley, R. C. (1998). Elements of petroleum geology (p. 470). Academic Press.

Chapter 9

Drilling and Logging

Once a petroleum prospect (hydrocarbon accumulation) is identified, the next step is to drill. Drilling is the sum total of all the complex operations necessary to construct wells of circular cross-section. The process of drilling is complex as several operations are carried out simultaneously. These involve overcoming the resistance of the rocks, removing the rock particles to the surface, maintaining the stability of the wellbore walls and maintaining the overpressure to stop the formation fluids from rushing into the wellbore. During drilling and after, geophysical information from the formations drilled needs to be collected to validate the geological prognosis, and to plan for the next course of action. Wellbore geophysical data, also called as well logs, help in building a mechanical model of the subsurface, which is essential for appraisal and production of hydrocarbons. A well log is a record of the properties of the formations encountered in a well. Well logs can be acquired, during and after the drilling process, and the process of its acquisition is known as borehole logging.

9.1 Drilling The earliest known oil wells were reported in China in the 6th century. At the time, the drill bits were tied to bamboo poles and the maximum depths they could achieve was about 200 m. The role of bamboo was not limited to be the drill pipe as it was also used to transport oil from the well to other locations. The first commercial well was drilled in Pennsylvania, USA in 1859. This was drilled using cable tool systems that were hoisted, raised and dropped to bore a well. The method was later known as percussion drilling. In 1901, the first known rotary drilling was used in the USA. In 1910, Shell drilled the first well, called the “the Grand Old Lady”, in Miri, East Malaysia (Fig. 9.1 left). Performance of drilling improved significantly with enhancements in drill bit design (Fig. 9.1 right) and with the advent of efficient and powerful of rotary drilling equipments.

© Springer Nature Singapore Pte Ltd. 2023 S. N. Kundu, Geoscience for Petroleum Engineers, https://doi.org/10.1007/978-981-19-7640-7_9

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Fig. 9.1 The First oil well in Miri, East Malaysia (left) and different types of bit (right)

9.1.1 Drilling Objectives At the onset, a well plan which includes all the technical requirements and the purpose of the well, is prepared. Planning a well in advance has its advantages as cost of equipment, time and manpower must be considered and a budget raised. The planning of a well begins after the subsurface is modelled using all the available exploration data, including interpreted seismic data. The motive of an exploratory well is to test and validate the geological prognosis around the petroleum system and the accumulation. To execute drilling of an exploratory well, information on the depth to the reservoir, the general stratigraphic and the lithological properties of the rocks above it, are must haves. Such information helps to foresee any potential problem that could arise during drilling operations. A typical well plan document includes the below; . . . . .

Well Location of surface Well Objective Target location and depth Stratigraphic and lithological profile Pore pressure gradient.

Post exploration and discovery, more wells can be drilled in vicinity of the area for field appraisal and development. During late production, more wells (e.g., injection wells) are drilled to enhance recovery. By design, a well can be vertical, directional, horizontal or a multilateral.

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9.1.2 Vertical and Directional Wells Traditional wells were all vertical as the prime objective was to reach the reservoir accumulation in the most direct and shortest way. Vertical wells were considered as they were simple to drill and also had major cost savings. In larger formations, multiple vertical wells were required to effectively produce oil & gas. This was necessary to negate some of the initial cost savings. In present day, directional well drilling technology is available, which has the advantage of entering the reservoir from many different directions. Directional drilling also allows to drill prospects where a vertical well is not possible due to difficult surface conditions. However, vertical wells are still mostly preferred for exploration wells and horizontal wells are considered at the field appraisal or field development stages. Different geometries of petroleum wells are provided in Fig. 9.2. Towards the end of the 20th century, technology was available to steer the well along a desired direction. This technology was actually developed to bring back vertical wells on track when they deviate naturally due to anisotropic stress fields in the subsurface. However, petroleum engineers found steerable drilling equipment useful to reach multiple subsurface targets from the same surface location. Directional well technology can deflect a well from any subsurface location towards another target. One can also can deviate the well horizontally, which was necessary to drain hydrocarbons from horizontal formations. Deviated and horizontal drilling has several advantages as below . Avoid locations that impede drilling performance or are environmentally sensitive along the path. . Drill an offshore target from onshore reducing the requirement of a costly marine drilling rig. . Drill horizontally and produce from a thin isolated reservoir. . Drill horizontally and hydraulically fracture formations with low permeability for production.

Fig. 9.2 Wells by geometry (MD = measured depth, VD = vertical depth)

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. Drill and place the well in the reservoir maximising contact with the reservoir for better production. . Assist in drilling infill wells to target locations where fluids are not being driven to the production well.

9.1.3 The Drilling Rig A drill rig is an assembly of mechanical components necessary to successfully drill a bore into the Earth’s subsurface (Fig. 9.3). For oil and gas exploration and production, rigs should be good enough to reach the target depth and to achieve the adequate diameter needed to produce hydrocarbons. The components of a drilling rig can be scaled up (or down) to suit the environment in which the well is being drilled. Several types of rigs are used for exploration and production of petroleum (Fig. 9.4). At the well planning stage, the choice of a rig is based on its cost and availability, topography and bathymetry of the location where drilling shall be undertaken, depth of the geological target, predicted formation pressures and prevailing weather in area of operation. A drilling rig must support all geological activities required during and after. It must have a mud logging setup to collect and examine the rock cutting that are brought to the surface. The drilling rig must support well logging activities during and after drilling where a string containing several sensors are lowered into the well to record various parameters of the formations around the well bore with respect to depth. In offshore environments, the drill rig is much more complex as it consists of facilities

Fig. 9.3 Components of Drilling Rig. a The Rig Mast, b Drill pipes, c Mud pump, d The Engine to provide rotary motion to the drill pipes

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Fig. 9.4 Types of Rigs and their operational environment

to accommodation both staff and equipment. Drilling ships must have a helipad for transportation of man and equipment. Land rigs, barge mounted rigs and Jack-up rigs are anchored to the ground at the base, but semi-submersible rigs and drilling ships require GPS based dynamic positioning to stabilize the rig during drilling operations.

9.1.3.1

Mud Logging

Mud logging is an essential activity during drilling. It involves monitoring the properties of drilling mud that is circulated through the drill pipe and involves the logging of the rock cuttings that are brought to the surface along with the return mud. The rock cuttings are examined and recorded against the depth from where it originates. This creates a geological log, which is studied alongside other data (e.g., rate of penetration, mud volume and temperature etc). Such studies help the driller with real-time information, which not only improves drilling operations but also helps in early detection and prediction of potential drilling hazards. Mud Logging also helps in identifying hydrocarbon bearing zones. as it involves analysis of dissolved gases in the return mud. Efficient rotary drilling requires continuous mud logging. The basic components of a mud logging set-up is illustrated in Fig. 9.5. Initial drilling muds were natural clays mixed with freshwater and sand to form a viscous solution. By design, the mud needs to be denser than the formation being drilled, so that the mud can float the cuttings to the surface during circulation. In addition, the mud forms a layer around the walls of the well bore, which prevents collapse of loose rocks from the walls of the well. Apart from removing debris, circulating mud also improves drilling rates. It controls formation pressures and extends the life of drilling bits by cooling and lubricating it. Some benefits of mud logging are listed as below

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Fig. 9.5 Components of Mud logging operations

. Collection of rock cuttings. . Lubrication of the drill string. . Cooling of the drill bit as it heats up from continuous abrasion against the rock formations. . Negotiates formation pressure. With time, several materials were experimented to be used as mud. We now have water-based (barites) muds, oil-based muds (OBM) and synthetic oil-based muds (SOBM). The mud type is selected based on the reservoir formation type and regional environment. Some generic characteristics considered while selecting a drilling mud include . . . . .

High density and viscosity Stability under high temperature and pressure Chemical inertness Environmental friendliness and cost Non-interference with measuring while drilling (MWD) and other logging tools.

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Loss of return mud while drilling indicates that formation being drilled has voids and fractures. Pressure jumps in the return mud is indicative of a fault zone where formation pressure is high. Gases trapped in the formation are released during drilling and they can betraced in the return mud. The concentration of such gases is indicative of the reservoir formation, its saturation and pore pressure. Monitoring temperature of the reverse mud provides us information on the geothermal gradient. High temperatures in return mud is indicative of hotspots or salt domes.

9.1.4 Geology Influenced Problems Common drilling problems are mostly mechanical in nature. When pressure is increased on the drill stem to achieve high Rate of Penetration (ROP), the drill pipe tends to buckle and rotate like a spindle. This widens the hole diameter and when uncontrolled, leads to sticking of the drill pipe to the side wall of the well. Pipe sticking resulting in downtime and therefore precautions must be taken to avoid it. Geological factors that result in drilling problems are: . Loss of circulated mud: This happens when the formation being drilled is vuggy (e.g., karst limestone) . Borehole deviation: Dipping beds force the drill string to deviate along zones of least resistance. . Return pressure (& blowouts): When formation pressure exceeds mud weight, the formation fluids flood into the well and gush out to the surface. If this is not wrested by the blow-out preventer (BOP), then it can destroy the drilling rig. Past events of rig blowouts have caused extensive damage to the life, property and environment. A vivid example is the Macondo disaster in the Gulf of Mexico. . Formation damage: The rotary motion of the bit alters the porosity and permeability of the formation along the skin of well walls. This impacts the producing reservoir section as reduced permeability impacts production rates. When formation damage happens, the well walls are stimulated post drilling to restore (and enhance) the permeability of the reservoir around the well bore.

9.1.5 Well Completions After a well is drilled, it must be completed for enabling the next operations. The completion methods deployed depend on the objective of the well. If it is an exploratory well, then the well must be completed to allow for wireline logging. If it was a dry well (no oil and gas was found), it must be plugged and abandoned. If it is a production well, then the producing zone must be cased and perforated to enable hydrocarbon production. At the initial stage of well completion for a production well, the wellbore is filled with brine (or completion fluid). Potassium chloride or “KCl water” is normally used

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Fig. 9.6 Types of Well Completion

as it secures the formation from swelling. Swelling impedes the perforation process. Some additives like filtration control agents, surfactants and corrosion inhibitors also are added. Selection of such additives depend on the well conditions and the material to be used for the well casing. Completion can either be open hole or cased hole. Cased holes completions use production liners, which are either slotted or require perforation (Fig. 9.6). The sequential steps for a typical well completion include . Casing: Installation of the casing to prevent the wall of the wellbore to collapse. Casing is not required where openhole completion is needed. . Cementing: Injection of cement to fill the voids between the casing and the well wall. This secure the casing to the formation. . Perforating: Piercing of holes through the casing into the formation to allow uninhibited flow of formation fluids into the well bore. . Gravel packing: Gravel packs are placed at the reservoir section to preserve permeability for continued production. . Wellhead Installation: Installation of a well head (also called Christmas Tree or Production Tree) from where smaller diameter pipes are connected to separate and distribute petroleum.

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9.2 Well Logging Mudding logging provides an imprecise record of the formations recovered from the well bore. Coring the well to recover an intact cylindrical sample is the most precise method to examine rock formations in a well. But, recovering cores is a very lengthy and expensive process, and therefore, it is not normally recommended for the whole well. Well logging, where indirect information from the formations in the well is collected by sensors, is a sensible trade-off between mud logging and coring. Well logging is a means to record the physical, acoustic and electrical properties of the rocks along the wall of a well bore. It is done by lowering a tool consisting of several sensors through a wireline to the bottom of the well. The tool sensors are switched on and then the string is pulled up to the surface at a constant rate. The greatest advantage of well logging is that, unlike rock cuttings and cores, it can measure the in-situ rock properties. Well logs provide a continuous picture of the subsurface along the well bore where both continuous and abrupt changes in geology can be detected. Top and bottom of reservoir formation and non-productive (impermeable) zones within the reservoir can also be measured with fair accuracy. This helps in a reasonable estimation of the hydrocarbon volume in the accumulation. Applications of well logs are plenty. They support detailed scale lithological characterisation and rock typing from simple electrical tools. They also support characterization and estimation of porosity and permeability from acoustic, electromagnetic and nuclear tools. The fluids in pores can be characterised from advanced logging tools like Nuclear Magnetic Resonance (NMR) tool, which can also determine the fluid type and its spatial distribution in the reservoir.

9.2.1 The Borehole Environment To be able to effectively design the logging tools and to accurately interpret well log data, it is important to understand the borehole environment. The process of drilling and mud circulation does impact the walls of the well, which impacts the performance of logging sensors. A well log interpreter must be aware of the borehole environment before the well logs can be interpretation. A typical borehole environment is explained in Fig. 9.7. The drilling mud, being heavier than formation pore pressure, floods into the rock formation leaving behind the mud substrate which forms a mud cake along the wall of the wellbore. The mud filtrate, which floods the formation, replaces the formation fluids in the formation. In the process, three zones around the bore are formed, as below . Invaded zone: In this zone, which is closest to the well bore annulus, has all the formation fluids displaced by the mud filtrate. . Mixed zone: In this zone, which deeper than the invaded zone, the formation fluids are partially displaced by the mud filtrate.

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Fig. 9.7 The Borehole Environment

. Intact zone: This is the zone, where the mud filtrate is not able to reach, the original formation fluids in the permeable rock are preserved. Mud cakes, invaded zones and mixed zones do not form along low permeability formations like shale and tight sandstone. A well logging plan needs to take into account the type of drilling mud used. This is because, water and oil based muds trigger different log responses from the same logging tool. Mud and salinity too alter the conductivity of the formation. The logging tool must be calibrated to achieve the necessary depth of penetration above and beyond the mud cake, so that properties of the invaded and mixed zones can be both measured.

9.2.2 Equipment, Types and Tools The logging equipment and set-up (Fig. 9.8 left), is essentially a recording vehicle that hosts a motorised winch to pull the wireline logging tool assembly from the well. The wireline logging tool uses the mast of the drilling rig. The wires connected to the tool assembly, transmit the data to the computer in the vehicle. The computer is used for recording and on-board processing. The tool is lowered into the bottom of the well bore. The tools are then switched on, tested before the recording is started when the winch starts pulling it up. This ensures that one has a good handle on the speed at which the tool is required to be pulled up. Traditional logging was done only after drilling has completed. However, as geoscientists were eager to gain real-time information on the formation, logging

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Fig. 9.8 Well logging equipment and tool-types with their resolution and penetration

was necessary while drilling. This gave rise to Logging while Drilling (LWD) tools where the logging tools were attached to the drill string so that data could be recorded during drilling. LWD was useful where there was a high risk of losing the drilled bole due to wall collapse or where the well trajectory was not suitable for wireline logging. Before LWD tools were used, the logged data was stored in a memory chip, which could only be downloaded when the drill string was brought to the surface. But now the data can be transmitted real-time during drilling operations. LWD is not as comprehensive as wireline logging. This is because several logging tools cannot be used as drilling interferes with the quality of data acquired. LWD tools, today, are capable of acquiring gamma ray logs, density logs, neutron logs, acoustic logs and resistivity logs. Nuclear Magnetic resonance, formation pressure and some other acoustic tools are also being developed to be used in LWD mode. After the completion of drilling in a hole section, the drill string is pulled out and the hole is logged using wireline tools. Wireline can be designed to be used in an open hole or in a cased hole. Cased hole logging tools are normally for production related data acquisition and include the likes of thermal decay tool, spectroscopy tool (Gamma Ray), production logging and cement bond log. At deviated sections, where conventional wireline tools cannot operate (as the toll assembly is long enough to negotiate the curvature of the hole), pile-conveyed logging is used. The logging tool assembly consists of various tools as required to meet the objectives of formation evaluation. Electrical logs (Induction, Laterolog and Microresistivity) are the fundamental tools which help in provide lithological variation and permeability characterization. Radioactive logs (e.g. Gamma Ray, Neutron and Density logs) help in estimating density, porosity and fluid content of the formation. Acoustic (or sonic) logs provide an estimate of density as well as velocity for seismic re-processing. Advanced logs like nuclear Magnetic Resonance (NMR) are

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used for fluid characterization and pore distribution. There are other logs (e.g. Caliper, Temperature, Image and Dip meter logs) which are used to measure the variations in the borehole diameter. Some tools image the well wall for any skin damage that may have resulted from drilling. Each tool has a resolution and penetration (Fig. 9.8 right). Therefore while interpreting the log in the whole section and reservoir, one must estimate whether the invaded zone or the mixed zone is influencing the readings. Electrical logs can be configured to investigate at different depths of penetration. They have the short and long Laterolog tools, which are visualized and interpreted together, to determine the depth of the invaded zone. As deep wells encounter a wide variety of rock formations, different logs are visualized together so that all the geological formations are discriminated with high degree of confidence.

9.2.3 Spontaneous Potential (SP) Log Also known as the self-potential log, the spontaneous potential log (or simply SP log) was perhaps among the earliest logging tool used for petroleum exploration. SP log detects the current which is created by the difference in concentration of electrolytes in the fluids contained in formations. This current is detected by running an electrode in the borehole against a reference electrode at the surface. When there is a change in electrolyte concentration, there is a flow of current that is reflected in the voltmeter. A change in concentration indicates a change in lithology which induces a change in voltage indicating a boundary between two formations that contain different electrolytes in their pore fluids. For the SP tool to function, there must be a conductive borehole fluid (i.e. a water based mud). The salinity of the borehole fluid (mud filtrate) must also be different to the formation fluid. When there is a porous and permeable formation (e.g. Sandstone) in between a low porosity or impermeable (e.g. shale) one then there shall be a relative change in the self-potential at the formation boundaries. This self-potential, measured in mV (millivolts), can be recorded by the SP log. SP logs can detect relative changes of 10 mV at formation boundaries. The SP log also indicates the proportion of shale in a formation and this is used to detect permeable formations and resistivity of water contained within the pores. SP logs are used for correlating sequences which support facies and depositional environment interpretations. Increase or decrease of shaliness of formations in a sequence can imply regressive and transgressive depositional environments respectively. SP logs also help in identifying reservoir sections and its quality (as increased proportion of shale makes a sandstone reservoir impure and less productive). A basic lithological interpretation using a SP log over a sandstone and shale alternating sequence is provided in Fig. 9.9. SP log can only be used where the borehole mud is conductive and where the resistivity of the mud is significantly different from the formation resistivity. It may not yield good results in thin laminated beds as it has low vertical resolution.

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Fig. 9.9 SP and Gamma ray logs over a Sand-Shale alternating section

9.2.4 Natural Gamma Ray (GR) Logs Shales consist of clay minerals which have radioactive isotopes of potassium, uranium, radium and thorium. These radioactive isotopes decay to emit gamma rays. A gamma ray detector can be used to detect the intensity of the emitted gamma rays which is reported in American Petroleum Institute (API) units. Like the SP log, GR logs are used to characterise shaliness of the formation. Therefore, both logs have high correlation. However, the resolution of the latter being high, it is used for detail scale stratigraphic correlations. Both SP and GR are used for lithology discrimination, shaliness estimation and for interpretation of facies and depositional environment. A striking advantage of GR over SP is that it is not influenced by resistivity of drilling muds and that it can be used for logging cased-hole sections of the well. Both logs complement each other for identifying sandstone (reservoir) and shale (non-reservoir) sections in the wellbore (Fig. 9.9). Some issues which influence GR logs are its anomalous values in well sections where the wall has caved. Hence, a caliper log data must be used alongside GR log data during interpretation to identify the caved sections. Density of mud can impact GR logs if the mud is based on potassium based clays. This is because, potassium is radioactive and can add to the gamma ray counts from the formation. In well sections with coal, dolomite and salts, GR logs can display anomalously high readings as they can contain absorbed uranium. A spectral gamma ray log is used to account for gamma emissions resulting from each radioactive element. This can be used to identify and remove the gamma counts emanating from the drilling mud.

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9.2.5 Resistivity Logs Resistivity is an electrical property, which is the inverse of conductivity. Measurement of conductivity, therefore, can be used to estimate resistivity. Rocks are insulators in general, but the fluids containing ions in the pores make it conductive. Low resistivity is therefore an indicator of a porous formation which contains fluids with dissolved ions. When a known current is passed through a formation, the electrical potential across it can be measured to derive resistivity. This is the principle of resistivity logs. Laterologs are a special type of resistivity logs, which consist of two guard and a central electrode. The distance between the electrodes govern the depth of penetration of the measurements from the well wall. This helps in obtaining the resistivity of the invaded zone (by Shallow Laterolog or LLS), the mixed zone (by Medium Laterolog or LLM) and the intact zone (by Deep Laterolog or LLD) in reservoir sections. Another resistivity log, which is also known as the induction tool, is used where the formation is induced with a current and the magnitude of induction achieved is measured. Components of a Induction log and the various laterologs are shown in Fig. 9.10. Detection and quantitative evaluation of hydrocarbon productive zones of a reservoir is possible using resistivity logs. Hydrocarbon bearing reservoir zones are highly resistive whereas reservoir sections with water or brine are more conductive. Gas is more resistive than oil. Hence resistivity logs can help us identify the oil-water contact (OWC) and the gas-oil contact (GOC). Laterologs support the calculation of the porosity of the formation when the resistivity of the mud in known. From a petroleum system perspective, source rocks can also be identified using resistivity logs. Resistivity of formation is impacted by formation temperature and therefore a temperature log of the well is always handy in correcting related anomalies in the

Fig. 9.10 Resistivity Logging Tools and log data

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resistivity measurements. As resistivity is strongly influenced by the invasion of drilling mud, LLD, LLM and LLS logs must be used to characterize these zones.

9.2.6 Neutron Porosity Log Indirect measurements of porosity can be achieved using several logging tools. Once such tool is the neutron porosity log, where neutrons are bombarded into the formation from a neutron source. The neutrons are absorbed as they elastically collide with hydrogen in the formation as they both have the same mass. The hydrogen atoms are mostly concentrated in the water or hydrocarbons contained in the pores. When neutrons collide with a heavier atom, it is reflected back and detected by the logging tool. Hence, the intensity of return neutrons is inversely proportional to the hydrogen concentration in the rock. The neutron tool, therefore, helps us measure the hydrogen concentration (or Hydrogen Index) in the rock, which is proportional to porosity of the rock. Shale is rich in clay minerals and is porous. It also has several hydrous minerals. Hence it displays high neutron porosity. Shaly sandstones, therefore record high porosity, despite the fact that most of its pores are clogged with clay minerals. This is called the shale effect in impure sandstones. As limestones contain very little hydroous minerals, neutron porosity works well for limestone reservoirs. In formations saturated with gas, the neutron porosity is anomalously low. This is because gas contains less hydrogen per unit volume as compared to water or oil. This low porosity anomaly is called the gas effect, which is very helpful in detecting gas saturated sections in the reservoir.

9.2.7 Gamma Density Log Unlike in the GR log, where the natural radioactivity of the formation is measured, the Gamma Density tool uses an active source of gamma rays. A caesium-137 or cobalt60 source is used to emit gamma rays into the formation and the detector records the return gamma rays. The gamma density tool, just like the neutron porosity tool, can be configured to have a near detector and a far one (Fig. 9.11). The gamma rays rebound elastically because of Compton scattering. They are detected based on photoelectric effect produced by the return gamma rays in the logging tool. High energy gamma rays are from the near detector and low energy ones are from the far detector. Low energy gamma rays help in lithological discrimination whereas high energy ones help in density estimation. The principle behind estimation is that rock with high bulk density has high number of electrons against which gamma rays elastically collide resulting in low return count. Conversely, a formation with low bulk density reflects more gamma rays.

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Fig. 9.11 Porosity logging tools. Neutron and gamma density (left) and acoustic (right)

9.2.8 Acoustic (Sonic) Porosity Log Sound travels through rocks at different speeds depending on the acoustic impedance of the formations. Acoustic impedance depends on density, which is inversely related to porosity. Thus, acoustic impedance can indirectly implicate the porosity of a formation. Moreover, in a porous rock, the fluids contained in the pores offer resistance to the propagation of acoustic waves. Hence, when porosity is known, the type of fluid contained in pores can also be inferred from acoustic logs. During acoustic logging, acoustic pulses are created. These refract along the wall of the rock to reach by a near and a far detector at the other end of the tool (Fig. 9.11). The travel times from the source to the near and far detectors are recorded, from which velocity of sound in the medium is calculated. As velocity of sound in water is far lower than its velocity in rock minerals, the measured velocity shall be inversely proportional to rock porosity.

9.2.9 Caliper Log The diameter of the well is not the same all along the well. It increases at well sections where the rock being brittle caves in. Moreover, the diameter is less in well sections where the mud has caked or where the formation has sloughed. It is important to have the diameter of the hole logged after drilling is completed as it provides essential information on the mechanical properties of the formation. More

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importantly, the well diameter impacts other logging tools and the hole diameter can be used for correction of the well log measurements. To measure the diameter of the hole, a caliper log is used. The borehole shape and diameter can be measured in many different ways and these are incorporated in the design of various caliper log instruments (Fig. 9.12 left). The basic mechanical caliper tool has extendible arms which are in contact with the hole wall. As the hole diameter becomes less or where the mud has caked, the arms move in. At sections where the formation is vuggy or brittle and has caved in, the arms move out (Fig. 9.12 right). These movements are converted into electrical signals from which the diameter is estimated. Apart from providing qualitative assessment of lithology, a caliper log helps in calculating the amount of cement required to install the casing. Most important application of a caliper log is to correct the readings of other logging tools, which are sensitive to hole diameter. There are other reasons where the well diameter could change. E.g. spiralling effect from the rotation of the drilling bit. Hence, one must eliminate such reasons to properly interpret caliper log data. In zones, where the stress fields are unbalanced, the formation may fracture or the hole section may deviate from a circle to being an oval. To measure these distortions in wellsections, a caliper tool with several arms arranged radially, is needed.

Fig. 9.12 Caliper log tool (left) and deviations from planned hole diameter in a well (right)

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Nuclear Magnetic Resonance (NMR) Log

As water, oil and gas contain hydrogen atoms with different bondings involving oxygen and carbon, its magnetic moment can be exploited to have an understanding on the type of fluid and its distribution in the reservoir. In Nuclear Magnetic Resonance (NMR) logging, the reservoir is exposed to a magnetic field and then the magnetic moment of hydrogen is recorded. The amplitude of the NMR signal correlated well with the quantity of hydrogen present in the reservoir and this is calibrated to estimate porosity. The advantage of NMR over other porosity logs is that this method only targets fluids and therefore is free from lithology effects. When the magnetic field is withdrawn, the rate of decay of the NMR signal amplitude helps in estimating permeability. NMR logs are used to distinguish water in the pores and the hydroxides in the minerals. NMR signatures of water bound to mineral surfaces are different and representative of the distribution of pores and pore size in water saturated reservoir. Therefore, it effectively responds to the volume, composition, viscosity, and distribution of reservoir fluids, thereby providing information on the properties and quantities of fluids present. This helps in quantitative estimation of . Volume (porosity) and distribution (permeability) of the rock pore space. . Type and quantity of fluid hydrocarbons. . Hydrocarbon production capability of the reservoir. The quality of NMR log data acquired in wells drilled using oil-based mud is superior to the ones where water-based mud is used. This is because the conductivity of oil based mud systems impacts less on the transmitter-receiver system of the logging tool.

9.2.9.2

Log Interpretation

Before the start of log interpretation, the rock physicist must gather the below information . The logging report which includes the type of logging method and tools involved, the total depth (TD) of the well and deviation data . The mud log report which includes the cutting description, gas reading and rate of penetration. These are needed to correlate and validate and detect inconsistencies. . Log data from nearby wells within the same basin (where available) and their interpretations as a guide and also for regional correlation. A basic quality control is necessary to check if the instruments recorded data well within the extreme ranges and whether there are any unusual data or data gaps. As the reference, the elevation of the derrick floor of the rig must be corrected to mean sea level. For geological formations, gravity data must range between 0 and 50 API, resistivity between 0.2 and 2000 .m (log scale). Similarly density must be between 1.95 and 2.95 g/cc and acoustic data should range between 140 and 40 μs/ft.

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The approaches to interpretation differ from person to person. However, it can be done sequentially (Fig. 9.13) to decipher progressive information on the formations. In order to identify the reservoir, permeable and non-permeable formations must first be identified. This is achieved using GR and SP logs, which correlate well to determine the shale and sand lines. Thus, the reservoir (permeable) and nonreservoir (non-permeable) sections can be differentiated (Fig. 9.14). A caliper log is also used to support reservoir (permeable) sections where the diameter of the hole is reduced because of mud caking. Once the permeable sections of the well are identified, the next step is to discriminate between the hydrocarbon and non-hydrocarbon (water) saturated zones. Oil, water and gas occur stratified in the reservoir observing a horizontal OWC and GOC.

Fig. 9.13 Sequence of Well log interpretation

Fig. 9.14 Basic Well log interpretation (NR: non-reservoir)

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The resistivity of the reservoir is heavily dependent on the saturation of fluids. Therefore electrical logs like induction and laterologs are helpful in segmenting the reservoir section into hydrocarbon and water-bearing zones. Water is conductive (low resistivity) as compared to hydrocarbons (high resistivity) and this helps in discriminating the zones (Fig. 9.14). The next step is to identify the GOC using the gas effect from porosity logs. As neutron porosity and bulk density logs are used to estimate porosity, albeit targeting the pores and mineral matter respectively, their response in gas saturated zones are opposite. Neutron porosity is low in gas rich reservoir sections where bulk density log values are high (Fig. 9.14).

9.2.9.3

Porosity Estimation

Porosity of a formation can be estimated from neutron, bulk density and sonic logs. The formation porosity (φ) can be determined from the bulk density log value (ρlog ) when we know the density of the rock matric (ρm ) and the density of the saturated fluid (ρf ) using the below equation φ=

(ρm − ρlog ) (ρm − ρ f )

The density of matrix of common reservoir rocks is well known and the density of fluids can be estimated based on brine. Therefore, an overestimation of porosity is likely when the formation is oil and gas saturated. The temperature log is used to correct the fluid densities as it is sensitive to temperature of the formation. As velocity of elastic waves through a rock is a function of its porosity, acoustic logs can be used to estimate porosity. The difference in arrival time from the near and far sensor (δlog ) from the acoustic log, the lab derived difference of arrival time in the known fluid (δf ) and known rock matrix (δm ) are used to estimate porosity using the below equation φ=

(δTlog − δTm ) (δT f − δTm )

Porosity values from different estimation methods does not necessarily yield similar values and therefore care must be taken to identify the best estimate from logs. As a general rule, the values obtained by different methods can be averages to yield a mean porosity value which can be used for deterministic reserve estimation.

9.2.9.4

Water Saturation Estimation

Electrical resistivity measurements can be used to derive hydrocarbon saturations. This is done using Archie’s law, which empirically related to fluid flow in clean and

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consolidated sandstones having intra-granular porosity. True formation resistivity (Rt ) can be derived from formation porosity (φ), water (brine) saturation (Sw ), water resistivity (Rw ) and deep resistivity (Rd ) as below: Rt = aφ −m Sw −n R w where a is the tortuosity factor, m is the cementation exponent (~2 for sandstones) and n is the saturation exponent (~2). The interpretation sequence is dependent on the regional geology of the sedimentary basin and in reality, the task is more complex than what is presented above. In real world, there are several other sedimentary rocks which are encountered in a well. These have peculiar log responses which are also used to interpret reservoir thickness and saturation. It is critical that estimation of reservoir thickness and saturation is fairly achieved from well logs as these are used for estimating reserves for the accumulation.

9.3 Summary Drilling and well logging are cost intensive activities, which are done to test the geological hypothesis around a petroleum accumulation. Success of a petroleum exploration project depends on intricate planning of these activities and on the selection of a drilling rig and the LWD and wireline logging tools. Mud logging and ensuing well logging, provide direct access to geological information at a resolution suitable for quantitative estimation of reservoir properties. Porosity, permeability, fluid type and saturation are important parameters, which are used to determine the volume of hydrocarbons contained in an accumulation. The finances of the whole project depends on a realistic estimation of the volume of hydrocarbons in the trap. Hence, the resolution of well log data, its qualitative and quantitative interpretation are important activities in an exploration project. Consistent quality in such interpretations from wells in an sedimentary basin is helpful in regional correlation. This can lead to further hydrocarbon discoveries in the same basin or in other analogous basins.

Further Readings 1. Bjorlykke, K. (2015). Petroleum geoscience, 2nd edn. Springer. 2. Darling, T. (2005). Well logging and formation evaluation (1st ed.). Elsevier.

Chapter 10

Hydrocarbon Reserves and Quality

10.1 Resources and Reserves A resource is the amount of a geologic commodity which occurs in-situ, irrespective of whether it is discovered or undiscovered. Whereas, a reserve is a resource that has been discovered, appraised and can be profitably recovered. E.g. the world’s estimated oil resource is three trillion barrels but the global reserves are only about a third of it. The concept and classification of resource is provided in Fig. 10.1. Of the total resources available for a commodity, a part is recoverable and the remainder is not. Of the recoverable resource, a part may be discovered and a part remains undiscovered. Once a resource is discovered, its in-situ volume must be confirmed before it can be called a reserve, else it is considered to be a probable resource. The part of a reserve, which has been extracted from its original in-situ location, is classified as cumulative production. From among the undiscovered resources, the ones whose geologic setting is considered highly favourable from an analogous proven resource, is called a probable resource. All the rest are termed as speculative resources. The amount of oil in the subsurface reservoir (proven reserves) is called Oil in Place (OIP). If the resource is gas, then it is called Gas in Place (GIP). As both oil and gas are referred to as hydrocarbons, OIP and GIP are often collectively known as ‘Hydrocarbon Initially in Place’ or HCIIP. Only a part of the HCIIP is recoverable and therefore, we practically use ‘Recoverable Reserves’ as the basis for economics of a resource. Recoverable reserves and HCIIP are related by a Recovery Factor (RF), which depends on the viscosity, permeability and reservoir drive (or pressure) of the hydrocarbon accumulation. Recoverable reserves can be maximised using Enhanced Oil Recovery (or EOR) methods, when the loss of pressure in the reservoir reduces production significantly.

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Fig. 10.1 The concept of Resource and Reserve for Hydrocarbons

10.2 Volumetric Estimation The estimation of the volume of oil or gas in place in the subsurface is a fundamental aspect of hydrocarbon exploration. Volumetric calculations form the basis of economic valuation, which is critical for decision making on exploration and production activities like drilling, relinquishment, development, farming in (or farming out) and purchase (or sale) of an asset. Volumetric estimation is a static measurement, which uses the geometry of the petroleum trap to describe the volume of hydrocarbons in the reservoir. For undrilled prospects, the only way to assess hydrocarbonsin-place is from the configuration of the trap and from the rough estimates of porosity and saturation. The fundamental purpose of volumetric estimation is to rank all the prospects in the project area for prioritizing drilling activities. Volumetric estimation uses computations from well logs, where available, to improve its accuracy and precision. Information on the thickness of the productive sections of the reservoir, pore volume and hydrocarbon saturation are more precise when well logs are used (as compared to seismic interpretations). Dimensions (shape and size) of the trap, at this stage, is the only parameter that is derived from seismic data. Regional correlation, using data from exploratory and appraisal wells, when available, are used too. In essence, a geological model of the hydrocarbon accumulation, which describes the gross reservoir volume, net pay volume, porosity, saturation and formation volume factor, is fed into the equation for reserve estimation (Fig. 10.2).

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Fig. 10.2 Gross Reservoir volume (GRV), Net Pay volume (NPV), Porosity (F) and Saturation (Sw, So)

10.2.1 Gross Reservoir Volume (GRV) and Net Pay Volume (NPV) Gross Rock Volume (GRV) is the volume of rock between the reservoir top and the postulated hydrocarbon-water contact in a hydrocarbon accumulation. GRV is the most significant parameter in estimating the in-place hydrocarbon volume for prospects. GRV is unique as there is no specific tool for its direct estimation. It is based on the geological interpretation of seismic data, the top of the trap, the depth map and the assumption of the trapping mechanism. As GRV estimation depends on depth estimation, a typical workflow for deriving depth maps includes; seismic time-section interpretation, gridding of horizons, timeto-depth conversion and the mapping of residuals. Each of these steps is not free from uncertainty as it involves interpolation methods. Net Pay Volume (NPV) is the GRV where the volume of non-productive rock intervals in between the saturated reservoir section are excluded. Alternatively, NPV is the summation of the volumes of all the reservoir sections from where hydrocarbons can be produced. The exclusions include non-permeable zones like shale (also referred to as ‘thief’ zones) in the reservoir.

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10.2.2 Porosity (Φ) Porosity of the producible zones of the reservoir is estimated from logging tools. The product of porosity with the NPV provides an estimate of the total volume of pores in the reservoir which are most likely to be saturated with hydrocarbons.

10.2.3 Fluid Saturation (Sw , Sg , So ) Fluid saturation in the porescan be determinde from well logs, specifically from resistivity logs. The pores are likely to be saturated with either water, oil, gas or a combination of all three. Once saturation of water (Sw ) is calculated, the saturation of hydrocarbons (Sg and So ) can be safely assumed. Saturation of hydrocarbons, multiplied with the pore volume, will yield the total amount of hydrocarbons which is contained in the reservoir.

10.2.4 Formation Volume Factor (FVF) The volume of hydrocarbons depends on its composition as well as the temperature and pressure conditions of the reservoir. Traps occur in the subsurface, which are at relatively high pressure and temperature as compared to surface conditions. The hydrocarbons will, therefore, expand in volume when it is lifted to the surface. As hydrocarbons are traded at surface tank conditions, it is important to have an estimate of this compression factor, so that the exact marketable volume can be calculated. This compression factor depends on the depth at which the reservoir is located and on the type of hydrocarbons contained in it. Gas can be compressed multiple times more than oil under same pressure and temperature conditions. Therefore, if the pores are filled by gas than the compression factor shall be much more than when it contains oil. This compression factor is called Formation Volume Factor (FVF) and is a measure of the ratio of the volume occupied by a fluid phase at reservoir conditions to its volume at surface conditions. The total volume of hydrocarbons in the pores divided by the FVF provides an estimate of the hydrocarbon volume at surface conditions.

10.2.5 Recovery Efficiency (RE) Ultimate Recovery (UR) is the fraction of hydrocarbons-in-place that can be economically recovered. The ration of UR to HCIIP is called Recovery efficiency (RE) and this varies from reservoir to reservoir. RE is the highest for water drive reservoirs and

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least in solution gas drive reservoirs. RE also shall be different for primary recovery methods and for EOR methods.

10.2.6 Hydrocarbons Initially in Place (HCIIP) The basic parameters required for estimating reserves are Gross Reservoir Volume (GRV), Net Pay Volume (NPV), Porosity (φ), Saturation (Sw , Sg , So ), Formation Volume Factor (FVF) and Recovery Efficiency (RE). The equation for calculation for calculating Hydrocarbons Initially in Place (HCIIP) is HC I I P =

(N P V × φ × (1 − Sw )) FVF

Ultimate Recovery (UR) is U R = RE − HC I I P Energy products are measured and traded in physical units based on their mass, volume and energy content. Therefore, the units of reporting and trading HCIIP are different for gas and oil. Crude oil is reported and traded by volume. As the volume of natural gas varies widely due to its compositional variations, trading by volume is not suitable. Calorific value, which signifies the energy value of gas, is practiced to quantify and trade gas. The oil industry dates back to the 19th Century when the whiskey industry was in vogue. Whiskey was traded in barrels and therefore its was conveniently used to quantify the volume of oil. A barrel, which is equivalent to 42 US gallons, thus became the standard unit of oil, which is used still to date. A barrel is equal to 4.972 Imperial gallons or 158.987 litres. Gas discoveries are reported in cubic feet (e.g. BCF-billion cubic feet—bcf, trillion cubic feet—tcf), but traded by its calorific value as determined at the point of its production. British thermal unit (BTU) is the standard unit used for calorific value of gas.

10.3 Uncertainty and Probabilistic Methods The calculation of volumetrics for a petroleum field involves the product and division of a number of parameters. These parameters are estimated based on interpreted data, which are acquired by tools with varying resolution and scale. Therefore, the estimates are not absolute and are highly uncertain. This introduces uncertainty in the reserve estimates and the HCIIP calculated from inputs from different data and tools, shall be different to each other. The GRV can be underestimated or overestimated depending on the resolution of the seismic data and the quality of its interpolation. The thief zones, too, are

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subjected to similar uncertainties as their estimates from different well logs widely vary. Porosity of reservoir rocks is not uniform throughout the formation as it changes with lateral and vertical facies changes. Saturation estimates too have their subjectivity. Hence, any deviation from the real or mean value for each parameter shall be propagated into the ultimate calculation of HCIIP. This carries a great risk of over-amplifying or underestimating the reserves. It is critical that the uncertainties for each parameter are quantified using sound statistical techniques. Else, the UR could be too large or too small and will impact the field development plans. This is why, a petroleum field is always evaluated not from just one exploratory well, but from several appraisal wells. Data from multiple wells help in quantifying and addressing uncertainty.

10.3.1 Probability Density Function (PDF) Uncertainty can be quantified using a probability density function (PDF). Simply put, PDFs are a function, table, or equation that shows the relationship between the outcome of an event and its frequency of occurrence. There are several types of probability distributions (Fig. 10.3) which are used to model different reservoir parameters. Probability distributions help in understanding the behaviour of reserve estimation parameters. They help in both analysing the parameters, and predicting the statistical outcomes. This makes PDFs a critical component of uncertainty analysis. Each parameter used in calculating HCIIP has a different behaviour which is accounted for its uncertainty analysis. For example, reservoir area and thickness

Fig. 10.3 Different probability density functions

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is modelled as a normal (Gaussian) distribution whereas porosity is modelled as a uniform distribution.

10.3.2 Monte Carlo Simulation Monte Carlo simulation iteratively estimates the value of a parameter based on its PDF. Each estimate of a parameter used for HCIIP calculation is done once which constitutes an instance or iteration of the reserve calculation. This process, which can be easily deployed using a computer program, can run iterations for over 1000 times, generating a cumulative PDF of the reserve estimates. The schematic of the workflow based on 4 independent variables is provided in Fig. 10.4. Several rounds of iterations are done to arrive at the optimal number needed to find the best central value for the reserve. This central value is used for the economic valuation of the asset and for the cost accounting of field development plans. Monte Carlo simulation can be performed in seconds on a computer, and the results are easily visualized and understood. More importantly, a sensitivity analysis can also be performed on the results, which helps in identifying overdependence on any particular parameter. The only drawback of this method is that, when the simulation is repeated, the end result may not be exactly identical although it tends to be around the central tendency. This makes the method difficult to audit. Even then, Monte Carlo method is still considered to be the best method for addressing uncertainty.

Fig. 10.4 The Monte Carlo simulation workflow

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10.4 Hydrocarbon Quality The composition of crude oil is critical to determine its quality. As oil is composed of paraffins, naphthenes, aromatics and asphaltics hydrocarbons, their respective proportions influence the specific gravity and viscosity of the crude. Impurities in crude, like sulphur content, are important quality criteria too. Average elemental composition and proportion of hydrocarbon crude is provided in Table 10.1. It has been observed that the geologic depth of occurrence and the age of the accumulation influences the proportion of paraffin, napthene and aromatic compounds and, the sulphur content in the crude. These are the components used for classification of crude (Fig. 10.5).

10.4.1 API Gravity Specific gravity of oil is an important quality parameter. It is expressed in degrees, as defined by the American Petroleum Institute (API) as below, Table 10.1 Composition of crude oil

Element

Weight %

Hydrocarbon Type

Weight %

Carbon

83–87

Paraffins

30

Hydrogen

10–14

Napthenes

49

Nitrogen

0.1–2

Aromatics

15

Oxygen

0.1–1.5

Asphalts

Fig. 10.5 Crude oil classification

6

10.4 Hydrocarbon Quality

155

( AP I =

141.5 ρ

) × 131.5

where, ρ is the specific gravity of oil (relative to water). As specific gravity of water is 1, its API gravity is 10 degrees. Crudes being lighter than water, their API gravity ranges from 15 (heavy crude) to 40 (light crude). A calibrated hydrometer is generally used to measure API gravity of oil.

10.4.2 Sulphur Content Sulphur is present naturally in crude. It is considered an impurity because it generates sulphur oxides which are undesirable in petroleum products. Most of the sulphur in crude oil is removed during its processing at the refinery. Sulphur can harm some of the catalysts used in the production of petroleum products. Crude oils with high sulphur content are referred to as sour crude and are traded at a lower value.

10.4.3 Dry Gas and Wet Gas Gas is generally classified into dry gas and wet gas. Dry gas is natural gas that consists mostly of methane with little amounts of condensable heavier hydrocarbons like propane and butane. In the USA, when a natural gas contains less than 0.1 gallon of condensables per 1,000 cubic feet, it is considered to be dry. Wet gas on the other hand contains a larger proportion of hydrocarbon compounds, (e.g. ethane, propane, butane etc.) which are heavier than methane. In the reservoir, wet gas exists in gaseous or liquid form, but when brought to the surface, the heavier condensable hydrocarbons are separated as natural gas liquids. Propane and other lighter compounds are marketed as liquefied petroleum gas (LPG) and the heavier ones are sold as petrol (or gasoline).

10.4.4 Processing of Crude Crude petroleum is a mixture of hydrocarbons which include paraffins, napthenes and aromatics. These exhibit different physical properties and have different boiling points. This property is used to separate the different hydrocarbons in crude into different petroleum products through a process called fractional distillation (Fig. 10.6). During fractional distillation, the crude is heated in a furnace to above 600 degrees centigrade and then the vapourised crude is gradually cooled to separate the fractions in different trays. The fractions, from highest to lowest boiling point,

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Fig. 10.6 Fractional Distillation and Petroleum Products

consisting of heavy gas oil, lubricating oil, gas oil and diesel, kerosene, gasoline, naphtha and LPG, are collected and marketed as petroleum products. The basic steps in a refinery are as below. . Heating of crude oil in a furnace to almost 600 °C, which vaporises most hydrocarbons. . The vaporized hydrocarbons are fed into the lower section of a distillation column, where the heavier ones (residual oil) sink to the bottom and are separated. . The remaining hydrocarbons rise up the column through distillation trays, where it is cooled as they pass through it. The hydrocarbons condense in different trays when the temperature is lower than their boiling point. . The temperature gradient and trays are organised in a way that each tray collects liquid fraction of hydrocarbons of similar boiling point. This is how the various components of crude are separated. The quantity of the petroleum products from fractional distillation depends on the quality of the crude that was fed into the refinery. Some petroleum products may be produced in excess and the others in low quantities. Hence, to balance the products to cater to the market demand, the end products are further processed to reconvert them into each other as needed. They are fed into a different processing unit, where longer

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hydrocarbons can be broken into smaller ones and vice versa. An alkylation unit can produce gasoline by combining two LPG-range molecules. A cracking unit can break heavy hydrocarbon into lighter ones by heat and pressure (and some catalysts). Cracking is the most important process for the commercial production of gasoline and diesel fuel. A catalytic reformer can convert naphthas (low octane) distilled from crude oil into high-octane reformates, which can then be used for blending stocks for high-octane gasoline. Crude oil from different parts in the world are different in terms of their constituent heavy and light hydrocarbons. As each refinery is designed to handle a specific type of crude, dependence on a single source of crude for its operation is both not sustainable and risky. Therefore, petroleum refineries source crudes from different parts of the world and blend them to achieve the consistency in its quality for their refinery.

10.5 Summary Quality and volume of oil and gas has tremendous economic impact on the field development. It governs the costs that can be recovered from the market by sale of the end products. It also must meet the requirement of the refinery to which it is shipped for refining and for producing products to meet the market demand. Accurate estimation of reserves in a discovery has a considerable impact on the economic aspects of the field development. Reserve estimation methods and reporting, therefore, has always been a rigorous process. Oil companies leave no stone unturned to obtain the best estimate of a discovery before the petroleum field can be developed. Reducing the uncertainty in the parameters used for reserve estimation, has always been and still is a major subject of interest. This is especially because it helps in optimizing resources and in maximizing profits from the asset in today’s competitive world. Probabilistic methods have always been preferred over deterministic ones as it addresses the risks and uncertainties in reserve estimation.

Further Readings 1. Fahim, M., Alsahhaf, T.A., & Elkilani, A. (2010). Fundamentals of petroleum refining. Elsevier Science. 2. Jahn, F. (1998). Hydrocarbon exploration and production. Elsevier.

Chapter 11

Production Geology

11.1 Reservoir Fluid and Production In the reservoir, the voids contain fluids. These fluids are multiphase, immiscible consisting of oil, gas and water in varying proportions. In the trap, these fluids occur in a stratified manner, with the least dense on top. Water being the densest, is always at the base and gas being the lightest is at the top. When both oil and gas occur in an accumulation, oil must be first produced. This helps in maintaining reservoir pressure because gas expands when oil is drained from the trap. Gas expansion helps in sustaining the reservoir pressure longer and drives oil to the production well. Thus, a steady rate of pressure decline in the reservoir is ensured that aids prolonged oil production. Gas being lighter, can always flow to the surface on its own, when oil is exhausted. The perforated production liner is therefore placed at a depth below the oil-gas contact to target the production of oil. As oil is produced, the oil–gas contact usually moves down because of expansion of gas. Where gas is absent, the oil–water contact moves up with depleting oil.

11.2 Reservoir Drive Mechanisms The prime aim of a production well is to produce hydrocarbons. This requires the hydrocarbons in fluid state to travel to the wellbore from where it can be pumped to the surface. As the subsurface accumulation is under huge lithostratigraphic pressure, hydrocarbons initially flows towards the well bore easily. This initial natural flow results from different forces called drive mechanisms. Three types of drive mechanisms are well defined (Fig. 11.1), although every reservoir has one predominant drive mechanism and an ancillary one.

© Springer Nature Singapore Pte Ltd. 2023 S. N. Kundu, Geoscience for Petroleum Engineers, https://doi.org/10.1007/978-981-19-7640-7_11

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Fig. 11.1 Reservoir Drive Mechanisms

11.2.1 Solution Gas Drive In a scenario, where the accumulation has a huge proportion of gas, which under pressure is in a liquid state dissolved with oil. The reservoir pressure drops as production starts and this allows the gas in solution to partially phase out from its liquid state and expand. The phasing out and expansion of gas maintains the reservoir pressure and aids oil production. Such a drive mechanism is called ‘solution gas drive’. During the production life, a solution gas drive reservoir exhibits typical changes in reservoir pressure from different gas-oil and oil–water production ratios in time. The pressure at which gas begins to phase out of solution to form bubbles is known as the bubble point. Above this pressure, there is no free gas in the reservoir. If the reservoir is initially undersaturated with gas, then the pressure drops rapidly. This eventually reduces oil flow and production rate. In such cases, the gas-oil ratio (GOR) remains constant until the reservoir reaches the bubble point. However, when the reservoir is saturated with gas, the gas constantly phases out after bubble point is reached, leading to a much slower rate of pressure decline in the reservoir. But the producing GOR rises quickly, and as a result, the oil production rates too fall quickly. Therefore, wells must be put on artificial lift early in their life. Water production, also, may start early from such wells, once the interstitial water saturation expands and exceeds the critical value for flow. Oil recovery from solution gas drive reservoirs is usually up to 30% of the oil-inplace (OIP). Therefore, enhanced oil recovery (EOR) strategies need to be planned early to sustain and prolong production from such wells.

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11.2.2 Gas Cap Drive Where the accumulation already has a gas cap above oil, the primary source of pressure results from this initial gas cap. As the drain point from the well bore is below this gas cap, production of oil moves the gas-oil contact lower because the gas cap expands, and this maintains the reservoir pressure. Compared to the solution gas drive, gas cap expansion sustains pressure in the reservoir and provides more prolonged production. The rate of decline of pressure is linked to the volume of the gas cap. The GOR increases slowly during the early stages of production, as the initial reservoir has more gas dissolved in oil. The GOR gradually climbs as the gas-oil contact moves down from its initial position. This results in more sustained pressure for oil production and the requirement of artificial lift is needed much later. Production of water is rare in a gas cap drive as the gas expansion keeps the oil–water interface almost at the same position as before. This is why, the recovery in a gas cap drive reservoir is usually up to 40% depending on the volume and proportion of the initial gas cap.

11.2.3 Water Drive In water drive reservoirs, the water underneath oil, which is normally connected to an aquifer, sustains the pressure in the reservoir during production. As oil is produced, the water from the aquifer exerts pressure by moving into the reservoir, displacing the void created by the produced oil. Water drive reservoirs are also aided by gas when the gas phases out from oil after bubble point is reached. In a water drive reservoir, the pressure response to production depends on the rate of production, volume and permeability of the aquifer. A slower rate of production always helps, as it allows the water move into the space created by the produced oil. This results in a lower rate of reservoir pressure decline. At high production rates, the pressure drop shall be rapid. Therefore, it is important that a steady production rate be decided, taking into account the size and permeability of the associated aquifer, for ensuring longer sustained production. Production rates of oil is strong in water drive reservoirs until the oil–water interface moves upward and touches the base of the wellbore. After this point, the water– oil ratio (WOR) increases in the produced fluids. Gas injection into the reservoir would then be needed to increase the pressure and to lower the oil–water contact. Oil recovery from water drive reservoirs is always above 30% and can reach up to 75% where the aquifer’s sweep efficiency, which is a measure of how effectively the encroaching water displaces oil, is high.

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11.3 Production Well Planning 11.3.1 Rate of Production The anticipated production rate from the reservoir is the prime factor behind production well planning. The reservoir pressure drop for a given well bore diameter is directly proportional to the production rate. Also for natural lift, the reservoir pressure must be good enough to bring the oil to the surface.

11.3.2 Drive Mechanisms A second factor for planning production wells is the drive mechanism as it determines how the pressure will behave with continued production. If the pressure is expected to drop early, the well plan must consider injection wells and artificial lift mechanisms. As solution gas and gas cap drive mechanisms require the presence of free gas in the reservoir, there is always a chance of high gas production. This would require setting up infrastructure at the surface to isolate gas from oil and manage both products. Similarly, water drive will end up with high production of water at some stage and the well completion must have the design to separate water and manage it.

11.3.3 Reservoir Geology The most important geological factor behind planning production wells is the reservoir geometry, its extent, its porosity and permeability and its producing horizons. Where there are multiple reservoir zones in a single well, producing from each individual zone needs to be planned and the well completed accordingly. Multiple oil–water interfaces must also be addressed in the production casing design. Higher production rates induce liquefaction, which makes the reservoir susceptible to migrating fines, leading to the production of sand with oil. Sand production and sand migration clogs the bottom of well and reduces the life of a well. Reservoirs with low permeability, on the other hand, require well stimulation to enhance permeability for smooth production of hydrocarbons.

11.3.4 Fluid Saturation, Wettability and Relative Permeability Wettability is defined as the affinity of a solid surface to a fluid phase. In reservoir rocks, the solid surface is composed of mineral grains, and the fluids are water, oil and gas. Some reservoirs are water-wet whereas others are oil-wet. There are mixed-wet

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163

Fig. 11.2 Wettability of oil (top) relative to different reservoir types (bottom)

reservoirs, where the affinity of both fluid phases is observed to be equal. As water, oil and gas are immiscible, the resulting surface tension from the molecules at a fluid interfaces are at higher compared to within the bulk of the fluids. Hence, a wetting fluid will always displace a non-wetting fluid in the reservoir. In an oil-wet reservoir (Fig. 11.2), the oil will stick to the grains and water shall roam free. The converse is true for a water-wet reservoir. In the case of two immiscible fluid phases (e.g. water and oil) in the reservoir, the permeability of oil and water are relative and depend on their saturation and wettability relationship with the rock minerals. The permeability of each fluid phase, in such a scenario, is referred to as relative permeability. In water-wet reservoirs, the relative permeability of oil shall be higher than that of water. However, as oil production starts, the saturation of oil decreases and that of water increases. As oil production continues, the relative permeability of water shall therefore continue to increase and that of oil shall reduce. At certain saturation level, water shall be co-produced with oil and the proportion of produced water shall keep increasing until a stage is reached where only water will be produced (Fig. 11.3). In reservoirs with the three phases (i.e. oil, water and gas), the influence of the relative proportions of the three phases on wettability will govern production.

11.4 Decline Curve Analysis (DCA) Decline Curve Analysis (or DCA) is an empirical technique that extrapolates trends in the historical production data of oil and gas wells. The purpose of DCA is to

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Fig. 11.3 Relative Permeability and Fluid Saturation in Oil-wet Reservoir

generate a forecast of future production rates and to determine the expected ultimate recoverable reserves. Owing to several reasons, like variations in reservoir pressure and changing relative proportions of fluids in the reservoir, production rates tend to decline with time. The production performance history is plotted against time and the trend is analysed. DCA cannot be performed if a definitive trend is not obtained from the decline curve. All production wells have initial transient flow period which is followed by a boundary-dominated stage. During the transient period, the reservoir pressure at the flow boundary is stable under the influence of initial reservoir pressure. As the flow boundary moves outward from the well through the reservoir, it experiences very high decline rates. When the flow boundary reaches an actual reservoir boundary, or meets with a flow boundary of another well, the reservoir pressure begins to decline and the well enters the boundary-dominated flow stage.

11.4.1 Forecasting Using Decline Curves Based on empirical observation of production history, the trends for future production are modelled. Today, commercial software programs are used for production

11.4 Decline Curve Analysis (DCA)

165

forecasting. The forecast models are based on exponential, hyperbolic or harmonic functions (Fig. 11.4). The exponential decline is the most common method used for DCA. Its trend line displays a more or less fixed slope, indicating a steady state of production decline with time. Hyperbolic decline model is less used, but is very common in low permeability and tight reservoirs. Here the trend line does not show a constant rate of decline, and the rate of decline reduces considerably with production time. A harmonic decline is a special case of the hyperbolic decline which is characterized by a very steep initial decline and a gentler later decline. Such models are usually used in horizontal production wells (shale gas wells). In several cases, both the hyperbolic and exponential models are assimilated to make a balanced prediction. As each individual curve tends to overestimate production and reserves, they can be averaged to moderate the overestimation. This combined model of forecasting, normally, has a transition point in the production life of the well, where the hyperbolic decline will transition to an exponential decline. All three decline equations fit nearly exactly in the first 2 years, but eventually produce noticeably different forecasts. If all three decline equations match the historical data, then the exponential decline forecast will show the most decline in production rates, making it the most conservative forecast.

Fig. 11.4 Decline Curves

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11 Production Geology

11.4.2 Decline Curves for Different Drive Mechanisms Drive mechanisms can be determined by analysing historical production data, reservoir pressure and the proportion of the produced fluids. The characterization of the drive mechanism, in an oil and gas accumulation, increases the ultimate recovery through meticulous production planning. The decline curves for different drive mechanisms are provided in Fig. 11.5. For solution gas drive, oil production rates fall quickly once the GOR increases. In a gas cap drive, the expansion of the gas cap wrests the fall in reservoir pressure. Hence, the pressure drops slowly as compared to a solution gas drive reservoir. In a water drive reservoir, the GOR remains fairly constant and therefore therefore the reservoir pressure is stable from water influx from the connected aquifer. A slower rate of production will allow the permeability of the formation to allow water to replace the voids emptied by the produced oil, simultaneously. This helps maintain reservoir pressure which is depicted by the slow rate of decline in oil production.

Fig. 11.5 Decline curves for various drive mechanisms

11.5 Enhanced Oil Recovery (EOR)

167

11.4.3 Reserve Estimation from Production History The process of estimating reserves for a producing field is continuous throughout the life of the field. There are several uncertainties in the parameters used for reserve estimation before its production. However, as production starts, production and pressure data from a producing field over time can be used for DCA, where material balance equations are used for reserve calculations. The use of production data and material balance equations provide more certain reserve estimation from a well or a field. A common decline curve relationship is the constant percentage decline (exponential) from low productivity wells. Hyperbolic or harmonic decline extrapolations can be used but they have a tendency to overestimate reserves. Hence, the concept of expectation values is used, where the reserve estimate is the average of three scenario; a low estimate (25% of cumulative production), a medium estimate (50% of cumulative production) and a high estimate (75% of cumulative production). The below equation is how the expectation value is derived from DCA. Expectation V alue =

(Low Estimate + Medium Estimate + High Estimate) 3

11.5 Enhanced Oil Recovery (EOR) Production of petroleum under natural drive conditions is called primary recovery. This includes natural flow under reservoir pressure or artificial lift using surface pumps. Lifting is said to be natural, when the reservoir fluids ascend spontaneously to the surface aided by natural reservoir pressure. Artificial lift is where the reservoir pressure is insufficient to bring the fluids to the surface, but is enough to force the fluids in the reservoir to the well bore. Normally, primary recovery accounts for about 30% production from the reserves on an average. A summary of the different oil recovery methods are provided in Fig. 11.6 Once reservoir pressure drops, the oil does not flow within the reservoir to the well bore. But, as there is still a lot of hydrocarbons left to be produced, enhanced oil recovery (EOR) methods are deployed. EOR is classified into two categories; Secondary recovery and Tertiary recovery. Post primary recovery, either secondary or tertiary recovery methods or both are deployed depending on the reservoir condition and the property of the remaining fluids. Injection wells are designed to inject various fluids to enhance reservoir pressure and flow. Injection wells are normally planned based on the reservoir geometry and structure (Fig. 11.7). A radial array of injections would be necessary in the case of a dome structural trap, whereas a linear array of wells is good for a dipping or gently folded reservoir.

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11 Production Geology

Fig. 11.6 Oil Recovery Methods

Fig. 11.7 Injection Well Patterns

11.5.1 Secondary Recovery Secondary recovery methods normally drive the fluids in the reservoir to the well bore by increasing reservoir pressure. The pressure is usually increased by injecting water or gas. Water (or gas) is injected into the reservoir to maintain pressure (Fig. 11.8). Usually, the co-produced formation water, which is separated and stored on surface,

11.5 Enhanced Oil Recovery (EOR)

169

Fig. 11.8 Secondary Recovery

is re-injected. Seawater is not used as it being saline can result in precipitation of sulphates and other salts in the reservoir. Salt precipitation would clog the pores in the reservoir which reduces its permeability. The prime issue with water injection is that, when water and formation fluids are incompatible, it leads to formation damage which alters the permeability of the reservoir. Gas (usually co-produced from the well) can also be injected into the reservoir to increase pressure. The co-produced gas is the solution gas that separates out from oil at the surface. This is collected and injected back through injection wells. This works well for an oilfield with low GOR where the produced gas is a noneconomic by-product. By re-injecting this gas, infrastructure on the surface to handle gas, like transportation, is not required. Since the gas is to be re-injected, there is no need to flare it. The prime issue with gas injection, at this stage, is that it is very likely to channel upwards bypassing the oil. This usually happens due to unfavourable mobility ratios of oil and gas in the reservoir.

11.5.2 Tertiary Recovery When secondary methods are no good for enhancing oil recovery, tertiary methods are used. Tertiary methods, in addition to increasing the reservoir pressure, aim to increase the effective permeability of the fluids in the reservoir. Effective permeability can be achieved by reducing viscosity and surface tension of oil. There are three primary methods of tertiary recovery; use of heat, use of gas and use of chemicals.

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11 Production Geology

Thermal recovery methods use heat to reduce the viscosity of oil, so that its flow to the wellbore is eased. Steam, injected into the reservoir through injection wells, increase the temperature of the formation. This causes expansion of fluids which increases the reservoir pressure, and at the same time it reduces the viscosity of oil as steam warms the oil. When oil is too viscous and tarry, in-situ combustion methods are used, where the reservoir is ignited to crack the oil to form lighter hydrocarbons. However, in-situ combustion is seldom used as it has long lasting environmental impacts. The gas injection method achieves the same objectives as the thermal method. In gas injection, the gases are chosen so that they mix with oil which reduces the viscosity of oil and also increases the reservoir pressure. The gases used are carbon dioxide, nitrogen, or natural gas. These mix very well with oil expanding it and pushing it to the wellbore. In the chemical flooding (Fig. 11.9), polymers and surfactants (chemicals which change the surface properties like surface tension) are flooded into the reservoir. These mix with organic acids in oil to form soapy matter which reduces the surface tension of oil. The reduction of surface tension allows oil to negotiate the capillary pressure of the fine pores, thereby moving it to the well bore. Different types of synthetic polymers are combined and used. This is because, some polymers mix with oil to reduce its surface tension and some mix with water to increase its viscosity. Thus the relative permeability of oil and water is altered and oil production is enhanced. Chemical flooding is carefully considered as it has several environmental concerns. It is therefore used in selective environments.

Fig. 11.9 Chemical Flooding

Further Reading

171

11.6 Summary Production geology mostly deals with the reservoir and its dynamic properties during production. The objective of production is to maximise ultimate recovery and therefore, it is necessary to monitor production data (volume and proportion of fluids being produced and the reservoir pressure). Variability of production data over time is a means to check the health of the well and the oilfield. With the dominant drive mechanism identified at the onset of production, predictive models will show the trend of future production. However, WOR and GOR ratio of the produced fluids must be monitored so that one can decide when to intervene and introduce EOR methods. When more gas or water is produced from a oil well, handing these must not become a unforeseen problem. Water disposal is an environmental issue and so is flaring gas from a platform. Hence, storing these for use during EOR must be considered. The price of oil plays a vital role on the rate of production and when a well is producing less than the expected commercial level, it is deemed uneconomic and the production is usually stopped despite the field having significant amount of unrecovered oil. Production is critical to the economics of the oil producing company and thus, it is of utmost importance that the geological characteristics of the reservoir and the accumulation are thoroughly modelled and monitored both before and during production.

Further Reading 1. Laudon, R.C., Principles of petroleum development geology. Prentice Hall (1996).

Chapter 12

Unconventional Hydrocarbons Resources

12.1 Introduction The previous chapters discussed hydrocarbons accumulated in conventional petroleum traps and were produced by drilling and pumping the oil to the surface. However, there are huge amounts of hydrocarbons, which lie trapped in petroleum system elements other than in the trap, and cannot be extracted by conventional means. These resources are called unconventional hydrocarbons. The classification of hydrocarbons, into conventional and unconventional, is based on the nature of the hydrocarbon, its geological occurrence and the method of its exploration and extraction. With population growth and urbanization, the demand for energy is ever increasing. As conventional resources are fast depleting, the focus has now shifted to explore alternative resources like unconventional hydrocarbons and renewables. As per the World Energy Council (WEC), fossil fuels comprise of about 85% of all our energy demands and renewables only account for the remaining 15%. In 2050, the energy mix of modern society is expected to be around the 80% fossil fuels and 20% renewables. As conventional fossil fuels are diminishing, the resulting energy deficit is likely to be filled by unconventional hydrocarbon resources. This is why, today, most countries are appraising their potential of unconventional hydrocarbons and many have even started producing them. The need to produce energy continuously for sustaining the requirements of modern society has shifted the focus to explore and produce from unconventional hydrocarbons.

12.1.1 Energy Returned on Energy Invested (EROEI) A major challenge in exploring and producing unconventional is its low energy return on the energy invested (or EROEI), which is defined as

© Springer Nature Singapore Pte Ltd. 2023 S. N. Kundu, Geoscience for Petroleum Engineers, https://doi.org/10.1007/978-981-19-7640-7_12

173

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Table 12.1 ERORI averages of various energy resources Energy resource

Year

EROEI

Fossil fuel (conventional oil)

1970

30

Fossil fuel (conventional oil)

2000

20

Fossil fuel (conventional oil)

2010

10

Fossil fuel (conventional gas)

2000

40

Fossil fuel (conventional gas)

2010

20

Fossil fuel (coal)

1950

80

Fossil fuel (coal)

2000

60 25

Fossil fuel (coal)

2010

Fossil fuel (unconventional—oil sand or tar sand)

After 2010

8

Fossil fuel (unconventional—oil shale)

After 2010

7

Fossil fuel (unconventional—shale gas)

After 2010

15

Fossil fuel (unconventional—tight gas)

After 2010

12

E RO E I =

U sable Acquir ed Energy Energy E x pended

To produce conventional hydrocarbons, the energy invested to extract is much lower than the energy produced from the extracted hydrocarbons. However, for unconventional hydrocarbons, the invested energy to produce them is very high. Unless the energy invested is lower than the energy that is produced, extracting unconventional is not commercially viable. Indicative EROEI for different fossil fuels are provided in Table 12.1.

12.1.2 Types of Unconventional Hydrocarbon Resources Unconventional hydrocarbons represent a failed petroleum system, where the hydrocarbons are scattered in its different elements other than the conventional trap. Most unconventional hydrocarbon resources are categorized on the basis of it being in solid, liquid or gas. Shale oil, and oil sands are solid state unconventional resources, whereas coal bed methane, shale gas and tight gas are in gaseous products. Their geological settings, in relation to conventional hydrocarbons, are shown in Fig. 12.1. Most unconventional fossil fuels occur in the source rock itself. Most source rocks are shale, which is a laminated, indurated rock consisting of clay sized minerals. Shale is abundant and comprises almost 50% of all sedimentary rocks found on Earth. Organic rich shales are good source of hydrocarbons. In sedimentary basins, there are two basic shale resource systems. One is gas producing and the other is oil producing. The gas trapped in shale, from its thermal maturation, is known as

12.2 Shale Gas

175

Fig. 12.1 Unconventional Hydrocarbons and Geological Settings

shale gas. Shale formations, where the kerogen has not matured to produce hydrocarbons, can be mined and cooked in a furnace to produce oil. Such shale formations are termed oil shales. In a sandstone reservoir, where oil has degenerated into the lighter hydrocarbons and has escaped, what remains behind is the viscous and heavy hydrocarbons. These heavy hydrocarbons are mostly tar and hence the reservoir formation is called tar sand. Tar sands are mined and heated to temperatures where the tar cracks into less viscous and simpler hydrocarbons. Anisotropical distribution of permeability in a conventional reservoir results in some gas trapped in the unconnected pores. This gas does not flow to the conventional well and is referred to as tight gas.

12.2 Shale Gas Natural gas, primarily methane, is trapped in shale formations and constitutes shale gas. Shale gas also consists of small amounts of other gases like ethane, propane and butane and several impurities in smaller quantities. As tiny pores within shale are not well connected, the gas can be extracted only when the pores are opened and the gas is channelled to a production well. Shale has a very wide occurrence and usually contains high organic content (~5%). It has abundant gas storage capacity, and once it thermally matures, its pores are saturated with gas. Shale is brittle and fissile and therefore, amenable for hydraulic fracturing. Developing shale gas is cost intensive, but it pays rich dividends as the resource has a long life as compared to conventional gas wells.

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Fig. 12.2 Trapped gas in shale and desorbed gas post hydraulic fracturing

In shales, gas can be trapped in several ways. Gas in the pore spaces is called free gas. Some gases lay adsorbed on the surface of the clay sized minerals and organic matter. Understanding the relative proportions of free and adsorbed gas is critical to planning gas production from a shale gas resource. Hydraulic fracturing of the shale opens the interstices and connects the pores. This helps convert the adsorbed gas into desorbed gas, which then flows to the production well along with free gas. Figure 12.2 illustrates the release of adsorbed gas in shale after hydrofracturing.

12.2.1 Exploration and Production As far as unconventional hydrocarbons are concerned, shale gas is way ahead of the rest in terms of its exploration and production. The geological risk of not finding gas in shale is low and its production in commercial quantities has been proven in many parts of the world. The boom in shale gas exploration, in recent times, was because of technological developments in drilling and hydraulic fracturing. As most shale formations are horizontally stratified, horizontal drilling technology immensely helped tap its vast resource potential. Once a horizontal well is in place, the casing is perforated using hydraulic fracturing, which creates fractures that connect the gas saturated pores and thus the trapped gases are released (Fig. 12.3). Horizontal drilling in shale gas wells can reach laterally up to 3 km and this helps over large borehole surface area in shale formations to maximize gas production. From a single well location at the surface, multiple horizontal wells that radiate from a common subsurface location are drilled to tap the resource to its maximum potential.

12.3 Oil Shale

177

Fig. 12.3 Horizontal Drilling and Hydraulic Fracturing in Shale Formations

12.3 Oil Shale Shale formations that are rich in kerogen but are not buried into the oil and gas window to thermally mature, can be mined and heated by artificial means to produce oil. This oil is called ‘shale oil’ and the thermally immature shale is called ‘oil shale’. Oil shales are mostly deposited in lacustrine and marine environment. Oil shales are rich in kerogen of type-I and type-II. Of the three main types of shale, namely marine shale, lacustrine shale and terrestrial shale, only the former two contain kerogen which are oil prone. Terrestrial shale has type-III kerogen, which is coaly and gas prone, and therefore cannot be used to produce oil. Shale source rocks mature between 60 and 120 °C, and these temperatures are naturally attained when the formation is buried between 2 and 4 km below the surface under normal thermal gradient. Humans have been using oil shale for thousands of years. Shale oil was used to pave roads in ancient Mesopotamia. During war, ancient Mongolians sent fire to their enemies through arrows dipped in shale oil.

12.3.1 Exploration and Production Oil shale is heated to extract oil. This process is expensive and has a very low EROEI. Still, several countries like Estonia, China, and Brazil, rely on oil shale for their energy

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12 Unconventional Hydrocarbons Resources

needs. Extracting oil from oil shale involves a process called pyrolysis where the kerogen is heated. Pyrolysis does not use oxygen, but temperatures much higher than the natural oil and gas windows are required. Pyrolysis is usually done in a furnace at the surface. It also can be achieved by insitu combustion. A generic workflow for oil shale processing is provided in Fig. 12.4. For the surface (ex-situ) process, oil shale is first mined from the subsurface. It is then crushed and then heated in an anoxic environment to produce oil. The produced oil would still contain impurities like sulphur, which is separated in later stages to improve the quality of oil. As mining and surface processing requires large facilities and produce lots of toxic products into the ambient environment, in-situ methods were developed. In-situ process does not require oil shale to be mined. In-situ oil shale is heated to the oil window by various methods. One such technology is volumetric heating, where the rock is heated directly using electricity. A electric heating element is injected through perforations into the horizontal section of the well. Once the shale is heated, oil oozes out into the well, which can then be pumped out. The internal

Fig. 12.4 Oil Shale Extraction Workflow

12.4 Tight Gas

179

combustion process uses a combination of gas and steam to heat the in-situ shale formation. The co-produced gas and water from steam is re-circulated. Mining for oil shale has damaging impacts on the environment, especially because it releases lots of trapped gases like carbon dioxide. Therefore, shale oil production is not commonly favoured by several countries.

12.4 Tight Gas Tight gas is found in reservoirs where the permeability is less than 0.1 millidarcies (mD). Low permeability makes it difficult to extract tight gas through conventional means. The geological environment, which favour the deposition of tight reservoirs, are deep ocean basins or river banks. Here the facies consist of fine sand and silty sedimentary formations. Tight reservoirs are also formed when the effective porosity is reduced by post-depositional processes selectively in a conventional reservoir. This leaves behind pockets of tight sand where gas lay trapped. Extensive cementation by authigenic clays in permeable reservoirs reduces the matrix permeability, making them tight reservoirs. Tight gas reservoirs can occur both in sandstone and carbonates. They normally have significant formation thickness and are under high overpressure. Gas production from tight gas reservoirs seldom co-produces water. They often contain anomalous pressures within the gas saturated layers. Unlike conventional accumulations, tight gas reservoirs do not require a seal rock or a structural closure to trap the gas. The permeability of unconventional reservoir is significantly lower than the conventional reservoir (Fig. 12.5).

Fig. 12.5 Hydrocarbon Reservoir Rocks and Permeability

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12 Unconventional Hydrocarbons Resources

12.4.1 Exploration and Production Tight gas is normally produced after other hydrocarbons present in the reservoir are extracted by conventional means. Extensive seismic data, which was gathered during conventional production, helps in detecting these non-permeable sections in the reservoir which contain tight gas. When these sections could be drilled directly, development wells are planned to extract the gas. Vertical wells are first considered as they are easier to drill and are less expensive. Horizontal wells are considered where deemed necessary. More number of wells are needed to produce gas from tight reservoirs as compared to the number of wells needed for conventional gas reservoirs. Moreover, each well requires stimulation to enhance permeability. Standard well stimulation methods used are matric acidization and fracture acidization (Fig. 12.6). During drilling operations, the wall of the well is abraded and this damages the formation reducing its permeability. Matric acidization process repairs this formation damage. The process involves pumping of acid into the well which dissolves and removes the pulverized fine materials. This unclogs the pores and enhances permeability. In the fracture acidization, pressurized acid-rich water is used with an aim to hydraulically fracture the formation around the wellbore. The acid helps in dissolving the fines and in clearing the pores for better flow of gas. Proppants are mixed with the pressurized acid water, and these occupy the void spaces created by the fractures and preserve the permeability of the formation. In some tight gas formations, the reservoir may contain small amounts of water which undermines the production process. Deliquifaction of the reservoir by artificial lift techniques removes the water from the reservoir. However, deliquifaction is seldom practiced, as its utility in tight gas production is not found to be very cost-effective.

Fig. 12.6 Well Stimulation methods for Tight Gas Reservoirs

12.5 Coal Seam Gas

181

12.5 Coal Seam Gas Large amounts of natural gas are found trapped in coal seams along with other gases like CO, CO2 and N2 . The gases are trapped in the coal seam as pressure from water saturated formations above the seam do not allow the cleats (natural cracks) to open. A large proportion of the contained gases are attached to coal macerals by weak covalent bonding forces (van der Waals force). Coal seam gas are recovered from the stratified coal seams when the pressure from overlying water is reduced and the weak covalent bonds are broken. The gas extraction from coal can be done before, during or after its mining operations. Coal seams, which are not minable, due to its deep location and high extraction costs, are usually the ones where gas extraction is targeted. Like shale gas and tight gas, coal seam gas is extracted using unconventional technology and therefore, it is considered as an unconventional hydrocarbons resource. Gas in coal is stored in several ways, as below . Free gas within the fractures and cracks of the coal bed. . Adsorbed gas molecules on the internal, at the surface micro pores and fractures within the coal bed. . Dissolved gas in solution (water) within the coal bed.

12.5.1 Exploration and Production Exploration of coal seam gas is done after deep seated coal seams are discovered. Test wells are then drilled to remove the water from the formations above the seams. This reduces the pressure and allows the cleats to open so that the trapped gas in the coal seam is released. Gas flow from the test wells are monitored and production volume with time is analysed before commercial production can commence. The schematic of coal seam gas production is provided in Fig. 12.7. Life of a coal seam gas well begins as water producing well. Substantial water is produced to reduce the hydrostatic pressure to a level where the fractures and cleats in the coal seams widen. The well then becomes a gas producing well when the absorbed and adsorbed gas in the seam are released. In some cases, hydraulic fracturing of the coal seam is done to expedite the production process. It is important to note that the amount of gas that can be stored in a cubic foot of coal seam is about six or seven times higher than the gas that is stored in a cubic foot of conventional gas reservoir. This makes coal seam gas a superior source of gas, as compared to conventional gas reservoirs.

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12 Unconventional Hydrocarbons Resources

Fig. 12.7 Coal Seam Gas Production

12.6 Oil (or Tar) Sands A natural mixture of heavy oil, water and sand constitute oil sands. Oil sands are either loose sands or are partially consolidated sandstones which contain dense and viscous hydrocarbons. Hydrocarbons contained in oil sands are bituminous and the ratio of sand to bitumen is about 90:10. Oil sands also contain small proportions of water and clays. Bitumen is semisolid and tarry and therefore, oil sands are also referred to as bituminous sands or tar sands. The organization of the various constituents of oil sand is depicted in Fig. 12.8. Oil sands are, in fact, heavily degenerated hydrocarbons in a conventional sandstone reservoir. As the reservoir’s seal is breached and the hydrocarbons are exposed to atmosphere, microorganisms act on the crude and break it down to release lighter hydrocarbons. This leaves behind the heavier bitumen in the reservoir.

12.6.1 Extraction and Production The oil is recovered from oil sands by both mining and in-situ drilling. The recovery method is chosen depending on the volume of the resource and its depth of occurrence. Oil is extracted from oil sand by separating the bitumen from the rock

12.7 Natural Gas Hydrates

183

Fig. 12.8 Organization of Oil Sand Constituents

minerals (sand and clay). This is done by injecting hot steam to reduce the viscosity of bitumen, which can then be separated from the sand. Generating steam for the separation process consumes large amounts of both energy and water. This reduces the EROEI and makes production from oil sand un-economic in many parts of the world. Mining is adopted when the oil sand resource is located at depths less than 250 ft below the surface. The oil sand is mined and processed in a plant to extract the bitumen. The bitumen is then refined and cracked to form smaller chained hydrocarbon products. The deeper oil sands cannot be economically mined and therefore the bitumen is extracted in-situ. Tertiary recovery methods for conventional hydrocarbons are used as primary recovery methods to recover oil from oil sands. These methods reduce the viscosity of bitumen using heat and surfactants. In Canada, where oil sand is produced, Cyclic Steam Stimulation (CSS) and Steam-assisted Gravity Drainage (SAGD) methods are used (Fig. 12.9) for in-situ extraction. These methods are preferred over mining as the latter has disastrous environmental impacts. In general, oil sand extraction has a very low EROEI and is carried out when oil is a strategic commodity to a nation.

12.7 Natural Gas Hydrates Natural gas hydrates are crystalline solids like ice. They are commonly referred to as gas hydrates or clathrates. Clathrates are chemicals that consist of a lattice which traps another molecule. In natural gas clathrates, a host water molecule traps a hydrocarbon gas molecule. Most common clathrate is the methane clathrate, where a methane molecule is trapped in an ice lattice. Methane clathrates burn with a flame just like camphor. Natural gas hydrates are widely distributed in ocean basins and permafrost regions. These provide an environment where the clathrates are stable. Deep-water

184

12 Unconventional Hydrocarbons Resources

Fig. 12.9 Cyclic Steam Stimulation (CSS) and Steam-Assisted Gravity Drainage (SAGD) methods

drilling for oil has encountered gas hydrate bearing marine sediments. Stability of a gas clathrate primarily depends on temperature and pressure, and on other variables like gas composition and ionic impurities. The estimated resource potential of natural gas hydrates is believed to be 10 times more that all conventional hydrocarbons resources taken together. With natural gas being a cleaner fossil fuel, natural gas hydrates are considered to be an important substitute of petroleum and coal.

12.7.1 Composition, Structure and Stability Water and natural gas, under low-temperature and high-pressure conditions, form an ice like crystalline compound, where a 3-dimensional lattice structure forms a cage inside of which a natural gas molecule is trapped (Fig. 12.10 left). The structure of gas hydrate is so compact and effective that, when one cubic meter of gas hydrate is brought to the surface, it releases 164 cubic meters of natural gas. Gas hydrates are sensitive to pressure and temperature conditions as they naturally dissociate to release methane when it is outside of its stability zone. Gas Hydrate Stability Zone (GHSZ) is a depth zone in the marine environment, where methane clathrates naturally exist (Fig. 12.10 right). Figure 12.11 shows a gas hydrate found in marine clay and the inset shows a burning clathrate.

12.7.2 Development and Production The natural gas trapped in the water lattice is released when the structure dissociates. Hence, to produce gas from clathrates, its stability needs to be altered. This can

12.7 Natural Gas Hydrates

185

Fig. 12.10 Structure of a Methane Clathrate (left) and its stability range (right)

be achieved either by lowering the pressure or increasing the temperature of the environment where it occurs. Doing so weakens the van der Waals force between methane and water and the trapped gas is released. The three conceptual techniques, which are being experimented for releasing the gas from the clathrate, are: . Thermal stimulation. . Depressurization. . Chemical inhibitor injection. Thermal stimulation involves drilling a well into the formation that hosts the gas hydrate and injecting steam or hot water through the well. This increases the temperature of the formation and destabilizing the gas hydrates. Electromagnetic heating has been found to be an effective method of heating gas hydrate bearing formations. However, thermal stimulation methods are challenged by problems arising from rapid heat loss, low efficiency of heating. At present, no efficient way of aggregating and producing the released gases exist. Depressurization is a method of reducing the pressure to destabilize the gas hydrate to effect its dissociation. The method involves drilling a borehole into the equilibrium pressure of a free gas zone underneath the gas hydrate bearing formation. Extraction of this free gas reduces the pressure which destabilizes the hydrate, thereby releasing the gas. This technique is suitable where gas hydrates are in vicinity of conventional gas reservoirs having good permeability which occur at depths greater than 750 m. Depressurization is economically more feasible than thermal stimulation method, but the process is slow. Moreover, this method has been found to be less effective in permafrost regions. Chemical inhibitors like brine, methanol and glycerine can effectively change the gas hydrate stability by impacting the environment’s phase equilibrium conditions. The method is easy and highly effective but comes at a greater cost. But, it is not suitable for hydrate formations which are under high pressure.

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12 Unconventional Hydrocarbons Resources

Fig. 12.11 Gas hydrate (white) in clay (black) found in deep-water drilling (inset: a burning clathrate)

Combining two or more methods is considered to be effective. But each combination has its advantages and drawbacks. Depressurization and thermal stimulation is by far the most effective design. Gas dissociation from natural gas hydrate has been successfully experimented, but channeling the released gas to a well from where it can be commercially extracted is still a major issue. A “three-phase control” exploitation theory was adopted in the South China Sea for gas production. This was the first trial for gas hydrate production where the methods focussed on formation stabilization, formation fluid extraction, well drilling and completion, reservoir stimulation and sand control. However, the production test lasted for only 60 days after which pressure from the formation diminished significantly. As of today, we do not have an economically feasible technology for production from gas hydrates. Research and development on gas hydrate production are still in their infancy, but hopefully mankind will make inroads to successful and sustainable production from gas hydrates.

12.8 Global Unconventional Resources and Distribution Unconventional hydrocarbons, which are currently under production, are oil sand, tight oil and gas, oil shale, shale gas and coal seam gas. With depleting conventional hydrocarbons, the footprint of unconventional hydrocarbons in the global energy mix, is steadily increasing. In 2015, it accounted for 9% of the global annual production

12.8 Global Unconventional Resources and Distribution

187

Fig. 12.12 Primary Energy Share from Fossil Fuels by Country (BP Statistical Review 2022)

of oil and 27% of the global annual production of gas. Production of tight oil in the US contributed to 45% of the total produced oil. Since 2000, agencies like United States Geological Survey (USGS) and Energy Information Administration (EIA) are constantly evaluating the potential of unconventional oil and gas resources across the globe. British Petroleum’s statistical review of world energy (2022), shows that the primary energy resource proportion from fossil fuels is still high for the world. Each country’s proportional share of the world’s energy mix is illustrated in Fig. 12.12.

12.8.1 Shale Gas Resource Distribution The global resource potential of shale gas is huge and known shale gas deposits worldwide has added over 40% to the technically recoverable natural gas resources. EIA based study, on the sedimentary basins assessed for shale gas reserves, stands at over 200 trillion cubic meters (Fig. 12.13), with China, US and Argentine leading the pack. Many other basins are less explored today. China has the largest known shale gas reserves in the world, though it is not producing much at the moment. Russia produces and exports 80% of the natural gas consumed in Europe and this includes little or no shale gas. In the US, shale gas in the most explored and is being produced. Even since its conventional oil peaked in 1970, US has always been exploring unconventional energy resources. Recent discoveries of gas reserves in

188

12 Unconventional Hydrocarbons Resources

Fig. 12.13 Global distribution of Shale Gas (in trillion cubic meters)

Eastern Europe may become an important unconventional source for the European market as the Russia–Ukraine war stifles the supply of Russian gas to Europe.

12.8.2 Tight Gas and Coal Seam Gas Resource Distribution Worldwide, tight gas sources are believed to occur significantly, but only in few countries like USA, Canada, China and Australia. Currently, about 50% of all the daily gas production in the US, is from tight and unconventional resources. Although tight gas and coal seam gas production has been reported from more than 35 countries, the actual gas production is low as compared to gas from conventional reservoirs on a per-well basis. The capital cost of unconventional gas production is high because of the added cost of hydrofracturing and co-produced water handling. The driving forces to increased unconventional tight gas production are . increased oil and gas prices, . decline in conventional oil and gas production, and . improvement in drilling, completion and hydraulic fracturing. Coal deposits are widespread in 70 countries of the world and it is a very affordable and reliable source of energy. Besides the minable coal reserve, vast deepseated deposits of coal have the potential to be exploited for coal seam gas. The prominent coal basins, where coal seam gas reserves have been evaluated, includes United States, Western Canada, United Kingdom, France, Germany, Poland, Czech Republic, Ukraine, Russia, China, Australia, India, and South Africa. These countries

12.8 Global Unconventional Resources and Distribution

189

Fig. 12.14 Global coal seam gas reserve estimates

produce 90% of global coal and account for nearly all of the coal seam gas produced. Since 3000 ft is the maximum depth from which coal can be mined, of all the coal resources of the world only about 1 trillion ton of coal can be mined. This leaves a vast reserve of coal resources with coal seam gas potential. The limiting factor is the present day capability of vertical drilling and hydraulic fracturing, which works only up to 3000–3500 ft depth and this is where the permeability ranges support coal seam gas production. Global estimates in TCF are provided in Fig. 12.14. Tight gas and Coal seam gas are important gas resources, which are set to replace the depleting conventional natural gas reserves. Some resource estimates for countries, where statistics were available, are provided in Table 12.2.

12.8.3 Oil Shale Resource Distribution Most oil shale deposits are either small (and uneconomic to recover under present circumstances) or are large (and potentially recoverable). Estimating oil shale reserves is highly uncertain as kerogen content within a shale formation is highly variable. The prevalent price of oil is the prime governing factor for the economic feasibility of oil shale. This is because it has a very low EROEI and only high oil price will render its extraction economic. More than 600 oil shale deposits are known to occur globally but only 33 countries have evaluated their oil shale resources from an economic perspective. The Green River deposits in the western United States, the Tertiary deposits in Queensland, Australia and the deposits in Sweden and Estonia are some of these well appraised deposits which collectively could potentially yield 40 litres of oil per metric ton of shale.

190 Table 12.2 Tight Gas and Coal Seam Gas resource estimates by Region

12 Unconventional Hydrocarbons Resources Region

Tight gas in TCF

Coal seam gas in TCF

North America

1371

3017

Former Soviet Union

901

3957

Centrally planned Asia and China

353

1215

Pacific (OECD)

705

470

Latin America

1293

39

823

0

Middle East and North Africa Sub-Saharan Africa

784

39

Western Europe

353

157

Other Asia Pacific

549

0

Central and Eastern Europe

78

118

South Asia Total

196

39

7406

9051

A conservative estimate, that was made in 2016, puts the reserves of oil shale at 6 trillion barrels of oil. This is huge compared to the 2 trillion barrels of conventional oil at the time. The annual production of some countries is provided in Fig. 12.15,

Fig. 12.15 Oil Shale Mined annually since 1950

12.9 Summary

191

Fig. 12.16 Inferred and Recovered Gas Hydrate locations

which provides an understanding of today’s shale oil production activity. As carbon content of shale is key to the volume of oil produced from oil shale, deposit wise break-up of information is currently not available from all countries.

12.8.4 Gas Hydrates Resource Distribution Resource occurrences and potential for gas hydrates are not comprehensively possible at the moment. Resource assessments are based on geological understanding of gas hydrate stability around the globe, and this has been partially validated by its encounters during drilling for conventional hydrocarbon. At the same time, the technology to commercially produce from gas hydrates, is not currently available. Therefore, it would be difficult to estimate how much of the in-place gas hydrates which are believed to exist and can actually be recovered commercially. Gas hydrates have been recovered from exploratory drilling. The understanding of its origin and stability at locations around the globe has led to the belief that the likelihood of its occurrence is very high. The locations where gas hydrates are inferred and recovered are provided in Fig. 12.16.

12.9 Summary A major transformation is happening in the global energy scene and this is driven by new supplies coming from unconventional fuels. The footprint of unconventional

192

12 Unconventional Hydrocarbons Resources

energy is steadily increasing in the energy mix of the world. As unconventional hydrocarbon resource potential far exceeds the conventional resources, its development and production is going to increase as more conventional resources deplete. Moreover, renewable energy resources are not expected to go beyond 20% of our energy mix and this would create a huge resource gap which only unconventional hydrocarbons can fill. The wide occurrences and huge in-place volumes of unconventional hydrocarbons are much higher compared to recoverable conventional hydrocarbons, and this indicates that it has a tremendous future potential. Shale gas production in the US, has not only made up for its energy deficit, but also has made US a gross exporter. With the Russia-Ukraine war threatening to shut energy supply to Europe, European countries shall now be forced to exploit their unconventional resources. This shall include overcoming all the regulatory environmental barriers for their production. Major policy changes are currently underway, to enable production from unconventional energy resources, to fill the energy deficit arising from geopolitical factors, technological advancements and global supply chain disruptions.

Further Readings 1. Zou, C. (2017). Unconventional petroleum geology (2nd ed.). Elsevier. 2. Naik, G. C. (2005). Tight gas reservoirs—An unconventional natural energy source for the future. 3. Law, B. E., & Curtis, J. B. (2002). Introduction to unconventional petroleum systems. AAPG Bulletin, 86, 1851–1852.