Latest Ordovician to Early Silurian Shale Gas Strata of the Yangtze Region, China 9819931339, 9789819931330

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Xu Chen Hongyan Wang Daniel Goldman Editors

Latest Ordovician to Early Silurian Shale Gas Strata of the Yangtze Region, China

Latest Ordovician to Early Silurian Shale Gas Strata of the Yangtze Region, China

Xu Chen • Hongyan Wang • Daniel Goldman Editors

Latest Ordovician to Early Silurian Shale Gas Strata of the Yangtze Region, China

123

Editors Xu Chen Nanjing Institute of Geology and Palaeontology Chinese Academy of Sciences Nanjing, Jiangsu, China

Hongyan Wang Petroleum Exploration and Development Petrochina Research Institute Beijing, China

Daniel Goldman Dayton University Dayton, OH, USA

ISBN 978-981-99-3133-0 ISBN 978-981-99-3134-7 https://doi.org/10.1007/978-981-99-3134-7

(eBook)

Jointly published with Zhejiang University Press The print edition is not for sale in China (Mainland). Customers from China (Mainland) please order the print book from: Zhejiang University Press. © Zhejiang University Press and Springer Nature Singapore Pte Ltd. 2023 This work is subject to copyright. All rights are reserved by the Publishers, 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 publishers, 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 publishers 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 publishers remain 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

Preface

Hydrocarbon, particularly natural gas, stored in marine black shale, is one of the important oil and gas resources in China. As a type of unconventional natural gas, shale gas represents a cleaner burning fossil fuel than coal, and thus is an especially valuable fuel source. In 2019 the annual output of shale gas in China was 154
108 m3, and in 2020 the shale gas annual production in China exceeded 200
108 m3. In the Sichuan Basin and its surrounding regions, the Upper Ordovician Wufeng Formation and the Lower Silurian Lungmachi Formation are the two major black shale-producing units, and are the main focus of shale gas exploration and development in China. In May 2014, at the Fuling Conference organized by China Petroleum & Chemical Corporation (SINOPEC), we proposed a detailed graptolite biozonation and correlation scheme for the Wufeng and Lungmachi formations in order to facilitate the precise sub-division and correlation of these two shale gas-bearing units. This graptolite-based biostratigraphic approach has been widely recognized as a key method for the division of shale gas formations. Later, young researchers from China National Petroleum Corporation (PetroChina), SINOPEC, and China Geological Survey organized as a shale gas biostratigraphic research group devoted to studying the Ordovician to Silurian shale gas-bearing strata in the Yangtze region. In the past 5 years, the research group has identified and studied the cores from more than 50 drilled wells and nearly 10 important outcrop sections in the Yangtze region. The group submitted timely consulting reports to various research departments of petroleum companies. Three training courses were held, field instruction was provided, and both sets of workshops received positive feedback. Based on this work, the stage-progressive distribution model of Ordovician-Silurian black shale in the Yangtze region and the circumjacent distribution model of the Yichang Uplift, as well as papers regarding the shale gas-bearing formations in key well locations and other important areas (Wang et al. 2015; Luo et al. 2017; Liang et al. 2016, 2017; Nie et al. 2017; Sun et al. 2018; Qiu et al. 2020) have been recently published (Chen et al. 2017, 2018). Based on 5 years of field and laboratory research, the present book summarizes the shale gas intervals in the black shales of the Wufeng and Lungmachi formations in the Yangtze region. We begin with an introduction to the geology of the Yangtze region from the Ordovician through Silurian periods and follow with a summary of the division and correlation of black shales in the Wufeng and Lungmachi formations. The global distribution of graptolitic black shale in the Ordovician and Silurian periods is discussed, compared, and evaluated. We examine the evolution of graptolite faunas and the accumulation of organic matter, analyze the evolution of the paleoenvironment between the Ordovician and Silurian, and summarize the “sweet spot” intervals for shale gas in late Ordovician to early Silurian strata in the Yangtze region. We conclude that the sweet intervals are from Dicellograptus complexus Biozone (WF2) to Demirastrites triangulatus Biozone (LM6), particularly between LM1–LM5. The distribution of K-bentonite layers above and below these “sweet spot” horizons, and the impact of volcanic eruptions (represented by K-bentonite beds) on the organic matter enrichment in adjacent paleo-sea areas are also discussed. In order to combine the framework of graptolite biostratigraphy with shale gas exploration, the relationship between gamma-ray logging response and graptolite biozonation is explained in this book. v

vi

Preface

Finally, key graptolites through Wufeng and Lungmachi formations are illustrated and described with plates and explanations. The writing and publishing of this book are jointly funded by the National Science and Technology Major Project of China “Shale Gas Enrichment Conditions, Favorable Region Evaluation and Application in the Sichuan Basin and Its Peripheral Areas” (Grant No. 2017ZX05035), National Natural Science Foundation of China (Grant No. 41972162), and Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences. The Research Institute of Petroleum Exploration and Development, PetroChina, SINOPEC Petroleum Exploration and Production Research Institute, Chengdu Institute of Geology and Mineral Resources, China Geological Survey, Qingdao Institute of Marine Geology, China Geological Survey, and Huzhou Laboratory of Geological Resources and Engineering, Zhejiang Province are thanked for their support. Nanjing, China Beijing, China Dayton, USA

Xu Chen Hongyan Wang Daniel Goldman

References Chen X, Fan JX, Wang WH, Wang HY, Nie HK, Shi XW, Wen ZD, Chen DY, Li WJ (2017) Stage-progressive distribution pattern of the Lungmachi black graptolitic shales from Guizhou to Chongqing, Central China. Sci China Earth Sci 60(6):1133–1146 Chen X, Chen Q, Zhen YY, Wang HY, Zhang LN, Zhang JP, Wang WH, Xiao CH (2018) Circumjacent distribution pattern of the Lungmachian graptolitic black shale (early Silurian) on the Yichang Uplift and its peripheral region. Sci China Earth Sci 61:1195–1203 Liang F, Bai WH, Zou CN, Wang HY, Wu J, Ma C, Zhang Q, Guo W, Sun SS, Zhu YM, Cui HY, Liu DX (2016) Shale gas enrichment pattern and exploration significance of Well Wuxi-2 in northeast Chongqing, NE Sichuan Basin. Petroleum Explorat Develop 43(3):350–358 (in Chinese with English abstract) Liang F, Wang HY, Bai WH, Guo W, Zhao Q, Sun SS, Zhang Q, Wu J, Ma C, Lei ZA (2017) Graptolite correlation and sedimentary characteristics of Wufeng–Longmaxi shale in southern Sichuan. Natural Gas Industry 37(7):20–26 (in Chinese with English abstract) Luo C, Wang LS, Shi XW, Zhang J, Wu W, Zhao SX, Zhang CL, Yang YX (2017) Biostratigraphy of the Wufeng to Longmaxi Formation at Well Ning 211 of Changning shale gas field. J Stratigrap 41(2):142–152 (in Chinese with English abstract) Nie HK, Jin ZJ, Ma X, Liu ZB, Lin T, Yang ZH (2017) Graptolite zones and sedimentary characteristics of Upper Ordovician Wufeng Formation–Lower Silurian Longmaxi Formation in Sichuan Basin and its adjacent areas. Acta Petrolei Sinica 38(2):160–174 (in Chinese with English abstract) Qiu Z, Zou CN (2020) Unconventional Petroleum Sedimentology: Connotation and prospect. Acta Sedimentologica Sinica 38(1):1–29 (in Chinese with English abstract) Sun SS, Rui Y, Dong DZ, Shi ZS, Bai WH, Ma C, Zhang LF, Wu J, Chang Y (2018) Paleogeographic evolution of the Late Ordovician–early Silurian in Upper and Middle Yangtze regions and depositional model of shale. Oil & Gas Geol 39(6):1087–1106 (in Chinese with English abstract) Wang HY, Liang F, Guo W, Zhao Q (2015) Biostratigraphy characteristics and scientific meaning of the Wufeng and Longmaxi Formation black shales at Well Wei 202 of the Weiyuan shale gas field, Sichuan Basin. J Stratigrap 39(3):289–293 (in Chinese with English abstract)

Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Xu Chen and Hongyan Wang 2 Geological Setting of the Ordovician and Silurian Strata of the Yangtze Platform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Xu Chen, Yuandong Zhang, Yue Li, and Junxuan Fan 3 Ordovician to Silurian Shale Gas-Bearing Strata from the Yangtze Region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Xu Chen, Hongyan Wang, Feng Liang, Qing Chen, Chao Luo, Zhi Zhou, Wenhui Wang, Jia Li, and Dexun Liu 4 Distribution Pattern of the Ordovician–Silurian Shale Gas-Bearing Strata in the Yangtze Region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Xu Chen, Hongyan Wang, Haikuan Nie, and Jin Wu 5 Regional and Global Correlation of the Latest Ordovician to Early Silurian Shale Gas-Bearing Strata . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Xu Chen, Qing Chen, Di Zhang, Hongyan Wang, Feng Liang, Jia Li, and Shasha Sun

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7

19

77

97

6 Paleogeography and Paleoenvironment Across the Ordovician–Silurian Transition in the Yangtze Region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 Qing Chen, Jitao Chen, Wenjie Li, and Zhensheng Shi 7 Gamma Log Responses Through the Ordovician–Silurian Black Shale Graptolite Zonal Succession in the Middle and Upper Yangtze Regions . . . . . 183 Qun Zhao, Chao Li, Shasha Sun, and Wei Guo 8 Volcanic Ash Deposition and Organic Matter Enrichment in the Black Shales of the Wufeng–Lungmachi Formations in the Yangtze Region . . . . . . . 195 Zhen Qiu and Xiangying Ge Appendix: Plates and Explanation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213

vii

List of Main Authors

Xu Chen

Hongyan Wang Qing Chen

Qun Zhao Zhen Qiu

State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology and Center for Excellence in Life and Paleoenvironment, Chinese Academy of Sciences, Nanjing, 210008, China, e-mail: [email protected] PetroChina Research Institute of Petroleum Exploration and Development, Beijing, 100083, China, e-mail: [email protected] State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology and Center for Excellence in Life and Paleoenvironment, Chinese Academy of Sciences, Nanjing, 210008, China, e-mail: [email protected] PetroChina Research Institute of Petroleum Exploration and Development, Beijing, 100083, China, e-mail: [email protected] PetroChina Research Institute of Petroleum Exploration and Development, Beijing, 100083, China, e-mail: [email protected]

ix

1

Introduction Xu Chen and Hongyan Wang

Abstract

1.1

Since 2010, the China’s shale gas industry has entered into a period of rapid development. It has gone through three stages: (1) cooperation and reference stage, (2) exploration and appraisal stage, and (3) scale production construction stage. The graptolite zonation method was introduced at the Fuling Conference and immediately received full support by the SINOPEC, PetroChina and Geological Survey of China. Keywords

Shale gas-bearing strata Yangtze region




Ordovician




Silurian




Geologists and petroleum engineers have long known that low permeability rocks such as shale and mudstone can be rich in hydrocarbons. These unconventional reservoirs require hydraulic fracturing and other enhancement processes in order to be exploitable deposits. Shale gas, a valuable clean energy source, is widely considered an economically feasible alternative to coal, and in recent years has become a major factor in the world’s energy development.

X. Chen (&) State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology and Center for Excellence in Life and Paleoenvironment, Chinese Academy of Sciences, Nanjing, 210008, China e-mail: [email protected] H. Y. Wang PetroChina Research Institute of Petroleum Exploration & Development, Beijing, 10083, China

Shale Gas Development in the United States

The first widespread discovery and exploitation of shale gas occurred in the United States, and the USA is currently the largest producer of it in the world. The overall development of shale gas in the United States can be divided into three stages: the scientific exploration stage, the technological breakthrough stage and the leapfrog development stage.

1.1.1 Scientific Exploration Stage (1821–1996) In 1821, Hart drilled the first onshore oil and gas well in the town of Fredonia, Southwest New York State, United States, and successfully obtained shale gas for the first time (Zou et al. 2015). In the 1940s, many companies began to explore shale gas as a productive unconventional oil and gas resource, and conducted development tests in shale gas units such as the Antrim (Devonian), Barnett (Mississippian) and Marcellus (Devonian) formations. Affected by the 1970s oil crisis, the United States government issued several policies to promote the development of unconventional oil and gas resources such as shale gas, including the launch of the Eastern Shale Gas Project in 1976 with an emphasis on the development and extraction of Devonian shale gas in the Michigan Basin, Illinois Basin, and Appalachian Basin.

1.1.2 Technological Breakthrough Stage (1997–2003) At this stage, the breakthrough of large-scale slick water fracturing technology enabled the economically advantageous and effective development of shale gas. The repeated fracturing and multi-stage fracturing of horizontal wells (as well as other technologies) have further improved the

© Zhejiang University Press and Springer Nature Singapore Pte Ltd. 2023 X. Chen et al. (eds.), Latest Ordovician to Early Silurian Shale Gas Strata of the Yangtze Region, China, https://doi.org/10.1007/978-981-99-3134-7_1

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X. Chen and H. Wang

efficiency of shale gas development. In 2002, the multi-stage horizontal well fracturing technology was successfully applied in a number of cases, demonstrating that it is an effective technology for shale gas development. After the initial breakthrough in the Barnett Shale gas field, the production of the Barnett Shale gas field increased rapidly, reaching 54  108 m3 in 2003, which accounts for 28% of the total production of the shale gas fields in the United States (Zou et al. 2015).

effective development. In 2017, the annual production of shale gas in the United States increased to 4744  108 m3, suggesting shale gas resources were efficiently developed (U.S. Energy Information Administration 2021).

1.2

Major Shale Gas Fields in the World

Zou et al. (2015) have reviewed the shale gas development in the United States and compared it with that in China in Tables 1.1 and 1.2.

1.1.3 Leapfrog Development Stage (Since 2004) The successful experience of shale gas development in the Barnett Shale has been repeated in the Haynesville, Marcellus, Utica and other shale gas fields. Shale gas production has grown rapidly and has become the main contributor of natural gas production in the United States. In 2007, the Fayetteville and Woodford shale gas fields achieved large-scale and effective development. In 2010, the Bakken and Eagle Ford shale gas fields created large-scale and

Table 1.1 Discovery time and resource distribution of major shale gas fields in the world (Zou et al. 2015)

Shale gas field

State

1.3

Shale Gas Development in China

The major development of China’s shale gas industry has occurred less than 10 years. Guo (2014) first noted that the main shale gas plays are concentrated in the Wufeng Formation (Latest Ordovician) and the Lungmachi Formation (early Silurian) from the Upper and Middle Yangtze regions

Valuable resources (1012 m3)

Year of discovery

Gas-bearing area (km2)

Depth (m)

Barnett

USA

1.22

1981

13,000

1980–2590

Marcellus

USA

7.40

2008

240,000

1220–3250

Haynesville

USA

7.10

2007

23,000

3200–4115

Fayetteville

USA

1.17

2003

23,000

305–2134

Woodford

USA

0.32

2003

28,500

New Albany

USA

0.54

1858

113,000

152–610

1829–3353

Antrim

USA

0.56

1940s

31,000

183–671

Lewis

USA

0.56

1998

10,000

915–1830

Horn River

Canada

3.79

1963

21,000

2500–3000

Barkken

Canada

0.06

1953

225,300

1524–2682

Colorado

Canada

1.21

1877

321,200

1524–3048

Vaca Muerta

Argentina

8.72

1931

30,000

30–1200

Weiyuan

China

0.45

2010

4216

1530–3500

Fushun– Yongchuan

China

0.85

2011

6670

3000–4500

Changning– Zhaotong

China

0.48

2011

3980

2300–4000

Jiaoshiba

China

0.40

2012

2304

2100–3500

Wuxi

China

0.15

2014

1660

1500–3200

Xiasiwan

China

0.23

2011

2400

1200–2000

1

Introduction

Table 1.2

3

Characteristics of major shale gas fields in the world (Zou et al. 2015)

Gas field

State

Geological setting and facies belt

Lithology

Strata

Origination

Output (108 m3)

Barnett

USA

Foreland basin, deep shelf facies

Siliceous and calcareous shale

C1

Thermogenic origin

462

Marcellus

USA

Foreland basin, deep shelf facies

Siliceous and calcareous shale

D2

Thermogenic origin

1330

Haynesville

USA

Craton basin, deep shelf facies

Siliceous and calcareous shale

J3

Thermogenic origin

617

Fayetteville

USA

Foreland basin, deep shelf facies

Calcareous shale

C1

Thermogenic origin

286

Woodford

USA

Foreland basin, deep shelf facies

Siliceous and dolomitic shale

D3

Thermogenic origin

161

New Albany

USA

Craton basin, deep shelf facies

Siliceous shale

D–C

Hybrid origin

–

Antrim

USA

Craton basin, deep shelf facies

Dolomitic and calcareous shale

D3

Hybrid origin

28

Lewis

USA

Foreland basin, littoral facies

Siliceous shale

K2

Thermogenic origin

–

Horn River

Canada

Foreland basin, deep shelf facies

Siliceous and calcareous shale

D2

Thermogenic origin

79

Barkken

Canada

Foreland basin, deep shelf facies

Siliceous and calcareous shale

D3–C1

Thermogenic origin

–

Colorado

Canada

Foreland basin, deep shelf facies

Clayey shale

K

Biogenic origin

–

Vaca Muerta

Argentina

Foreland basin, deep shelf facies

Carbonate shale

J3–K1

Thermogenic origin

4–6

Weiyuan

China

Craton platform, deep shelf facies

Siliceous and calcareous shale

O3w–S1l, Cm

Thermogenic origin

2.43

Fushun– Yongchuan

China

Craton platform, deep shelf facies

Siliceous and calcareous shale

O3w–S1l

Thermogenic origin

1.78

Changning– Zhaotong

China

Craton platform, deep shelf facies

Siliceous and calcareous shale

O3w–S1l

Thermogenic origin

6.60

Jiaoshiba

China

Craton platform, deep shelf facies

Siliceous and calcareous shale

O3w–S1l

Thermogenic origin

30.6

Wuxi

China

Craton platform, deep shelf facies

Siliceous and calcareous shale

O3w–S1l

Thermogenic origin

–

Xiasiwan

China

Major depression, lake facies

Siliceous and calcareous shale

T3 y

Associated gas

0.05

based on the first high productive shale gas well drilling (Well Jiaoye 1 (JY 1) at Fuling of Chongqing). Shale gas exploration and development in China, which began in 2005 and included continuous key research projects for over 10 years, has also gone through three main stages.

1.3.1 Cooperation and Reference Stage (2005–2009) As early as 2003, some Chinese scholars began to use the successful experience of the United States as a reference to

introduce the concept of shale gas exploration and make predictions on the prospect of China’s shale gas resources. Since 2005, China has used the successful North American experience as a template for shale gas exploration and development, and applied it to different types of shale with different geological characteristics. Extensive research on China’s shale gas formations and their geologic history, prospect evaluation and optimization, and “sweet spot” area evaluation has been carried out. Many strategic regional appraisal wells for shale gas were drilled. For example, the wells Changxin 1, Yuye 1, Wei 201, Ning 211 (N211), Jiaoye 1, and Wuxi 2 (WX2) were drilled in the Sichuan Basin and its peripheral areas, and the Well Zhao 101 in the

4

Zhaotong region of northeastern Yunnan, the Well Xiangye 1 in western Hunan, the Well Xuanye 1 in the Lower Yangtze region, and the Well Liuping 177 in Ordos Basin. Consequently, shale gas was successfully discovered in Cambrian, Ordovician–Silurian, Carboniferous–Permian, and Triassic–Jurassic horizons in South China, and in Triassic, and Carboniferous–Permian horizons in the Ordos Basin. Based on detailed evaluation and optimization, the Sichuan Basin and its peripheral areas and the Ordos Basin are considered favorable for shale gas exploration and development in China. A number of favorable shale gas targets such as Weiyuan, Changning–Zhaotong, Fushun– Yongchuan, Fuling, Wuxi, and Ganquan–Xiasiwan have been confirmed.

1.3.2 Exploration and Appraisal Stage (2010–2013) This stage is the period for industrialized exploitation assessment of marine shale gas, and the exploration and appraisal stage for shale gas between transitional and terrestrial facies. Oil companies evaluated shale gas resource potential and development prospects based on drilling, and established the development status of marine shale gas of the Wufeng and Lungmachi formations in the Upper and Middle Yangtze regions. Two shale gas fields were discovered in Shunan (South Sichuan) and Fuling, laying a solid foundation for shale gas development on a large scale. Since 2010, China has discovered high-yielding shale gas resources in the Weiyuan–Changning, Fushun–Yongchuan, Zhaotong and Fuling blocks of the Sichuan Basin, and established three marine shale gas industrial production demonstration regions.

1.3.3 Scale Production Construction Stage (Since 2014) Through June 2014, the shale gas daily output in 28 wells of the Jiaoshiba (Fuling) gas field reached 322  104 m3, with 11  108 m3/a of productive capacity (Wang 2015). In July 2014, experts from the Ministry of Land and Resources reviewed the Jiaoye 1 to Jiaoye 3 wells in the Fuling shale gas field, which possesses a gas producing area of 106.45 km2 and has geological reserves of 1067.50  108 m3. Geologists think that the Fuling field is uniform in both the reservoir quality and its available thickness. The reservoir strata are concentrated in continuous sandstone, carbonate or siliceous interlayers in an 80–150 m interval of Lungmachi Formation black shale (Guo 2014). Successful exploitation of the Jiaoshiba (Fuling) shale gas field greatly has promoted further exploration and

X. Chen and H. Wang

development of shale gas in China. In the spring of 2014 the author Chen Xu was invited to participate in the Fuling Conference by the SINOPEC leadership. The Fuling Conference not only documented the successful experience of exploration and development of shale gas plays, but also discussed the further development of the shale gas-bearing strata of the Wufeng and Lungmachi formations in the entire Sichuan Basin. Problems related to the black shale are highlighted herein, and are the main topics addressed in this book. The correlation difficulties are due to the uniform lithology of the black shale, and the lack of unique index layers. In the subsurface, the geophysical logging curves in the black shales of the Wufeng and Lungmachi formations cannot provide the detailed correlation required for exploration of shale gas-bearing layers. Lithostratigraphy, biostratigraphy and chronostratigraphy are the three major branches of stratigraphy. Generally, lithostratigraphy has been less useful than biostratigraphy in black shale correlation because of the inability to recognize time-significant subdivisions within the uniform lithology. In the Yangtze region, the lithological characteristics of the black shale in the Wufeng and Lungmachi formations are very similar. These units, although spanning a significant amount of geologic time, are not easily differentiated, particularly if the Kuanyinchiao muddy limestone is missing. This is especially true in subsurface drill core litho-logs. Thus, biostratigraphy is a more useful tool for correlation in black shale sequences. In sequences of Ordovician and Silurian black shale, graptolites are the most important index fossils, and graptolite biozones represent a correlation standard. Recently, we published a regional Yangtze graptolite zonation through the Wufeng and Lungmachi formations that can easily be correlated with other black shale successions around the world (Fig. 1.1; Chen et al. 2015). In order to facilitate the use of this zonation by petroleum geologists and other non-graptolite specialists, we coded the graptolite zones. WF represents the Wufeng Formation zones, LM— the Lungmachi Formation zones and N—the Nanjiang Formation zones. Use of these codes by the Chinese petroleum companies in discussion and publication avoids the difficulty of using the Latin graptolite species and genera names. The graptolite zonation method was introduced at the Fuling Conference and immediately received full support by the leadership of SINOPEC and PetroChina. Mr. Wang Zhigang, the vice president of SINOPEC, concluded that the sub-division and correlation based on graptolites would be an index scale or “ruler” for the exploration of the black shales in the Wufeng and Lungmachi formations. Since 2014, we have spent a substantial amount of time working on the stratigraphy and biostratigraphy of the black shales in the Wufeng and Lungmachi formations. The research area

1

Introduction

5

Fig. 1.1 Graptolite Biozonation (with abbreviation codes) of the Wufeng Formation (Ordovician) to Lungmachi and Nanjiang formations (Silurian) in the Yangtze region (Chen et al. 2015). The ages are from Gradstein et al. (2012). Abbreviations: Se., Series; St., Stage; Hir., Hirnantian; Tel., Telychian; Fm., Formation; K., Kuanyinchiao Bed; Paraorthogr. = Paraorthograptus; Diceratogr. = Diceratograptus; Ch. = Chientsaokou

includes more than 30 drill cores in the Sichuan Basin and its surrounding regions. Reports of the stratigraphic division and correlation of these drill cores have been submitted to the two petroleum companies. Additionally, field trips in the Yangtze Gorges and two graptolite biostratigraphy workshops have been conducted. A stratigraphy working group including more than 10 young members from the institutes of PetroChina, SINOPEC and Geological Survey of China worked together on the Wufeng and Lungmachi formations (Chen et al. 2015, 2017; Wang et al. 2015; Luo et al. 2017; Liang et al. 2017). Based on the study of a large number of sections and drill cores, we recognized two patterns of distribution for the black shales of the Wufeng and Lungmachi formations on the Upper and Middle Yangtze platforms (Chen et al. 2017, 2018). In November of 2017, a conference entitled “Origin and Distribution of Shale Gas in South

China” was held in Nanjing. Important scientific exchanges between scientists and experts of petroleum companies aided and promoted China’s shale gas exploration and development. To date, large reserves with approximately a trillion cubic meter scale of shale gas provinces have been discovered in the Sichuan Basin. These provinces, which include the Weiyuan, Changning, Jiaoshiba, Weirong and Luzhou fields, are characterized by deep burial, high formation pressure in old rock units, and a high degree of thermal evolution. Their reservoirs are the Wufeng–Lungmachi formations, with proved geological reserves of 10,454  108 m3 and a cumulative production of shale gas of more than 300  108 m3. China produced more than 200  108 m3 of shale gas annually by 2020. Early Silurian oil and gas fields in North Africa (e.g., Morocco, Algeria, and Libya) and the Arabian Peninsula

6

(e.g., Jordan) are important hydrocarbon resources (Alsharhan and Nairn 1997; Lüning et al. 2000, 2003, 2005). These reservoirs are referred to as Hot Shales, and are stratigraphically and lithologically similar to the Yangtze Lungmachi Formation black shale (LBS). The precise age of the black shales, geological distribution pattern and certain geochemical parameters, such as total organic carbon (TOC) and natural gamma (gamma-ray), are remarkably similar between the Middle Eastern Hot Shale and the Lungmachi black shale. One important difference, however, is that the extractable product of the Yangtze region is solely shale gas, whereas both petroleum and natural gas are produced from the Middle Eastern Hot Shale. This may be due to the fact that the Yangtze Platform experienced several orogenic movements in its geological history, and the shale gas in LBS is in a post-mature stage. Nevertheless, comparisons with the Hot Shale of North Africa and Arabian Peninsula provide important geological information for the present LBS study. The main purpose of this book is to compile, summarize, and present the accumulated data, knowledge, and current geological understanding of the main Chinese shale gas fields.

References Alsharhan AS, Nairn AEM (1997) Sedimentary basins and petroleum geology of the middle east. Elsevier, Amsterdam, p 978 Chen X, Fan JX, Zhang YD, Wang HY, Chen Q, Wang WH, Liang F, Guo W, Zhao Q, Nie HK, Wen ZD, Sun ZY (2015) Subdivision and delineation of the Wufeng and Lungmachi black shales in the subsurface areas of the Yangtze Platform. J Stratigr 39(4):351–358 (in Chinese with English abstract) Chen X, Fan JX, Wang WH, Wang HY, Nie HK, Shi XW, Wen ZD, Chen DY, Li WJ (2017) Stage-progressive distribution pattern of the Lungmachi black graptolitic shales from Guizhou to Chongqing, Central China. Sci China Earth Sci 60(6):1133–1146

X. Chen and H. Wang Chen X, Chen Q, Zhen YY, Wang HY, Zhang LN, Zhang JP, Wang WH, Xiao CH (2018) Circumjacent distribution pattern of the Lungmachian graptolitic black shale (early Silurian) on the Yichang Uplift and its peripheral region. Sci China Earth Sci 61:1195–1203 Gradstein FM, Ogg JG, Smith AG, Ogg GM (2012) The geologic time scale 2012. Elsevier, Amsterdam, p 1176 Guo XS (2014) Rules of two-factor enrichment for marine shales gas in southern China—understanding from the Longmaxi Formation shale gas in Sichuan Basin and its surrounding area. Acta Geol Sin 88(7):1209–1218 (in Chinese with English abstract) Liang F, Wang HY, Bai WH, Guo W, Zhao Q, Sun SS, Zhang Q, Wu J, Ma C, Lei ZA (2017) Graptolite correlation and sedimentary characteristics of Wufeng-Longmaxi shale in southern Sichuan Basin. Nat Gas Ind 37(7):20–26 (in Chinese with English abstract) Lüning S, Craig J, Loydell DK, Štorch P, Fitches B (2000) Lower Silurian ‘hot shales’ in North Africa and Arabia: regional distribution and depositional model. Earth-Sci Rev 49:121–200 Lüning S, Archer R, Craig J, Loydell DK (2003) The Lower Silurian ‘hot shales’ and ‘double hot shales’ in North Africa and Arabia. The geology of northwest Libya (Ghadamis, Jifarah, Tarabulus and Sabratah basins): tripoli. Earth Sci Soc Libya 3:91–105 Lüning S, Shahin YM, Loydell D, Al-Rabi HT, Masri A, Tarawneh B, Kolonic S (2005) Anatomy of a world-class source rock: distribution and depositional model of Silurian organic-rich shales in Jordan and implications for hydrocarbon potential. AAPG Bull 89 (10):1397–1427 Luo C, Wang LS, Shi XW, Zhang J, Wu W, Zhao SX, Zhang CL, Yang YX (2017) Biostratigraphy of the Wufeng to Longmaxi Formation at Well Ning 211 of Changning shale gas field. J Stratigr 41(2):142–152 (in Chinese with English abstract) U.S. Energy information Administration (2021) Dry shale gas production estimates by play. Release date, 17 July 2021 Wang ZG (2015) Breakthrough of Fuling shale gas exploration and development and its inspiration. Oil Gas Geol 36(1):1–6 (in Chinese with English abstract) Wang HY, Guo W, Liang F, Zhao Q (2015) Biostratigraphy characteristics and scientific meaning of the Wufeng and Longmaxi Formation black shales at Well Wei 202 of the Weiyuan shale gas field, Sichuan Basin. J Stratigr 39(3):289–293 (in Chinese with English abstract) Zou CN, Dong DZ, Wang YM, Li XJ, Huang JL, Wang SF, Guan QZ, Zhang CC, Wang HY, Liu HL, Bai WH, Liang F, Lin W, Zhao Q, Liu DX, Yang Z, Liang PP, Sun SS, Qiu Z (2015) Shale gas in China: characteristics, challenges and prospects (I). Pet Explor Dev 42(6):689–701 (in Chinese with English abstract)

2

Geological Setting of the Ordovician and Silurian Strata of the Yangtze Platform Xu Chen, Yuandong Zhang, Yue Li, and Junxuan Fan

Abstract

Boundaries separating different parts of the Yangtze Platform during the Ordovician and Silurian are defined on evidence of tectonic evolution. Constraint factors of distribution patterns, lithofacies, biofacies and the relationship with their surrounding units are provided. Keywords

Geological setting pattern




Tectonic evolution




Distribution

Geological setting is an internationally accepted general term used to depict the geological system of a specific region or block in a specific time period. It commonly includes information such as distribution patterns, lithofacies, biofacies, and the relationship with its surrounding units. It is more appropriate than other terms such as “geological environment” and “sedimentary environment”, because the word “environment” is commonly used to explain the relationship between the various biological and physical conditions of the modern earth. Environmental science is closely related to the interaction of human beings with their physical surroundings. In the present book, the boundary of the Yangtze Plate is overlay on a simplified Chinese geological map to identify its general position with respect to geological background features (Fig. 2.1). The Yangtze Platform, Chiangnan transitional belt, and Zhujiang Basin have been defined as the

X. Chen (&)
Y. D. Zhang
Y. Li State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology and Center for Excellence in Life and Paleoenvironment, Chinese Academy of Sciences, Nanjing, 210008, China e-mail: [email protected] J. X. Fan School of Earth Sciences and Engineering, Nanjing University, Nanjing, 210093, China

three major parts of the South China Plate that share a common Yangtze basement structure (Chen and Rong 1992; Chen et al. 2010). Thus, using the term Yangtze Plate might be more appropriate than using South China Plate. Moreover, the Yunkai Block, which was previously included in the South China Plate, is no longer considered to be part of the Yangtze Plate, although geographically it is situated in modern South China (Chen et al. 2010). The Qinling Orogens are located to the north of the Yangtze Platform. They have experienced a long and complicated orogenic history from the Paleozoic to the Mesozoic. During this process, Qinling was directly or indirectly involved in collision with different parts of the Yangtze Plate. The Mian–Lue suture zone is the boundary between the Yangtze Platform and Qinling, which geographically is the border area between Shaanxi and Sichuan provinces. To the west, the Mian–Lue suture zone is connected with Zhouqu county, which was considered as part of West Qinling Orogen (Lin et al. 1984). A Cyrtograptus-bearing graptolite fauna collected at Zhouqu shows similarity to one collected at Ziyang on the northeast margin of the Upper Yangtze Platform (Mu et al. 1982). Thus, Zhouqu is considered to belong to the marginal belt of the Yangtze Plate. The border between West Qinling and the Yangtze Platform is truncated by the Ningshan fault. The border between the Dabashan (Daba Mountains) belt (or the Yangtze north marginal belt) and East Qinling is along the east end of the Ningshan fault in the area between Shangnan and Xichuan. This border merges with the Shangdan suture zone, which is the south border of North Qinling (Fig. 2.2, after Fig. 2 of Meng 2017). Geographically, the northern border of the Yangtze Platform extends from Zhouqu– Lueyang–Mianxian to Ningshan. After connecting with the Shangdan suture zone, the northern border of the Yangtze Platform turns southeastwards along the south side of the Tongbai and Dabie Mountains. It then turns northeastwards to the south of Hefei, through Chaohu–Lianyungang and extends into the Yellow Sea (Fig. 2.2).

© Zhejiang University Press and Springer Nature Singapore Pte Ltd. 2023 X. Chen et al. (eds.), Latest Ordovician to Early Silurian Shale Gas Strata of the Yangtze Region, China, https://doi.org/10.1007/978-981-99-3134-7_2

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Fig. 2.1 Boundary of the Yangtze Platform in Ordovician and Silurian (based on a geological map edited by Institute of Geology, Chinese Geological Academy 2005)

107 o E

110 o E

113 o E

35 o N

35 o N Tianshui

North China Plate

Weihe Basin

South marginal belt of North China Plate

Huashan

Baoji Xi’an

34 o N

Luonan

Taibaishan

Huixian

Luanchuan

North Qinling West Qinling

Chengxian

Luan

Shangdan s u t u r e z o n e ( f a u l t b e l t )

Ophiolitic melange

N in g sh a n

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fault

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Erl

Shangnan

34 o N chua

ang

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pin

go

ph

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33 o N

Lushan

n fau lt iol

itic

me

lan

ge

33 o N

Ningqiang Ziyang

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Fangxian

32 o N

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Daba

shan

ter ra

ne

Xiangfan

32 o N

fa u lt

0 107 o E

ang

110 o E

Fig. 2.2 The northern Yangtze Platform and the Qinling Orogens (after Fig. 2 of Meng 2017)

100 km 113 o E

2

Geological Setting of the Ordovician and Silurian Strata of the Yangtze Platform

The Yangtze Plate and Qinling Orogens had been partially subducted during the collision process between Yangtze Plate and Qinling in the Triassic. Additionally, part of the northern Yangtze Platform is truncated by the Ningshan fault belt. The west part of this fault belt spreads between Yangpingguan and Ningqiang (Fig. 2.2). One of the authors (Chen) found the graptolite species Tetragraptus bigsbyi Hall in the metamorphosed slates near Yangpingguan. Furthermore, specimens of the graptolite Expansograptus and some of the trilobite Ptychopyge have been found near Chenjiaba of Guangyuan (Sichuan), a region in the same tectonic belt of Yangpingguan. All of these fossils are also found on the Yangtze Platform, which indicates that the area between Mian–Lue suture zone and Ningshan fault belt belongs to the marginal belt of the Yangtze Platform. The central part of the Ningshan fault belt between Nanzheng and Ningshan was partially incorporated into the Yangtze Platform. A typical Yangtze-aspect Ordovician– Silurian succession including the Zhaojiaba, Zhongliangsi, Pagoda, Nanzheng, Lungmachi and Cuijiagou formations is well developed in the Zhongliangsi section of Nanzheng. Similar Ordovician–Silurian sequences including the Datianba, Pagoda, Chientsaokou, Wufeng, Lungmachi and Nanjiang formations are present at Sanlangpu of Xixiang. All of these locations are close to the Ningshan fault (Fig. 2.2). East Qinling is a narrow tectonic belt. Its southern border, adjacent to the Yangtze Platform, is located between Shangnan and Xichuan. Metamorphic phyllites and slates are well developed at Shangnan, within the East Qinling. However, carbonates from the Lower–Middle Ordovician Bailongmiao Formation, Zuoxiu Formation, and the Upper Ordovician Sigang and Shiyanhe formations from Xichuan bear a Yangtze-aspect conodont fauna (Wang et al. 1996). In the Llandovery, the weathered yellow-gray shale of Xichuan contains an Aeronian graptolite fauna which is similar to that found on the Yangtze Platform. Although Wang and Xue (1986) subdivided the Aeronian graptolite fauna into four graptolite zones, a recent reexamination of the fauna by Chen Xu suggests only one biozone—the Demirastrites triangulatus Biozone (=LM6), similar to the contemporary fauna in the Yangtze region. As mentioned above, Meng (2017)’s evolutionary geologic model for the development of the Qinling Orogens (Fig. 2.3) has led to a much greater understanding of the geologic setting of the northern border of the Yangtze Plate. The Yangtze Plate was separated from the North China Plate by the Shangdan Ocean (Fig. 2.3a, b), which closed in the early Silurian (Fig. 2.3c), and an unnamed allochthonous terrane. The Yangtze Plate then drifted towards the North China Plate from late Paleozoic onwards (Fig. 2.3d). In the late Ordovician to early Silurian, North Qinling, an island-arc belt derived from the subduction of the Shangdan

9

oceanic crust, was amalgamated with the allochthonous terrane (Fig. 2.3c). In middle to late Triassic, the Yangtze Plate was amalgamated completely with North China along the Mian–Lue suture zone (Fig. 2.3e), forming the Dabashan tectonic belt (Fig. 2.3f). Both the Dabashan belt (i.e., the Yangtze north marginal belt) and the Yangtze Platform experienced lateral tectonic movement and as a result a lateral fault zone truncates the isopachs of the Wufeng and Lungmachi formations (Fig. 2.4). Based on the biostratigraphic evidence mentioned above, it is clear that different areas along the northern Yangtze Platform joined the tectonic belts of the Qinling through an extended period of geological time. The southern margin of the North Qinling represents the border of the Qinling and the Yangtze Platform. This borderline extends southeastwards from Xichuan along the south side of the Tongbai and Dabie Mountains and then turns northeastwards to south of Hefei. The Hefei– Lianyungang border line coincides with the Tan–Lu fault zone, which also represents the border between the Lower Yangtze Platform and the North China Platform. Few Paleozoic outcrops are distributed southwest of the Xichuan–Tongbai–Dabie line. However, Late Ordovician rocks do crop out north of the Dabie Mountains. Belodina compressa (Branson and Mehl) and some other Late Ordovician conodont taxa were collected from the Baidashan Group at Baidashan, Huoqiu, Anhui (Zhang et al. 2014). Similar conodont faunas have been recorded in the Late Ordovician Fengfeng, Taoqupo and Beiguoshan formations of North China and the Lianglitage Formation of Tarim. This new information indicates the existence of a marginal belt of the North China Platform in the Huoqiu area. It also implies that the border between the North China and Yangtze platforms is along the Tongbai–Dabie line. The Mesozoic Tan–Lu fault zone cuts off the western margin of the Lower Yangtze Platform. The Tan–Lu fault passes through the area between Chuzhou and Jiashan (Mingguang) (Institute of Geology, Chinese Geological Academy 2005). Chuzhou belongs to the Lower Yangtze since the Ordovician rocks and faunas in Chuzhou show high similarity with those of Nanjing, where typical Yangtze faunas and rock units are developed (Zhu et al. 1984). Thus, Chuzhou does not extend beyond the margin of the Lower Yangtze Platform. Similarly, Lianyungang is located on the Yangtze Platform as evidenced by the well-developed Lower Palaeozoic strata of Yangtze features (Lü 1997). Li (1994) recognized that the eastern border of the Yangtze Platform might locate about 700 km east of the Tan–Lu fault belt. Chen et al. (2010) noted that the Tan–Lu fault belt cut off not only the western marginal belt of Yangtze Platform but also the eastern North China Platform. Thus, the Tan–Lu fault does not represent precisely the northeast border of the

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(a) Neoproterozoic

Shangdan Ocean North China Plate

Allochtonous terrane

Crust

Lithosphere mantle Asthenosphere

(b) Late Ordovician

Island arc system

Passive continental margin

Arc back basin North Qinling

Allochtonous terrane

Shangdan suture zone

(c) Early Silurian Allochtonous terrane

(d) Permain

Active continental margin

North Qinling

North China Plate

Erlangping suture zone

North China Plate

Active continental margin Qinling Ocean

Yangtze Plate

Allochtonous terrane

North Qinling

North China Plate

(e) Middle–Late Triassic Early Mesozoic Qinling Orogens

Allo

Yangtze Plate

chto

nous

North Qinling

terra

North

North China Plate

ne

Mian–Lue suture zone Shangdan suture zone Erlangping suture zone

Oceanic crust

(f) Late Jurassic Epoch Dabashan

North Qinling

Island arc magma North Qinling

Yangtze Plate

Ophiolitic melange

Late Mesozoic Qinling Orogens

Lateral escape

North China Plate

Allochtonous terrane Continental subduction

Fig. 2.3 An evolutionary geologic model of the Qinling Orogens (after Fig. 3 of Meng 2017)

2

Geological Setting of the Ordovician and Silurian Strata of the Yangtze Platform

11

109°E

108° E 32° 20′

110° E

0

Langao

10

20 km

32° 20′ N

Erosion area

60–70 m

80 m

Miaoziba 31.5 m

30

40

m

m

Wangyuan 50 m

Yanhe

41.6 m

40–50 m

68.1 m

50–60 m

74.0 m

Borehole and thickness

32° 00′ N

43.0 m

Chengkou 51.1 m

Jiuyuan

60 m

71.7 m

40 m

Miaoziwan 77.6 m

Dongan

77.8 m

70 m

Dukou

31° 40′ N

Zhengping

68.1 m 74.0 m

32° 00′ N

Section and thick

Houping 73.9 m

48.1 m

Yidu

Erosion area

30.3 m 31° 40′ N

Chahe 55.8 m

Zhongliang

70.8 m

42 m

Banxi

46.8 m 46.3 m

50 m

60 m

Chaoyang

80

m

156.4 m 81.3 m

Bancang

59.2 m

Wuxi

Dangyang

31° 20′ N

Muyu 40 m 31° 20′ N

Sangping

Kaixian

70

m

Heyan

47.2 m

Chuyang

Dashu 50 m

Fengjie 31° 00′ N 108° E

Wushan

Badong 31° 00′ N

109° E

110° E

Fig. 2.4 Distribution and isopach map of the black shales in the Wufeng and Lungmachi formations in Dabashan area (after Fig. 15 of Xiong et al. 2017)

Yangtze Plate. The Yangtze Plate was evidently much larger than its earlier interpretation. Recently, an important discovery of fossil plants and spores from a borehole in the South Yellow Sea extends the northeast boundary of the Yangtze Platform from Lianyungang to the Korean Peninsula. A late Devonian plant fossil Archaeopteris and medium-sized spore Apiculiretusispora were found in the CSDP-2 borehole in South Yellow Sea by Guo et al. (2017) (Fig. 2.5). The fossils in the CSDP-2 borehole represent a common assemblage of the Wutong Formation (Upper Devonian) near Nanjing and Chaohu of the Lower Yangtze. Hsü et al. (1990) extended and merged the Tan–Lu–Qingdao suture zone into the Korean Imjingang belt. Later, Ree et al. (1996) demonstrated the existence of this tectonic belt based on tectonic, lithologic and chronostratigraphic evidence. The Gyeonggi Massif of central Korean Peninsula may also be an extension of the Lower Yangtze Platform. The Su–Lu tectonic belt and the Imjingang belt are recognized as the border separating the North and South Yellow Sea (Zhao et al. 2017). Xiao et al. (2005) recognized that the Paleozoic basement of the North Yellow Sea is similar to that of North China Platform. However, a very recent report by Kim et al. (2018) revealed an important discovery—Silurian strata with an abundant fossil fauna occurred in the middle of the northern Korean Peninsula. It brings a new

understanding of North Korean geology and the Paleozoic basement of the North Yellow Sea. According to Kim et al. (2018), the discovery of Silurian strata from the Pingnan Basin of northern Korean Peninsula near Pyongyang indicates a very different stratigraphic architecture from that of the North China Platform. Silurian and Devonian strata are all absent on the North China Platform. Interestingly, there are two Llandovery biozones in the Gushan Formation in the Pingnan Basin. The fossils, including brachiopods Striispirifer shiqianensis, Howellella tinga, and corals Heliolites fenggangensis, are all well known Yangtze taxa. In the Yueyangli Formation of Wenlock age in the Pingnan Basin, two characteristic species of the third biozone, Leptostrophia guizhouensis and Striispirifer hsiehi, are Yangtze taxa as well. Thus, the Pingnan Basin and the connected North Yellow Sea basement may be part of the Yangtze Platform. Based on a recent study of Silurian corals, Kido (2009) also extended the range of Yangtze Platform to Kyushu, and Shikoku, Japan. Two characteristic Yangtze coral genera Nanshanophyllum and Shensiphyllum are recorded in Kyushu and Shikoku. Thus, the Silurian Kurosegawa Terrane is considered as a part of the marginal belt of Yangtze (Kido 2009). The Yangtze Platform is bounded to the west by the Longmenshan Block, where a thick Devonian carbonate

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Fig. 2.5 A sketch map of the Yangtze Platform and its adjacent South Yellow Sea (after Fig. 5 of Guo et al. 2017)

Shenyang

an S Jap

Korean Peninsula CHINA

ea lt be ng a g jin Im Gyeonggi Massif

Pyongyang

Beijing

Seoul

Bohai Sea

u –L Su

slip kestri Sea llow t Ye Eas

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lt be c i ton tec

Tsingtao

Korean Peninsula

Yellow Sea

Tan–Lu fault

lt fau

Lianyungang

Baoying

GSDP-2

Yangtze

Nappe of Nanjing Hills Hefei

Borehole CSDP-2

Nanjing Shanghai

Qinlin g tecton –Dabie ic belt

Chaohu

Upper Devonian plants National boundary and coastline Tectonic belt and presumed tectonic belt

Yangtze

sequence was deposited. An angular disconformity separates the Devonian rocks from the underlying Maoxian Group at Linxi High School of Beichuan County. Very recently, Li Yue and Stephen Kershaw recorded the Silurian coral Paleofavosites in the Maoxian Group. As an allochthonous massive, the Upper Paleozoic Longmenshan Block was shifted by a substantial thrust fault far to the east to its current location. Based on a structural interpretation of a seismic profile that runs from the thick Ruo’ergai Basin to the Longmenshan Block, Guo et al. (2013) demonstrated that the basement of the Longmenshan Block belongs to the Yangtze Plate. The metamorphosed Silurian rocks of the Maoxian Group represent the marginal belt of Yangtze. It appears that the western boundary of the Yangtze Plate is along the Longriba fault, which also represents the border

between the Yangtze Plate and the Ruo’ergai Basin (Fig. 2.6). The Longriba fault belt runs from Aba to Daofu in a north–south direction, and might be the western boundary of the Yangtze Plate. However, there is a substantial area between Aba and Zhouqu, where no evidence of the Yangtze boundary are available. A further study in this area is necessary, although this high mountain district is difficult to access for geological investigation. The Ordovician biogeography of South China was first investigated by Mu (1974) using graptolite distribution patterns. He subdivided the Ordovician biogeography of South China into the Central China-type (including Yangtze) and South China-type biotopes. Lu et al. (1976) also described the Ordovician biogeography of South China, but subdivided it into a Yangtze Realm and a Southeast Realm

Geological Setting of the Ordovician and Silurian Strata of the Yangtze Platform

13

SGT (eastern Tibetan Plateau) 3.5(km)

Fig. 2.6 A seismic profile from the Ruo’ergai Basin to the Longmenshan Block (after Fig. 5 of Guo et al. 2013)

Topography

2

0 4

Ruo’ergai Basin

Flysch sediments

Sichuan Basin

Longmenshan Block

Sedimentary cover

8 12 16 20

Moho

Moho Yangtze crust （highly thrusted）

Moho Yangtze crust （relatively rigid）

(including the Chiangnan (Jiangnan) and Zhujiang depositional regions) based on trilobite, graptolite, and nautiloid faunas. Lu’s biogeographical subdivision was the basis for proposing the Yangtze region, the Zhujiang region and the Chiangnan transitional belt (Chen and Rong 1992). These three biofacies are accepted as a model of platform–slope– basin transition. However, along with the process of Kwangsian Orogeny this platform to basin transition

was altered and complicated during the late Ordovician Dicellograptus complanatus Biozone by the rise of the Cathaysian Old Land (Fig. 2.7, after Fig. 2 of Chen et al. 2014). The southern border of the Yangtze Platform coincides with the boundary between the Yangtze Platform and Chiangnan transitional belt. The border extends from Xianlin near Hangzhou through Banqiao of Lin’an, turns to Hanggai

Fig. 2.7 Changing boundaries of the Yangtze Platform plus Chiangnan transitional belt and the northwestward advancing coast line of Cathaysian Old Land over the late Ordovician (after Fig. 2 of Chen et al. 2014). Abbreviation: Mts., Mountains. A, boundary at the end of Nemagraptus gracilis Biozone (Sandbian); B, Climacograptus bicornis Biozone (Sandbian); C, placanthograptus spiniferus Biozone (early Katian); D, Dicellograptus complexus Biozone (late Katian); E,

Akidograptus ascensus Biozone (earliest Rhuddanian) to Parakidograptus acuminatus Biozone (Rhuddanian); F, Cystograptus vesiculosus Biozone (Rhuddanian) to Coronograptus cyphus Biozone (Rhuddanian). G–H refer to the southern boundaries of the two Telychian Red Beds on the Yangtze Platform, in early and late Telychian respectively

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120°E

115°E

Xi’an

North

China

Nanjing Hexian

Hefei Chengkou

Shanghai

Shennongjia

Jingshan

Changyang

Hangzhou

Wuhan

30°N

Yichang 30°N

Chongqing

South

China Nanchang Xin’gan Changsha Nanfeng Anfu

Central Jiangxi Old Land

? Tectonic boundary Facies boundary

Guiyang

Xingguo

Shicheng

Yong’an

Carbonate (platform) Graptolitic shale (slope, basin) Land

25°N

Coarse and fine clastics

100 km 110°E

115°E

Fig. 2.8 A facies distribution map of the Yangtze Platform and Chiangnan transitional belt in Sandbian of Late Ordovician (after Fig. 2 of Chen et al. 2012)

of Anji, passes through Ningguo, Jingxian, Yixian, and then extends to the north end of the Poyang Lake, Xiushui County, and the Dongting Lake. The boundary further extends along the north side of Wulingshan to Shimen–Cili– Dayong–Jishou of southern Hunan Province. It then turns southwestwards to Guizhou Province, extending along the Fenghuang–Tongren–Zhenyuan–Kaili–Danzhai line to Sandu (Fig. 2.8, after Fig. 2 of Chen et al. 2012). The southwest boundary of the Yangtze Platform coincides with the north margin of the Central Guizhou Old Land. It connects with the west end of the Chiangnan transitional belt at Danzhai–Sandu of Guizhou Province. There was a narrow Yangtze marginal belt at Zunyi along the

northern margin of the Central Guizhou Old Land. Silty shales that correlate with the Coronograptus cyphus Biozone (LM5) overly the Kuanyinchiao Bed and disconformably underly Lower Permian rocks (Chen et al. 2017). The Daduhe Formation, which correlates with the Wufeng Formation, crops out in the southwest corner of the Yangtze Platform. Tang et al. (2017) provided a distribution map of the area (Fig. 2.9). The Qujing–Kunming area in eastern Yunnan Province also belonged to the Yangtze Platform during the Early Palaeozoic, although it was separated from the Central Guizhou Old Land by a narrow bay. Both the Lower Paleozoic rocks and faunas of this area are different from those of

2

Geological Setting of the Ordovician and Silurian Strata of the Yangtze Platform

Fig. 2.9 Distribution of late Ordovician strata in the southwest corner of the Yangtze Platform (after Fig. 7 of Tang et al. 2017)

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N

Nanjing

Zhushan

Nanjiang

Yichang Luding

Chengdu Hongya

Wuhan Susong

Yangtze Platform

Hanyuan Weiyuan Changning

Zunyi

Yanjin

Butuo Ningnan

Bijie

Guiyang

se a X iz a n g Yu n n a n –

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an ensh

Old

Modified from Mu et al. (1981) and Ma et al. (2009)

80 160 km

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Leshan

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Weiyuan

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Butuo 33 32

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Daqing 44 43 Formation area 45 46

Weixin

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Puge

City

the Upper Yangtze, containing no Wufeng Formation or Lungmachi Formation black shales. The west margin of the Central Guizhou Old Land passed through Huili–Yanbian, an area between Zhaotong and Kunming, extending northwards along the Dadu River (Daduhe) and connecting with

Daduhe Formation area

Yuexi

39

Dechang

Zigong

5

34

35

Tiezufeike Formation area

Kham–Dian Old Land

Shimian

Wufeng Formation area

2

Bijie Hezhang 48

Section

Central Guizhou Old Land Land

Facies boundary

the Longriba fault belt near Daofu (Fig. 2.10). Thus, the western boundary of the Yangtze Platform during the Lower Paleozoic was located far to the west of the Longmenshan Block (Fig. 2.10).

16

X. Chen et al.

Fig. 2.10 The boundaries of the Yangtze Platform in the Ordovician

References Chen X, Rong JY (1992) Ordovician plate tectonics of China and its neighbouring regions. In: Webby BD, Laurie JR, Chen X, Rong JY (eds) Global perspectives on Ordovician geology. Balkema, Rotterdam, pp 277–291 Chen X, Zhou ZY, Fan JX (2010) Ordovician paleogeography and tectonics of the major paleoplates of China. Geol Soc Am Spec Papers 466:85–104 Chen X, Zhang YD, Fan JX, Tang L, Sun HQ (2012) Onset of the Kwangsian Orogeny as evidenced by biofacies and lithofacies. Sci China Earth Sci 55(10):1592–1600 Chen X, Fan JX, Chen Q, Tang L, Hou XD (2014) Toward a stepwise Kwangsian Orogeny. Sci China Earth Sci 57(3):379–387 Chen X, Fan JX, Wang WH, Wang HY, Nie HK, Shi XW, Wen ZD, Chen DY, Li WJ (2017) Stage-progressive distribution pattern of the Lungmachi black graptolitic shales from Guizhou to Chongqing, Central China. Sci China Earth Sci 60(6):1133–1146 Guo XY, Gao R, Keller GR, Xu X, Wang HY, Li WH (2013) Imaging the crustal structure beneath the eastern Tibetan Plateau and implications for the uplift of the Longmen Shan range. Earth Planet Sci Lett 379:72–80 Guo XW, Xu HH, Zhu XQ, Peng YM, Zhang XH (2017) Discovery of Late Devonian plants from the southern Yellow Sea borehole of China and its palaeogeographical implications. Palaeogeogr Palaeoclimatol Palaeoecol 531:1–7 Hsü KJ, Li JL, Chen HH, Wang QC, Sun S, Şengör AMC (1990) Tectonics of South China: key to understanding west pacific geology. Tectonophysics 183:9–39

Kido E (2009) Nanshanophyllum and Shensiphyllum (Silurian Rugosa) from the Kurosegawa Terrane, Southwest Japan, and their paleobiogeographic implications. J Paleontol 83(2):280–292 Kim BS, Wang XL, Kang JG, Li ZJ, Li B, Ho CL, Kim M (2018) Characteristics of the Silurian System in the middle part of the Korean Peninsula and their significance. Earth Sci Front 25(4):23– 31 (in Chinese with English abstract) Li ZX (1994) Collision between the North and South China blocks: A crustal-detachment model for suturing in the region east of the Tanlu fault. Geology 22:739–742 Lin BY et al (1984) The Silurian System of China. Geological Publishing House, Beijing Lu YH, Zhu ZL, Qian YY, Zhou ZY, Chen JY, Liu GW, Yu W, Chen X, Xu HK (1976) Ordovician biostratigraphy and Paleozoogeography of China. Mem Nanjing Inst Geol Palaeontol Acad Sin 7:1–83 (in Chinese) Lü HG (1997) Introduction. In: Bureau of geology and mineral resources of Jiangsu Province. Stratigraphy (Lithostratic) of Jiangsu Province. China University of Geosciences Press, Wuhan, pp 1–7 (in Chinese) Ma YS, Chen HD, Wang GL (2009) Paleogeography and sequence stratigraphy of South China. Science Press, Beijing, pp 1–603 (in Chinese) Meng QR (2017) Origin of the Qinling Mountains. Sci Sin Terrae 47:412–420 (in Chinese) Mu AT (Mu EZ) (1974) Evolution, classification and distribution of graptoloidea and graptodendroids. Sci Sin 17(2):227–238 Mu EZ, Li JJ, Ge MY, Chen X, Ni YN, Lin YK (1981) The Late Ordovician palaeogeographical map in Central China and its synopsis. J Stratigr 5(3):165–170 (in Chinese)

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Geological Setting of the Ordovician and Silurian Strata of the Yangtze Platform

Mu EZ, Song LS, Li JS, Xu BZ, Zhang YK (1982) Hemichordata Graptolithina. In: Xi’an Institute of Geology and Mineral Resources. An atlas of paleontology from Northwest China (Shaanxi, Gansu and Ningxia), pre-Cambrian to early palaeozoic. Geological Publishing House, Beijing, pp 294–347 Ree JH, Cho M, Kwon ST, Nakamura E (1996) Possible eastward extension of Chinese collision belt in South Korea: the Imjingang belt. Geology 24:1070–1074 Tang P, Huang B, Wu RC, Fan JX, Yan K, Wang GX, Liu JB, Wang Y, Zhan RB, Rong JY (2017) On the Upper Ordovician Daduhe Formation of the Upper Yangtze region. J Stratigr 41 (2):119–133 (in Chinese with English abstract) Wang XF, Xue ZJ (1986) Early Silurian graptolites from southwestern Henan. Bull Chin Acad Geol Sci 12:35–49 (in Chinese with English abstract) Wang XF, Chen X, Chen XH, Zhu CY (1996) Lexicon of Chinese stratigraphy: Ordovician. Geological Publishing House, Beijing, pp 1–126 (in Chinese) Xiao GL, Sun CH, Zheng JM (2005) Pre-Mesozoic basement characteristics in the eastern depression of the North Yellow Sea

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Basin. Geoscience 19(2):261–266 (in Chinese with English abstract) Xiong GQ, Wang J, Li YY, Yu Q, Men YP, Zhou XL, Xiong XH, Zhou YX, Yang X (2017) Lithofacies palaeogeography of the Early Paleozoic block rock series in Dabashan region and their shale-gas geological significance. J Palaeogeogr 19(6):966–986 (in Chinese with English abstract) Zhang CL, Bi ZG, Gong WL, Zha SX, Ma GM, Xia Q, Lu S (2014) Discovery of Upper Ordovician conodonts from the Baidashan Group in Huoqiu County, Anhui. J Stratigr 38(2):200–203 (in Chinese with English abstract) Zhao SJ, Li SZ, Suo YH, Guo LL, Dai LM, Jiang SH, Wang G (2017) Structure and formation mechanism of the Yellow Sea Basin. Earth Sci Front 24(4):239–248 (in Chinese with English abstract) Zhu ZL, Xu HK, Chen X, Chen JY, Jiang LF, Wu SJ, Zhou GX (1984) Early Paleozoic strata of Chuxian-Quanjiao and Nanjing-Luhe areas. Bull Nanjing Inst Geol Palaeontol Acad Sin 7:306–338 (in Chinese with English abstract)

3

Ordovician to Silurian Shale Gas-Bearing Strata from the Yangtze Region Xu Chen, Hongyan Wang, Feng Liang, Qing Chen, Chao Luo, Zhi Zhou, Wenhui Wang, Jia Li, and Dexun Liu

Abstract

Forty-three reference sections and boreholes subdivided into latest Ordovician to early Silurian graptolite biozones are included in the present chapter. These lithologic and biologic columns are the basic data for defining the Ordovician–Silurian shale gas-bearing strata correlation across the Yangtze Platform and its marginal belts, from which the distribution of most potential shale gas-bearing graptolite strata might be easily identified. Keywords








Reference sections and boreholes Graptolite biozonation Wufeng formation Lungmachi formation

X. Chen (&)
Q. Chen State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology and Center for Excellence in Life and Paleoenvironment, Chinese Academy of Sciences, Nanjing, 210008, China e-mail: [email protected] H. Y. Wang
F. Liang
D. X. Liu PetroChina Research Institute of Petroleum Exploration and Development, Beijing, 100083, China C. Luo Shale Gas Research Institute, PetroChina Southwest Oil and Gasfield Company, Chengdu, 610051, China Z. Zhou Oil and Gas Resources Survey Center, China Geological Survey Bureau, Beijing, 100083, China W. H. Wang School of Geosciences and Info-Physics, Central South University, Changsha, 410012, China J. Li Academician Studio, Zhejiang Nuclear Industry Geological Survey Team, Huzhou, 313002, China

Graptolite faunas from forty-three reference sections and boreholes are assigned to latest Ordovician to early Silurian graptolite biozones, and are described in the present chapter. These lithologic and paleontologic columns comprise the basic data for correlating the Ordovician–Silurian shale gas-bearing strata through the Yangtze Platform and its marginal belts. From these data the distribution of the highest potential shale gas-bearing graptolite strata are easily defined (Fig. 3.1). Chen et al. (2015, 2017) concluded that the shale gas-bearing strata with the greatest potential belong to nine graptolite biozones, WF2–WF3 of the Wufeng Formation (latest Ordovician) and LM1–LM6 of the Lungmachi Formation (early and Middle Llandovery, Silurian). These bio-units are recognized in the 43 stratigraphic columns and borehole logs without a layer-by-layer description. However, in some measured sections and borehole logs the graptolite biozones are only recognized by a few characteristic forms. In these sections and boreholes the boundaries between zones could not be located on the first appearances of their zonal species due to limited sampling and small chip surface areas (drill cores). Thus, we use dashed lines to correlate the possible boundaries in the stratigraphic columns. In some published boreholes, TOC and gamma-ray (GR) curves are included. In these stratigraphic borehole logs, global graptolite biozonation may assist in the correlation of isotopic TOC and gamma-ray logs. In fact, these valuable correlations between boreholes and sections promoted and accelerated the shale gas exploration by both SINOPEC and the PetroChina in recent years. Figure 3.1 illustrates the 43 sections and boreholes in a northwest to southeast transect of the Yangtze region. A few sections located on the Yangtze marginal belts are also included. All the identification lists of downhole graptolites in this book are based only on the graptolites preserved on the top and bottom sections of the core samples. Because the oil company does not allow the core samples to be split, so the graptolite lists obtained from the cores are very limited.

© Zhejiang University Press and Springer Nature Singapore Pte Ltd. 2023 X. Chen et al. (eds.), Latest Ordovician to Early Silurian Shale Gas Strata of the Yangtze Region, China, https://doi.org/10.1007/978-981-99-3134-7_3

19

20

X. Chen et al. Xi’an

Suqian

Shangluo

Zhouqu Lueyang

Ningshan

Aba

Xichuan

Zizhong

WD4

WD3 17

15 WD2

Nanjiang Qiaoting

WD6 Weiyuan WD5 20 Well Weiyuan 1 18 19 Zigong Fushun

Chengkou Tianba Wuxi Tianba Bailuzhen Wenfengzhen WX1

Muchuan

Daofu 11 N203

Yanjin

Borehole

Huarongshan Sanbaiti

nta

Hefei

in

Jianshi Longping

Tangshan 40 39 Jurong Ganggangshan

Nanjing 38

Hexian Sinianpan

bie

Jingshan Daozimiao

Mo

unt

ain

s

36

Ningguo Jingshan u 42 Anji ho 41 gz Hanggai an

Jingxian

Wuhan

H

Yixian

Chongqing

26

JY1

Laidi 1

Jiaoshiba Pengshui Lujiao

31

28

Qijiang

24 Changning 25 Gaoxian Ren’ai Qilongcun Guanyinqiao 13 Ning211 Shuanghe Shizishan 12 Hanjiadian 23 11 Changningnan Tongzi 22 YJ1 N203 Honghuayuan Yanjin Donggongsi Yongshan Wanhe 21

37

Changde

30

Yongshun Kelisha

n Yu a

jian

gR

i

Xiushui

ver

Xinkailing Wuning

Jingdezhen

43

Taojiang

Jiangshan Putang kou (Changshan)

Changsha

29

Songtao Ludiping Tongren

Zunyi

Zhaotong Huili

Weining

Longhui Dongkou Liupanshui

Taijiang

Guiyang Dongchuan

Danzhai Sandu

Legend

Yanbian

Huoqiu

ou

27

Weiyuan

Xichang

Wangjiawan 34 Fenxiang 33 Yichang

32

Kangding Luding Yuanyangyan

ai M

Da

Town

Chengdu Profile position

gb

Shennongjia Bajiaomiao

WX2

Legend

Changning 10 YJ1

To n

Dazu

Neijiang

Shaoxing

14 WD1 16

Leshan

35

N in g

Songpan

guo

Ningqiang Zhongliangsi Nanzheng Fucheng

Border of the Yangtze Platform Location of the section Location of the borehole

Fig. 3.1 Distribution of the reference sections and boreholes containing latest Ordovician Wufeng Formation to earliest Silurian Lungmachi Formation in the Yangtze region

Note that with the official adoption of the Pinyin rather than Wade-Giles spellings for Chinese words in the late 1970s, the English spellings of many Chinese geographic and stratigraphic names were changed into Pinyin. However, some of stratigraphic unit names published earlier still remain. For example, stratigraphic unit name of the Kuanyinchiao Bed was published before 1970. However, the name of its type location should be changed to Guanyinqiao followed Pinyin system.

3.1

Upper Yangtze: Sichuan Basin and Periphery

1. Nanzheng and Lungmachi Formations in the Zhongliangsi Section, Nanzheng, Shaanxi The Nanzheng Formation and its overlying and underlying strata in the Zhongliangsi section were first measured by Lu

(1943). His collections of graptolite specimens from the section were described and published by Chen Xu in 1984. Among Lu’s collections, the graptolite Normalograptus angustus (Perner) and the trilobite Dalmanitina occur in the Nanzheng Formation (Fig. 3.2). This indicates an Upper Ordovician age for the Nanzheng Formation. The Zhongliangsi section is located near the Hanzhong Old Land, situated in the northwest marginal belt of the Yangtze Platform. The yellow-gray shale and siltstone of the Nanzheng and Lungmachi formations represent mainly nearshore paleoenvironments. Common graptolite taxa of the Wufeng Formation are present in the section. The overlying Nanzheng Formation includes WF4 to LM1. Both the base and top of the Nanzheng Formation are diachronous in sections around the Hanzhong Old Land. Increasing silty material and a thinning of the strata, as well as the low diversity of the graptolite and shelly faunas, may indicate a nearshore platform margin environment.

21

Thickness Age Lithology Ranges of important graptolites Biostratigraphy (m) (Ma)

89.6

6.05

Yellow-gray shale with siltstone

Dalmanitina nanzhengensis

Yellow-gray or green-gray shale

Spirograptus guerichi

Rastrites guizhouensis

15.1

Coronograptus cyphus

17.8

Campograptus communis

10.75

Normalogr. angustus

Lungmachi

3.5

Pagoda Nanzheng

Aeronian Katian

Hir. U. Ord.

Ordovician

Rhuddanian

Llandovery

Silurian

3.3

Streptograptus plumosus

Formation

Cuijiagou

Stage

Telychian

Series

Ordovician to Silurian Shale Gas-Bearing Strata from the Yangtze Region

System

3

LM9

438.49 LM7—LM8?

439.21

LM6

440.77

LM5

441.57

LM2—LM4 WF4—LM1

443.83 445.16

Under sea water shrinkage limestone

Fig. 3.2 Stratigraphic column of the Nanzheng and Lungmachi formations with a few key graptolite taxa in the Zhongliangsi section (Chen 1984). Abbreviation: U. Ord., Upper Ordovician

upper part

Formation Thickness Lithology (m)

Ranges of important graptolites

Ka.

Wufeng

>0.25

Black shale

Shale with fine grained sandstone

Streptograptus plumosus

Rastrites linnaei Spirograptus turriculatus Torquigraptus planus Streptograptus filiformis

Nanzheng

Coronograptus leei Rastrites guizhouensis Demirastrites cf. triangulatus

Hir.

Paraorthograptus spp. Diceratograptus mirus Metabolograptus ojsuensis Metabologr. persculptus Parakidogr. acuminatus Cystograptus vesiculosus

R.

1.27 0.05 0.07 0.15 >0.04

Lituigraptus convolutus

Aeronian

Lungmachi

3.03

Stimulograptus sedgwickii Spirograptus guerichi Pseudoretiolites daironi

lower part

89.04

3.19

Upper Ordovician

Age (Ma)

185.73

18.81

Ordovician

Biostratigraphy

Cuijiagou

Llandovery

Silurian

Telychian

Stage

X. Chen et al.

Series

System

22

LM8–LM9

438.76

LM7 439.21

LM6 440.77

LM2–LM5 LM1

WF4

443.83 443.43 445.16

WF3 447.02

Silty shale

Fig. 3.3 Stratigraphic column of the Wufeng and Cuijiagou formations with characteristic graptolite taxa from the Fucheng section, Nanzheng, Shaanxi (after Wang 1988). Abbreviations: R., Rhuddanian; Ka., Katian

2. Wufeng and Cuijiagou Formations in the Fucheng Section, Nanzheng, Shaanxi Fucheng is a small town in the southeast part of Nanzheng County, near the border of Shaanxi and Sichuan provinces. From the Zhongliangsi section to the Fucheng section, the lithofacies and biofacies change from platform margin siltstone with a low diversity graptolite fauna to platform basin black shale with a high diversity graptolite fauna (Fig. 3.3).

The graptolites Paraorthograptus spp. and Diceratograptus mirus Mu (top of WF3) occur at the top of the Wufeng Formation in the Fucheng section. Their occurrences suggest that the boundary between the Wufeng and Nanzheng formations at Fucheng might coincide with the main phase of the latest Ordovician extinction event (Chen et al. 2004), and during the maximum expansion of the South Pole ice sheet and global sea level fall. The Nanzheng Formation at Fucheng may correlate with the Kuanyinchiao

3

Ordovician to Silurian Shale Gas-Bearing Strata from the Yangtze Region

Bed of the Wangjiawan section, Yichang, which is the Global Stratotype Section and Point (GSSP) of the Hirnantian Stage (Chen et al. 2005, 2006). Metabolograptus ojsuenesis (Koren’ and Mikhailova) from the lower Nanzheng Formation may extend from WF4 and upwards. The overlying bed, the base of the Lungmachi Formation may represent LM1 (Metabolograptus persculptus Biozone). Thus, the top of the Nanzheng Formation at Fucheng is the same age as the top of the Kuanyinchiao Bed at Yichang and may correlate with LM1. The Lungmachi Formation in both the Zhongliangsi and Fucheng sections spans an interval from LM2 to LM9. However, the thickness of the Lungmachi Formation thins from 50.45 m (at Zhongliangsi) to 26.35 m (at Fucheng). Thus, the rock accumulation rate has decreased by half from nearshore to basin. 3. Nanjiang Formation in the Qiaoting Section, Nanjiang, Sichuan The Qiaoting section of Nanjiang is the type locality of the Nanjiang Formation. The Nanjiang Formation consists of a Telychian (LM9/N1 to N2) black graptolitic shale sequence (Fig. 3.4) that crops out along the Daba Mountains (Fig. 2.2). The lower part of the Nanjiang Formation includes 30 m of black shale (Fig. 3.4). However, its distribution is limited to a range of 40 km from Qiaoting of Nanjiang westwards to Dalianghui of Wangcang. This implies a reduced potential for shale gas exploitation of the Nanjiang Formation. The lower Nanjiang Formation comprises a continuous deposition of black graptolitic shale with a highly diverse graptolite fauna that spans the Spirograptus guerichi Biozone (LM9/N1) to the Spirograptus turriculatus Biozone (N2) (Fig. 3.5). Loydell (1992) subdivided these two graptolite biozones into six sub-biozones based on a turbidite sequence in the Welsh borderland. However, graptolite-rich layers within turbidite sequences are interlayered with non-fossiliferous intervals and it can be difficult to recognize the first appearance of the zonal species. Thus, we prefer using the two undivided graptolite biozones in both Wales and Nanjiang as opposed to the more detailed subdivisions.

23

4. Lungmachi Formation in the Tianba Section, Chengkou, Chongqing The Lungmachi Formation was recorded by Ge (1990) from Tianba, Chengkou of Chongqing. However, the section has never been described in detail. The present study recognizes some of key taxa within the lower 37 m black shale, which corresponds to beds 1–5 of Ge’s Tianba section. Ge (1990) combined this 37 m black shale with the overlying thick non-black shale and siltstone as a new litho-unit, the Shuanghechang Formation. We accept only the lower 37 m black shale as Lungmachi Formation since it fully agrees with the original definition of the Lungmachi Formation (Fig. 3.6). 5. Ordovician–Silurian Boundary Strata in the Tianba Section of Wuxi, Chongqing The Ordovician–Silurian boundary strata in the Tianba section of Wuxi, Chongqing were investigated by Chen Xu with colleagues from the PetroChina Research Institute of Petroleum Exploration & Development in 2015. The strata through the Ordovician and Silurian boundary in the Tianba section have been measured along the rural road (Fig. 3.7). 6. Stratigraphic Logs of the Wufeng and Lungmachi Formations in Wells WX2 (Near the Town of Wenfeng) and WX1 (Near the Town of Bailu), Wuxi, Chongqing These closely spaced boreholes were arranged for investigating the shale gas potential of the Wufeng and Lungmachi formations in the entire Wuxi area. Chen Xu and his colleagues of the PetroChina investigated the logs in the summer of 2016. WF4 (Metabolograptus extraordinarius Biozone) is recognized from the top of the Wufeng Formation in two boreholes. The LM1 (Metabolograptus persculptus Biozone) to LM9 (Spirograptus guerichi Biozone) interval is well developed in these two boreholes as well. Black shale was continuously deposited along with some siltstone layers, which occur in the upper part of the

Ranges of important graptolites

Lithology

Biostratigraphy

N2

Spirograptus turriculatus Black shale

Sinodiversogr. multibrachiatus

1.5

Spirograptus guerichi

Kuanyinchiao

U.Ord.

Ordovician

18.2

Streptogr. plumosus

16.4

Pseudoplegmatograptus obesus

Nanjiang

Telychian

Llandovery

Silurian

Rastrites distans

134.5

Hir.

Age (Ma)

Streptograptus linealis

Thickness (m)

Formation

Stage

Series

X. Chen et al.

System

24

438.13

LM9/N1

438.49

Weathered gray shale

Fig. 3.4 Stratigraphic column of the Nanjiang Formation with a few key graptolite taxa from Nanjiang, Sichuan (after Chen 1984)

3

Ordovician to Silurian Shale Gas-Bearing Strata from the Yangtze Region

25

Thickness (m)

Formation

Telychian

Series Stage

Lithology

Ranges of important graptolites

406.00

Biostratigraphy Age(Ma)

LM9

Hir.

Ord.

Stimulograptus sedgwickii

Coronograptus leei

5.65

Akidograptus ascensus

8.94

Neodiplograptus modestus

4.20

Parakidograptus acuminatus

Aeronian Lungmachi

8.00

Rhuddanian

Silurian Llandovery

87.32

1.50 1.60 Black shale

Spirograptus guerichi

438.49

U. Ord.

Fig. 3.6 Stratigraphy column of the Lungmachi Formation with key graptolite taxa in the Tianba Section, Chengkou, Chongqing (after Ge 1990). Abbreviations: Ord., Ordovician

System

Fig. 3.5 Nanjiang Formation and a graptolite-bearing slab from the Qiaoting section, Nanjiang

LM8 438.76

LM7 439.21 LM5—LM6

441.57 LM4?

442.47 LM3 LM2 LM1

Silty shale

443.40 443.83

Thickness (m)

Formation

Stage

Series

X. Chen et al.

System

26

Ranges of important graptolites

Lithology

Biostratigraphy

Age (Ma)

Korenograptus gracilis

LM4

442.17

LM2—LM3

443.83 LM1

Dicellograptus ornatus

1.10

Appendispinograptus leptothecalis

0.17

Normalogr. mirnyensis

Normalogr. angustus

0.10

Paraorthograptus pacificus

Wufeng

Katian

Upper Ordovician

Ordovician

Kuanyinchiao

5.30

Avitograptus avitus

Lungmachi

Rhuddanian

Llandovery

Silurian

0.42

Cystograptus sp.

Covered

444.43

WF3

Black shale 447.02

Covered

Fig. 3.7 Ordovician–Silurian boundary strata with key graptolite taxa in the Tianba section, Wuxi, Chongqing

Lungmachi Formation. The Wufeng–Lungmachi formations in these two boreholes are similar to that of the Bajiaomiao section of Shennongjia, Hubei. Thus, from Wuxi to Shennongjia the black shales of the two formations are uniformly

distributed along the Dabashan high mountain ranges. However, secondary geologic structure might be a disadvantage for shale gas exploration in the Dabashan area (Figs. 3.8, 3.9).

Fig. 3.8 Stratigraphic logs of the Wufeng–Lungmachi formations in four boreholes in Wuxi County, Wenfeng Town (after Fig. 3 of Liang et al. 2016)

3 Ordovician to Silurian Shale Gas-Bearing Strata from the Yangtze Region 27

Ranges of important graptolites

Lithology

Spirograptus guerichi

(m)

Thickness

Formation

Stage Telychian

Series

X. Chen et al.

System

28

10.49

Biostratigraphy Age(Ma)

LM9

Stimulograptus cf. utilis

438.49

Wufeng

3.16

3.59

Covered

Anticostia macgregorae

Black shale

LM8

438.76

LM7 439.21

LM6 440.77

LM4—LM5

442.47

Huttagraptus sp.

1.7

Tangyagr. sp.

Hir.

?

Katian

Upper Ordovician

Ordovician

?

Metabologr. extraordinarius

11.45

5.0

?

Cystograptus penna

Rhuddanian

4.35

Campograptus sp.

7.15

Rastrites sp.

Lungmachi

Aeronian

Llandovery

Silurian

14.83

LM3

WF4

443.40 445.16

WF3 447.02

WF2

Black shale with siltstone

Fig. 3.9 Lithostratigraphic log of the Wufeng and Lungmachi formations with key graptolite taxa in the Bailu borehole, Wuxi, Chongqing (Investigation by Chen Xu and colleagues of the PetroChina in 2016)

Thickness (m)

Lithology

29

Biostratigraphy Age(Ma)

Ranges of important graptolites

7.5

Stimulograptus sedgwickii

Petalolithus folium

Demirastrites sp.

Spirograptus guerichi

438.49

LM8

438.76

LM7 439.21

LM6 440.77

LM4—LM5

Coronograptus gregarius

Avitograptus avitus

Neodiplograptus modestus

Korenograptus jerini

Neodiplograptus daedalus

0.5

Akidograptus ascensus

4.5

Parakidograptus acuminatus

1.53

Coronograptus cyphus

4.03 Cystograptus vesiculosus

Rhuddanian

10.93

Rastrites guizhouensis

Lungmachi

4.70

Lituigraptus convolutus

6.03

Llandovery

Silurian

Aeronian

11.20

Spirograptus andrewsi

LM9

Rastrites linnaei

Stage

Formation

Telychian

Series

Ordovician to Silurian Shale Gas-Bearing Strata from the Yangtze Region

System

3

LM3

443.40

LM2 LM1

Black shale

443.83 444.43

Siltstone

Fig. 3.10 Stratigraphic column of the Lungmachi Formation in the Bajiaomiao section, Shennongjia, Hubei (section provided by Fan Junxuan and Chen Qing with the key graptolite taxa identified by Chen Xu)

7. Lungmachi Formation in the Bajiaomiao Section, Shennongjia, Hubei The LM1 to LM9 graptolite biozones are well developed in the Bajiaomiao section and yield high diversity graptolite faunas. However, a small fault has cut out part of the middle Parakidograptus acuminatus Biozone (LM4) resulting in the absence of the Hirsutograptus fauna. The Hirsutograptus fauna occurs in the middle of LM4 in the Wangjiawan North section, the Hirnantian Stratotype Section and Point of the Hirnantian Stage, and in the same interval in the Shizishan section, Changning, Sichuan.

The upper Lungmachi Formation is commonly replaced by siltstone or sandy shale above the LM6 (Demirastrites triangulatus Biozone) strata in this area. This indicates that coarser clastic material is provided from the neighboring terrestrial area in the post-LM6 interval (Figs. 3.10, 3.11, 3.12). 8. Xingou and Yuanyangyan Formations in the Yuanyangyan Section, Erlangshan, Sichuan The Yuanyangyan section is located along the east side of the large Daduhe fault zone in the Erlang Mountain (Erlangshan)

GR (API)

Lithology

Shallow investigation laterolog

(Ω • m) 500 50

0

All gas hydrocarbon (%)

2500 0

1

Graptolite zones

Depth (m)

Formation

Stage

Series

X. Chen et al.

System

30

300

350

Lungmachi

Llandovery

Silurian

Aeronian

LM8

400 LM7

LM6

U. Ord.

Ord.

Rhuddanian

450

Hir.

LM5

LM4 LM1—LM3 Hirnantia WF3—WF4 WF2

Kuanyinchiao

Wufeng

Katian Linhsaing

500

Argillaceous limestone

Siliceous shale

Fig. 3.11 Parameter curves in the Shennongjia borehole (after Song et al. 2018)

Mudstone

3

Ordovician to Silurian Shale Gas-Bearing Strata from the Yangtze Region

31

Fig. 3.12 Lungmachi Formation in the Bajiaomiao section, Shennongjia, Hubei

range. The Xingou Formation is composed of dolomite intercalated with some siltstone. Graptolites in the formation are similar to those of the Wufeng Formation (WF2–WF3). Lithologically, the Xingou Formation is similar to the Daduhe Formation of the Yongshan–Butuo area, belonging to the southwest marginal belt of the Yangtze Platform. The Yuanyangyan Formation differs from the Xingou Formation, however, by containing black shale with siltstone interlayers, which includes LM1 to LM8. The Llandovery Yuanyangyan black shale is present in this younger interval (LM7, LM8) and geographically extends to the late Ordovician Yangtze marginal belt, indicating that the early Silurian global sea level rise has spread across the Yangtze Platform (Fig. 3.13). 9. Daduhe and Lungmachi Formations in the Wanhe Section, Yongshan, Yunnan There is a continuous sequence of strata across the Ordovician–Silurian boundary in the Wanhe section (Fig. 3.14). The Daduhe Formation is composed of shallow water muddy limestone and mudstone. Wufengian (WF2–

WF4) graptolites have been collected from the shale intercalations. This demonstrates that the Wufengian graptolite fauna may have reached the marginal belt of the Yangtze Platform. The Hirnantian Stage includes two graptolite biozones, Metabolograptus extraordinarius and Metabolograptus persculptus. The overlying Lungmachi Formation black shale only contains graptolites from LM2 to the base of LM4. Higher in the section, yellow-gray silty shale replaces the black shale (Fig. 3.15). This demonstrates that shallow water coarse clastic material fills the nearshore marginal belt earlier than in the basin. During the LM4 interval, shallower-facies siltstone occupied the platform margin, while the deeper-facies black shale was still distributed widely in the Yangtze Platform basin.

10. Stratigraphic Log from the Wufeng Formation to the Lungmachi Formation in the Well YJ1, Yanjin, Yunnan The Ordovician Wufeng Formation and Kuanyinchiao Bed, and the overlying Silurian strata are continuous

Lithology

Thickness (m)

Formation

Stage

Series

X. Chen et al.

System

32

Biostratigraphy

Ranges of important graptolites

LM8

Dolomite with siltstone

Normalogr. angustus

Conodonts

Metabologr. cf. ojsuensis

Paraorthograptus

5.87

Appendispinograptus venustus

XG

0.5

Appendispinograptus longispinus

Katian

U.Ord.

Ordovician

17.5

Parakidograptus acuminatus

18.01

Cystograptus vesiculosus

Rhuddanian

14.02

Mudstone

438.76

Stimulograptus sedgwickii

Campograptus communis

Coronograptus gregarius

140.74

Demirastrites

Yuanyangyan

Aeronian

Llandovery

Silurian

23.18

Paramonoclimacis

44.63

Hir. EL

Age (Ma)

LM7 439.21

LM6

440.77 LM4—LM5 442.47 LM3 443.40 LM1—LM2 WF4

444.43 445.16

WF2—WF3

Black shale with siltstone

Fig. 3.13 Stratigraphic column of the Xingou and Yuanyangyan formations in the Yuanyangyan section, Erlangshan, Sichuan (after and summarized from Jin et al. 1989). Abbreviations: XG, Xingou; EL, Erlangshan

3

Ordovician to Silurian Shale Gas-Bearing Strata from the Yangtze Region

33

Fig. 3.14 Daduhe and Lungmachi formations in the Wanhe section, Yongshan, Yunnan

deposited in the Well YJ1 (Fig. 3.16, after Fig. 3 of Liang et al. 2017). The LM1 to LM7 are all composed of black shale with a particularly thick LM7 interval in the borehole. At the level of 820 m in the borehole log, marine red beds occur. These may correspond to the marine red beds of the Sifengya Formation at Huanggexi, Daguan, Northeast Yunnan. The red beds in the Well YJ1 and the Sifengya Formation of Huanggexi may correlate with LM9. The Lungmachi Formation black shale is thinner in the Well YJ1 than in the Huanggexi section. However, the depositional center in the Yongshan–Yanjin– Daguan area, the southwest corner of the Yangtze Platform, was in Yanjin during the late Ordovician to Silurian (Fig. 3.16).

11. Stratigraphic Log of the Wufeng and Lungmachi Formations in the Well N203, Changning, Sichuan The black shales of the Wufeng and lower Lungmachi formations are 0.43 m and less than 20 m thick, respectively, in

the Well N203. Above LM5, there are coarse clastic sediments that still yield graptolites. Thus, the Well N203 is not ideal for shale gas exploration since the black shales of the Wufeng and Lungmachi formations are thinner than expected (Fig. 3.16).

12. Wufeng and Lungmachi Formations in the Shizishan Section, Shuanghe, Changning, Sichuan Zones from WF2 of the Wufeng Formation to LM8 of the Lungmachi Formation are continuously fossiliferous in the Shizishan section with a covered interval in LM4. The thickness of WF2–WF4 of the Wufeng Formation is 7.6 m of uniform black shale. This interval and the lower Lungmachi Formation (Rhuddanian) are potential candidates for shale gas exploration. The top of the Lungmachi Formation black shale reaches the lower part of LM8 (lower Stimulograptus sedgwickii Biozone). The lithology becomes mudstone or siltstone above LM8. The Shizishan section may serve as a reference section for the

Hirnantian Kuanyinchiao 0.15 6.05

2.0 1.2 0.5

7.05

Mudstone

7.05 8.15

10.05

12.05

Rhuddanian

13.05

Black shale

Hirsutograptus sinitzini

Parakidograptus acuminatus

Paraclimacograptus innotatus

Korenograptus lungmaensis

Atavograptus antiquatus

Metabolograptus bicaudatus

Normalogr aptus trifilis

Akidograptus ascensus

Normalograptus ajjeri

Lungmachi

Llandovery

Silurian

26.3

?

14.05

Cystograptus vesiculosus

Thickness (m)

Formation

Stage

Series

System

Lithology

Normalograptus mirnyensis

Korenograptus guantangyuanensis

Metabolograptus ojsuensis Normalograptus angustus

Daduhe

Upper Ordovician

Ordovician

34 X. Chen et al.

Ranges of important graptolites Biostratigraphy

Covered

Age(Ma)

LM4 442.4

LM3

443.4

LM2

LM1 443.83 445.16

Hirnantia Fauna

Siltstone

Fig. 3.15 Stratigraphic column of the Daduhe and Lungmachi formations with a few graptolite ranges at Wanhe, Yongshan, Yunnan (after Tang et al. 2017, with additional data from the present authors)

Ordovician to Silurian Shale Gas-Bearing Strata from the Yangtze Region

35

GR (API) 0

500 0

Well N203 TOC (%) 10

Depth (m)

Mineral composition

Stage

Lithology

Graptolite zones

Depth (m)

Stage

Well YJ1

LM7

Graptolite zones

3

GR (API)

Lithology 50

300

Mineral composition

LM7 2310

1480

Aeronian

2320 1470

2330

LM6

Aeronian

1460

LM6

2340 1490

2350 1500

LM5

1530

Rhuddanian

1520

LM5 2370 LM4

TOC> 2%

2380

LM1— LM3

1540

Katian

Kuanyinchiao

2390

Rhuddanian

2360 1510

LM4 LM3 LM1— LM2 Kuanyinchiao

Katian WF2 —WF4

WF3

2400

1550

Mineral composition

Siliceous mineral

Carbonate mineral

Clay mineral

Fig. 3.16 Stratigraphic logs of the Wufeng and Lungmachi formations in the Yanjin (Well YJ1) and Changning (Well N203) boreholes (after Fig. 4 of Liang et al. 2017)

Wufeng and Lungmachi formations in South Sichuan (Figs. 3.17, 3.18).

13. Stratigraphic Log of the Wufeng and Lungmachi Formations in the Well N211, Renyi, Gaoxian, Sichuan The Well N211 is located on the south limb of the Changning anticline. The Wufeng Formation comprises 5.87 m of black shale. Metabolograptus extraordinarius occurs in the lower Kuanyinchiao Bed and Avitograptus avitus at the base of the Lungmachi Formation (LM1). Zonal fossils from LM2 to LM4 (Rhuddanian) are all present in the drill core, in a 35 m thick interval. Above LM5, the shale becomes dark gray with siltstone intercalations and contains a lower diversity

graptolite fauna. The highest horizon in the borehole belongs to LM8, the Stimulograptus sedgwickii Biozone (Fig. 3.19). The graptolite succession agrees with that of the surface outcrops in Changning County, e.g., the Shizishan section.

14. Stratigraphic Log of the Wufeng and Lungmachi Formations in the Well WD1, Weiyuan, Sichuan In the Well WD1, WF2–WF4 consist of black shale with a thickness of 8 m. However, the complete sequence of Lungmachi Formation black shale from LM1 to LM9 is unfortunately thinner than elsewhere in Yangtze. Thus, the thinner shale gas-bearing beds in the Weiyuan shale gas field will result in a lower productivity potential for the gas field (Fig. 3.20).

Anticostia lata

K.

Black shale

Wufeng 5.7

0.16 0.8 0.7 1.2

6.05 Covered

>0.3

Yellow-gray shale with siltstone

19.6

25.12

Muddy limestone

61.03

Stimulograptus sedgwickii

Pristiograptus xiushanensis

Petalolithus minor

Lituigraptus convolutus

Campograptus communis

Coronograptus gregarius

Rastrites guizhouensis

Demirastrites triangulatus

Pseudorthograptus mutabilis

Petalolithus praecursor

Coronograptus cyphus

Coronograptus annellus

Hirsutograptus sinitzini

Parakidograptus acuminatus

Metabolograptus bicaudatus

Lungmachi >73.85 Spirograptus andrewsi

Aeronian

Llandovery

Silurian

Thickness (m)

Formation

Stage

Series

System

Lithology

Korenograptus longus

Rhuddanian

Covered

Normalograptus wangjiawanensis

Metabolograptus ojsuensis

Metabolograptus extraordinarius

Paraorthograptus pacificus

Dicellogr. complexus

Hir.

Katian

Upper Ordovician

Ordvician

36 X. Chen et al.

Ranges of important graptolites Biostratigraphy Age (Ma)

LM8

438.76

LM7

439.21

LM6

440.77

LM5

441.57

LM3

443.40

LM2

443.83 LM1 444.43 Hirnantia Fauna WF4 445.16 WF3 447.02 WF2

447.62

Mudstone

Fig. 3.17 Stratigraphic column from the Wufeng to Lungmachi Formation in the Shizishan section, Shuanghe Changning, Sichuan (measured by Fan Junxuan and Chen Qing et al., graptolites identified by Chen Xu)

3

Ordovician to Silurian Shale Gas-Bearing Strata from the Yangtze Region

37

Fig. 3.18 Wufeng Formation to the base of the Lungmachi Formation (where people are standing) in the Shizishan section, Shuanghe, Changning, Sichuan

15. Stratigraphic Log of the Wufeng and Lungmachi Formations in the Well WD2, Weiyuan, Sichuan

16. Stratigraphic Log of the Lungmachi Formation in the Well WD4, Weiyuan, Sichuan

The Well WD2 has a similar stratigraphic log as WD1. The log spans the Lungmachi Formation (LM1–LM9, 33 m), the Kuanyinchiao Bed and the Wufeng Formation (WF2–WF3, 5 m) (Fig. 3.20). The thinner black shale units with a lower diversity of graptolite species in this borehole implies that the locality was near the Central Sichuan Old Land during the Early Paleozoic. However, nearshore deposits have not yet been found in the boreholes of the Weiyuan area. Coarser clastics first occur in LM9. This indicates that the Weiyuan–Changning shale gas field is situated in a stable but nearshore environment. The black shales of the Wufeng and Lungmachi formations were deposited in a stable and stagnant environment with a low depositional rate.

The top of the Lungmachi Formation black shale may correlate with LM8 and is then overlain by non-black shale (Fig. 3.21).

17. Stratigraphic Log of the Lungmachi Formation in the Well WD3, Weiyuan, Sichuan A precise correlation between the wells WD3 and WD4 was demonstrated by Liang et al. (2017), which is reproduced herein as Fig. 3.21. Liang et al. (2017) selected the Well WD2 as the reference standard for their correlation. The spikes of gamma-ray curves (HSGR/API) within these three

Formation

Linhsiang

0

0

% 10

GR

Depth Lithology (m)

API 500

2340

C

C

Kuanyinchiao

C

Wufeng

C

2360

C

C

C

C

2200

LM8

LM6

2300

2320

LM5

?

C

C

C

LM4

LM3

Stimulograptus sedgwickii (Portlock)

Graptolite zone

Quartz+Feldspar 0 (%) 100 Clay 0 (%) 100 Carbonate 0 (%) 100 Pyrite 0 (%) 100

2220

LM7

2240 C

C C

C

2260 C

Rastrites sp. Petalolithus minor (Elles) Monograptus sp. Lituigraptus convolutus (Hisinger)

TOC

LM2 LM1 Hirnantia Fauna WF4 WF3 WF1-WF2

Paraorthograptus pacificus (Ruedemann) Dicellograptus ornatus (Elles et Wood) Metabolograptus extraordinarius (Sobolevskaya) Metabolograptus ojsuensis (Koren’ et Mikhailova) Korenograptus sp. Avitograptus avitus (Davies) Korenograptus bicaudatus (Chen et Lin) Korenograptus anjiensis (Yang) Akidograptus ascensus (Davies) Parakidograptus acuminatus (Nicholson) Cystograptus sp. Cystograptus penna (Hopkinson) Paraclimacograptus innotatus (Nicholson) Demirastrites sp.

Lungmachi

38 X. Chen et al.

2280

Shale

Sandstone

Carbon shale

Legend

Calcareoussandy mudstone

Siltstone

Silty mudstone

Muddy limestone

C

C

Silty shale

Muddy siltstone

Fig. 3.19 Stratigraphic log of the Wufeng and Lungmachi formations in the Well N211 at Renyi, Gaoxian, Sichuan (after Fig. 3 of Luo et al. 2017)

Stage

Formation

Telychian

Series

Ordovician to Silurian Shale Gas-Bearing Strata from the Yangtze Region

System

3

Lithology

39

Depth(m)

Ranges of important graptolites Biostratigraphy Age (Ma)

2539.15 2540

Spirograptus guerichi Loydell et al.

2541.63

Glyptograptus nanjiangensis Chen

LM9 438.49

LM8 2546.79

Llandovery

439.21

LM6

2558.58 2560

Campograptus cf.communis (Hisinger)

2561.60

Rastrites guizhouensis Chen et Lin

2565.60

Coronograptus gregarius (Lapworth)

2567.34

Monograptus sp. Coronograptus gregarius (Lapworth)

2569.54 2570 2571.40 2571.79

Pseudorthograptus sp. Korenograptus laciniosus (Churkin et Carter) Avitograptus ex gr.avitus (Davies)

2573.10

Mucronaspis (Songxites) sp.

2574.52

Appendispinograptus leptothecalis (Mu et Geh)

Wufeng Katian

Upper Ordovician

Lituigraptus convolutus (Hisinger) Cephalograptus cometa (Geinitz)

Lungmachi

Rhuddanian

Silurian

Aeronian

LM7 2550 2551.08

Hir. Kuanyinchiao

Ordovician

438.76

Monograptus priodon (Bronn)

440.77

LM5

441.57 LM1–LM4 444.43

WF2–WF3

Paraplegmatograptus sp. 2578.06

Appendispinograptus supernus (Elles et Wood)

447.62

2580 Linhsiang

Shale

Limestone

Fig. 3.20 Stratigraphic log of the Wufeng and Lungmachi Formations in the Well WD2 at Weiyuan, Sichuan (after Fig. 3 of Wang et al. 2015)

Depth(m)

C

C

C C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

Lithology

Shale

1 100

HSGR /gAPI

Graptolite zones

C

C C

C

Pagoda

WF2–WF4?

Hirnantia Fauna

LM1–LM4

LM5

LM6

LM7

LM8

LM9

Mineral composition Carbonaceous shale

TOC> 2%

C C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

Limestone

/gAPI 1 100

HSGR

C C

Lithology C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C C

C

C

C

Graptolite zones WF2–WF4?

LM1? – LM3?

LM4

LM5–LM6

LM7

LM8

LM9

Mineral composition

Siliceous mineral

1 100

HSGR /gAPI

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

Carbonate mineral

Mineral composition Clay mineral

Pagoda

Hirnantia Fauna WF2 –WF4?

LM1–LM3

LM4 –LM5

LM6

LM7

LM8

LM9

Mineral composition

Fig. 3.21 Black shales of the Wufeng and Lungmachi formations in the Well WD4 at Weiyuan, Sichuan (after Fig. 4 of Liang et al. 2017)

2585 2580 2575 2570 2565 2560 2555 2550 2545 2540 2535 2530

Stage

Telychian

Aeronian

Rhuddanian

Katian

Depth(m) 3540 3535 3530 3525 3520 3515 3510 3505 3500 3495 3490 3485 3480

Depth(m) 3185 3180 3175 3170 3165 3160 3155 3150 3145 3140 3135

WD3 Lithology

WD4

Samples uncollected

Graptolite zones

WD2

40 X. Chen et al.

3

Ordovician to Silurian Shale Gas-Bearing Strata from the Yangtze Region

wells are somewhat different (Fig. 3.21). In the Well WD2 three spikes occur at the base of the Lungmachi Formation, between LM4 and LM5, and in upper LM5, respectively. Only one spike, near the base of the Lungmachi Formation occurs in the wells WD4 and WD3 (Fig. 3.21).

41

21. “Lungmachi Formation” in the Donggongsi Section, Zunyi, Guizhou

Liang et al. (2017) established a correlation of the Wufeng and Lungmachi formations among the wells WY1, WD5 and WD6 (Fig. 3.22). Among these drill cores, the data from the Well WY1 is the sparsest, with only a few records from WF2–WF3 and LM4.

The “Lungmachi Formation” at Donggongsi is composed of non-black shale belonging to LM5 (Coronograptus cyphus Biozone), and occurs above a disconformity that spans all the zones down to the underlying Kuanyinchiao Bed (Fig. 3.23). It indicates the maximum global sea level rise after the latest Ordovician mass extinction event. On the Yangtze Platform the LM5 deposits occurring along the marginal belt consist of silty shale and siltstone and have a low diversity graptolite fauna. Chen et al. (2017) explained this sea level rise and transgression on the Yangtze Platform as a stage-progressive distribution pattern.

19. Stratigraphic Log of the Lower Lungmachi Formation in the Well WD5

22. Wufeng and Lungmachi Formations in the Honghuayuan Section, Tongzi, Guizhou

Normalograptus mirnyensis (Obut and Sobolevskaya) occurs at the base of Lungmachi Formation in the Well WD5. This taxon ranges from LM1 to LM3, but is particularly abundant in LM2. The late Katian Pagoda Formation (or Linhsiang Formation) unconformably underlies the Lungmachi Formation. Thus, both of the Wufeng Formation and the Kuanyinchiao Bed are absent at this locality (Fig. 3.22). Liang et al. (2017) explained this stratigraphic break as the result of uplift between the wells WY1 and WD6, or more broadly between the Weiyuan and Changning counties. This uplift was called the Neijiang–Zigong uplift event (Liang et al. 2017).

The Wufeng Formation in the Honghuayuan section contains 4 graptolite biozones, WF1–WF4 (Figs. 3.24, 3.25). The Kuanyinchiao Bed yields the graptolite index species Metabolograptus extraordinarius and M. ojsuensis, in addition to trilobite, brachiopod and coral faunas (Fig. 3.26). However, the base of the Lungmachi Formation is absent, with LM1 (M. persculptus Biozone) to LM3 (P. acuminatus Biozone) missing. Rong et al. (2011) suggested there was a short northward expansion of the Central Guizhou Old Land, which occupied the Tongzi area during this short interval.

18. Stratigraphic Log of Wufeng and Lungmachi Formations in the Well WY1, Weiyuan, Sichuan

20. Stratigraphic Log of the Wufeng and Lungmachi Formations in the Well WD6, Weiyuan, Sichuan The Wufeng and Lungmachi formations are deposited without a break. Unfortunately, the records are not complete in the lithologic log (Fig. 3.22).

23. Wufeng and Lungmachi Formations in the Hanjiadian Section, Tongzi, Guizhou The Wufeng Formation, the Kuanyinchiao Bed and the Lungmachi Formation are conformably deposited in the Hanjiadian section, which is also the type location of the Hanjiadian Formation. The top of the Lungmachi Formation

Si

C

Lithology

C

C

C

C

Graptolite zones

Suggested boundary

Clay mineral

Carbonate mineral

Siliceous mineral

WF2–WF3

LM4

Shale C

C

C

C

/gAPI 1 100

HSGR

C

C

C

C

C C C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

Ka.

R.

WF 2?–WF?

LM4–LM6 LM ? –LM3?

LM7 – LM9?

Graptolite zones

Carbonaceous shale

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

Lithology

WD5 Mineral composition

Limestone

TOC> 2%

/gAPI 1 100

HSGR

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

WD6

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

Fig. 3.22 A correlation of the Wufeng and Lungmachi formations among the wells WY1, WD5 and WD6 (after Fig. 5 of Liang et al. 2017). Abbreviations: W., Wufeng; L., Linhsiang; P., Pagoda

L.–P.

W.

Depth (m)

3585 3588

C

C

C

Depth (m) 3710 3705 3700 3695 3690 3685 3680 3675 3670 3665 3660

Formation

Lungmachi

Stage Telychian Aeronian

Depth (m) 3800 3795 3790 3785 3780 3775 3770 3765 3760 3755 3750 3745

Lithology

WY1 Graptolite zones LM4 – LM5 LM ?– LM3? WF2 – WF4?

LM6 – LM8

LM9

42 X. Chen et al.

Mineral composition

Lungmachi Chihsia Formation

Thickness (m)

Biostratigraphy Age (Ma)

P1

2.65

2

Limestone

Hirnantia Fauna WF3–WF4 447.02 WF2 447.62

Pararetieograptus sinensis

Anticostia lata

1

Coronograptus tenellus

LM5

Dicellogr. complexus

Rhuddanian

Black shale

Ranges of important graptolites

Lithology

0.7 Ch.

43

Coronograptus cyphus

Wufeng

0.2

Stage

Hir. K.

Katian

Series Llandovery Lower Per.

Ordovician

Upper Ordovician

System Per.

Ordovician to Silurian Shale Gas-Bearing Strata from the Yangtze Region

Silurian

3

Mudstone

Dark-gray silty shale

Silty shale

Fig. 3.23 Stratigraphic column of the Wufeng and “Lungmachi” formations in the Donggongsi section, Zunyi, Guizhou (after Zhang et al. 1964). Abbreviation: Per., Permian

black shale is within LM6 (Demirastrites triangulatus Biozone) (Figs. 3.27, 3.28). Above the upper part of LM6, the Lungmachi Formation black shale is replaced by the shallow water Shiniulan limestone, which yields a shelly fauna (Chen et al. 2017).

section may extend to the top of LM6. The strata in LM7 are not typical black shale although a few graptolites are still present (Figs. 3.29, 3.30). Chen et al. (2017) concluded that the belt from Qijiang to Huayingshan has the greatest potential for shale gas productivity.

24. Wufeng Formation, Kuanyinchiao Bed and Lungmachi Formation in the Guanyinqiao Section, Qijiang, Chongqing

25. Stratigraphic Log of the Wufeng and Lungmachi Formations in Qilongcun borehole, Qijiang, Chongqing

As in the Hanjiadian section, these three formations are conformable in the Guanyinqiao section. The Guanyinqiao section is located on the opposite (north) limb of a big anticline from the Hanjiadian section (south limb). However, the top of the Lungmachi Formation black shale in this

The Qilongcun borehole is located about 10 km south of the Guanyinqiao section. The strata spanning the latest Ordovician to early Silurian in the Qilongcun borehole is similar to those units at the Guanyinqiao section (Fig. 3.31).

Thickness (m)

Ranges of important graptolites

Hirnantia Fauna

b

Black shale

Diceratogr. mirus

Metabologr. extraordinarius

Dicellogr. anceps

Tangyagr. typicus

Dicellogr. tenuiculus

Dicellogr. complexus

Dicellogr. complanatus

b

Appendispinogr. longispinus

1.1

Appendispinogr. venustus

Wufeng 3.0

1m

441.57 LM4 442.47

Hirnantia Fauna

WF4

4.3

L.

Atavogr. atavus

5.51

Cystogr. vesiculosus

b b

Metabologr. bicaudatus

1.3

Coronogr. tenellus

LM5

b b

Katian

Biostratigraphy Age (Ma)

1.07

Normalogr. madernii

Stage Rhuddanian

Lungmachi Formation Kuanyinchiao

Series Llandovery

Lithology

1.6

Upper Ordovician

Ordovician

Hirnantian

System

X. Chen et al.

Silurian

44

Yellow-gray mudstone with limestone lens

445.16

WF3

447.02

WF2 447.62 WF1

Dark-gray shale and silty shale

Fig. 3.24 Stratigraphic column of the Wufeng and Lungmachi formations at Honghuayuan, Tongzi, Guizhou (after Chen et al. 2000). *The LM6 graptolite Demirastrites cf. triangulatus occurs at 12.8 m above the base of the Lungmachi Formation; b represents K-bentonites

D. complanatus

Linhsiang D. complexus Zone AFA275

Hirnantia Bed

Shelly Bed

C. A. vesicu. atavus

Lungmachi Fm.

Rhuddanian

?Llandovery (Silurian)

AFA319

AFA318

AFA317 AFA316 AFA315 AFA314 AFA313

AFA311a AFA310

AFA312

AFA309

AFA308

AFA307 AFA306 AFA305 AFA304 AFA302 AFA301 AFA300

AFA299

AFA298

AFA297 AFA296 AFA295

AFA294AFA292

AFA289 AFA288 AFA287 AFA286 AFA290 AFA291

D. mirus

AFA283 AFA284

AFA282

AFA280 AFA281

AFA279

AFA278

AFA277

AFA276

Legend

AFA274

AFA273

AFA272 AFA271

AFA270

AFA269

AFA268

AFA267

100 cm

50

0

1 2 5 6 3 4 Amplexogr. latus Appendispinogr. longispinus Leptograptus sp. Dalmanitina sp. Rectograptus socialis Leonaspis sp. Ontarion sp. Appendispinogr. hvalross Trilobites Climacograptus sp. ex gr. C. gmaeus Lower Dicellogr. complanatus T. typicus Subz. Plegmatograptus sp. Triplesia sp. ? Dalmanella Subzone Rectogr. intermedius testudinaria Dicellograptus complexus ? Paromalomena polonica Pararetiograptus sp. atrypid Anticostia uniformis Hindella crassa Dicellograptus ornatus incipiens Cliftonia? sp. nov. Dicellograptus tenuiculus Eostropheodonta parvicostellata Paraplegmatograptus connectus Mirorthis mira Rectograptus uniformis Hirnantia sagittifera Appendispinograptus supernus Orbiculoidea sp. Kinnella? sp. Normalograptus sp. ? Parareteograptus magnus Fardenia sp. Leptaena trifidum Anticostia fastigata strophomenoid ? Paraorthograptus sp. Eospirifer sp. Dicellograptus complanatus arkansasensis Plectothyrella sp. Dicellograptus sp. aff. D. complanatus Brachiopods Nalivkinia sp. Parareteograptus sinensis Brevilamnulella? sp. Rectograptus abbreviatus Hyattidina? sp. Yinograptus disjunctus Palaeoglossa? sp. Pseudopholidops sp. Climacograptus hastatus Trucizetina? sp. Manosia yichangensis Dicellograptus graciliramosus Dicellograptus minor rhynchonelloid Leptograptus annectans amplectens Rectograptus songtaoensis Appendispinograptus venustus Dicellograptus tumidus Leptograptus extremus Leptograptus macer Leptograptus planus Orthograptus maximus Paraorthograptus pacificus Paraplegmatograptus uniformis Pleurograptus lui Appendispinograptus leptothecalis Dicellograptus anceps Paraorthograptus brevispinus Kassinella sp. Tangyagraptus remotus Tangyagraptus typicus Brachiopods Appendispinograptus supernus sinicus Climacograptus tatianae Diceratograptus mirus Normalograptus normalis Rectograptus obesus Normalograptus mirnyensis Metabolograptus extraordinarius Metabolograptus ojsuensis Paraplegmatograptus sp. Normalograptus angustus Normalograptus laciniosus Normalograptus? lungmaensis Normalograptus madernii Atavograptus gracilis Cystograptus vesiculosus Dimorphograptus malayensis Graptolites Dimorphograptus sp. Metaclimacograptus robustus Neodiplograptus bicaudatus Normalograptus premedius Normalograptus rectangularis Normalograptus tortithecatus Paraclimacograptus innotatus Sudburigraptus? sp. nov. Sudburigraptus? lingulatus Metaclimacograptus hughesi Atavograptus atavus Monograptus sp. Atavograptus strachani Coronograptus tenellus Monoclimacis? lunata Normalograptus sp. aff. N. angustus Normalograptus ugurensis Monograptus sp. aff. M. intermedius Normalograptus minutus Glyptograptus sp. aff. G. incertus Glyptograptus tamariscus linearis

Cover 2m

P. pacificus Zone

Kuanyinchiao Bed

Hirnantian M. extraordinariusM. ojsuensis

Upper Ordovician

Wufeng Formation

Katian

3 Ordovician to Silurian Shale Gas-Bearing Strata from the Yangtze Region

26. Stratigraphic Log of the Wufeng–Lungmachi Formations in the Well JY1, Jiaoshiba, Fuling, Chongqing

The Well JY1 was determined by Guo and Liu (2013) to be the most productive shale gas borehole in the Yangtze region. Thus, the Wufeng–Lungmachi formations in the Well JY1 log need to be precisely correlated with the other nearby boreholes. We were first invited to provide a graptolite biozonation table for the core from this well in 2014 (Chen et al. 2015) (Figs. 3.32, 3.33, 3.34). Chen et al. (2018b) indicated that the TOC and gamma-ray curves in the Well JY1 represent a stable high value curve from the

45

Subzone

1

2

AFA285, K-bentonite

Dark-brown mudstone

3

AFA289a

Black shale, siliceous shale and silty shale

4

AFA303

Argillaceous limestone concretion

5

AFA311b

AFA268a, K-bentonite

6

AFA311c

Fig. 3.25 Stratigraphic column of the Wufeng and Lungmachi formations at Honghuayuan, Tongzi, Guizhou (after Chen et al. 2000). Abbreviations: vesicu., vesiculosus; Subz., Subzone

Rhuddanian to the Aeronian. The curves are different from values measured from the Well N211 and the North Africa Hot Shale boreholes which have high TOC and gamma-ray values only in the lower Rhuddanian (Lüning et al. 2005).

27. Wufeng–Lungmachi Formations at Sanbaiti (Three Hundred Steps), Huayingshan, Sichuan

The Sanbaiti section (Yanwanggou section) was firstly measured by Lu Yanhao. His graptolite collections were studied by Mu Enzhi who erected two graptolite zones, Pleurograptus lui (lower) and Dicellograptus szechuanensis

46

X. Chen et al.

Fig. 3.26 Base of the Kuanyinchiao Bed at Honghuayuan, Tongzi, Guizhou

(upper) based on Lu’s collections (Mu 1950). The latter zone was revised and renamed the Dicellograptus complexus Biozone (WF2) by Chen et al. (2000) (Fig. 3.35). The Wufeng Formation, the Kuanyinchiao Bed and the Lungmachi Formation are conformable in this section. The top of the Lungmachi Formation black shale may correlate with LM9, indicating the youngest exposure of Llandovery black shale in the Central Yangtze region. Chen et al. (2017) considered that the Qijiang–

Huayingshan belt had the greatest potential for shale gas productivity. 28. Wufeng and Lungmachi Formations Lujiaozhen Section, Pengshui, Chongqing

in

the

The Kuanyinchiao Bed and the LM1 to LM2 interval of the Lungmachi Formation are absent in the Lujiaozhen section (Fig. 3.36).

T hickne ss ( m)

Formation

Sta ge

Series

Ordovician to Silurian Shale Gas-Bearing Strata from the Yangtze Region

S yste m

3

Lithology

47

Ranges of important graptolites

Biostratigraphy Age (Ma)

LM6

Muddy limestone

Black shale and gray shale

Campog r. communi s

Rhaphidograptus minutus

Cyst og r. vesiculosus

Normalo g r.

Dicellogr. magnu s

Dicellogr. turgidus

Ch.

Appendispinogr. supernus

3.75

Tangyagraptus

3.25

Dicellogr. ornat u s

W u f en g

K.

Katia n

U pper O r dovicia n

Ordovicia n

H.

1.3 0.5

Korenogr. b icaudat u s

13

Demirast rit es sp.

Hu t t ag r. l unat a

R h u d d a ni a n

31

Rastr ites guizhouensi s

114

Lungmachi

Llandovery

Siluria n

Aeronian

SN.

Siliceous blac k shale

440.77

LM5 441.57

LM4 442.47

LM1–LM3 444.43 Hirnantia Fauna WF3–WF4 447.02

WF2 447.62

Calcareous mudstone

Fig. 3.27 Stratigraphic column of the Wufeng and Lungmachi formations at Hanjiadian, Tongzi, Guizhou (after Zhang et al. 1964; Chen and Lin 1978). Abbreviation: SN., Shiniulan

48

X. Chen et al.

Fig. 3.28 Lower part of the Lungmachi Formation at Hanjiadian, Tongzi, Guizhou

29. Wufeng–Lungmachi Formations in the Ludiping Section, Songtao, Guizhou The Ludiping section includes the Wufeng Formation, the Kuanyinchiao Bed and the basal Lungmachi Formation. The top of the Lungmachi Formation in the section yields LM3 graptolites. The Wufeng and Lungmachi formations are thicker at Ludiping than they are in the central part of the Yangtze region (Fig. 3.37). 30. Wufeng–Lungmachi Formations in the Kelisha Section, Qingpingzhen, Yongshun, Hunan This section was recently measured by Wang Wenhui et al. (Fig. 3.38). 31. Stratigraphic Log through Wufeng–Lungmachi Formations in the Well LD1, Lianghekou, Laifeng, Hubei The Well LD1 is within the Yichang Uplift region. LM2 (Akidograptus ascensus Biozone) sits directly and

unconformably on WF3 of the Wufeng Formation. Thus, the Hirnantian Stage is completely absent (Chen et al. 2018b). The strata younger than LM3 are all non-black shale (Fig. 3.39).

3.2

Middle Yangtze: Three Gorges Region

32. Lungmachi Formation in the Longping (Ehongdi) Borehole at Longping, Jianshi, Hubei The Longping borehole is also in the Yichang Uplift region. The Lungmachi Formation black shale is only 10 m thick with LM4 (Cystograptus vesiculosus Biozone) at its base. It unconformably overlies WF4 (Metabolograptus extraordinarius Biozone). Hence, the Kuanyinchiao Bed and the base of the Lungmachi Formation (LM1–LM3) are all absent. The interval with the greatest shale gas potential in the base of the Lungmachi Formation black shale is missing (Fig. 3.40).

Thickness (m)

Formation

Lithology

49

Ranges of important graptolites

Biostratigraphy

Age (Ma)

Hubeigraptus arcuata

Qiaogou

Stage

Series

Ordovician to Silurian Shale Gas-Bearing Strata from the Yangtze Region

System

3

0.40

Campograptus communis

Coronograptus cyphus

Cystograptus vesiculosus

1.49

Black shale

Gray calcareous shale

Demirastrites triangulatus

WF K.

Hir. K.

U. Ord.

Ordovician

1.60

Coronograptus leei

1.20

Parakidograptus acuminatus

21.0

Paraorthograptus pacificus

119.40

Tangyagraptus typicus

Rhuddanian

Lungmachi

Llandovery

Silurian

Aeronian

17.90

LM7?

439.21

LM6

440.77 LM5

441.57 LM4

442.47 LM2–LM3 ? Hirnantia Fauna ? WF3 ? Argillaceous limestone

Fig. 3.29 Stratigraphic column of the Wufeng–Lungmachi formations in the Guanyinqiao section, Qijiang, Chongqing (after Jin et al. 1982)

50

X. Chen et al.

Fig. 3.30 Wufeng Formation to Lungmachi Formation at Guanyinqiao, Qijiang, Chongqing. The person on the left is at the level of the Kuanyinchiao Bed

33. Wufeng–Lungmachi Formations in the Fenxiang Section, Fenxiang, Yichang, Hubei The Wufeng–Lungmachi formations are all conformable in the Fenxiang section (Fig. 3.41). The section was densely sampled (Chen et al. 2000).

fossils from multiple groups. Wang et al. (1987) measured the section from Wangjiawan to Dazhongba through the Ordovician and Silurian (Fig. 3.42). Later, Chen et al. (2006) published their Ordovician–Silurian boundary section (Figs. 3.43, 3.44) and proposed the Wangjiawan North section as the Stratotype Section for the Hirnantian Stage.

34. Wufeng–Lungmachi Formations in the Wangjiawan Section, Yichang, Hubei

35. Llandovery Strata at Zhangwan, Xichuan, Henan

The Wangjiawan section has long been the Ordovician and Silurian boundary reference section in China. The section lacks any obvious unconformity and also yields abundant

As the mentioned above, the Llandovery age yellow-gray shale yields graptolites of LM6. This lithology indicates the north marginal belt of the Yangtze Platform.

Lithology

Upper Lungmachi

250

C

C

C

C

C

C

150

C

C

C

C

C

QL-26

Argillaceous limestone, calcareous shale

QL-25

Black shale

QL-24

Black carbonaceous shale

QL-23

Mudstone Carbon mudstone Calcareous shale

QL-22 QL-21 QL-20

C

C

C

C

(S 1 +S 2 )/(mg/g)

0.6

0

T/C

1000

C

C

C

C

C C

C Ca

Si

Ca Si

Si Si

Si

Si

Si

Si

Si

Si

Si

Si

Si

C

C

C

QL-16

Sandy shale

QL-15

Shale

QL-14

Carbonaceous siltstone

QL-13 QL-12

Carbonaceous siltstone

C

C C

C

QL-17

Argillaceous siltstone

C

C C

C

QL-18

Silty siltstone

Carbonaceous, argillaceous siltstone

C

C

C

QL-19

Si

C

C

Siliceous shale Silty fine sandstone

Si

Si

Si

C C

C

Si

C

C C

C

Si

Si

Si

C

C

Si

Si

C

C

C

Ca

Si

Black calcareous, argillaceous siltstone

C

Black shale Black shale Silty shale C

C

C

e

Kuanyinchiao

C

C

C

0

C

C

C

C C

e

e

C

C

C

QL-06

Black shale

QL-05

C

C C

C

Carbonaceous shale Biotic shell limestone

e

C C

C

QL-11 QL-10 QL-09 QL-08 QL-07

C

C C

e

C C

C

e

e

e C C C

C C

e

C

Wufeng

C

C

C

C

C

C

C

C

Upper Ordovician

0

C

C

Si Si

C

C

C

C

Ca

Pagoda

7

C

C

C

Si

Chientsaokou

TOC(%)

C

C

C

C

C

Lower Lungmachi

C

C

C

C

C

C

C

C

C C

C

C

C C

C

C

50

C

C

C C C

100

C

C C

C

Si

C C

C

Lungmachi

Silty shale

0

51

200

C

Lower Sulurian

Lithology description

Sample

Depth (m)

Formation

Ordovician to Silurian Shale Gas-Bearing Strata from the Yangtze Region

Series

3

C

C

C

C

C

C

Black carbonaceous shale

QL-04 QL-03 QL-02

Gravel chalky clay Breccioid limestone

QL-01

Fig. 3.31 Stratigraphic log from the top of the Pagoda Formation to the lower part of the Lungmachi Formation in the Qilongcun borehole (after SINOPEC internal report)

Formation

52

X. Chen et al.

Hydrocarbon

GR/API Depth 0

200

Lithology

(m)

(%) 0

5

Quartz (%)

Gas

TOC(%) 0

(m 3 /t)

5 0

6

0

100

Clay (%) 0

Lithology description

Graptolite biozones

100

Muddy siltstone

Mudstone and shale

2330

2340 C

C

C C

Wufeng–Lungmachi

triangulatus

C

2350 C

C C C

2360

C C

Silty mudstone

C

2370

2380 C C C C C

Carbonaceous shale

2390

C

2400

C C C C C C

2410

C C

cyphus/leei

vesiculosus ascensus– acuminatus

C C

Limestone

C C

Argillaceous siltstone

Shale

Carbonaceous shale

Nodular limestone

Silty mudstone

Fig. 3.32 Stratigraphic log through Wufeng–Lungmachi formations in the Well JY1 (After Guo and Liu 2013; Chen et al. 2015)

Formation

Stage

Series

Ordovician to Silurian Shale Gas-Bearing Strata from the Yangtze Region

System

3

Lithology

Depth(m)

Ranges of important graptolites

2333.63

Oktavites circularis

53

Biostratigraphy

Age (Ma)

LM8

2344.19

Stimulograptus sedgwickii

438.76

LM7 439.21

Lituigraptus convolutus

Aeronian

2348.22

LM6

Lungmachi

Llandovery

Silurian

Rastrites guizhouensis

2367.43

Demirastrites triangulatus

440.77

Rastrites longispinus

Coronograptus cyphus

LM5

Rhuddanian

Dimorphograptns confertus

2390.58

441.57

Coronograptus cf. cyphus

LM4 Cystograptus vesiculosus

2404.27

Hir. K.

Katian

Upper Ord.

Ordovician

2407.76

WF. L. Gray shale

Cystograptus vesiculosus Korenograptus jerini Akidograptus ascensus Akidograptus sp.

2411.00 2411.10 2411.74

Paraorthograptus pacificus Dicellograptus turgidus

2414.15 2415.20 2415.36

Dicellograptus complexus Appendispinograptus longispinus

442.47

LM2—LM3 443.83

LM1 Hirnantia WF2—WF4

Black shale

Fig. 3.33 Stratigraphic log from the Well JY1, Fuling, Chongqing (investigated by the present authors)

447.62 Limestone

54

X. Chen et al.

Fig. 3.34 A corner of the Fuling shale gas field

Fig. 3.35 Linhsiang and Wufeng formations in the Sanbaiti section, Huayingshan, Sichuan

36. Ordovician–Silurian Section at Daozimiao, Jingshan, Hubei

3.3

The Wufeng Formation (3.5 m thick) contains WF2 and WF3 graptolite faunas. The Hirnantian strata are absent. The base of the Lungmachi Formation yields Korenograptus bicaudatus (Chen and Lin), which indicates LM2. The overlying strata are covered (Figs. 3.45, 3.46).

37. Wufeng and Lishuwo Formations in the Xinkailing Section, Wuning, Jiangxi

Lower Yangtze Platform and Chiangnan Transitional Belt

The stratigraphy of the Wufeng and Lishuwo formations in the Xinkailing section was first published by Yu et al. (1976) (Fig. 3.47). The region from Wuning of Jiangxi to the

Lithology

55

Biostratigraphy Age (Ma)

Ranges of important graptolites

Atavograptus sp.

Thickness (m)

Formation

Stage

Series

Ordovician to Silurian Shale Gas-Bearing Strata from the Yangtze Region

System

3

0.6

0.3

Covered

Black shale

Parakidograptus acuminatus

LM3

443.40?

Anticostia lata

Wufeng

Katian

Upper Ord.

Ordovician

0.9

Avitograptus avitus

Normalograptus wangjiawanensis

0.6

Neodiplograptus modestus

Lungmachi

Rhuddanian

Llandovery

Silurian

0.01– 0.02

Normalograptus rectangularis

0.25

WF2–WF3

Clay

Fig. 3.36 Wufeng and Lungmachi formations in the Lujiaozhen section, Pengshui, Chongqing (after Appendix of Chen et al. 2018a, b)

D. complanatus D. complexus Zone P. pacificus Zone

Wufeng Formation

Katian

Upper Ordovician

M. Hirnatia Fauna persculptus

Kuanyinchiao Bed

Hirnantian

A. ascensus Zone

Lungmachi Formation

Rhuddanian

Llandovery (Silurian)

AFA429

AFA428 AFA427

AFA426

AFA425

AFA394 AFA393 AFA392 AFA391

AFA387

AFA386

AFA385

AFA384

AFA383

AFA382

AFA381

AFA380 AFA379

100 cm

50

0

Brachiopods

AFA424

AFA423

AFA422 AFA421 AFA420 AFA419

AFA416

5 4

AFA415 -414

3

D. mirus Subzone

AFA407

AFA405 AFA406

AFA404

T. typicus Subzone

AFA403

AFA402 AFA401

AFA399 AFA400

Lower Subzone

AFA398

AFA397 AFA396

AFA395

2

Graptolites

AFA389 AFA390

Legend

AFA388

Black shale, siliceous shale and silty shale

1

Amplexogr. latus Appendispinogr. hvalross Dicellogr. complanatus Normalogr. angustus Normalograptus sp. Pseudoclimacograptus chiai Rectograptus socialis Anticostia fastigata Anticostia uniformis Appendispinogr. supernus Dicellogr. tenuiculus Parareteogr. magnus Dicellogr. complanatus arkansasensis Leptograptus extremus Orthograptus maximus Dicellograptus complexus Appendispinograptus? fibratus Dicellogr. sp. aff. D. complanatus Parareteograptus sinensis Rectograptus songtaoensis Appendispinograptus venustus Dicellograptus anceps Dicellograptus ornatus aff. Rectograptus abbreviatus Dicellograptus graciliramosus Paraorthograptus brevispinus Dalmanitina sp. Trilobites Paraorthograptus pacificus Platycoryphe sp. Pleurograptus lui Paraplegmatograptus connectus Climacograptus hastatus Manosia inconstanta Yinograptus disjunctus Dalmanella testudinaria Dicellograptus minor Cliftonia sp. Paraplegmatograptus uniformis Fardenia sp. Rectograptus uniformis Hirnantia sagittifera Dicellograptus tumidus Plectothyrella crassicosta Hindella crassa incipiens Arachniograptus connectus Leptaena trifidum Leptograptus macer Eostropheodonta cf. parvicostellata Leptograptus planus Paromalomena polonica* Tangyagraptus flexilis Kinnella kielanae* Tangyagraptus? grandis Appendispinograptus leptothecalis Paraorthograptus longispinus Diceratograptus mirus Normalograptus normalis Normalograptus laciniosus Sudburigraptus? angustifolius Normalograptus? guizhouensis Akidograptus ascensus Neodiplograptus modestus Neodiplograptus shanchongensis Normalograptus wangjiawanensis Neodiplograptus bicaudatus Normalograptus lubricus Normalograptus ugurensis Normalograptus? sp. aff. N. trifilis Hirsutograptus sinitzini Parakidograptus acuminatus

P. acuminatus

56 X. Chen et al.

AFA430

Dark-brown mudstone

1

AFA386a, K-bentonite

2

AFA390a, K-bentonite

3

AFA408–AFA413

4

AFA417

5

AFA418

*

Collected from Huangfan, Songtao

Fig. 3.37 Stratigraphic column of the Wufeng–Lungmachi Formations in the Ludiping section, Songtao, Guizhou (after Chen et al. 2000)

Thickness (m)

Stage

Formation

Series

Ordovician to Silurian Shale Gas-Bearing Strata from the Yangtze Region

System

3

Lithology

57

Ranges of important graptolites

Biostratigraphy Age (Ma)

Petalolithus minor

Lungmachi

Coronogr. tenellus

Coronogr. annellus

Coronogr. cyphus Appendispinogr. venustus

Wufeng

LM5

LM4 LM3 LM1–LM2 Hirnantia

Cystogr. vesiculosus

0.84 0.3 0.53 0.1

Parakidogr. acuminatus

K.

H.

Rhuddanian

7.08

Covered Katian

Upper Ordovician

LM6

440.77

1.54

Ordovician

Campogr. communis

13.70

Aeronian Llandovery

Silurian

Rastrites guizhouensis

Covered

441.57 442.47 443.40

WF3–WF4

Limestone Black shale

Fig. 3.38 Stratigraphic column of the Wufeng and Lungmachi formations in the Kelisha section, Qingpingzhen, Yongshun, Hunan (after Wang Wenhui et al., in Chen et al. 2018b, appendix)

Formation

X. Chen et al.

Series Stage

System

58

Depth Lithology GR (gAPI) (m)

30

TOC (%)

300 0

Graptolite occurrence

Biostratigraphy Age (Ma)

5

Aeronian Lungmachi

Silurian Llandovery

LM7

900

Paramonoclimacis chengkouensis

905.18

Lituigraptus convolutes

439.21

Rickardsograptus tortithecatus

Katian Rhuddanian Pagoda Linhsiang Wufeng

Upper Ordovician

Ordovician

LM6 Coronograptus gregarius Rastrites peregrinus Demirastrites raitzhainiensis

928.65

Akidograptus sp. Appendispinograptus venustus Dicellograptus ornatus Dicellograptus tumidus Rectograptus socialis

943.08 449.10

950

Limestone

Nodular limestone

Silty shale

440.77 LM2–LM5

443.83 WF2–WF3

447.62

Shale

Fig. 3.39 Stratigraphic log of the Wufeng and Lungmachi formations in the Well LD1, Lianghekou, Laifeng, Hubei (after Appendix of Chen et al. 2018b)

Ningzhen Mountains (Nanjing) of Jiangsu was geologically referred to as the Lower Yangtze Platform. The Wufeng Formation black shale is thicker here than it is in the Upper Yangtze, but the graptolite fauna is similar. Interestingly, the brachiopod fauna in the Xinkailing Bed is characterized by Paromalomena and other taxa that are similar to the Hirnantia fauna in the Kuanyinchiao Bed (Rong and Chen 1987; Rong et al. 2018). The Lishuwo Formation corresponds to the Lungmachi Formation but with thinner black graptolitic shale. The top of the Lishuwo Formation black shale corresponds to part of the Rhuddanian.

38. Wufeng and Kaochiapien Formations in the Sinianpan Section, Hexian, Anhui The Wufeng Formation is thicker here than in the Upper Yangtze sections, but has a short unconformity spanning WF4 to LM2 (Fig. 3.48). The late Ordovician and early Silurian are continuously deposited in the most of the Lower Yangtze region. Even in the Hexian area, no erosional surfaces or weathering crusts between the Ordovician and Silurian have been clearly observed.

Formation

Stage

Series

Ordovician to Silurian Shale Gas-Bearing Strata from the Yangtze Region

System

3

Depth Lithology (m)

GR 0

59

Ranges of important graptolites

Biostratigraphy Age (Ma)

500

1740

LM8

Lungmachi

Aeronian

Llandovery

Silurian

1750

1760

Stimulograptus cf. sedgwickii (Portlock)

Rhuddanian

Pribylograptus incommodus (Törnquist)

1770

Rastrites guizhouensis Chen et Lin Huttagraptus praestrachani (Hutt et Rickards)

Wufeng

Katian–Hirnation

Appendispinograptus supernus (Elles et Wood)

L M 5–LM4

WF4

440.77 442.47 445.16

WF2–WF3

Styracograptus tatianae (Keller)

1780

Rectograptus cf. socialis (Lapworth)

Pagoda– Linhsiang

Upper Ordovician

LM6–LM7

Coronograptus gregarius (Lapworth)

Metabolograptus extraordinarius (Sobolevskaya)

Ordovician

438.76

Monograptus gemmatus (Barrande)

Shale

Limestone

Fig. 3.40 Stratigraphic Log of the Wufeng and Lungmachi formations in the Longping borehole at Longping, Jianshi, Hubei (provided by Zhou Zhi and Tong Chuanchuan, after Appendix of Chen et al. 2018b)

39. Wufeng–Kaochiapien Formations in the Tangshan Section, Nanjing, Jiangsu

40. Wufeng–Kaochiapien Formations in the Ganggangshan Section, Jurong, Jiangsu

Zhang and Jiao (1985) measured the section at Tangshan (Fig. 3.49). The Wufeng Formation at Waigangou near Tangshan is 9.17 m thick with a few graptolites. The Wufeng Formation occurs in several locations near Tangshan but yields graptolites from different biozones (WF2–WF4).

The Wufeng Formation is not well exposed in the Ganggangshan section, but the overlying Kaochiapien Formation is continuous from LM1 to LM6 with a thickness about 40 m. This section is the most complete one in the Ningzhen Mountains (Figs. 3.50, 3.51).

60

X. Chen et al. Upper Ordovician Katian

Hirnantian

Llandovery

?

Rhuddanian

Kuanyinchiao Bed Lungmachi Formation

Wufeng Formation D. complexus Zone

?

M. extraordinariusM. persculptus Zone- P. acumiM. ojsuensis Zone HB A. ascensus Zone natus Zone

P. pacificus Zone

AFA154 AFA153 AFA152 AFA151

AFA150

AFA149 AFA148

AFA146

AFA147

AFA145 AFA144

AFA143

AFA141

AFA142

AFA140 AFA139

AFA138

AFA137 AFA136 AFA135 AFA134

AFA133

AFA132 AFA131 AFA130 AFA129a AFA128a

AFA126a AFA125a AFA124a

AFA120-2

AFA120-1

AFA120-4

AFA120-3

AFA120-5 AFA120-6 AFA120-7 AFA120-8 AFA120-9 AFA120-10

AFA120-11 20 cm

10

0

1

Amplexograptus latus Appendispinogr. supernus Trilobites Climacograptus hastatus Dalmanitina yichangensis Dicellograptus minor Dicellograptus ornatus Leptograptus extremus Paraplegmatogr. connectus Pseudolingulopsis sp. Parareteograptus sinensis Acanthocrania sp. Arachniograptus connectus Onniella yichangensis Dicellograptus complexus Dalmanella testudinaria Dicellogr. sp. aff. D. complanatus Lower Subzone Dysprosorthis sinensis Rectograptus abbreviatus Draborthis caelebs Yinograptus disjunctus Hirnantia sagittifera Appendispinogr. leptothecalis Kinnella kielanae Appendispinogr. supernus sinicus Eostropheodonta parvicostellata Appendispinograptus venustus Paromalomena polonica D. mirus Dicellograptus tenuiculus Leptaena trifidum Subzone Dicellogr. complanatus arkansasensis Triplesia yichangensis Orthograptus maximus Hirdella crassa incipiens Yangzigraptus yangziensis T. typicus Manosia sp. Leptograptus macer Brachiopods Subzone Rostricellula? sp. Pleurograptus lui Rectograptus uniformis Anticostia fastigata Anticostia uniformis Paraorthograptus brevispinus Paraplegmatograptus uniformis Pseudoreteograptus nanus Rectograptus songtaoensis Graptolites Dicellograptus graciliramosus Leptograptus planus Paraorthograptus pacificus Parareteograptus parvus Appendispinograptus? fibratus Tangyagraptus remotus Tangyagraptus typicus Pseudoclimacograptus chiai Normalograptus sp. Paraorthograptus longispinus Sunigraptus regularis Dapsilodus mutatus Yinograptus dubius Conodonts Climacograptus tatianae Normalograptus angustus Metabolograptus normalis Diceratograptus mirus ?Neurograptus sp. Climacograptus tubuliferus Metabolograptus ojsuensis Neodiplograptus charis Normalograptus laciniosus Paraorthograptus uniformis Normalograptus avitus Metabolograptus extraordinarius Normalograptus? lungmaensis Normalograptus mirnyensis Normalograptus parvulus Metabolograptus persculptus Normalograptus wangjiawanensis Neodiplograptus modestus Normalograptus ugurensis Legend Neodiplograptus shanchongensis AFA127a Akidograptus ascensus 1 Neodiplograptus bicaudatus Normalograptus premedius Black shale, siliceous shale and silty shale Normalograptus rectangularis Parakidograptus acuminatus Sudburigraptus? illustris Cystograptus? sp. Dark-brown mudstone Glyptograptus temalaensis Normalograptus jideliensis Normalograptus sp. aff. N. imperfectus Talacastograptus elongatus Hirsutograptus sinitzini Paraclimacograptus innotatus

Fig. 3.41 Stratigraphic column of the Wufeng–Lungmachi formations in the Fenxiang section, Yichang, Hubei (after Chen et al. 2000)

3

Ordovician to Silurian Shale Gas-Bearing Strata from the Yangtze Region

61

Fig. 3.42 Stratigraphic column of the Wufeng–Lungmachi formations in the Wangjiawan–Dazhongba section, Yichang, Hubei (after Wang et al. 1987)

62

X. Chen et al. Llandovery (Silurian)

Upper Ordovician Hirnantian

Lungmachi Formation

20 cm

0 10 ex gr.

D. mirus Subzone

aff. Amplexograptus latus 1 2 Appendispinogr. supernus Appendispinogr. venustus Climacograptus hastatus aff. Dalmanitina yichangensis Dicellograptus minor Leonaspis sp.* Dicellograptus ornatus aff. Platycoryphe sp.* Trilobites Leptograptus extremus Rectograptus abbreviatus Rectograptus socialis Tangyagraptus sp. Paraorthograptus pacificus Appendispinogr.? fibratus Appendispinogr. leptothecalis Dalmanella testudinaria Appendispinogr. supernus sinicus Onniella? yichangensis Dicellogr. sp. aff. D. complanatus Mirorthis mira Dicellograptus tenuiculus Dysprosorthis sinensis Dicellograptus turgidus Draborthis caelebs Leptograptus macer Hirnantia sagittifera Orthograptus maximus aff. Kinnella kielanae Paraplegmatogr. connectus Triplesia yichangensis Paraplegmatogr. uniformis Cliftonia obovata Pararetiograptus sinensis T. typicus Aegiromena convexa Anticostia fastigata Subzone Paromalomena polonica Dicellograptus graciliramosus aff. Leptaena trifidum Rectograptus uniformis Eostropheodonta parvicostellata Tangyagraptus flexilis Plectothyrella crassicosta* Paraplegmatogr. connectus ovalis Hindella crassa incipiens Pleurograptus lui Yinograptus disjunctus Manosia yichangensis Brachiopods Pseudoclimacograptus chiai Tangyagraptus typicus Anticostia uniformis Appendispinograptus hvalross Climacograptus tatianae Paraorthograptus brevispinus Paraorthograptus longispinus Tangyagraptus remotus Amorphognathus ordovicicus Dicellograptus mirabilis Rectograptus songtaoensis Conodonts Diceratograptus mirus Normalograptus normalis Sunigraptus sp. Normalograptus angustus Normalograptus imperfectus Normalograptus laciniosus cf. Paraorthograptus tenuis Appendispinograptus sp. Climacograptus tubuliferus Metabolograptus extraordinarius aff. Normalograptus mirnyensis Metabolograptus ojsuensis Yinograptus? sp. Paraorthograptus uniformis Normalograptus avitus Normalograptus? guizhouensis Normalograptus sp. aff. N. indivisus Metabolograptus persculptus Normalograptus ugurensis Normalograptus rhizinus Normalograptus? lungmaensis Neodiplograptus shanchongensis Sudburigraptus? angustifolius Neodiplograptus charis Neodiplograptus modestus Normalograptus parvulus Normalograptus wangjiawanensis Akidograptus ascensus Glyptograptus sp. nov. Normalograptus madernii Normalograptus? sp. aff. N. trifilis Paraclimacograptus sp. aff. P. innotatus Neodiplograptus bicaudatus Parakidograptus acuminatus Sudburigraptus? illustris Atavograptus primitivus Normalograptus nanjingensis Normalograptus premedius Legend Sudburigraptus? sp.nov. Glyptograptus lungshanensis Black shale, Neodiplograptus diminutus apographon siliceous shale AFA101b 1 Neodiplograptus parajanus and silty shale Normalograptus jideliensis aff. Normalograptus medius Normalograptus sp. nov. 1 Dark-brown Glyptograptus sp. nov. 1 AFA106 2 mudstone Hirsutograptus sinitzini Hirsutograptus villosus Collected from Argillaceous Neodiplograptus sp. nov. Huanghuachang, Paraclimacograptus sp. nov. limestone * Normalograptus rectangularis Yichang concretion Cystograptus vesiculosus Talacastograptus sp.

Fig. 3.43 Wangjiawan North section, Yichang, Hubei (after Chen et al. 2006)

Graptolites

ex gr.

AFA120

AFA119

AFA118

C. vesiculosus Zone AFA116

AFA115

AFA114

AFA113

AFA111

AFA110

AFA109

P. acuminatus Zone

AFA117

A. M. persculptus ascensus AFA107 AFA108

AFA99

AFA100

AFA98

AFA96

AFA95

AFA94

AFA93

AFA92

AFA91

AFA90

AFA89

AFA88

AFA87

AFA86

AFA85

AFA84

AFA97

M. extraordinanusHB M. ojsuensis

AFA101a AFA102 AFA103 AFA104 AFA105

Wufeng Formation P. pacificus Zone AFA83

Rhuddanian

Kuanyinchiao Bed

AFA112

Katian

3

Ordovician to Silurian Shale Gas-Bearing Strata from the Yangtze Region

63

Ranges of important graptolites

Korenogr. bicaudatus

Neodiplogr. modestus

Normalogr. mirnyensis

Avitograptus avitus

Diceratograptus mirus

Parareteogr. sinensis

443.83?

WF3

447.02

WF2

Dicellograptus sp.

Wufeng Linhshang

Katian

Upper Ordovician

Appendispinogr. supernus

LM2

3.5

Ordovician

Biostratigraphy Age (Ma)

Covered

Dicellograptus ornatus

0.8 0.3 0.18

Lithology

Anticostia uniformis

Stage Rhuddanian

Thickness (m)

Series Llandovery

Lungmachi Formation

System Silurian

Fig. 3.44 Hirnantian GSSP at Wangjiawan North Section, Yichang, Hubei

447.62

Mudstone

Black shale

Fig. 3.45 Ordovician–Silurian section at Daozimiao, Jingshan, Hubei (a field excursion of the present authors in 2007)

64

X. Chen et al.

Fig. 3.46 Ordovician to Silurian strata in the Daizimiao section, Jingshan, Hubei

41. Xinling and Anji Formations at Jingshan, Ningguo, Anhui The Xinling Formation was erected by Li (1984) with its type location at Xinling of Jingxian, Anhui. It is composed of shale and siltstone, contains graptolites, and corresponds in age to the Wufeng Formation. However, the thickness of the Xinling Formation is much greater than that of Wufeng,

indicating deposition in a marginal or slope facies of the Yangtze Platform. The Xinling Formation is 370 m thick with WF2 graptolites in the lower part and WF3 graptolites (Paraorthograptus pacificus Biozone) in the upper. It is important to note that Diceratograptus mirus Mu occurs at the top of the formation (upper WF3). The Lower Silurian rocks at this location were named the Xiaxiang Formation

Climacograptus hastatus

LM1—LM2

444.43 WF4 445.16

WF3

447.02

Mudstone

WF2

Dicellograptus tenuiculus

Dicellograptus complanatus

4.5

Dicellograptus complexus

Wufeng 15.22

Huangnehkang

Katian

Upper Ordovician

Ordovician

7.14

Paraorthograptus pacificus

Tangyagraptus typicus

1.08

Biostratigraphy Age (Ma)

Paramalomena Fauna

Ranges of important graptolites

Dalmanitina sp.

Thickness (m)

Lithology

65

Paraorthograptus sp.

Formation

Lishuwo

Stage

Rhuddanian

6.49

Xinkailing

Series

Llandovery

Hirnantian

System

Ordovician to Silurian Shale Gas-Bearing Strata from the Yangtze Region

Silurian

3

447.62

WF1

Black shale, mudstone

Fig. 3.47 Stratigraphic column of the Wufeng and Lishuwo formations in the Xinkailing section, Wuning, Jiangxi (after Yu et al. 1976)

Thickness (m)

Formation

Stage

Series

X. Chen et al.

System

66

Lithology

Ranges of important graptolites

Biostratigraphy Age (Ma)

LM4

>0.6

Black shale

Cystograptus vesiculosus

Neodiplograptus modestus

Normalograptus angustus

Parakidograptus acuminatus

Dicellograptus ornatus

Anticostia lata

4.32

Dicellograptus sp.

Wufeng 7.18

Tangtou

Katian

Upper Ordovician

Ordovician

4.31

Paraorthograptus

6.65

Dicellograptus anceps

Kaochiapien

Rhuddanian

442.47

LM3

443.40 WF3 447.02

WF2

447.62

Yellow-gray shale

Fig. 3.48 Stratigraphic column of the Wufeng and Kaochiapien formations in the Sinianpan section, Hexian, Anhui (after Qi 1989, and field investigation by Chen Xu et al. in 1994)

Thickness (m)

Formation

Stage

Series

Ordovician to Silurian Shale Gas-Bearing Strata from the Yangtze Region

System

3

Lithology

67

Biostratigraphy Age (Ma)

Ran ges of important graptolites

Covered

Demirastrites cf. triangulatus, etc. >350

Rast rites guizhouensis, etc.

Kaochiapien

Aeronian

Llandovery

Silurian

440.77

Coronograptus cf. cyphus, etc. 43.72

LM3–LM5

Cystograptus vesiculosus, etc.

Akidograptus ascensus

LM2

443.40

Xinkailing

Hirnantian

Upper Ordovician

Ordovician

0.72

Muddy limestone lens

Weathered dark-gray shale

Yellow-green shale with siltstone

Fig. 3.49 Stratigraphic column of the Xinkailing Bed and the Kaochiapien Formation in the Tangshan section, Nanjing, Jiangsu (Zhang and Jiao 1985)

Formation

Stage

Series

X. Chen et al. System

68

Depth (m)

Lithology

Graptolite biozones

TOC (%) 0

0

5

1.3 2.2

Aeronian

4.1 5.2

Demirastrites triangulatus

7.3 8.2 10.1

12.7

15.3 16.1

Coronograptus cyphus

Kaochiapien

19.2

Rhuddanian

Silurian

Llandovery

18.2

20.2

25.2 27.2 29.1 30.2

Cystograptus vesiculosus 33.2

Parakidograptus acuminatus 36.1

40.2

Wufeng

Upper Ord. Hirnantian

Ordovician

Akidograptus ascensus

44.3

Tangtou

46.2

Calcareous mudstone

Shale

Mudstone

Siltstone

Limestone Yellowish mudstone

Fig. 3.50 Stratigraphic column of the Wufeng–Kaochiapien formations in the Ganggangshan section, Jurong, Jiangsu (after Fig. 6 of Wang et al. 2017)

3

Ordovician to Silurian Shale Gas-Bearing Strata from the Yangtze Region

69

Fig. 3.51 The Wufeng Formation to the base of the Kaochiapien Formation in the Ganggangshan section, Jurong, Jiangsu

(Qi 1989). However, the type location of this unit is only 40 km west of that of the Anji Formation. In all practical aspects, these two rock units are the same. The Anji Formation is much better studied than the Xiaxiang Formation, and more importantly, the Anji section was documented as a reference section for the Hirnantian Stage in the Lower Yangtze region (Fig. 3.52).

42. Changwu, Wenchang and Anji Formations in the Hanggai Section, Anji, Zhejiang The Changwu, Wenchang and Anji formations in the Hanggai section are composed of siltstone, fine grained sandstone and shale. Wang et al. (2016) subdivided these rock unit biozones. These strata are interpreted as a

136

37.8

Shale and siltstone

1.5 7

30

1

Akidograptus sp.

Normalograptus mirnyensis

Diceratograptus mirus

Styracograptus tatianae

Paraorthograptus pacificus

Rectograptus abbreviatus

Stage Formation Thickness (m)

Rhuddanian Anji

Series

Lithology

Anticostis lata

Appendispinograptus leptothecalis

Hir.

Dicellograptus sp.

Katian Xinling

System

Silurian Llandovery

>120

Dicellogr. graciliramosus

Ordovician Upper Ordovician

70 X. Chen et al.

Ranges of important graptolites Biostratigraphy Age (Ma)

Covered

LM1–LM2 444.43

WF4

WF3

447.02

156

WF2

447.62

Siltstone and fine grained sandstone

Fig. 3.52 Stratigraphic column of the Xinling and Anji formations in Jingshan, Ningguo, Anhui (Mu et al. 1980; Qi 1989)

Aegiromenella planissime Songxites wuningensis

378.48

Wenchang

Hirnantian

Thickness(m) >64.20

Lithology

Akidograptus ascensus Biozone

C

C

Songxites

C

C

C

sandstone–finesandstone–siltstone

Sponge

Trilobite

C

C

C

Ranges of important graptolites

Metabolograptus persculptus Biozone

C C

C

Brachiopoda

Neodiplograptus parvulus Neodiplograptus shanchongensis Normalograptus aff. indivisus Avitograptus avitus Korenograptus zhui Normalograptus madernii Normalograptus acceptus Glptograptus aff. tamariscus Nicholson Normalograptus angustus Akidograptus ascensus

Normalograptus rhizinus Metabolograptus persculptus

Formation

Stage

Series

System

Anji

Rhuddanian

Silurian

C

Amplexograptus disjunctus yangtzensis Climacograptus hastatus Yinograptus disjunctus Paraplegmatograptus uniformis Dicellograptus extremus Paraorthograptus pacificu Styracograptus chiai Paraorthograptus angustus Metabolograptus extraordinarius Neodiplograptus charis Paraclimacograptus innotatus Normalograptus mirnyensis Normalograptus normalis Metabolograptus? cf. persculptus Metabolograptus ojsuensis Korenograptus laciniosus

Appendispinograptus venustus Amplexograptus suni Appendispinograptus supernus

>187.23

Changwu

Katian

Ordovician

Fig. 3.53 Stratigraphic column of the Changwu and Anji formations in the Hanggai section, Anji, Zhejiang (after Wang et al. 2016)

Upper Ordovician

3 Ordovician to Silurian Shale Gas-Bearing Strata from the Yangtze Region 71

Carbonaceous shale

Chitinozoan

δ 13 C

Isotope

(‰)(-VPDB) -31 -27 -23 -5

δ 34 S

Graptolite

(‰)(-VPDB) 5

C

Aegiromenella Biozone

Metabolograptus extraordinarius Biozone

Paraorthograptus pacificus Biozone

C

Dicellograptus complexus Biozone

Silty mudstone

15 25

72

X. Chen et al.

Fig. 3.54 Changwu, Wenchang and Anji formations in the Hanggai section, Anji, Zhejiang

slope facies sequence on the Yangtze marginal belt (Figs. 3.53, 3.54). 43. Sanqushan Formation at Putangkou, Changshan, Zhejiang The Sanqushan Formation at Putangkou, Changshan corresponds to the lower Wufeng Formation (WF1–WF2). It is

interesting that the Sanqushan clastics occur as a typical slump structure (Figs. 3.55, 3.56). The Chiangnan transitional belt was broken up and transferred to the marginal belt of the Yangtze Platform during the early Wufengian (Chen et al. 2018a). The Sanqushan slump structure might indicate deposition on a slope in the Yangtze marginal belt (Li et al. 2018).

Thickness (m)

Member

Formation Changwu

Stage

73

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Biozonation ( Li et al.,2018)

18-06

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180

17-06 17-05

170

17-04 17-03

160

17-02

150

17-01 16-10

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Sanqushan

130 120

Dicellograptus complexus

16-08

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Slump 3

110

Katian

Upper Ordovician

16-07

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90

16-02 16-01

80 Huangnehkang

70

15-09

Slump 2

60 50

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Slump 1

Upper

13-03 13-02 13-01 12-05 12-06 12-03 12-04 12-01~ 12-02 11-07 ~ 10-09 11-01 ~ 10-01 ~ ~ 09-08 ~ 09-01 ~ 08-14 ~ 08-01 07-04 ~ ~ 07-01 ~ ~ 06-07 06-01~ 05-07 ~ 05-01 ~ ~ 04-08 04-01 03-10 03-04 03-08 03-02 03-06 02-01 A4-04 01-01 A4-02 A4-03 A4-01 A3-05 A3-02 A2-11

30 20

Lower

Yenwashan

~ 15-01 14-04 14-03

40

Sandbian

Fig. 3.55 Stratigraphic column of the Sanqushan Formation at Putangkou, Changshan, Zhejiang (after Li et al. 2018)

Series

Ordovician to Silurian Shale Gas-Bearing Strata from the Yangtze Region

Legend

3

10

Nankinolithus nankinensis (Trilobite Zone)

Sinoceras chinense (Nautiloid Zone)

A2-09 A1-01

0 MCMLMW P S F

Mudstone

Slump structure

Nodular limestone

Calcirudite

Breccia

Purple calcareous Covered mudstone

Fine grained sandstone

Gray-green calcareous mudstone

74

X. Chen et al.

Fig. 3.56 Slump structure in the Sanqushan Formation at Putangkou, Changshan, Zhejiang

References Chen X (1984) Silurian graptolites from southern Shaanxi and northern Sichuan with special reference to classification of Monograptidae. Palaeontol Sin whole number 166, new series B 20:1–102 (in Chinese with English summary) Chen X, Lin YK (1978) Lower Silurian graptolites from Tongzi, northern Guizhou. Mem Nanjing Inst Geol Palaeontol Acad Sin 12:1–76 (in Chinese) Chen X, Rong JY, Mitchell CE, Harper DAT, Fan JX, Zhan RB, Zhang YD, Li RY, Wang Y (2000) Late Ordovician to earliest Silurian graptolite and brachiopod biozonation from the Yangtze region, South China with a global correlation. Geol Mag 137 (6):623–650 Chen X, Fan JX, Melchin MJ, Mitchell CE (2004) Patterns and processes of latest Ordovician graptolite extinction and survival in South China. In: Rong JY, Fang ZJ (eds) Mass extinction and recovery—evidences from the palaeozoic and triassic of South China. University of Science and Technology of China Press, Hefei, pp 9–54, 1037–1038 (in Chinese with English abstract) Chen X, Fan JX, Melchin MJ, Mitchell CE (2005) Hirnantian (Latest Ordovician) graptolites from the Upper Yangtze region, China. Palaeontology 48:235–280 Chen X, Rong JY, Fan JX, Zhan RB, Mitchell CE, Harper DAT, Melchin MJ, Peng PA, Finney SC, Wang XF (2006) The Global Boundary Stratotype Section and Point (GSSP) for the base of the Hirnantian Stage (the uppermost of the Ordovician System). Episodes 29(3):183–196 Chen X, Fan JX, Zhang YD, Wang HY, Chen Q, Wang WH, Liang F, Guo W, Zhao Q, Nie HK, Wen ZD, Sun ZY (2015) Subdivision and delineation of the Wufeng and Lungmachi black shales in the subsurface areas of the Yangtze Platform. J Stratigr 39(4):351–358 (in Chinese with English abstract) Chen X, Fan JX, Wang WH, Wang HY, Nie HK, Shi XW, Wen ZD, Chen DY, Li WJ (2017) Stage-progressive distribution pattern of

the Lungmachi black graptolitic shales from Guizhou to Chongqing, Central China. Sci China Earth Sci 60(6):1133–1146 Chen Q, Fan JX, Zhang LN, Chen X (2018a) Paleogeographic evolution of the Lower Yangtze region and the break of the “platform-slope-basin” pattern during the Late Ordovician. Sci China Earth Sci 61(5):625–636 Chen X, Chen Q, Zhen YY, Wang HY, Zhang LN, Zhang JP, Wang WH, Xiao CH (2018b) Circumjacent distribution pattern of the Lungmachian graptolitic black shale (early Silurian) on the Yichang Uplift and its peripheral region. Sci China Earth Sci 61 (9):1195–1203 Ge MY (1990) Silurian graptolites from Chengkou, Sichuan. Palaeontol Sin whole number 179, new series B 26:1–157 (in Chinese with English summary) Guo TL, Liu RB (2013) Implications from marine shale gas exploration breakthrough in complicated structural area at high thermal stage: taking Longmaxi Formation in Well JY1 as an example. Nat Gas Geosci 24(4):643–651 (in Chinese with English summary) Jin CT, Ye SH, He YX, Wan ZQ, Wang SB, Zhao YT, Li SJ, Xu XQ, Zhang ZG (1982) The Silurian stratigraphy and paleontology in Guanyinqiao, Qijiang, Sichuan. People’s Publishing House of Sichuan, Chengdu, pp 1–84 (in Chinese with English abstract) Jin CT, Ye SH, Jiang XS, Li YW, Yu HJ, He YX, Yi YE, Pan YT (1989) The Silurian stratigraphy and Paleontology in Erlangshan district, Sichuan. Bull Chengdu Inst Geol Miner Resour Chin Acad Geol Sci 11:1–224 (in Chinese with English abstract) Li JJ (1984) Graptolites from the Xinling Formation (Upper Ordovician) of South Anhui. Mem Nanjing Inst Geol Palaeontol Acad Sin 20:145–194 (in Chinese with English abstract) Li WJ, Zhang YD, Chen JT, Yuan WW (2018) Characteristics of the Upper Ordovician lithofacies in the Putangkou section, Changshan, western Zhejiang Province, South China. J Stratigr 14(4):393–407 (in Chinese with English abstract) Liang F, Bai WH, Zou CN, Wang HY, Wu J, Ma C, Zhang Q, Guo W, Sun SS, Zhu YM, Cui HY, Liu DX (2016) Shale gas enrichment pattern and exploration significance of Well WuXi-2 in Northeast

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Ordovician to Silurian Shale Gas-Bearing Strata from the Yangtze Region

Chongqing, NE Sichuan Basin. Pet Explor Dev 43(3):1–9 (in Chinese with English abstract) Liang F, Wang HY, Bai WH, Guo W, Zhao Q, Sun SS, Zhang Q, Wu J, Ma C, Lei ZA (2017) Graptolite correlation and sedimentary characteristics of Wufeng-Longmaxi shale in southern Sichuan Basin. Nat Gas Ind 37(7):20–26 (in Chinese with English abstract) Loydell DK (1992) Upper Aeronian and lower Telychian (Llandovery) graptolites from western mid-Wales. Part 1. Monogr Palaeontogr Soc 146(589):1–55, pl. 1 Lu YH (1943) The Ordovician and Silurian strata in Nanzheng, Shaanxi. Geol Rev 8(Z1):149 (in Chinese) Lüning S, Shahin YM, Loydell DK, Al-Rabi HT, Masri A, Tarawneh B, Kolonic S (2005) Anatomy of a world-class source rock: distribution and depositional model of Silurian organic-rich shales in Jordan and implications for hydrocarbon potential. AAPG Bull 89(10):1397–1427 Luo C, Wang LS, Shi XW, Zhang J, Wu W, Zhao SX, Zhang CL, Yang YX (2017) Biostratigraphy of the Wufeng to Longmaxi Formation at Well Ning 211 of Changning shale gas field. J Stratigr 41(2):142–152 (in Chinese with English abstract) Mu EZ (1950) On the occurrence of Pleurograptus in China. Palaéontol Novit 7:1–4 Mu EZ, Ge MY, Chen X, Ni YN, Lin YK (1980) New observations of the Ordovician strata in southern Anhui. J Stratigr 4(2):81–86 (in Chinese) Qi DL (1989) Anhui stratigraphy: Ordovician. Anhui Science and Technology Press, Hefei, pp 1–234 (in Chinese) Rong JY, Chen X (1987) Faunal differentiation, biofacies and lithofacies pattern of Late Ordovician (Ashgillian) in South China (in Chinese with English summary). Acta Palaeontol Sin 26(5):507– 535 Rong JY, Chen X, Wang Y, Zhan RB, Liu JB, Huang B, Wu RC, Wang GX (2011) Northward expansion of Central Guizhou Oldland through the Ordovician and Silurian transition: evidence and implications. Sci Sin Terrae 41(10):1407–1415 (in Chinese) Song T, Chen K, Bao SJ, Guo TX, Lei YX, Wang Y, Meng FY, Wang P (2018) The discovery of shale gas in Wufeng-Longmaxi

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Formation at Hongdi-1Well on the northern limb of Shennongjia anticline in northwestern Hubei Province. Geol China 45(1):195– 196 (in Chinese) Tang P, Huang B, Wu RC, Fan JX, Yan K, Wang GX, Liu JB, Wang Y, Zhan RB, Rong JY (2017) On the upper Ordovician Daduhe Formation of the upper Yangtze region. J Stratigr 41 (2):119–133 (in Chinese with English abstract) Wang YZ (1988) Research on the Silurian in Fucheng, Nanzheng, Shaanxi. Xi’an Institute of Geology, Xi’an (in Chinese) Wang XF, Ni SZ, Zheng QL, Xu GH, Zou TM, Li ZH, Xiang LW, Lai CG (1987) Biostratigraphy of the Yangtze Gorge Area 2: Early Palaeozoic Era. Geological Publishing House, Beijing, pp 1–641 (in Chinese) Wang HY, Guo W, Liang F, Zhao Q (2015) Biostratigraphy characteristics and scientific meaning of the Wufeng and Longmaxi Formation black shales at Well Wei 202 of the Weiyuan shale gas field, Sichuan Basin. J Stratigr 39(3):289–293 (in Chinese with English abstract) Wang LW, Zhang YD, Zhu CH, Zhang JF, Liu FL, Chen JH, Xu SH (2016) Geological characteristics of the Hanggai section in Anji, Zhejiang (the reference section of Upper Ordovician Hirnatian stage for the Lower Yangtze region) and their implications. J Stratigr 40 (4):370–381 (in Chinese with English abstract) Wang WH, Hu WX, Chen Q, Jia D, Chen X (2017) Temporal and spatial distribution of Ordovician-Silurian boundary black graptolitic shales on the Lower Yangtze Platform. Palaeoword 26:444– 455 Yu JH, Xia SF, Fang YT (1976) The Ordovician in Xiushui basin, Jiangxi Province. J Nanjing Univ (nat Sci) 12(2):57–77 (in Chinese) Zhang QZ, Jiao SD (1985) The new success in the study of Silurian of Tangshan, Nanjing, Jiangsu Province. Bull Nanjing Inst Geol Miner Resour Chin Acad Geol Sci 6(2):97–111 (in Chinese with English abstract) Zhang WT, Chen X, Xu HK, Wang JG, Lin YK, Chen JY (1964) Silurian of the northern Guizhou. Acad Sin 79–110. Paleozoic rocks of northern Guizhou. Nanjing Institute of Geology and Palaeontology, Nanjing (in Chinese)

4

Distribution Pattern of the Ordovician– Silurian Shale Gas-Bearing Strata in the Yangtze Region Xu Chen, Hongyan Wang, Haikuan Nie, and Jin Wu

Abstract

The present chapter provides a distribution pattern of shale gas strata through the Ordovician–Silurian of the Yangtze Platform. Progressive distribution and circumjacent distribution patterns of the black shales are defined. Shale gas enrichment and highly productive intervals in the Yangtze region are demonstrated with two case studies of the Weiyuan–Changning and Fuling shale gas fields. Keywords







Progressive distribution pattern Circumjacent distribution pattern Weiyuan–Changning and Fuling shale gas fields Since 2010, PetroChina and SINOPEC have successfully discovered and produced shale gas from the uppermost Ordovician to the lowest Silurian black shales in the Weiyuan–Changning, Fuling, Fushun–Yongchuan, and Zhaotong gas fields. Notably, highly productive shale gas flow was obtained in Well Jiaoye 1 in Jiaoshiba near Fuling, and in Well Ning 201-H1 in the Changning area of Yibin. It was symbolic of the major breakthroughs of shale gas development in China.

X. Chen (&) State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology and Center for Excellence in Life and Paleoenvironment, Chinese Academy of Sciences, Nanjing, 210008, China e-mail: [email protected] H. Y. Wang
J. Wu PetroChina Research Institute of Petroleum Exploration & Development, Beijing, 100083, China H. K. Nie SINOPEC Petroleum Exploration and Production Research Institute, Beijing, 100083, China

Downhole shale gas-producing horizons are usually calibrated by the shale thickness of the wellbore, a property that cannot be easily used to precisely correlate with the same producing horizon in neighboring wells. The black shale intervals of the Wufeng and Lungmachi formations are subdivided lithologically. Unfortunately, it is impossible to recognize marker beds through the continuous black shales. Similarly, the peak value in a specific geochemical or geophysical borehole log is also difficult to confidently correlate into neighboring wells. Therefore, having an index or standard correlation tool for the black shales of the Wufeng and Lungmachi formations is extremely important for shale gas exploration. Graptolite biozonation of these two rock units is recognized as a correlation standard and is essential for Ordovician–Silurian shale gas exploration. Chen et al. (2015) proposed a standard reference graptolite biozonation of the Wufeng to Lungmachi formations. The utility of this biozonation was demonstrated in the cores from Well Jiaoye 1 by verifying that the recognition and correlation precision of graptolite biozones would fully meet the requirements of shale gas exploration. Thus, the proposal convinced the leadership of both SINOPEC and PetroChina, and this biozonation was accepted as the standard for subdivision and correlation in the shale gas exploration of the Wufeng and Lungmachi formations. We correlated the Wufengian and Lungmachian graptolite biozones of northern Guizhou and southern Sichuan (Chen and Lin 1978) with Well Jiaoye 1’s stratigraphic log, which was prepared by Guo and Liu (2013). We also established the succession of graptolite biozones in Well Jiaoye 1 as shown in Fig. 3.32, which was subsequently revised and improved (Fig. 3.33). In Well Jiaoye 1, we found that the most productive and rich shale gas production came from the graptolite biozones WF2 to lower LM6. In order to support PetroChina and SINOPEC in shale gas exploration and development, we set up a shale gas biostratigraphic research team to conduct relevant activities. Since 2015 we have performed three training workshops on

© Zhejiang University Press and Springer Nature Singapore Pte Ltd. 2023 X. Chen et al. (eds.), Latest Ordovician to Early Silurian Shale Gas Strata of the Yangtze Region, China, https://doi.org/10.1007/978-981-99-3134-7_4

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X. Chen et al. 105° 00' E

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GUANGDONG 110° 00' E

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Fig. 4.1 Distribution of key wells and field sections of the Wufeng to Lungmachi formations in the Yangtze region. The blue triangles represent sections: 1. Yuanyangyan in Erlangshan; 2. Shuanghe in Changning; 3. Guanyinqiao in Qijiang; 4. Tianba in Wuxi; 5. Bailu in Wuxi; 6. Wangjiawan in Yichang; 7. Hanggai in Anji; 8. Huangnitang in Changshan. The red dots represent Wells: 1. Yandi 2; 2. Xingdi 2; 3. Wei 201; 4. Wei 202; 5. Wei 204; 6. Wei 203; 7. Wei 205; 8. Zi205; 9. Zi 202; 10. Zi 201; 11. Zi 204; 12. Yi 202; 13. Minye 1; 14. Xindi 1; 15. Suiye 1; 16. Xindi 2; 17. Yanjin 1;18. Yanjin 2; 19. Ning 211; 20.

Ning 206; 21. Ning 203; 22. Ning 201; 23. YS108; 24. YS111; 25. YS113; 26. Ning 209; 27. YS118; 28. Renye 1; 29. Anye 1; 30. Dingye 1; 31. Lu201; 32. Yang 101H3-8; 33. Yang 101H2-7; 34. Huang 202; 35. Zu203; 36. Zu 201; 37. Zu 202; 38. Huadi 1; 39. Nanye 1; 40. Jiaoye 8; 41. Jiaoye 7; 42. Jiaoye 4; 43. Jiaoye 2; 44. Jiaoye 1;45. Bao 201; 46. Tianye 1; 47. Qianjiang 1; 48. Qianjiang 2; 49. Laiye 3; 50. Laiye 2; 51. Laiye 1; 52. Laidi 1; 53. Long 1; 54. Yongye 3; 55. Yongye 1; 56. Yongye 2; 57. Wuxi 2; 58. Xi 202; 59. Wuxi 1; 60. Jiandi 1; 61. Jing 102; 62. Jing101; 63. Gudi 1; 64. Dongshen 1

graptolites in shale gas-bearing. The research team activities in graptolite identification, graptolite biozone division, and correlation have increased substantially. Sixty-four shale gas-drill cores have been investigated with consulting reports and maps prepared for the corresponding organizations/companies (Fig. 4.1).

Lungmachi Formation in the Yangtze region, particularly in the Middle and Upper Yangtze regions (Tables 4.1, 4.2). Based on the data contained in these reports, we conclude that the most important shale gas-bearing interval in the Ordovician and Silurian black shales in the Yangtze region is WF2–LM6, especially LM1–LM5. This is the sweet and rich interval for shale gas production. In defining this rich shale gas interval, two temporal-spatial distribution patterns of shale gas were described and illustrated by Chen et al. (2017, 2018). The richest Ordovician–Silurian shale gas production is primarily from black graptolite-rich shales distributed in the Sichuan Basin. Therefore, the Upper Yangtze region is selected as a reference area for identifying the temporalspatial distribution pattern. Fortunately, type sections of the Wufeng and Lungmachi formations are present and accessible along the roads and railways between Zunyi and Chongqing (Fig. 4.2).

4.1

Sweet Beds and Their Distribution of the Ordovician to Silurian Shale Gas Strata in the Yangtze Region

Since 2015 we have investigated 45 wells and 7 field sections (out of 64 localities), and submitted stratigraphic reports. Graptolites in the sections and wells were identified and the graptolite biozones were outlined. Finally, we determined the reliable sweet beds in the shale gas strata in the Ordovician Wufeng Formation through the Silurian

4

Distribution Pattern of the Ordovician–Silurian Shale Gas-Bearing Strata in the Yangtze Region

Table 4.1

79

Forty core observation results of shale gas exploration wells (including well logging) in the Yangtze region

Date

Well

Location

Dominant shale gas intervals

Report compilers

Nov. 2014

Jiaoye 1

Jiaoshiba, Fuling

WF2–LM6

Chen Xu, Fan Junxuan, Chen Qing

Nov. 2014

Dingye 1

Datong Town, Qijiang

WF2–LM6

Chen Xu, Fan Junxuan, Chen Qing

Dec. 2014

Jiaoye 4

Tianxing Village in Jiaoshiba, Fuling

WF3 and LM4–LM5 (Discontinuous)

Chen Xu, Zhang Linna

Dec. 2014

W-201 (Weiyuan)

Laochang Village, Xinchang Town, Weiyuan

LM1–LM2 and LM6 (Discontinuous)

Chen Xu, Zhang Linna

Dec. 2014

W-204 (Weiyuan)

Longhui Town, Weiyuan

LM8 and strata below not observed

Chen Xu, Zhang Linna

Mar. 2015

Wuxi 2

Wenfeng Town, Wuxi

WF2–LM6

Chen Xu, Fan Junxuan et al.

Mar. 2015

Wuxi 1

Bailu Town, Wuxi

WF2–LM6

Chen Xu, Fan Junxuan et al.

Apr. 2015

Wei 202

Xinchang Town, Weiyuan

Only 10.19 m in LM2–LM5

Chen Xu, Wang Hongyan et al.

Apr. 2015

Wei 204

Longhui Town, Weiyuan

No complete core samples, only 26 m in LM4–LM9 (not as high-quality shale reservoir)

Chen Xu, Wang Hongyan et al.

May 2015

Jiaoye 7

Dongquan Village in Jiaoshiba, Fuling

Lower part of WF2–LM6

Chen Xu, Chen Qing et al.

May 2015

Jiaoye 8

Shuijiang Town, Nanchuan

WF2–LM6

Chen Xu, Chen Qing et al.

July 2015

Laidi 1

Lianghekou, Laifeng

Hiatus between WF3 and LM2, only 12 m in LM2– LM5

Chen Xu, Wang Hongyan, Xiao Zhaohui et al.

July 2015

Jing 101

Hekou Township, Yuan’an

Small thickness in lower part of WF2–LM6

Chen Xu, Wang Hongyan et al.

July 2015

Jing 102

Maopingchang Town, Yuan’an

Small thickness in lower part of WF2–LM6

Chen Xu, Wang Hongyan et al.

July 2015

Yanjin 1

Yanjin County

WF3–LM5

Chen Xu, Wang Hongyan, Liang Feng et al.

Oct. 2015

Zi 201

Shuangshi Town, Rong County

Complete graptolite belt in the Lungmachi Formation, favorable interval below LM6

Chen Xu, Wang Hongyan et al.

Oct. 2015

Wei 203

Northwestern Neijiang City

Complete graptolite belt in the Lungmachi Formation, favorable interval below LM6

Chen Xu, Wang Hongyan et al.

Mar. 2016

Ning 211

Renyi Township, Gao County

WF2–LM4 (LM5)

Chen Xu, Fan Junxuan, Shi Xuewen, Luo Chao et al.

Apr. 2016

Dongshen 1

Shanlian Village, Donggang Town, Wuxi

WF3 non-black shale, thin LM1–LM2 in eastern Yangtze Platform

Chen Xu, Li Fei

May 2016

Minye 1

Xiaxi Township, Pingshan County

Continuous graptolite belt in WF2–LM7-8, with low TOC

Chen Xu, Chen Qing, Wen Zhidong et al.

May 2016

Nanye 1

Shiqiang Town, Nanchuan

WF2–LM7, mainly in WF2–LM2

Chen Xu, Chen Qing, He Guisong et al.

May 2016

Renye 1

Gulin Town, Gulin County

LM1–LM5

Chen Xu, Chen Qing, He Guisong et al.

May 2016

Tianye 1

Nantian Town, Fengdu

Lower part of WF2–LM6

Chen Xu, Wei Xiangfeng, Wen Zhidong et al.

May 2016

Yongye 1 (Chongqing)

Laisu Town, Yongchuan District of Chongqing City

LM1–LM5

Chen Xu, Chen Qing, Wei Xiangfeng et al.

Aug. 2016

Anye 1

Anchang Town, Zheng’an

WF2–LM4 thin black shale

Chen Xu, Wang Hongyan et al.

June 2016

Jiandi 1

Longping Township, Jianshi

WF2–WF4, LM4, LM1–LM3 missing

Chen Xu, Zhou Zhi, Tong Chuanchuan

Nov. 2016

Ning 209

Shangluo Town, Changning

WF2–LM4-5

Chen Xu, Wang Hongyan et al.

Feb. 2017

Yongye 3

Shiping Town, Yongshun County

LM1–LM4, thin

Chen Xu, Chen Qing, Wang Hongyan et al.

Feb. 2017

Yongye 1

Kesha Township, Yongshun County

WF3–LM4, thin

Chen Xu, Chen Qing, Wang Hongyan et al.

Feb. 2017

Yongye 2

Shidi Town, Yongshun County

LM1–LM3, thin

Chen Xu, Chen Qing, Wang Hongyan et al.

(continued)

80 Table 4.1

X. Chen et al. (continued)

Date

Well

Location

Dominant shale gas intervals

Report compilers

May 2017

YS113

Jiucheng Town, Weiyuan County, Zhaotong

WF2–LM6 partially

Chen Xu, Wang Hongyan et al.

May 2017

YS118

Gulin

WF2–LM4-5

Chen Xu, Wang Hongyan et al.

Sep. 2018

Huang 202

Huangguashan in Hejiang District of Chongqing

WF2–LM5

Chen Xu, Wang Hongyan et al.

Sep. 2018

Zu 201

Yongxi Town in Dazu, Chongqing

WF2–LM5

Chen Xu, Wang Hongyan et al.

Mar. 2019

Z-204 (Weiyuan)

Guoshui Town, Rong County

No complete records below LM6

Chen Xu, Wang Hongyan et al.

Mar. 2019

Z-205 (Weiyuan)

Niufo Town, Da’an District of Zigong

WF3–LM9, no complete records in WF3 and LM3

Chen Xu, Wang Hongyan et al.

Apr. 2019

Suiye 1

Nansui, Suijiang County

LM1–LM4

Chen Xu, Liu Guoheng, Zhou Zhi et al.

Dec. 2019

Yang 101H3-8

Qifeng Town, Lu County

WF2–LM5

Chen Xu, Wang Hongyan, Lin Changmu et al.

Dec. 2019

Yang 101H2-7

Xuantan Town, Lu County

WF2–LM5

Chen Xu, Wang Hongyan, Lin Changmu et al.

Dec. 2019

Zu 203

Huaxing Town, Tongliang District of Chongqing

Complete graptolite belt, LM1–LM3

Chen Xu, Wang Hongyan, Lin Changmu et al.

Table 4.2

Thirteen sections and graptolite zonation identified and delivered by Chen et al.

Date

Section/Well

Dominant shale gas-bearing horizon

Provider

Mar. 2016

Wanhe section in Yongshan, Yunnan

LM2–LM4, possible hiatus between its lower part and Kuanyinchiao Bed

Chen Qing

Mar.–Apr. 2018

Well Suye 1 in Lunshan, Jurong

LM6–LM5; primarily the lower part of LM3–WF3

Hu Wenxuan, Yang Shengchao

Apr. 2018

Lunshan section in Jurong

WF4–LM2

Hu Wenxuan, Yang Shengchao

July 2018

Wuxing Village, Xiaoyang Town, Zhenba County, Shaanxi

Totally 11 m from LM1 to the bottom of LM6; Three layers of K-bentonite in LM3; Four layers of K-bentonite in LM5

Ge Xiangying

2018

Well Handi 1 in Qingxi Town, Hanshan

Lower part of WF2–LM6

Wang Zhongpeng et al. 2020

2018

Shizishan section in Shuanghe, Changning

WF2–LM8, dominated by LM2–LM6

Chen Xu, Fan Junxuan, Chen Qing

2018

Bajiaomiao section in Shennongjia

LM2–LM9

Chen Xu, Fan Junxuan, Chen Qing

Apr. 2018

Well Gudi 1 in Qigu Village, Chaohu

WF2–WF3, faults between LM5 and LM6

Chen Xu, Wang Wenjuan

May 2019

Jiaodingshan section in Hanyuan

WF3–LM6 bottom, 25 m Lungmachi Formation, no continuous black shale between Ordovician and Silurian

Zhang Di

May 2019

Well Yandi 2, Gesala Township, Yanbian

LM2–LM9

Zhang Di

May 2019

South to Well Yongdi 2 in Yongshan

WF2––LM8 between Ordovician and Silurian

Zhang Di

May 2019

Gaolun Village in Jurong

WF3–LM1

Wang Wenjuan

June 2019

Well Ziye 1, Yanglinqiao Town, Zigui County

WF3–LM7

Zhou Zhi

4

Distribution Pattern of the Ordovician–Silurian Shale Gas-Bearing Strata in the Yangtze Region

Fig. 4.2 Key sections of Lungmachi Formation along the highway from Guizhou–Chongqing–Sichuan (after Fig. 2 of Chen et al. 2017). 1. Dingshan in Qijiang; 2. Guanyinqiao in Qijiang; 3. Hanjiadian in Tongzi; 4. Liangfengya in Tongzi; 5. Honghuayuan and Daijiagou in Tongzi; 6. Banqiao in Zunyi; 7. Jiadangwan, Donggongsi, Zunyi; 8. Huanghuachong and Yegouchong No. 2 sections in Wudang, Guiyang

The black shale in the LM1–LM3 interval of the Lungmachi Formation expands southward from the fourth terrain in the center of the Upper Yangtze Sea Basin to the Central Guizhou Old Land, accompanying the early Silurian global sea level rise. LM1–LM3 can be traced southward to Liangfengya in Tongzi of the third terrain (Fig. 4.3). However, LM1–LM3 are absent in Honghuayuan–Daijiagou and other places in Tongzi of the second terrain. Black shale did not cover the entire second terrain until LM4. Continued sea level rise through LM5 resulted in the Yangtze Sea extending to Donggongsi–Banqiao in Zunyi of the first terrain. Thus, the black shale distribution pattern exhibits a progressive expansion from LM1 to LM5 (Chen et al. 2017; Fig. 4.3). After the deposition of LM6 age strata, the entire Central Guizhou Old Land underwent continuous uplift and provided detrital materials to the north of the Oldland. The first terrain was filled up at that time. Terrigenous sediments were continuously deposited in the Second and third terrains, but these consisted of coarser, non-black shale and siltstone. Only the fourth terrain in the center of the Yangtze Basin was continuously filled with a complete sequence of the Lungmachi Formation black shale. The progressive distribution pattern of the black shale explains why the major shale gas fields

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such as Fuling of Chongqing, Weiyuan–Changning, and Luzhou occur in the fourth terrain (Fig. 4.3). The Kwangsian Orogeny occurred during the Ordovician–Silurian boundary interval in South China (Chen and Mitchell 1996; Chen et al. 2001, 2010, 2012, 2014). In the southwest of Hubei Province and the northwest of Hunan Province, a regional Yichang Uplift corresponded to the Kwangsian Orogeny on the Middle Yangtze Platform. This uplift caused an unconformity from the latest Ordovician to the early Silurian. The hiatus is most pronounced at Xiaohe Village near Wufeng County, where LM7 sits directly on the Linhsiang Formation. Hence, the entire Wufeng Formation and LM1 to LM6 of the Lungmachi Formation are absent at that locality. There is a temporal and spatial distribution pattern to the Yichang Uplift, which might be one of the important controlling factors for the preservation and distribution of shale gas in this region. We demonstrated that shale gas production is generally less promising in the areas where the Ordovician–Silurian boundary unconformity encompasses more of the basal part of the Lungmachi Formation (Chen et al. 2018; Fig. 4.4). Therefore, a better understanding of the circumjacent distribution pattern of the Yichang Uplift may provide important guidance for shale gas exploration. Examining the effects of the circumjacent distribution pattern of the uplift along with the progressive distribution pattern of the black shale, allows us to recognize that the areas from the fourth terrain (IV in Fig. 4.5) of the Sichuan Basin to the Yangtze Gorges have the most potential for productive shale gas fields (Fig. 4.5). The biozonation and correlation of the PetroChina and SINOPEC cores over the past five years (2014–2019) demonstrated that in the Yangtze region, with no exceptions, the shale gas sweet beds are all WF2 to LM6 in age (mostly LM5). Recently, Well Jiaoye 6–2 obtained high-yield shale gas flow, following Well Jiaoye 1 in Jiaoshiba of Fuling. It greatly enlarged the high-yield shale gas area in Fuling of Chongqing (Nie et al. 2020). Well Jiaoye 6–2 also produces shale gas from WF2–LM4. In addition to the Fuling shale gas field, the same interval (i.e., WF2–LM4) was confirmed as the optimal shale gas production interval in the Weiyuan–Changning shale gas field. The results are the same in recent observations of exploration wells in Luzhou (Figs. 3.19, 3.21, and 3.22; Table 4.1; Luo et al. 2017; Liang et al. 2017). Based on the data discussed above, potential Ordovician– Silurian shale gas fields in the Yangtze region should be evaluated according to the following criteria: (1) Presence of strata from WF2 to LM6 (particularly the presence of LM5); (2) TOC > 3%; and (3) Porosity of organic matter > 4%.

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Fig. 4.3 Progressive distribution pattern of Lungmachi Formation black shale from Guizhou to Chongqing (according to Fig. 5 of Chen et al. 2017)

4.2

Early Silurian Disaster Graptolites and Their Rich Organic Horizons in the Yangtze Region

Near the base of the Lungmachi Formation of the Yangtze region, particularly in LM1–LM3, a number of graptolite species are concentrated in a single layer or a few black shale layers. Most of these graptolites are common species of the group Neograptina which radiate explosively after the latest Ordovician extinction. Such species are called “disaster species” (Figs. 4.6, 4.7). During and after the latest Ordovician extinction, over 80% of the DDO graptolite fauna (Dicranogratidae– Diplograptidae–Orthograptidae fauna, Melchin and Mitchell 1991, =Diplograptid fauna, Štorch et al. 2011) was rapidly replaced by the neograptine graptolite fauna (N or Normalograptid fauna of Chen et al. 2004). The extinction created a vacancy in the marine surface water ecological

space, where the DDO graptolites lived. This new vacancy likely provided open ecological space for the opportunistic Normalograptid fauna to expand into. Two disaster species, Hirsutograptus comanitis (Chaletzkaya) and Korenograptus laciniosus (Churkin and Carter), were typical elements of the radiating Normalograptid fauna. Interestingly, these two taxa exhibited two different life strategies. Hirsutograptus comanitis (Chaletzkaya) is a very short-ranging species in LM3. Indeed, the genus Hirsutograptus itself was quite short-lived. Its existence might be predicated upon some specific, ephemeral food supply supporting this taxon. The other taxon, Korenograptus laciniosus (Churkin and Carter) shown in Fig. 4.7, is a long-lasting species. In the Yangtze region, it occurs in LM2–LM5, just extending into the range of “sweet beds” of shale gas. Thus, the ecology of Rhuddanian disaster species, is likely linked to specific types of available food supply. Regardless of the ecology of the individual disaster species, their presence indicates the production of specific microbial populations at the

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Distribution Pattern of the Ordovician–Silurian Shale Gas-Bearing Strata in the Yangtze Region

Fig. 4.4 Spatial and temporal distribution of the Lungmachi Formation sweet beds around the Yichang Uplift (after Fig. 2 of Chen et al. 2018). 1. Tianba, Wuxi, Chongqing; 2. west of Wenfeng, Wuxi, Chongqing; 3. Bailu, Wuxi, Chongqing; 4. Bajiaomiao, Shennongjia, Hubei; 5. Maliangping, Baokang, Hubei; 6. Taiyanghe, Enshi, Hubei; 7. Longping, Jianshi, Hubei; 8. Siyangqiao, Badong, Hubei; 9. Longmaxi, Xintan, Zigui; 10. Wangjiawan, Yichang, Hubei; 11. Huanghuachang, Yichang, Hubei; 12. Xiaohe, Wufeng, Hubei; 13. 1.5 km to the west of Wufeng County, Hubei; 14. northeast of Dayan, Changyang, Hubei; 15. Huaqiao, Changyang, Hubei; 16. Panjiawan,

bottom of the food chain, which in turn provides the organic matter necessary for the development of units rich in shale gas. Using an extensive database (Geobiodiversity Database, GBDB) compiled from Chinese sections, Fan et al. (2020) reexamined genus and species level diversity patterns across the Ordovician–Silurian boundary and confirmed the presence of a graptolite mass extinction event at the end of the Ordovician. They also affirmed that an immediate recovery and radiation occurred among graptolites from the Rhuddanian to the Telychian in the early Silurian. The black shales in the early Silurian were widespread, and the radiating disaster species lived above anoxic waters, which were themselves the products of the biological recovery and radiation process.

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Maohutang, Yidu, Hubei; 17. west of Wantan, Wufeng, Hubei; 18. Longcihe, Nishi, Shimen, Hunan; 19. Muchanggou, Qiliao, Shizhu, Chongqing; 20. west of Miaolinwan, Shaba, Qianjiang, Chongqing; 21. Lianghekou, Laifeng, Hubei; 22. Gaoluo, Xuan’en, Hubei; 23. Guanwu, Taiping, Hefeng, Hubei; 24. Lujiao, Pengshui, Chongqing; 25. east of Hongdu Bridge, Pengshui, Chongqing; 26. north of Zhangjia, Huanyuan, Youyang, Chongqing; 27. Keli, Shidi, Yongshun, Hunan; 28. Qingping, Yongshun, Hunan; 29. Yumidu, Bodu, Cili, Hunan. Abbreviations: LM, Lungmachi Formation; WF, Wufeng Formation; LX, Linhsiang Formation

4.3

Shale Gas Enrichment and Highly Productive Interval in the Yangtze Region

Zou et al. (2015) noted that in the Wufeng–Lungmachi formations the shale gas sweet beds are composed of black siliceous and calcareous shales and comprise a thickness of 10–40 m. This interval generally encompasses the graptolite biozones WF2–LM4 (Fig. 4.8). Four basic geological conditions are required for development of these shale gas sweet beds. These are as follows: (1) Organic-rich deposits are developed in the anoxic deep water shelf environment, which is necessary for extensive shale gas formation.

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Fig. 4.5 Distribution and locations of the major study sections and wells of the Lungmachi Formation black shale in the Sichuan Basin and adjacent areas (after Fig. 3 of Chen et al. 2018). Wells 1 to 35 were investigated by the present authors. The Lungmachian graptolite biozonations recognized in the subsurface succession agree with the outcrops in the same area. We follow the practice of the petroleum companies and avoid marking serial numbers on the wells. The sections around the Yichang Uplift (green triangles) area are recorded in

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Fig. 4.4. The blue triangles are respectively: 36. Meigu area; 37. Yongshan area; 38. Laomucheng, Yanjin; 39. Shuanghe, Changning; 40. Shanbaiti, Huaying; 41. Guanyinqiao, Qijiang; 42. Hanjiadian, Tongzi; 43. Xijiu, Xishui; 44. Honghuayuan, Tongzi; 45. Donggongsi, Zunyi. Areas with different temporal completeness of the Lungmachi Formation: I (first terrain), only LM5; II (second terrain), LM4– LM6; III (third terrain), LM1–LM6; IV (fourth terrain), LM1–LM7-8 (after Chen et al. 2017)

Fig. 4.6 Disaster species Hirsutograptus comanitis (Chaletzkaya) in LM3 of the Wangjiawan, Yichang, Hubei (Fan et al. manuscript)

(2) A nano-pore throat system is developed in the organic matter, which is necessary for shale gas storage. (3) Closed roof and floor are developed in a relatively stable deep water shelf environment, which is good for shale gas preservation. (4) The rate of deposition in laminated and siliceous deposits is low, which have well-developed microfractures. This facilitates shale gas exploration.

Fig. 4.7 Disaster species Korenograptus laciniosus (Churkin and Carter) in LM2 of Well Eziye 1, Zigui, Hubei (provided by Zhou Chu)

The strata in graptolite biozones WF2–LM4 in the Sichuan Basin meet all of the above-mentioned conditions to comprise the shale gas sweet beds (Fig. 4.8).

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Distribution Pattern of the Ordovician–Silurian Shale Gas-Bearing Strata in the Yangtze Region

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Fig. 4.8 Distribution of graptolite biozones of the shale gas sweet beds in the Wufeng– Lungmachi formations in the Sichuan Basin (Zou et al. 2015)

Jin et al. (2018) also pointed out that the shales from the Wufeng Formation to the lower Lungmachi Formation (WF2–LM4) in the Sichuan Basin are characterized by slow deposition, a good type of organic matter, high TOC, and strong hydrocarbon generation potential. They suggested these were appropriate strata for shale gas generation. They also considered that the organic matter from planktonic algae is conducive to massive hydrocarbon generation and extensive organic pores. The high organic carbon content enables the existence of abundant organic pores and the formation of a three-dimensional interconnected network of organic pores, thereby providing high porosity and a flow path for natural gas (Fig. 4.9). They further indicated that the WF2– LM4 graptolite shale interval is very thick, and that the shale gas is concentrated by overlying caprocks of Middle–Lower Triassic gypsum-salt rocks and mudstones. Moreover, the WF2–LM4 interval has excellent conditions for the enrichment and preservation of shale gas. Liu et al. (2018) indicated that bituminous nano-pores in shale are the primary storage space of shale gas. They believed that the formation of bituminous nano-pores in shale is important to gas generation and is produced by crude oil cracking. The residual bitumen from crude oil cracking is conducive to the preservation of vesicles, with ultra-low water saturation in pores. The sphericity of the pores is

related to the pressure in the pores. Higher pore sphericity is concentrated in shale gas overpressure enrichment areas. Liu et al. (2018) reproduced the process of shale gas generation and vesicle-pore conversion through simulations. The process of nano-pore formation and evolution is divided into three stages: crude oil generation, vesicle formation, and vesicle-pore conversion (Fig. 4.10). The origin of these organic nano-pores in black shale has become increasingly clear in recent studies.

4.4

Case Study of Weiyuan–Changning Shale Gas Field

All data in this chapter are provided by Wang Hongyuan based on his institute of PetroChina.

4.4.1 Organic Matter Abundance Abundant organic matter is one of the key factors for shale gas accumulation and the material basis for hydrocarbon generation. The organic matter abundance is directly proportional to the gas content of shale reservoirs. The greater the number of organic pores created after hydrocarbon

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Fig. 4.9

Fig. 4.10

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Stratigraphic log and favorable exploration zones of shale gas in Well Jiaoye 1 (Jin et al. 2018)

The three evolutionary stages of vesicle-pore conversion (according to Fig. 2 of Liu et al. 2018)

generation, the more conducive the rock unit to shale gas accumulation. Effective shale reservoirs with a low degree of evolution generally have TOC > 2%, while those with a high degree of evolution, especially Type I organic matter-bearing shales, usually have TOC > 1%. In North America, the shales under commercial recovery usually have TOC > 2%: 0.3–25% in Antrim and New Albany, and 0.45– 4.7% in the Ohio, Barnett and Lewis fields (Zou et al. 2013). In China, the organic-rich shales of the Lungmachi Formation in the Sichuan Basin possess a TOC of 0.51–4.88%, and the organic-rich shales of the Lower Cambrian Qiongzhusi Formation have a TOC of 1.0–11.07% (Zou et al. 2013). In the Weiyuan field, the measured TOC of core samples ranges from 2.1 to 2.9%, with an average of 2.5% (Fig. 4.11). In a single well of the Weiyuan field, the TOC of

high-quality shale (TOC > 2%) from the Wufeng Formation to LM6 is 2.6–3.6%, with an average of 3.2% according to logging data. The organic matter abundance in the same interval is relatively stable, and the high-quality shale interval has small variations in an organic carbon content, which tends to increase to the northeast and gradually decrease on both sides of the Weiyuan field. In the Weiyuan field strata, LM1 possesses the highest TOC, followed successively by LM5, WF2–WF4, LM2– LM4, and LM6. The measured TOC and logging-based TOC are 1.0–4.5% (average 2.6%) and 2.6–4.5% (average 3.8%), respectively for the Wufeng Formation. In the Lungmachi Formation values are 2.8–6.7% (average 4.9%) and 3.3–7.7% (average 5.7%) for LM1, 2.1–2.9% (average 2.6%) and 2.1–4.2% (average 3.0%) for LM2–LM4,

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Fig. 4.11 Contour map of organic carbon content in the Wufeng–Lungmachi formations in the Weiyuan field

Fig. 4.12 Contour map of organic carbon content in the Wufeng–Lungmachi formations in the Changning field

2.3–3.7% (average 2.8%) and 2.0–4.2% (average 3.3%) for LM5, and 1.1–2.9% (average 2.1%) and 2.0–2.9% (average 2.5%) for the LM6 interval. The analysis of shale core samples in the Changning field shows that the average TOC of Wufeng Formation through LM6 in a single well is 3.0–4.2%. From highest to lowest, the LM1 interval exhibits the highest TOC (4.46–6.60%), followed by LM5 (4.10–5.47%), LM2–LM4 (3.42–3.98%),

the Wufeng Formation (2.64–4.10%), and lastly LM6 (2.26–3.12%) (Fig. 4.12).

4.4.2 Organic Matter Type Different types of organic matter may all produce natural gas. However, the different types of organic matter affect not

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only gas production in the source rocks, but also the adsorption capacity of organic matter. Different types of organic matter are characterized by kerogen type and kerogen carbon isotope values. The measured kerogen carbon isotope values of the Wufeng to Lungmachi formations (below LM6) in the key wells of the Changning shale gas field range between –27.92‰ and –30.78‰. According to the classification from Huang et al. (1989), in the Changning field the carbon isotope value of Type I kerogen is 4%, hydrocarbon generation terminates. According to the analysis of laboratory data of PetroChina, the maturity of the source rocks from the Wufeng Formation to the lower Lungmachi Formation in the Weiyuan field is 1.78–2.26%, suggesting a highly mature to overmature stage, with the generation of dry gas as the dominant hydrocarbon. Vertically, the Ro value increases slightly from top to bottom. The Ro value is 2.1–2.5% within the Weiyuan field, but in general, it gradually increases from west to east with an increasing burial depth. The organic

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matter in the Changning area has experienced a high degree of evolution, with Ro values mostly between 2.6 and 3.2%, suggesting that the organic matter has reached the overmature stage, with a dominance of dry gas generation. The thermal evolution of organic matter is the highest in the core of the Changning field, and it gradually decreases toward the northwest.

4.4.4 Mineral Composition Shale reservoirs are usually rich in clay minerals (kaolinite, illite, smectite), as well as feldspar, quartz, calcite, and pyrite. The mineral composition of the shale can have a great impact on the seepage capacity of gas reservoirs. Shale reservoirs are relatively tight, and the artificial fracturing technology is usually required to improve the seepage capacity of reservoir during recovery. Therefore, the higher the content of brittle minerals, the easier it is to create fractures. The content of quartz in organic-rich shale in North America is usually >40% and it is of biological siliceous origin. The content of brittle minerals (e.g., quartz and calcite) in the Lungmachi and Qiongzhusi formations in the Sichuan Basin is more than 40%, and the content of clay minerals (e.g., illite) is mainly between 31 and 51%. The lithology of the Wufeng Formation through LM6 in the Weiyuan area is mainly black carbonaceous and siliceous shale, black shale, gray-black shale, and black silty shale, with foliation and rich fossils (e.g., graptolite, gastropod, brachiopod, trilobite, siliceous radiolaria, and sponge spicules). The mineralogy of the Wufeng Formation through LM6 is mainly composed of quartz, plagioclase, calcite, dolomite, clay minerals and pyrite (Fig. 4.13). The measured brittle mineral content of the Wufeng–Lungmachi formations in the Weiyuan area gradually decreases from bottom to top, in the following order: LM1 > Wufeng Formation > LM2–LM4 > LM5 > LM6. The measured brittle mineral content is 66.2–92.3% (average 79.4%) for the Wufeng Formation, 77.0–95.6% (average 83.3%) for LM1, 61.0–84.8% (average 74.1%) for LM2–LM4, 62.9–78.1% (average 70.6%) for LM5, and 59.3–74.1% (average 65.7%) for LM6. The content of brittle minerals is generally stable in distribution, and the average for well areas is generally greater than 60%, showing good reservoir compressibility. The Wufeng Formation–LM6 rock lithologies in the Changning area are mainly black carbonaceous shale, black shale, siliceous shale, black mudstone, black silty mudstone and gray-black silty mudstone. The whole-rock X-ray diffraction analysis shows that mineralogically it primarily contains quartz, feldspar, calcite, dolomite, clay minerals (e.g., illite, illite/smectite, and chlorite), and pyrite. The core analysis reveals that the mineral composition characteristics of Types I+II reservoirs in the Wufeng Formation–LM6

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Distribution Pattern of the Ordovician–Silurian Shale Gas-Bearing Strata in the Yangtze Region

Fig. 4.13

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Mineral composition of black shale in the Wufeng–Lungmachi formations in a typical well in the Weiyuan and Changning areas

interval are basically identical. From top to bottom, the content of brittle minerals gradually increases, with high values recorded in LM1 (up to 82.3%) and LM2–LM4. The average brittle mineral content of the Wufeng Formation– LM6 interval in the Changning area is larger than 55%, with high values found in the northeastern part and slightly lower values in the southern part, and with consistent distribution in all layers.

4.4.5 Physical Properties of Reservoirs The key physical properties of shale reservoirs include permeability, porosity and gas content. The permeability of shale reservoirs is usually low and has a certain positive correlation with porosity. The GRI matrix permeability of shale reservoirs in North America is (50–1000)  10–9 lm2, while the permeability of the Lungmachi and Qiongzhusi formations in the Sichuan Basin is (1000–110,000)  10–9 lm2 (Wang et al. 2009). Pores are the primary storage space of shale reservoirs—the greater the shale porosity, the better

the gas storage capacity. The porosity of shale gas reservoirs developed commercially in the United States is mainly between 2 and 10%, while that of marine organic-rich shale in China is between 2 and 12%. The storage space of the Wufeng and Lungmachi shales in the Weiyuan field is complex and diverse. The pores can be divided into four types according to their genesis, namely, organic pores, intergranular pores, intercrystalline pores and intracrystalline dissolved pores. The shale of the Lungmachi Formation has a large specific surface area and pore volume, which is conducive to the adsorption of shale gas. The shales in LM1–LM5 have the largest specific surface area, while the LM6 shale has the smallest specific surface area. The shale in LM6 with high organic carbon content contains the most nano-pores. The porosity of the Wufeng Formation through LM6 in the Weiyuan field is generally high, with the measured average of 4.7–7.4% in a single well and the logging average of 5.0–8.2%. In general, the average measured porosity of each layer is above 4.0%, of which LM1 and LM5 have the highest porosity, followed by LM6, LM2–LM4 and the Wufeng Formation.

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The measured porosity and logging-based porosity are 2.0–5.1% (average 4.1%) and 3.5–7.4% (average 5.1%) in the Wufeng Formation of the Weiyuan shale gas field. In the Lungmachi Formation, the measured porosity and logging-based porosity are respectively 3.1–6.9% (average 5.8%) and 4.9–9.1% (average 6.5%) in LM1, 3.9–6.8% (average 5.5%) and 4.9–8.0% (average 5.7%) in LM2–LM4, 4.8–7.4% (average 5.8%) and 5.2–7.5% (average 6.2%) in LM5, as well as 4.5–7.4% (average 5.7%) and 5.4–7.8% (average 5.9%) in LM6. The porosity distribution of wells is relatively stable geographically. The porosity of the Wufeng Formation to LM6 in the Weiyuan shale gas field is small in range, and is generally greater than 5.0%. The shale matrix permeability ranges from 1.06  10–5 to 6.14  10–4 mD, with an average of 1.60  10–4 mD. According to the horizontal and vertical permeability measurements, the horizontal permeability is much higher than the vertical one, a characteristic which is related to the development of horizontal bedding in the shale. The measured matrix permeability of the major production area is 1.06  10–5–5.25  10–4 mD, with an average of 1.50  10–4 mD. Vertically, LM1 exhibits the highest matrix permeability, followed by LM5, LM2–LM4, LM6, and then the Wufeng Formation. Structural analysis reveals a strong positive correlation between porosity and matrix permeability. The shale reservoirs of the Wufeng Formation through LM6 have relatively high gas saturation, ranging from 53.7 to 76.4%, with an average of 62.2%. The gas saturation of the Wufeng and Lungmachi formations is longitudinally heterogeneous. Generally, LM1 has the highest gas saturation (up to 73% on average), in contrast to LM2–LM4 (average 67.1%), the Wufeng Formation (average 65.4%), LM5 (average 61.5%) and LM6 (average 60.6%). The shale of Wufeng Formation to Lungmachi Formation in the Changning shale gas field is rich in organic pores. Pores include organic-hosted pores, biological pores, and inorganic pores. In other words, the shale contains intergranular pores, intragranular dissolved pores, intracrystalline dissolved pores, intercrystalline pores, and biological pores. As measured by the liquid nitrogen adsorption method, the specific surface area of pores in LM1 and LM5 are the largest, followed by the Wufeng Formation, and LM2–LM4, LM6, and LM7. Pores from the shales above LM7 have poor specific surface area. The longitudinal porosity distribution characteristics in the Wufeng and Lungmachi formations are similar. The measured porosity of the Wufeng Formation to LM6 is 2.0–6.8% (average 5.53%). Above LM6 and in younger strata the average porosity is 3.96%. According to well log interpretation, the porosity of Types I+II reservoirs in the Wufeng Formation to LM6 is 3.6–7.3%, generally suggesting a relatively high porosity, with LM5 and LM6 having the largest porosity vertically. The porosity is the

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greatest in the major production area of the Changning shale gas field, and is slightly lower in its periphery. The layer porosity and overall porosity have a uniform lateral distribution. The measured matrix permeability in the Wufeng Formation through LM6 in a single well of the Changning field is 0.714  10–4–1.48  10–4 mD, with an average of 1.02  10–4 mD. The reservoir of the Wufeng Formation to LM6 shale has relatively high gas saturation. The average gas saturation measured in a single well is 50–70%. Vertically, LM1 has the largest gas saturation, followed by LM2– LM4, LM5, and LM6. The Wufeng Formation has the smallest gas saturation.

4.4.6 Rock Mechanical Properties The main parameters of rock mechanics of shale reservoirs include Young’s modulus, Poisson’s ratio, and brittleness index. Favorable exploration areas for shale gas usually exhibit a Young’s modulus of greater than 2  104 MPa and a static Poisson’s ratio of less than 0.25 MPa. The evaluation of rock mechanical properties helps determine the fracture-forming capacity of shale reservoirs. According to the rock mechanics tests that were conducted on the Lungmachi Formation shale in the Weiyuan shale gas field, the tri-axial compressive strength is 97.7– 281.6 MPa, with an average of 213.90 MPa; the Young’s modulus is 1.1  104–3.3  104 MPa, with an average of 2.1  104 MPa; and the Poisson’s ratio is 0.17–0.29, with an average of 0.20. Through the analysis of logging interpretation, LM1 and LM2–LM4 have a larger Young’s modulus, the lowest Poisson’s ratio, and the highest brittleness index, making them the most conducive to stimulation. In the Wufeng and Lungmachi formations of the Changning shale gas field, the tri-axial compressive strength of shale is 181.73–321.74 MPa, with an average of 238.648 MPa; the Young’s modulus is 1.548  104– 5.599  104 MPa, with an average of 2.982  104 MPa; and the Poisson’s ratio is 0.158–0.331, with an average of 0.211. Clearly, the shales of Wufeng and Lungmachi formations demonstrate a higher Young’s modulus and a lower Poisson’s ratio, suggesting good compressibility. The brittleness index is generally high, and shale reservoirs have better brittleness characteristics.

4.4.7 Reservoir Pressure and Gas Content At the same burial depth, the greater the formation pressure coefficient, the better the preservation conditions and the higher the gas content of shale reservoirs. In North America, the Antrim, Ohio, New Albany, and Lewis shales occur at relatively shallow depths (20

Thickness (m)

supernus

Ordovician

131

acuminatus

188—198

Hirnantian

Fig. 5.42 Uppermost Ordovician strata on Novaya Zemlya (after Koren’ et al. 1997)

Rhuddanian

Regional and Global Correlation of the Latest Ordovician to Early Silurian Shale Gas-Bearing Strata

Silurian

5

8805-2;8 8804-1 8804-2

Blackstone River section correlates with the most productive shale gas-bearing strata in Yangtze. Isotopic analyses of eNd and d13C indicate that sea level fall during the Hirnantian Stage is both preceded and followed by transgressive intervals and black shale deposition (Fig. 5.50). In Arctic Canada, the Cape Phillips Formation comprises Ordovician to lower Devonian graptolitic shale and coarser clastic rocks (Thorsteinsson 1958). A continuous sequence

Pristiograptus concinnus

Rastrites sp. Diversograptus sp.

Demirastrites decipiens valens

Globosograptus crispus

Streptograptus cf. exiguus Streptograptus plumosus

M onograptus rickardsi M onograptus veles Stimulograptus halli Stimulograptus sedgwickii

Monograptus (?) cf. praetestis

Campograptus communis communis

8801-1

Rhaphidograptus vicinus

8801-1/3 8802

Normalograptus angusts Normalograptus normalis Normalograptus aff. rectangularis Glyptograptus ex gr. tamariscus

42 50 55 70 79

8801-3/4

34 40

8801-9/7 8801-8/2 8801-4/4 8801-4/3

>75

acumi- vesiculosus– triangu- convolutus – turriculatus sedgwickii natus cyphus latus (s.l.)

8801-11/3 8801-11/3a

persculptus

LM7–LM8 LM9–N2 LM4–LM5 LM6 LM2–LM3

LM1

Aeronian

Hirnantian

U.Ord.

Ord.

Rhuddanian

Llandovery

Silurian

Telychian

spiralis/ grandis

8312-2

Coronograptus sp. Stomatograptus candensis Monograptus acus Monograptus marri Monograptus priodon

Retiolites angustis simus Stomatograptus grandis grandis

282

centrifu-

8803-7/4

Cystograptus vesiculosus Parakidograptus acuminatus

sakma- gus–riccar ricus tonensis

8803-16/1

Monograptus (Penerograptus) cf. austerus praecousor

Thickness (m)

biozones

Neodiplograptus modestus

Yangtze biozones Russian

Sheinwoodian

Wenlock

System Series Stage

Pristiograptus aff. regularis Spirograptus turriculatus Oktavites conspectus Torquigraptus proteus Oktavites spiralis Cyrtograptus lapworthi

X. Chen et al. Monoclimacis asiatica Monoclimacis cf. linnarssoni Monoclimacis ex gr. vomerina Pristiograptus ex gr. dubius Pristiograptus nudus

132

Fig. 5.43 Llandovery stratigraphic column of Novaya Zemlya, Russia (after Sobolevskaya and Koren’ 1997)

of Llandovery strata along the Peel River was described by Lenz (1982), and the Diceratograptus mirus Biozone (corresponding to the top of WF3) was subsequently recorded by Chen and Lenz (1984). Melchin (1987, 1989) established a graptolite zonation for the uppermost Ordovician through Silurian strata, which is now used as a global reference standard (Figs. 5.51, 5.52, 5.53). From the stratigraphic columns illustrated above, the intervals with the greatest potential for shale gas recovery are relatively thin. With respect to the Yangtse Platform strata, the fastigatus Zone correlates with WF2–WF3, the atavus

Zone to LM4, the acinaces and cyphus zones to LM5, the curtus Zone to LM6, and the convolutus Zone to LM7. 2. West Margin of the Laurentian Continent The reference sections for the latest Ordovician black shale in this region are the Vinini Creek section and the Martin Ridge section of Eureka County, Nevada, USA. (Figs. 5.54, 5.55). Unfortunately, there is an unconformity between the upper Ordovician and the Llandovery, with the Rhuddanian and part of the Aeronian missing.

B

B

9

B

70

L-D gray shale

B

Covered

(LM8 – LM9) 65

Covered

L-D gray shale

25

14

Covered

Black shale

(LM1) Monogr.acinaces -35

Dalmanitina Beds

1m

L-D gray shale

Covered

31

L-D gray shale

covered L-D gray shale

85

covered L-D gray shale

60

L-D gray shale

covered 24

L-D gray shale

(LM7)

L-D gray shale

55

B

B

30

covered

Covered

Cephalogr. cometa

Monogr. convolutus

30

Metabolograptus persculptus

covered

13

Black shale

Light shale

L-D gray shale

L-D gray shale

covered L-D gray shale

23

Covered

L-D gray shale

-30

-18

Covered

15

L-D gray shale

B

4

Dark-gray limestone

L-D gray shale

Covered

B

(LM2 – LM3)

L-D gray shale

L-D gray shale

35

Monogr. acinaces

Covered

L-D gray shale

Monogr. turriculatus

Covered L-D gray shale

L-D gray shale

L-D gray shale

16

Covered L-D gray shale

Covered

L-D gray shale

26

L-D gray shale L-D gray shale

Akidograptus acuminatus

-15

(LM8-LM9?)

Covered

L-D gray shale

B

40

Monogr. revolutus

L-D gray shale

B

27

Covered

5

Covered

L-D gray shale

Monogr. triangulatus

(Lm6)

Covered

6

Monogr. gregarius

7

(Lm6)

L-D gray shale

B

Covered

L-D gray shale

8

L-D gray shale

Cyrtograptus lapworthi

45 L-D gray shale

-10

L-D gray shale

-25

80

L-D gray shale

L-D gray shale

22

L-D gray shale

20

21

29

L-D gray shale

28

Monogr.spiralis

B

17

Covered

Monogr. pectinatus

L-D gray shale

Monograptus spiralis

10

Covered

Covered

L-D gray shale

L-D gray shale

L-D gray shale

Covered

Covered

B

-5

Covered

11

(Lm6)

L-D gray shale

Monogr. acinaces

Maraine

Covered

Monogr. gregarius

B

L-D gray shale

133

-20

Moraine and shale fragments (not primary)

19

Covered

12

L-D gray shale

Covered

L-D gray shale

50

Monogr. griestoniensis

25 Covered

0

Post glacial sand

Regional and Global Correlation of the Latest Ordovician to Early Silurian Shale Gas-Bearing Strata

Monogr. crispus

5

Covered L-D gray shale

Covered L-D gray shale

18

Fig. 5.44 Silurian black graptolitic shale in Bornholm Basin, Denmark (after Figs. 4, 5 of Bjerreskov 1975). Abbrevaitions: L–D gray shale, lightto dark-gray shale; B, K-bentonites

134

X. Chen et al.

sO cean etu

Lap

i-La Per

a

G a n d e ri a

ia Amazon Gondwana W es tA f

Ava

Lau

ren

tia

a

ure

ntia

B a lt ic

loni

~5 30 Ma

a

ric

a

b

~5 10 Ma

c

? ~4 90 Ma

d

?

?

~4 70 Ma

?

e

Rheic Ocean

3. Eastern Margin of the Laurentian Continent Ordovician and Silurian strata on Anticosti Island, Canada, are located at the eastern margin of the Laurentian continent and exhibit continuous, shelly carbonate lithology. Sparse graptolites occur above and below the Ellis Bay Formation, which contains a rich shelly fauna. Unfortunately, the graptolites present are all long-ranging species (Riva 1988). In general, the Ellis Bay Formation correlates with the Wufeng Formation (Fig. 5.56). 4. Scottish Uplands in the Laurentian Marginal Belt

?

Sea of Exploits

As shown in the figures, the latest Ordovician black shale corresponds to that of the Wufeng Formation, but the base of the Llandovery is absent. This indicates that the beds that correspond in age to the Hot Shale of North Africa are absent in Nevada. Another location of the uppermost Ordovician strata in the west margin of the Laurentian continent is at the Trail Creek area of central Idaho (Carter and Churkin 1977). Pleurograptus linearis and Dicellograptus ornatus biozones present in this area, which are overlaid by Lituigraptus convolutus Biozone. Thus, most Llandovery strata are missing in the Trail Creek section. P. linearis Biozone is only about 2.4 m in thickness and the true thickness of the D. ornaus Biozone is difficult to determine (Carter and Churkin 1977). However, these two graptolite biozones mainly correspond to those of WF2 in the Yangtze region.

?

Fig. 5.45 Migration of the Avalonia Block during the Early Paleozoic (after Fig. 2 of Waldron et al. 2014)

The Scottish Uplands were located in the complicated Caledonia tectonic zone (Fig. 5.57, Waldron et al. 2014). Charles Lapworth, one of the founders of graptolites research, and the geologist who erected the Ordovician System, spent three years in his “Lapworth house” studying graptolites at Dob’s Linn. Later, two of his successors, Drs. Elles and Wood, spent 20 years completing their great

5

Regional and Global Correlation of the Latest Ordovician to Early Silurian Shale Gas-Bearing Strata

135

Thickness(m) 177.825 176.25

sedgwickii Zone

174.105 173.60 172.05

Thickness(m)

169.65 51.11 167.1175 166.85 165.225 turriculatus Zone

162.1025

159.875 94.74 39.7

36

triangulatus Zone

32

LM6 28

24

80.84

75.94

145.075

140.11

73.114 cyphus Zone

20 136.125

LM5 17

LM9–LM10 acinaces Zone

LM4 atavus Zone

13

9 124.1025 123.725 5

58.11

2

117.30

acuminatus Zone

LM2–LM3

131.1025

turriculatus Zone (maximus Subzone)

115.50 51.11

Limestone

Pre-sedgwickii Zone graptolitic

Graptolitic mudstone

Ash and ashy mudstone

Modified Ashgill shale mudstone

Fig. 5.46 Llandovery black graptolitic shale in the Spengill section, northern England (after Fig. 8 of Rickards 1970)

136

X. Chen et al. Thickness(m) 15.7

Non-graptolitic mudstone ± nodules

14.3

13.0

Mixed beds atavus Zone

(LM 4)

13.7

Graptolitic mudstone

Thickness(m)

12.0 11.9 11.2 10.6 10.1

cyphus

9.4 9.3

(LM 5)

Zone

24 22.3 22.2 Zone

acinaces Zone

21.7

Thickness(m)

21.4 21.0 20.9

74/37

(LM 7)

3.2

2.2

17.9 17.6

1.2

17.0 16.7

0.4 0.3

persculptus Zone

(LM 4)

atavus

19.6 19.3

Zone

31.2

3.6

convolutus Zone

argenteus Zone 29.4

74/36

magnus Zone

28.2 74/35 F

(LM1)

Fig. 5.47 Llandovery stratigraphic column of the Yewdale Beck section, Lake District (after Fig. 7 of Hutt 1974)

(LM 6)

4.9

acuminatus

5.3

LM 2–LM3

6.2

22.5 22.2

Regional and Global Correlation of the Latest Ordovician to Early Silurian Shale Gas-Bearing Strata 140°W Ordovician (Upper Caradoc and Ashgill) Lithofacies

142°W

138°W

136°W

134°W

132°W

Blackstone River

subd uction

Panthalassa Ocean

100 km

68°N

Equator

-4

68°N

subd uctio n

Anther Arc?

PP

1000 km

-6 -8

Laurentia Willston Basin

rt Vinini Creek

MP 66°N

Monitor Range

bt 30°S

OP

YUKON

ME 140°W

138°W

137

136°W

subduction

5

134°W

132°W

onic Tac

y gen Oro

Lapetus Ocean

Surface relief Epicratonic seas Low Shallower High Deeper

Fig. 5.48 Distribution of the Ordovician–Silurian black graptolitic shale on the Laurentian continent and its west marginal belt (after Fig. 2.1 of Loxton et al. 2011) Fig. 5.49 Ordovician to Silurian black graptolitic shale in the Blackstone River section, Yukon, Canada (after Fig. 2.2 of Loxton 2017)

138

X. Chen et al.

Fig. 5.50 Ordovician–Silurian isotopic analyses of eNd and d13C in the Blackstone River section (after Fig. 2.3 of Loxton 2017)

monographs on graptolite taxonomy (Elles and Wood 1913) (Figs. 5.58, 5.59). These classic works are a standard of graptolite taxonomy and biostratigraphy. Zalasiewicz et al. (2009) recently revised the British graptolite zonation. The correlations between graptolite biozones in the Yangtze of China and Britain is illustrated in Figs. 5.60 and 5.61.

Great Britain, including England, Scotland and Northern Ireland, is part of the Caledonian orogenic zone. Thus, the Ordovician and Silurian black shales are deformed by lateral tectonic deformation.

5

Regional and Global Correlation of the Latest Ordovician to Early Silurian Shale Gas-Bearing Strata

120°W

0

100°W

139

80°W

60°W

200 km

100

80°N

AR

CT

IC

OC

EA

N

9

GH OU in TR Bas N d Carbonate ZE rve Buildup HA Sta

7 75°N

MELViLLE I.

PL

CAPE

BASIN

F AT

OR

M

8

6 BATHURST I. PHILLIPS

10

11 ELLESMERE I.

5

4

3 2 1

DEVON I.

CORNWALLIS I.

Late Llandovery paleogeography and locations of sample sections: 1.Snowblind Creek,75°11'N,93°47'W; 2.Cape Manning,75°27'N,94°21'W;3.Cape Phillips,75°37'N,94°30'W;4.Rookery Creek,75°22'N,95°46'W; 5.Truro Island, 75°18'N,98°08'W;6.Twilight Creek,76°10'N,99°10'W;7.Middle Island,75°53'N,111°54'W;8.Cape Becher,76°17'N, 95°25'W;9.Trold Fiord,78°36'N,84°37'W;10.Huff Ridge,78°34'N,83°32'W;11.Irene Bay,79°04'N,82°15'W

Fig. 5.51 Locations of sections with Upper Ordovician to Wenlock graptolitic shale in Canadian Arctic islands (after Fig. 1 of Melchin 1989)

140

X. Chen et al.

Fig. 5.52 Sections on Cornwallis, Bathurst and Ellesmere islands in the Canadian Artic that correlate with the Wufeng and Lungmachi formations (after Fig. 2 of Melchin 1987)

5

Regional and Global Correlation of the Latest Ordovician to Early Silurian Shale Gas-Bearing Strata

Fig. 5.53 Llandovery to Wenlock age sections in Canadian Arctic islands (after Fig. 2 of Melchin 1989)

141

ornatus(=WFl–WF2) 5

pacificus (=WF3) 15

10

(=WF4)

extraordinarius 20

25

(=LM7)

conv.

Dicellograptus ornatus

Yinograptus disjunctus

Phormograptus connectus

Styracograptus tatianae

Diplograptus rarithecatus Paraplegmatograptus uniformis Parareteograptus sinensis Pleurograptus lui Rectograptus abbreviatus

Climacograptus hastatus Styracograptus mississippiensis

Anticostia fastigata

Anticostia lata

(=LM1)

AER.

30

Dicellograptus cf. complanatus Dicellograptus minor

Anticostia tenuissima Appendispinograptus longispinus

mirus Subzone

persculptus

Hirnantian

Thickness (m)

Anticostia uniformis Dicellograptus cf.anceps Appendispinograptus leptothecalis Appendispinograptus supernus Paraorthograptus pacificus Paraorthograptus uniformis Phormograptus sp. Dicellograptus tumidus Parareteograptus parvus Dicellograptus turgidus Dicellograptus mirabilis Parareteograptus turgidus Diplograptus rigidus Anticostia macgregorae Diceratograptus mirus Appendispinograptus sp. Styracograptus sp. Metabolograptus extraordinarius Metabolograptus ojsuensis Normalograptus ajjeri Normalograptus mirnyensis Normalograptus angustus Normalograptus persculptus Normalograptus elegantulus Normalograptus sp.

Anticostia thorsteinssoni

Katian K

142 X. Chen et al.

Diplograptina

0

Neograptina

Graptolite record

Hirnantian DDO lazarus taxa

Black shale and siliceous lime mudstone with grainstone layers Brown mudstone Lime mudstone

Shale Gray calcareous shale

Lime mudstone with grainstone layers

Fig. 5.54 Katian and Hirnantian graptolite biostratigraphy from the Vinini Creek section, Eureka, Navada (after Fig. 3 of Štorch et al. 2011)

75

50

25

0 105

100

95

Dicellograptus ornatus

Styracograptus mississippiensis

Clayey lime mudstone

Eureka quartz sandstone

Fig. 5.55 Late Katian graptolite succession in the Martin Ridge section, Eureka, Navada (after Fig. 4 of Štorch et al. 2011) 90

85

Graptolite record

Cherty lime mudstone

Interbedded lime mudstone and calcareous mudstone

Yinogaptus disjunctus

Paraplegmatograptus uniformis

Paraotrthograptus uniformis

Diplograptus rarifhecatus

Paraorthograptus pacificus

Appendispinograptus supernus

Styracograptus tatianae

Appendispinograptus longispinus

Rectograptus abbreviatus

Pleurograptus lui

Parareteograptus sinensis

Phormograptus connectus

Parareteograptus parvus

Anticostia tenuissima

110

Anticostia lata

110

Dicellograptus minor

125 Climacograptus hastatus

pacificus (=WF3)

100

ornatus (=WF1–WF2)

Katian

5 Regional and Global Correlation of the Latest Ordovician to Early Silurian Shale Gas-Bearing Strata 143

Thickness(m) 140

Thickness (m)

Lithology

Member 3 2 1

200 m 186 m

Soviet Zones

Brilish Zones

262 m 250 m

Rhabdinoporo sp.

5 4

Dendroides

300 m

Scalarigraptus normalis

2

Scalarigraptus angustus

Becscie Ellis Bay

No graptolite zones

Regional biozones

Ranges of graptolite species

67

5 4

150 m

C. supernus

Dicellogr. anceps

Desmograptus sp.

Orthograptus sp.

Rectogroptus abbreviatus

2

Peirograptus tallax

34 m

Paraclimacograptus decipiens

50 m

Amplexograptus prominens

3

Amplexogroptus latus

100 m

Glyptograptus ct G. hudsoni

Vaureal

135 m

A. prominens

Upper Ordovician

Lower Silurian

Zone

Formation

X. Chen et al.

Series

144

Fig. 5.56 Stratigraphic column across the Ordovician–Silurian boundary on Anticosti Island, Canada (after Fig. 1 of Riva 1988). Scalarigraptus in the figure has been revised to Normalograptus

Laurentia Deformed margin Peri-Laurentian terranes

Baltica Deformed margin Peri-Baltic terranes

GONDWANA Greenland

Nova Scotia

Ganderia Avalonia 500 km Megumia Carolinia Peri-Gondwanan arcs

Scotland Ireland Newfoundland Wales

Fig. 5.57 Scottish Uplands in the Caledonia tectonic zone (after Fig. 1 of Waldron et al. 2014)

BA LT IC A

Peri-Gondwanan terranes

5

Regional and Global Correlation of the Latest Ordovician to Early Silurian Shale Gas-Bearing Strata

Fig. 5.58 Lapworth house at Dob’s Linn (photograph from Chen Xu and Fan Junxuan in 2006)

Fig. 5.59 A correlation between the Wufengian–Lungmachian and Dob’s Linn graptolite biozones (Elles and Wood 1913)

145

146

X. Chen et al.

Fig. 5.60 The latest Ordovician graptolite biozone correlation between Yangtze and Britain (after Fig. 1 of Zalasiewicz et al. 2009)

Fig. 5.61 The early Silurian graptolite biozone in Britain (after Fig. 2 of Zalasiewicz et al. 2009)

System

Series

Zalasiewicz et al., 2006

Age (Ma)

Rickards(1976) Zones Monograptus crispus

Telychian

Monograptus turriculatus (Ra. maximus Subzone) 436

Llandovery

Monograptus sedgwickii Aeronian

439

Rhuddanian

Subzones

Streptograptus sartorius

Torquigraptus carnicus Torquigraptus proteus Spirograptus turriculatus Streptograptus johnsonae Stimulograptus utilis Spirograptus guerichi Pristiograptus gemmatus Paradiversograptus Stimulograptus halli runcinatus Stimulograptus sedgwickii ‘Monograptus’ crispus

Monograptus convolutus

Lituigraptus convolutus

Pribylograptus leptotheca

Pribylograptus leptotheca

Diplograptus magnus

Neodiplograptus magnus

Monograptus triangulatus

Monograptus triangulatus

Coronograptus cyphus

Monograptus revolutus

Lagarograptus acinaces

Huttagraptus acinaces

Atavograptus atavus

Atavograptus atavus 443.7 Orthograptus acuminatus Akidogr. ascensus-Parakidogr. acuminatus

References Amjad S, Hashmi MA, Shali AD, Khan TMA (1995) Impact of monsoonal reversal on zooplankton abundance and composition in the northwestern Arabian Sea. In: Thompson MF, Tirnizi NM (eds) The Arabian Sea. Rotterdain: A. A.Baikema, pp 497–508 Apollonov MK, Bandaletov SM, Nikitin IF (1980) The OrdovicianSilurian Boundary in Kazakhstan. Nauka Kazakh SSR Publishing House Berry WBN (1998) The Arabian Sea: a modern analogue for North African-Southern European Silurian organic-rich graptolite-bearing shales? In: Gutoerrez-Marco JC, Rabano I (eds) Proceedings of the sixth international graptolite conference of the GWG (IPA) and the SW Iberia field meeting 1998 of the international subcommission on Silurian stratigraphy (ICS-IUGS). Mineros: Temas GeologicoMineros, ITGE, vol 23, pp 57–59

Bjerreskov M (1975) Llandoverian and Wenlockian graptolites from Bornholm. Fossil Strata 8:1–93 Boucot AJ, Chen X, Scotese CR, Morley RJ (2013) Phanerozoic paleoclimate: an atlas of Liuthologic Indicators of Climate. SEPM (Society for Sedimentary Geology), Tulsa Branisa L, Chamot GA, Berry WBN, Boucot AJ (1972) Silurian of Bolivia. In: Berry WBN, Boucot AJ (eds) Correlation of the South American Silurian Rocks. The geological society of America, special paper, vol 133, pp 21–31 Carter C, Churkin M (1977) Ordovician and Silurian Graptolite succession in the Trail Creek area, Central Idaho–a graptolite zone reference section. Geological survey professional paper, vol 1020 Chen X, Lenz AC (1984) Correlation of Ashgill graptolite faunas of Central China and Arctic Canada, with a description of Diceratograptus cf. mirus Mu from Canada. In: Nanjing Institute of geology and palaeontology, academia sinica stratigraphy and palaeontology of systemic boundaries in China, Ordovician-

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Regional and Global Correlation of the Latest Ordovician to Early Silurian Shale Gas-Bearing Strata

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Regional and Global Correlation of the Latest Ordovician to Early Silurian Shale Gas-Bearing Strata

Zonenshain LP, Kuzmin MI, Natapov LM (1990) Geology of the USSR: a plate tectonic synthesis. Geodynam series, vol 21. American Geophysical Union Zou CN, Dong DZ, Wang YM, Li XJ, Huang JL, Wang SF, Guan QZ, Zhang CC, Wang HY, Liu HL, Bai WH, Liang F, Lin W, Zhao Q,

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Paleogeography and Paleoenvironment Across the Ordovician–Silurian Transition in the Yangtze Region Qing Chen, Jitao Chen, Wenjie Li, and Zhensheng Shi

Abstract

Paleogeographical and Paleoenvironmental studies across the Ordovician–Silurian transition in the Yangtze region are based on the GBDB database and isopach maps. Sedimentary processes during the Late Ordovician to early Silurian of the Yangtze Platform are reviewed. Microfacies analyses on the shale gas drill cores in Weiyuan and Luzhou are provided as additional data. Keywords










Paleogeography and paleoenvironment GBDB database Isopach maps Microfacies analyses

6.1

Research History

Paleogeographic studies on the Ordovician–Silurian transition (the time of deposition for the Wufeng to Lungmachi formations) in South China can be traced back to the 1950s, when Liu (1955) first utilized paleontology and sedimentary facies data to outline a distribution map of terrestrial and marine areas in the Late Ordovician and Early Silurian. Later, in the 1980s, Mu et al. (1981) drew six paleogeographic maps with the lithofacies and biofacies of the Wufeng Formation at the “graptolite biozone” level. With the development of Paleogeography as a field of study, Chinese scholars compiled a series of Chinese paleogeographic atlases based on sedimentary facies, biological Q. Chen (&)
J. T. Chen
W. J. Li State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology and Center for Excellence in Life and Paleoenvironment, Chinese Academy of Sciences, Nanjing, 210008, China e-mail: [email protected] Z. S. Shi PetroChina Research Institute of Petroleum Exploration and Development, Beijing, 100083, China

facies, and plate tectonics. In these atlases, the paleogeographic maps describe the distribution of land and sea, the evolution of sedimentary facies, and the paleogeographic patterns of the Late Ordovician and early Silurian (Guan et al. 1984; Wang 1985; Liu and Xu 1994). In the 21st century, paleogeographic mapping has developed into an era of quantification and interdisciplinary integration. Feng et al. (2001) compiled a paleogeographic map of South China during the depositional period of the Wufeng Formation, with the “single factor analysis and comprehensive mapping method”. Ma et al. (2009) compiled structure-sequence lithofacies maps for the SS7 supersequence of South China that span the Late Ordovician to Silurian. In addition, comprehensive studies on biofacies-lithofacies palaeogeography gradually developed with the establishment of high-resolution time scales and the accumulation of fossil data. Chen et al. (2004) compiled lithofacies and biofacies palaeogeographic maps of South China for three stages (late Katian, Hirnantian, and early Rhuddanian) during the Ordovician–Silurian transition. Rong et al. (2003) compiled eight lithofacies and biofacies maps of South China from the latest Ordovician to Wenlock. These include the Hirnantian, Late Ordovician (M. extraordinarius Chron), early Rhuddanian (P. acuminatus Chron), early Aeronian (D. triangulatus Chron), middle and late Aeronian (D. convolutus–S. sedgwickii Chrons), early Telychian (S. turriculatus–M. crispus Chrons), late Telychian (O. spiralis Chron), Wenlock, and late Ludlow, Silurian. Since 2010, along with the exploration of shale gas from the Wufeng and Lungmachi formations, a series of related paleogeographic research results emerged. Mou et al. (2011, 2014, 2016a, b) compiled a series of palaeogeographic maps from the Late Ordovician to the Early Silurian (corresponding to the depositional period of the Wufeng and Lungmachi formations) in the Middle and Upper Yangtze regions based on lithofacies analyses. Mou studied the relationship between sedimentary facies and geological

© Zhejiang University Press and Springer Nature Singapore Pte Ltd. 2023 X. Chen et al. (eds.), Latest Ordovician to Early Silurian Shale Gas Strata of the Yangtze Region, China, https://doi.org/10.1007/978-981-99-3134-7_6

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conditions favorable for shale gas formation and recovery, and pointed out the spatio-temporal distribution of the favorable facies belt for the hydrocarbon source rocks. Wang et al. (2015) completed two palaeogeographic maps during the early and late depositional period of the Lungmachi Formation in the Sichuan Basin and its peripheral areas based on stratigraphic sequences and sedimentary characteristics. Zou et al. (2015) drew three palaeogeographic maps of the late Katian, Hirnantian and Rhuddanian–early Aeronian (corresponding to the depositional periods of the Wufeng Formation, the Kuanyinchiao Bed and the Lungmachi Formation, respectively) in the Sichuan Basin and its peripheral areas, and proposed a depositional pattern for marine organic-rich shale: a stable ocean basin with low subsidence rate, high sea level, semi-enclosed water body, and low deposition rate. Zhou et al. (2014, 2017) constructed five lithofacies paleogeographic maps from the Sandbian (Late Ordovician) to Telychian (early Silurian) in the Middle and Upper Yangtze regions according to the concept of “structures control basins; basins control facies; facies control basic geology of oil and gas”, and summarized the spatio-temporal distribution and sedimentology of source-reservoir-cap rocks. Based on sedimentary characteristics and stratigraphic thickness from shale gas wells and sections, Nie et al. (2017) completed three paleogeographic maps of the late Katian, Hirnantian, and early-middle Rhuddanian in the Sichuan Basin and its peripheral areas. Sun et al. (2018) constructed four paleogeographic maps of

Q. Chen et al.

the late Katian, Hirnantian, Rhuddanian, and Aeronian, and studied the sedimentary process and depositional model for the black shales. It is worth mentioning that graptolite biozonation was employed in the last two studies for stratigraphic subdivision and correlation, which greatly improved the precision of the time scales. In recent years, with the development of the Internet, geological databases, and Geographic Information System (GIS), paleogeographic research and mapping have met new opportunities and challenges. Paleogeographic studies based on the above-mentioned technologies have been gradually applied to the Ordovician–Silurian transition in South China. Compared with traditional qualitative or hand-drawn paleogeographic maps, the new generation of quantitative and comprehensive paleogeographic maps produced from databases and GIS technology has many advantages, which are mainly reflected in the following aspects. (1) They cover a wider range of scientific information from various disciplines, such as mineral petrology, stratigraphy, paleontology, paleoecology, tectonics, and paleomagnetics. (2) The amount of data used in map construction is much larger, often involving hundreds of sections and hundreds of different types of geological data items (Figs. 6.1, 6.2). (3) The resolution of the data and maps is higher, and at the same time, based on the current database, network, and computer technology, the collection, sorting and analysis of the massive data set can be completed quickly. (4) Using the Internet, experts in multi-disciplinary fields can collaborate

Fig. 6.1 Distribution of the sections of the Wufeng Formation in the GBDB database. The number of sections is 433, covering an area of approximately 527,800 km2. The data comes from: http://www.geobiodiversity.com/

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Paleogeography and Paleoenvironment Across the Ordovician–Silurian Transition in the Yangtze Region

Fig. 6.2 Distribution of the sections of the Lungmachi Formation in the GBDB database. The dots indicate the Lungmachi Formation, and the squares indicate the Kaochiapien Formation. The number of

to facilitate data collection and quality evaluation, thereby ensuring the efficiency of data integration and data quality (Fan et al. 2016). Based on the GBDB database and GIS technology, Chen et al. (2014a) constructed a high-resolution paleogeographic lithofacies map for the Wufeng Formation and stratum thickness maps of different graptolite biozones in South China. Based on paleoecological, stratigraphic and sedimentological data, and using databases and GIS methods, Zhang et al. (2014, 2016) calculated paleo-water depths of different areas in the Upper Yangtze region during the Hirnantian (depositional period of the Kuanyinchiao Bed), and quantitatively reconstructed the two- and three-dimensional paleotopography at that time. In summary, in order to meet the needs of oil and gas exploration, the major research trend is to focus on important biological, environmental and geological events, and utilize the integrated data from stratigraphy, sedimentology, geochemistry, oil and gas geology, and the techniques of database and GIS. These data are used to produce a high-resolution geochronologic framework constructed by biostratigraphy and sequence stratigraphy, to carry out temporal and spatial high-precision, high-resolution, large-scale, practical and quantitative studies on the paleogeography of South China during the Ordovician–Silurian transition.

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sections is 538, covering an area of approximately 606,200 km2. The data comes from: http://www.geobiodiversity.com/

6.2

Distribution Patterns of Black Shales During the Ordovician–Silurian Transition in South China

6.2.1 Geological Setting The distribution of the Ordovician marine strata in South China basically followed the Cambrian platform-slope-basin pattern, which is the Yangtze Platform, Chiangnan Slope and Zhujiang Basin. In the Early–Middle Ordovician, the Yangtze Platform was dominated by shallow-water carbonate rocks, which are distributed in the south of Kangding– Guangyuan–Hanzhong–Chengkou–Xiangfan–Wuhan–Jiujiang–Hefei, and in the north of Wenshan–Duyun–Jishou– Yueyang–Shitai–Hangzhou. To the southeast, transitional facies and the deep-water shales are characteristic of the Chiangnan Slope and Zhujiang Basin, respectively. In the Late Ordovician, the platform–slope–basin paleogeographic pattern gradually changed to the platform–depression pattern (Fig. 6.3). The black shales of the Wufeng and Lungmachi formations were deposited on the Yangtze Platform from the late Late Ordovician to the early Silurian.

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Fig. 6.3 Schematic diagram of the transformation of the paleogeographic pattern in South China from the Ordovician to Silurian. a the platform–slope–basin pattern in the Early and Middle Ordovician; b the platform–depression pattern from the latest Ordovician to the early Silurian

6.2.2 Lithofacies and Biofacies from the Late Katian to Early Hirnantian The Late Katian to early Hirnantian in the Late Ordovician was the depositional interval of the Wufeng Formation, which was equivalent to the D. complexus–M. extraordinarius biozones (Chen et al. 2000). During this period, due to the development of the Kwangsian Orogeny and the advancement of the Cathaysian Old Land from south to Fig. 6.4 Distribution of the Wufeng Formation and correlative sediments in South China (Chen et al. 2014a)

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north, South China became separated by land areas in the west, south and southeast, forming a vast and semi-enclosed Yangtze epicontinental sea (Chen et al. 2014a). Except for the Hunan–Hubei Submarine High, the seafloor was mainly flat. The stable and semi-restricted marine environment led to an abundance of plankton in seawater and a large area of anoxia near the seafloor, providing the critical redox conditions for the formation and widespread distribution of the graptolite-bearing Wufeng Formation black shale (Chen et al. 1987; Rong and Chen 1987). Based on the stratigraphic data from 389 sections and related analyses of paleoecological and sedimentary characteristics, Chen et al. (2014a) constructed a paleogeographic lithofacies and biofacies map during the late Late Ordovician in South China, and identified five regions with different sedimentary facies (Fig. 6.4). In Fig. 6.4, the red line indicates the northern boundary of the South China Block; the black line indicates the boundary between areas with or without deposition; the black dashed line indicates the uncertain boundary; and the thin dotted line indicates the boundary between different lithofacies. The yellow dots indicate the sections where the Wufeng Formation or coeval strata are absent; the green triangles indicate sections with deposition of sandstones or siltstones; the green squares indicate sections with deposition of limestones or limestones interbedded with shales, the red stars indicate key sections with the typical black shale deposition of the Wufeng Formation, which also contain precise biostratigraphic sub-division and thickness data. Dark-blue diamonds indicate sections with typical deposition of the Wufeng Formation and the total thickness data; blue dots indicate sections only known to contain typical Wufeng

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Paleogeography and Paleoenvironment Across the Ordovician–Silurian Transition in the Yangtze Region

Formation black shale, but no further data. A, B, C, D and E indicate regions where the Wufeng Formation crops out or with correlative strata of different sedimentary facies; and F indicates areas without sediment in the study interval.

(A) Black shale deposition area: The Wufeng Formation, composed mainly of black carbonaceous and siliceous shales with abundant graptolites, is widely distributed in the Upper and Lower Yangtze regions. Although its thickness varies from place to place, it is generally less than 10 m thick. The Wufeng Formation is characterized by its wide distribution, small and consistent thickness, and uniform biofacies and lithofacies, which collectively indicates an environment with slow deposition rate, flat seafloor, and weak hydrodynamics during deposition. In consideration of the paleogeography and paleoecology from the Katian to Hirnantian stages, all the evidence indicates that the Wufeng Formation was deposited in a shallow-water, platform environment. Radiolarians in the Wufeng Formation were also shallow-water species, rather than pelagic forms (Wang and Zhang 2011). As a consequence, the semi-restricted paleogeography of the Yangtze epicontinental sea, its stagnant seawater, and severely anoxic seafloor led to the formation and development of the Wufeng Formation black shale. In addition, there are many K-bentonite beds in the Wufeng Formation, indicating volcanic activity during that period. Nitrogen fixation by volcanic activity provided sufficient nutrients for photosynthetic autotrophic organisms, which promoted primary producers such as diatoms and cyanobacteria, and provided a high organic matter source for the black shale (Ohkouchi et al. 2015). Fortey and Cocks (2005) considered that a global warming event (Boda Event) has occurred in the late Katian, which might also have had a positive impact on the plankton productivity and black shale deposition of the Wufeng Formation. Chapter 8 of this book also discusses the relationships among volcanic activity, plankton productivity and organic matter enrichment in black shale. (B) Mixed facies area: The mixed biological facies area in southern Shaanxi is mainly distributed along the periphery of the Hannan and Central Sichuan old lands. For example, in the Nanzheng area of southern Shaanxi, the Nanzheng Formation is composed of fine siliciclastic rocks that are dominated by mudstone and siltstone. A diverse fossil fauna with organisms of different environmental preferences are found in this formation, including benthic taxa (brachiopods, trilobites, bivalves, gastropods, ostracods, bryozoans, and crinoids), nektonic taxa (nautilus, conodonts and trilobites) and

(C)

(D)

(E)

(F)

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planktonic (graptolite) taxa (Li and Cheng 1988), indicating a normal-marine, shallow-water environment. Mixed facies area: In southeast Sichuan and northeastern Yunnan, the Daduhe (Hu 1980; Mu et al. 1981; Tang et al. 2017) and Tiezufeike formations (Mu et al. 1979) contain mixed limestone and shale facies. The two formations consist mainly of limestones interbedded with shale or sandy shale, and graptolites occur in the shales. The shallow-water limestones are rich in benthic algae, which may have improved the anoxic seawater environment through photosynthesis (Li et al. 2002). This area, which existed as a bay, was surrounded by lands in the north, west, and south, all of which served as a relatively stable supply of terrigenous siliciclastics. Benthic algae formed a small and limited carbonate platform. Flysch facies area: Hundreds of meters of flysch were deposited from central and southern Hunan to northeastern Guangxi, and include strata of the Tianmashan and Tianlingkou formations. The Tianmashan Formation, in central and southern Hunan, is a set of dark-gray, slightly metamorphosed fine-grained quartz sandstone, siltstone and slate, with a thickness of 900– 1000 m (Liu and Fu 1984). The Tianlingkou Formation is mainly distributed in northeastern Guangxi, and has also a package of sandstone and slate with flysch characteristics, and a thickness of more than 700 m (Chen et al. 1981; Tang et al. 2013). These thick strata were deposited in the southern Hunan–northern Guangxi depression area, which was surrounded by lands to the east, south, and west, and represent a rapid, low-maturity clastic accumulation with rapid basement subsidence. Carbonate and flysch facies area: This facies is distributed at the conjunction area of Anhui, Zhejiang and Jiangxi. The lithostratigraphic units in southern Anhui, northeastern Jiangxi and northwestern Zhejiang include, in ascending order, the Sanqushan, Xiazhen, Changwu, Xinling and Yuchien formations (Li et al. 1984; Zhan and Fu 1994; Zhang et al. 2007). The depositional history is very obvious in this area, representing a continuous evolution from shallow-water carbonate platform to slope and finally deep-water turbidite and flysch basin. Most of these formations are hundreds of meters thick, indicating continuous, rapid subsidence through the Ordovician and Silurian transition (Rong and Chen 1987; Rong et al. 2010b). Weathering and denudation area: The western and southern margins of the entire South China depositional area were surrounded by a series of old lands during the Late Ordovician, including the Hannan, Chengdu, Qianzhong (Central Guizhou), and Cathaysia land

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masses. The older strata, middle to lower Ordovician, or Cambrian, are exposed in some of these old lands. On the Hunan–Hubei Submarine High in the center of the Upper Yangtze basin, the Wufeng Formation is also entirely or in part missing. It is not clear whether this area was uplifted above the sea surface and exposed as a topographic high, or was a submarine disconformity. In order to examine the temporal and spatial distribution patterns of the Wufeng Formation, Chen et al. (2014a) modeled the dynamic changes in the geographic distribution of the Wufeng Formation black shale based on the stratigraphic data from 389 sections (Fig. 6.5), and used ArcGIS software to calculate their sedimentary variables, such as distribution area, rock volume and mean thickness. The total volume of the Wufeng Formation black shale is about 6  1012 m3, and the average thickness is about 5.87 m. Construction of the isopach maps by graptolite biozones shows that the distribution area of the Wufeng Formation was the largest in its earliest depositional stage, i.e., the period from the D. complexus Zone to the lower subzone of the P. pacificus Zone in the late Katian. It covered nearly the entire Yangtze Platform, and was thickest in three depositional centers in Northwest Hunan, North Chongqing, and North Hubei. In the abovementioned three zones, the distribution area of the Wufeng Formation gradually decreased, and some areas with no shale deposition appeared in the center of the epicratonic sea and expanded over time. In the early Hirnantian, the period of the M. extraordinarius Biozone, the distribution area of the Wufeng Formation black shale was the smallest, and was subsequently overlain by Fig. 6.5 Distribution and isopach maps of the Wufeng Formation in four graptolite biozones (Chen et al. 2014a). The isopach maps were interpolated via the Ordinary Kriging method in the Geostatistical Wizard tool in ArcGIS. Red lines indicate the northern margin of the South China Block; black lines indicate the area enclosing all sections of the Wufeng Formation distribution area

Kuanyinchiao limestone (Chen et al. 2014a). The areas where the Wufeng Formation is missing are mainly in the Hubei–Hunan conjunction area (Hunan–Hubei Submarine High) (Mu et al. 1981; Chen et al. 2001; Fan et al. 2011; Chen et al. 2014a), in northeastern Hunan, eastern Hubei to southern Anhui (Chen et al. 2018a), and in northern Guizhou along the northern margin of the Central Guizhou Old Land (Rong et al. 2011). However, in southeastern Sichuan to southern Chongqing, which is the core area of the shale gas exploration at this stage, sedimentation was continuous without any obvious discontinuity throughout the late Ordovician. The reduction in the distribution area of the Wufeng Formation was most likely a result of the expansion of the Gondwana continental ice sheets that occurred at the end of the Ordovician and the accompanying glacio-eustatic sea-level fall (Rong 1984; Sheehan 2001; Chen et al. 2004, 2005; Fan et al. 2009).

6.2.3 Lithofacies and Biofacies in the Middle Hirnantian The deposition of the Kuanyinchiao Bed occurred in the mid-Hirnantian Stage (late M. extraordinarius–early M. persculptus graptolite biozones). The first large-scale glacial event in the Phanerozoic also occurred at this time. The global climate became colder and sea level dropped sharply, precipitating a series of biological and geological events. During this period, due to the superimposed effects of global sea level fall and the Kwangsian Orogeny, the Yangtze Platform was divided into two parts (Chen et al. 2018a). At

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the same time, ocean circulation increased significantly due to climatic cooling, which refreshed the stagnant and anoxic state of the seafloor. A set of thin but widely distributed argillaceous limestones, the Kuanyinchiao Bed, which is rich in brachiopods and trilobites and represents a cold water and oxygen-rich environment, was deposited. The Kuanyinchiao Bed is only a few meters thick in South China, and although it is thicker in northern Guizhou, it is still commonly less than 10 m (Rong et al. 2010a). Based on the data from 342 Hirnantian sections (outcrops and drill cores) from the online Geobiodiversity Database, Zhang et al. (2016) reconstructed the geographic distribution of the Kuanyinchiao Bed in the Upper Yangtze region using GIS software (Fig. 6.6a). The results show that the Upper Yangtze region in the mid-Hirnantian was surrounded by four major old lands: the Central Sichuan Old Land in the west, the Dian–Qian–Gui Old Land in the southwest, and the Cathaysian and Jiangnan old lands in the east. The Kuanyinchiao Bed is widely distributed in most areas of the Upper Yangtze region, ubiquitously in northeastern Yunnan, northern Guizhou, southeastern Sichuan, Chongqing and western Hubei. Coeval strata with different lithologies only occur in the border area of the Upper Yangtze region. The Kuanyinchiao Bed is thickest to the north of the Dian-Qian-Gui Old Land and parallel to the paleo-shoreline (Fig. 6.6b). Zhang et al. (2016) also reconstructed the palaeotopography of the mid-Hirnantian Upper Yangtze region by inferring the palaeo-water depth values of each section according to its sedimentologic, palaeontologic and palaeoecologic characteristics (Fig. 6.6c). The result shows an apparent palaeogeographic pattern of “one uplift and three depressions”. The uplift represents the Hunan–Hubei Submarine High, while the three depressions comprise the southeastern Sichuan, the central Hunan and the northern Hubei depressions. The semi-restricted and stagnant palaeogeographic setting is one of the key factors that influence the deposition of the organic-rich black shales of the Wufeng and Lungmachi formations in the Upper Yangtze region. The southeastern Sichuan depression and its neighboring area are presently the major areas for shale gas exploration, and where three important gas fields have been found—the Jiaoshiba, Changning and Weiyuan areas. The submarine high, which appeared in the late Katian and disappeared in the mid-Rhuddanian, acted as a barrier restricting water exchange between the depressions and the open ocean. As a consequence, the isolated and stagnant water in the depression promoted production of organic-rich sediment. Similar to the facies variation observed in strata equivalent to the Wufeng Formation, depositional areas exhibiting disparate lithofacies and biofacies are equivalent in age to the Kuanyinchiao Bed. For example, in northeastern Sichuan and southwestern Shaanxi, the upper part of Nanzheng Formation is about 1.35 m thick, is composed of sandy

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shale, siltstone and shale, and contains conulariids, sponge spicules, brachiopods, lamellibranchs, cephalopods, trilobites, ostracods and graptolites (Liu 1986). In northeastern Yunnan and southwestern Sichuan, the upper parts of the Daduhe Formation and the Tiezufeike Formation are composed of limestone, calcareous mudstone interbedded with shale and siltstone, and contain trilobites and graptolites. In northeastern Guangxi and central Hunan, the upper parts of the Tianmashan Formation and the Tianlingkou Formation are hundreds of meters thick, mainly composed of feldspar-quartz sandstone, siltstone and mudstone, and contain turbidite sedimentary features, such as the Bouma sequences, groove casts and flute casts (Liu and Fu 1989, 1990; Chen et al. 1981; Tang et al. 2013). In the middle Hirnantian, in the central part of the Upper Yangtze region, the Hunan–Hubei Submarine High was uplifted and extensively distributed across the area. Hirnantian strata are absent at many sections in Enshi, Xuan’en, Changyang, Wufeng in Hubei, and Zhangjiajie in Hunan. Regolith of various thicknesses occurs beneath the unconformity surface (Wang et al. 2011, 2013). The coastline of northern Guizhou also moved northward during the middle Hirnantian, and at the same time an archipelago formed (Rong et al. 2011).

6.2.4 Lithofacies and Biofacies from the Late Hirnantian to Aeronian This period spans the late graptolite M. persculptus Biozone to the S. sedgwickii Biozone, and represents the major time of deposition for the Lungmachi Formation black shale. The beginning of the Silurian was an important time geologically, when the southern hemisphere glacial event ended, global sea level rose rapidly, and marine life began to recover quickly. During this period, black shale was developed globally, and is well known from North America, Siberia, Baltica, Kazakhstan, Czech, Middle East and South China (Melchin et al. 2013). The Lungmachi Formation was widely distributed in the Upper Yangtze region from the late Hirnantian to Aeronian, except in northern Guangxi and southern Hunan, where the strata are dominated by sandstone and siltstone of the Zhoujiaxi Formation (Fig. 6.7). There are, however, some stratigraphic gaps in the lower part of the Lungmachi Formation in different locations across the Hunan–Hubei Submarine High (Fan et al. 2011). In the Lower Yangtze region, the Anji Formation, which is mostly composed of sandstone and siltstone, was deposited in southern Anhui and North Zhejiang; and the Kaochiapien Formation, which is composed of black shale and silty shale, was deposited in middle-southern Anhui and the Ningzhen Mountains in the southern Jiangsu (Zhang et al. 2007).

158 Fig. 6.6 Paleogeographic lithofacies map (a), isopach map (b) and palaeotopographic map (c) of the Kuanyinchiao Bed in the Upper Yangtze region during the mid-Hirnantian (Zhang et al. 2016)

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Fig. 6.7 Early Rhuddanian palaeogeographic map with the distribution of litho- and biofacies on the Yangtze Platform. Solid circles indicate continuous deposition in the early Rhuddanian, whereas open circles indicate absence of the stratigraphic interval (modified from Chen et al. 2004)

The Lungmachi Formation is widely distributed in the Upper Yangtze region, and its stratigraphic distribution patterns can be explained as resulting from the stage-progressive distribution pattern from Guizhou to Chongqing (Chen et al. 2017), and from the circumjacent distribution pattern in the area bordering East Chongqing, West Hubei and Northwest Hunan (Chen et al. 2018a). The stage-progressive distribution pattern of the Lungmachi Formation black shale is based on the biostratigraphy of well-studied sections in the Guizhou, Chongqing and Sichuan areas. The distribution of the Lungmachi Formation black shale can be subdivided into four geographic terrains from south to north, with the lower boundary of the Lungmachi Formation black shale gradually becoming older from south to north, and the upper boundary gradually becoming younger (Fig. 4.3). The fourth geographic terrain from Dingshan to Huayingshan, is the area with the greatest potential for shale gas exploitation due to the widest distribution area and greatest thickness of black shale. The changes in basin water depth occur in two major stages. The first stage is named the transgressive distribution stage, which ranged from the Metabolograptus persculptus Biozone (LM1) to the Coronograptus cyphus Biozone (LM5), an interval mostly controlled by global sea level rise. The second stage, ranging from the Demirastrites triangulatus Biozone (LM6) to the Spirograptus guerichi Biozone (LM9), is named the regressive shrinking stage, during which the black shales were gradually replaced by mixed-facies or carbonate facies from the south to the north, representing the effects of the persistent uplift of the Central Guizhou Old Land. In the area bordering East Chongqing, West Hubei and Northwest Hunan, stratigraphic hiatuses of variable

durations occur within the Rhuddanian to early Aeronian (Llandovery, Silurian). The gap distribution suggests the existence of a local uplift, traditionally named the Yichang Uplift. The diachronous nature of the basal black shale of the Lungmachi Formation across different belts of the Yichang Uplift indicates different stages of uplift development. Overall, the closer a section is to the rising center of the Yichang Uplift (near Wufeng County, Hubei), the larger its stratigraphic gaps are, which also become smaller toward the peripheral areas, and gradually disappear as the stratigraphic contact between the Wufeng and Lungmachi formations becomes conformable. From the middle-late Aeronian to the early Telychian, the black shale in the lower part of the Lungmachi and Kaochiapien formations in the Upper and Lower Yangtze regions of South China gradually changes to gray (gray-green, yellow-green after weathering) shales in the upper part. The abundance and diversity of graptolites and the total organic carbon content of rocks decrease significantly. Above these beds, the upper part of the Lungmachi and Kaochiapien formations comprise other lithostratigraphic units with different lithofacies, such as the Shihniulan, Hsiaohopa, Lojoping and Houjiatang formations. The large-scale black shale deposition characteristics of the early Silurian in South China ended.

6.3

Microfacies Analyses of the Shale Gas Drill Cores in Weiyuan and Luzhou Areas

In order to study the detailed lithological characteristics of the black shales in the Wufeng Formation and the lower part of the Lungmachi Formation, five drill cores from wells near

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Luzhou and Weiyuan in the Sichuan Basin were selected to carry out the microfacies research. The five drill cores are: Lu205, located *20 km northwest of Luxian County; Lu202, *38 km southwest of Luzhou City; W210, *38 km northeast of Weiyuan County; W231, *55 km northeast of Weiyuan County; W204, *15 km east of Weiyuan County. A generalized stratigraphic description of the five cores is as follows: the Linhsiang Formation is mainly composed of bioclastic and bioturbated calcareous mudstone; the Wufeng Formation consists mainly of homogeneous, discontinuously laminated, silty mudstone with radiolarian siliceous laminae, indicating a quiet environment where deposition is dominated by settling from suspended mud; the Lungmachi Formation is mainly composed of mudstone, laminated silty mudstone, and banded siltstone, indicating relatively deep water as a result of the global sea level rise in the post-glacial period. Vertical lithofacies changes within the Wufeng Formation are relatively small, whereas the Lungmachi Formation shows strong vertical facies changes through different graptolite zones. Detailed photomicrographs and descriptions of microfacies from these five cores are presented in Figs. 6.8, 6.9, 6.10, 6.11, 6.12, 6.13 and 6.14. For the codes of the graptolite biozonation, please refer to Fig. 1.1 of this book.

Fig. 6.8 Typical microfacies of the Wufeng Formation in the core Lu205. a depth 4041.25 m, WF2; b depth 4037.93 m, WF2; c depth 4034.97 m, WF4; d depth 4033.87 m, WF4

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6.3.1 Core Lu205 The Wufeng Formation (Fig. 6.8): discontinuous, lenticular muddy laminae with radiolarian siliceous bands; occasionally weak bioturbation and grading; mainly homogeneous with faint lamination; few fossils including radiolarians and lm-sized needle-like bioclasts. A few trilobite fragments occur at the bottom of this formation. The overall microfacies analysis indicates a quiet, anoxic environment with a relatively stable and slow deposition rate. The bottom of the Wufeng Formation shows similar sedimentary features to those of the underlying Linhsiang Formation, suggesting a gradual transition in depositional settings. LM1–LM3 (Fig. 6.9a, b): laminated silty mudstone, horizontal laminae of siltstone and silty mudstone, partly with grading and few bioclasts, indicating low-energy, anoxic environments with periodic input of terrigenous fines. LM4 (Fig. 6.9c, d): laminated silty mudstone with laminae of bioclasts. The bioclasts are mainly composed of brachiopods and radiolarians; brachiopods are preserved as single shells and arranged horizontally, either concave or convex upwards, suggesting quiet, anoxic environments with intermittent rapid deposition. LM5–LM6 (Fig. 6.9e, f): laminated silty mudstone to mudstone with silt, slightly bioturbated; quartz particles becoming less common upward,

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Fig. 6.9 Typical microfacies of the Lungmachi Formation in the core Lu205. a depth 4031.5 m, LM1–LM2; b depth 4029.12 m, LM3; c depth 4026.01 m, LM4; d depth 4025.12 m, LM4; e depth 4023.75 m, LM5; f depth 4006.90 m, LM6; g depth 3969.04 m, LM7; h depth 3895.55 m, LM7

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Fig. 6.10 Typical microfacies of the Wufeng and Lungmachi formations in the core Lu202. a depth 4331.20 m, WF2; b depth 4327.05 m, WF3–WF4; c depth 4324.09 m, WF3–WF4; d depth 4321.88 m, WF3–WF4; e depth 4321.54 m, Kuanyinchiao Bed; f depth 4321.05 m, LM1

with sporadic bioclasts, indicating reduced water energy upward. LM7 (Fig. 6.9g, h): laminated bioclastic silty mudstone, homogeneous mudstone, and cross-laminated siltstone, suggesting an increase in the terrigenous input, deposition rate and water energy.

6.3.2 Core Lu202 WF2–WF4 (Fig. 6.10a–d): homogeneous mudstone, faintly laminated silty mudstone, with occasionally lm-scale bioclasts that are composed mainly of brachiopod shells and echinoderm fragments. The shells are arranged horizontally, either concave or convex upward, indicating a quiet, anoxic environment with insufficient terrigenous input and low

deposition rate; sediment deposited mainly from suspension settling. The Kuanyinchiao Bed (Fig. 6.10e): bioclastic calcareous mudstone, with uncommon bioclasts that are supported by matrix. The bioclasts are mainly composed of brachiopod and trilobite fragments, with a few corals, and the bioclasts are randomly arranged, indicating quiet and oxic environments that are suitable for the benthic organisms survival, no reworking by currents or waves posterior to deposition. The Lungmachi Formation LM1 (Fig. 6.10f): horizontally laminated silty mudstone with no bioclasts or bioturbation, suggesting a low-energy, anoxic and deep-water environment with periodic terrigenous input, corresponding to the global sea level rise with the contraction and melting of the Gondwanan ice sheets.

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Fig. 6.11 Typical microfacies of the Wufeng Formation in the core W210. a depth 3251.54 m, WF2; b depth 3251.36 m, WF2; c depth 3249.56 m, WF3; d depth 3249.15 m, WF3

6.3.3 Core W210

6.3.5 Core W204

WF2–WF3 (Fig. 6.11a–d): homogeneous, flaser- or horizontal-laminated mudstone and silty mudstone. Few quartz grains, a small number of bioclasts, and weak bioturbation can be seen at the bottom; a few randomly arranged bioclasts and weak bioturbation occur upward. LM5–LM8 (Fig. 6.12a–d): parallel-laminated argillaceous siltstone. The terrigenous clastics are dominated by quartz grains which are significantly increased compared to the Wufeng Formation; bioclasts are rare, indicating an increased deposition rate and terrigenous input. LM9 (Fig. 6.12e–f): horizontally laminated silty mudstone with no bioclasts.

WF2–WF3 (Fig. 6.14a, b): homogenous mudstone with a small number of bioclasts and weak bioturbation at the bottom, suggesting quiet-water, suboxic conditions. The Kuanyinchiao Bed (Fig. 6.14c, d): argillaceous bioclastic wackestone and packstone, with relatively high bioclastic components, matrix- to grain-supported. The bioclasts are composed mainly of echinoderms, brachiopods and trilobites, which are randomly arranged; quartz particles are present. The microfacies overall suggest an oxic environment which is suitable for benthic organisms, with a relatively rapid deposition rate, and no subsequent reworking. LM1–LM9 (Fig. 6.14e–h): horizontally laminated silty mudstone, with radiolarian siliceous laminae. The microfacies of the Wufeng and Lungmachi formations in this core are very similar, and are dominated by massive mudstone, with few coarse-grained terrigenous clastics and some planktonic radiolarians, indicating a relatively deep, quiet-water, and anoxic environment away from the terrigenous input.

6.3.4 Core W231 WF2 (Fig. 6.13a, b): massive, with weak horizontal-laminated mudstone, with a small number of bioclasts and weak bioturbation, and uncommonly with silt. LM5–LM7 (Fig. 6.13c, d): horizontal-laminated mudstone, occasionally with weak low-angle cross-lamination. The microfacies suggest upward increase in hydrodynamics and terrigenous input.

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Fig. 6.12 Typical microfacies of the Lungmachi Formation in the core W210. a depth 3247.67 m, LM5; b depth 3247.42 m, LM5; c depth 3246.41 m, LM6; d depth 3228.44 m, LM8; e depth 3226.39 m, LM9; f depth 3210.82 m, LM9

6.4

Sedimentary Processes During the Late Ordovician to Early Silurian

The end of the Ordovician through the beginning of the Silurian is a significant period of transition in the Earth’s surface system. A series of important biotic, climatic, and environmental events all occurred during this interval (Fig. 6.15). This geologic time interval is also an important one for oil and gas resources in China, and globally, with the development of widely distributed black shale with high organic matter. The end-Ordovician extinction was the first of the five main mass extinction events of the Phanerozoic,

and also the first mass extinction of metazoans. Approximately 26% of families, 49% of genera and 86% of the species became extinct at that time (Sepkoski 1981; Brenchley et al. 2001; Sheehan 2001). This is also the only mass extinction that is directly related to climate crash and glaciation. Therefore, a comprehensive and interdisciplinary study of this period of critical Earth transition will help us understand the co-evolution of organisms and paleoenvironments, and will also have important theoretical significance for the exploration and production of the unconventional oil and gas resources. The end-Ordovician extinction was thought to have occurred in two primary pulses, each of which is directly

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Fig. 6.13 Typical microfacies of the Wufeng and Lungmachi formations in the core W231. a depth 3887.58 m, WF2; b depth 3887.40 m, WF2; c depth 3886.30 m, LM5; d depth 3885.11 m, LM6–LM7

related to the onset and demise of glaciation (Sheehan 2001). The first pulse occurred in the late Katian to the early Hirnantian, and was initially attributed to the onset and development of the Gondwanan glaciation, which led to a rapid decline in temperature and global sea level, and, in turn, to a reduction in habitat and biodiversity (Brenchley et al. 1994; Finnegan et al. 2012). However, the latest sedimentological and sequence-stratigraphic studies suggest that this pulse was instead related to global warming and sea level rise during the deglacial period (Ghienne et al. 2014). The second pulse, which occurred in the late Hirnantian, was primarily caused by ocean anoxia (even euxinic conditions) as suggested by various geochemical proxies, including sulfur isotopes, statistics of pyrite grain size, redox sensitive elements, iron speciation, and uranium isotopes (Zhang et al. 2009; Jones and Fike 2013; Bartlett et al. 2018; Zou et al. 2018). The global occurrence of black shale during this period is also direct evidence of an anoxic seafloor. However, recent studies show that both the patterns of extinction and recovery (two pulses versus one pulse), and their mechanisms (e.g., cold versus warm climate, sea level rise versus fall, anoxic versus oxic seafloor) are controversial. In this section, we focus on the latest studies on sedimentary facies, palaeogeography, palaeoclimate and palaeo-ocean environment from the Late Ordovician to

early Silurian in South China, and discuss controls on the deposition and distribution of black shales in the Wufeng– Lungmachi formations.

6.4.1 Transition Between the Linhsiang Formation and the Wufeng Formation In most areas of the Yangtze Platform, the Wufeng Formation black shale overlies limestone strata (Fig. 6.16). These strata include: the widely distributed Linhsiang Formation, which is composed mainly of thin-bedded to nodular limestones; the Chientsaokou Formation, which is composed of argillaceous limestone, nodular limestone and calcareous mudstone, and distributed in northern Guizhou; and in the Lower Yangtze region, the Tangtou Formation, which is mainly composed of nodular limestone, argillaceous limestone and mudstone. The conspicuous lithological transition between the Wufeng Formation black shale and the underlying carbonate in most parts of the Yangtze area (Fig. 6.16) represents the demise of the carbonate platform that existed from the Sandbian to middle Katian, and which was replaced by fine-grained siliciclastic rocks in the late Katian. There are several reasons for the demise of the carbonate platform, including shutdown of carbonate factories due to marine

166 Fig. 6.14 Typical microfacies of the Wufeng and Lungmachi formations in the core W204. a depth 3360.07 m, WF2; b depth 3359.31 m, WF3; c depth 3358.22 m, Kuanyinchiao Bed; d depth 3358.22 m, Kuanyinchiao Bed; e depth 3357.91 m, LM1; f depth 3356.68 m, LM2; g depth 3353.74 m, LM4; h depth 3313.25 m, LM9

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Fig. 6.15 Stratigraphy, with biological and environmental events during the Ordovician–Silurian transition (after Brenchley et al. 2006; Harper et al. 2014)

Fig. 6.16 Contact relationship between the Wufeng Formation black shale and the underlying limestone of the Linhsiang Formation. a Laomucheng section, Yanjin, Sichuan; b Huangying section, Wulong, Chongqing

anoxia, exposure of the carbonate platform due to sea level fall followed by inundation by rapid sea level rise, and a large amount of terrigenous input. There are numerous sedimentological studies on the Upper Ordovician in the Yangtze area of South China, many of which, however, reach different conclusions regarding the disappearance of the carbonate platform that existed from the Pagoda Formation to the Linhsiang Formation. Chen and Qiu (1986) thought that the Wufeng Formation black shale formed in a semi-enclosed stagnant basin environment on the Yangtze Platform, rather than in the deep sea. Yan et al. (2011) proposed that the transition from the Linhsiang Formation to the Wufeng Formation represented an important inundation event of the carbonate platform, resulting from relative sea level rise caused by tectonic deflection. Mou et al. (2014) believed that deposition of the Wufeng Formation was related to the expansion of the uplift region and the intensified structural enclosure from the late Katian to the Hirnantian under the intra-plate collision and compression of South China. We thought that the deposition of the Wufeng Formation was related to the paleogeographic evolution of the Yangtze Platform. In the latest Ordovician, the Kwangsian Orogeny led to continuous progression of the Cathaysia Block (Chen et al. 2014b). To the southeast, the open sea of South China was sub-aerially exposed, making the Yangtze Platform a semi-closed, stagnant-water environment. In addition, the global climate might have been in a relatively warm period prior to the Hirnantian glaciation (Finnegan et al. 2011), and

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South China was located in low latitudes, both of which were negative factors for seawater thermohaline circulation. This may have resulted in stagnant water circulation, rise of the redox interface, and the wide distribution of the Wufeng Formation black shale. On the other hand, prolonged weathering of the old lands surrounding the Yangtze Sea during the Ordovician might have provided fine siliciclastics, which led to the formation of a shale- and silt-dominated lithofacies. The Daduhe Formation, characterized by mixed carbonate rocks and mudstone, was deposited in the southwest corner of the Upper Yangtze Sea (Tang et al. 2017), indicating that there might have been small carbonate factories in the coastal area. The anoxic sea water in most parts of the Upper Yangtze region was an extremely unfavorable environment to benthic organisms (e.g., corals and brachiopods), and therefore, carbonate production could not be maintained. However, surface water still exchanged freely with atmosphere and thus remained oxic, which had little impact on planktonic organisms (e.g., graptolites). Therefore, there are abundant graptolite fossils in the Wufeng Formation. Furthermore, the contemporaneous deformation structures developed in the Wufeng Formation on the southern margin of the Yangtze Platform and in the coeval Sanqushan Formation in the Zhewan Depression also suggest that paleogeographic evolution affected the depositional processes of the Wufeng Formation black shale. Zhao et al. (2014) identified contemporaneous deformation structures in the Wufeng Formation near the Dianzhong Old Land, which mainly include small folds and stepped faults between undeformed layers. These structures show certain trends along the Changning–Qijiang–Xiushan and Gulin–Tongzi– Songtao transects, which are roughly consistent with the west–east strike of the Central Guizhou Old Land. The contemporaneous deformation structures in the Wufeng Formation mainly occur in the Dicellograptus complexus– Paraorthograptus pacificus graptolite biozones, whereas the underlying and overlying layers often contain K-bentonites, indicating volcanic activity. The aforementioned contemporaneous deformation structures were thought to have formed on a steep slope under multiphase crustal movement triggered by volcanic events. The ancient seafloor topography was controlled by the Central Guizhou and Yichang uplifts caused by the collision between the Yangtze and Cathaysia blocks in the Late Ordovician (Zhao et al. 2014). It is worth noting that in slump structures, stepped faults generally appear near the proximal part of the slump sheet (Lewis 1971), and nearly upright plunging folds are common in the head and translational zones (Cardona et al. 2020). Therefore, according to Zhao et al. (2014), the slump and deformation structures in the Wufeng Formation might have moved from the east to the west along the Central Guizhou Old Land. The deformation structures within the

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Wufeng Formation may also be related to the interlayer dislocation caused by tectonic compression in the same or late periods. We identified some folds in the Wufeng Formation in the Honghuayuan section in Tongzi, Guizhou and the Huangying section in Wulong, Chongqing. These deformation structures also developed between intact stratigraphic layers interbedded with K-bentonites (Fig. 6.17). However, they generally lack soft sediment deformation features. These deformation structures are accompanied by the dislocation and deformation of the overlying strata, which suggests a close relationship with the later tectonic uplift. However, the deformation structures of the upper Katian strata in South China were not limited to the Wufeng Formation near the Central Guizhou Old Land. In the Jiangshan–Changshan–Yushan area (JCY area) of Zhejiang and Jiangxi, a variety of contemporaneous deformation structures occurred in the Huangnehkang and Sanqushan formations in the late Katian, including slump folds, plastically deformed debris flow deposits, and contemporaneous S-shaped cracks. These structures were likely controlled by the uplift of the Cathaysia Old Land, which resulted in a steepening slope and related seismic activity (Li et al. 2019). The stepwise uplift of the Cathaysia Old Land and the formation of the Hunan–Hubei Submarine High were both closely related to the Kwangsian Orogeny (Chen et al. 2014b), and the subsequent transformation of the paleogeography partially controlled the deposition processes of the Wufeng Formation.

6.4.2 Depositional Processes of the Kuanyinchiao Bed The Kuanyinchiao Bed, which sits between the Wufeng Formation and Lungmachi Formation black shales in South China, consists mainly of argillaceous limestone or calcareous mudstone (Fig. 6.18). It is widely distributed in the continuous Ordovician–Silurian transitional succession of the Upper and Lower Yangtze regions. Although the South China Block was located at lower latitudes during the Hirnantian, the brachiopod fauna in the Kuanyinchiao Bed was a cold-water fauna, indicating glaciation and climate cooling at the end of the Ordovician. The age of the Kuanyinchiao Bed is interpolated by the graptolite biozones of the overlying and underlying strata. The Kuanyinchiao Bed correlates with the upper part of the M. extraordinarius Biozone (WF4) to the lower part of the M. persculptus Biozone (LM1) in South China, although its base could extend down to the top of the P. pacificus Biozone (WF3) in some nearshore sections (Fig. 6.19, Rong et al. 2002). The global sea level fall that resulted from the expansion of Gondwanan glaciation during the Hirnantian

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Paleogeography and Paleoenvironment Across the Ordovician–Silurian Transition in the Yangtze Region

Fig. 6.17 Deformation structures in the Wufeng Formation of Huangying Section, Wulong, Chongqing. Yellow arrow indicates the dislocation and deformation of the strata overlying the deformed layer; white arrow indicates the K-bentonite layers

Fig. 6.18 Sedimentary features of the Kuanyinchiao Bed in outcrops and thin section. a Shuanghe section in Changning, Sichuan—the Kuanyinchiao Bed is 15 cm thick and contains a large number of brachiopods; the Kuanyinchiao Bed in this outcrop is combined with black shale to form a uninterrupted natural layer. b Wangjiawan section in Yichang, Hubei—the Kuanyinchiao Bed is 17 cm thick and contains fragmentary trilobites and brachiopods; the argillaceous content of the Kuangyinchiao Bed is high. c, d Brachiopods in the Kuanyinchiao Bed of Wangjiawan section. e Brachiopod shells in the Kuanyinchiao Bed of Huangying section in Wulong, Chongqing. The shells are arranged almost horizontally (yellow arrow), and mostly preserved by single shell. f Wanhe section in Yongshan, Yunnan—the bioclastics of the Kuanyinchiao Bed is composed of brachiopods and trilobites which are arranged with a certain orientation; small corals and a few silt-size quartz particles can be found

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Fig. 6.19 Age and correlation of the Kuanyinchiao Bed at different localities in South China and other regions or countries (Rong et al. 2002). 1. Ganxi, Yanhe, NE Guizhou; 2. Ludiping, Songtao, NE Guizhou; 3. Yangliugou and Shichang, Renhuai, NW Guizhou; 4. Shanwangmiao, Honghuayuan, Tongzi, N Guizhou; 5. Gusong, Xingwen, S Sichuan; 6. Shuanghe and Helong, Changning, S Sichuan; 7–8. Wangjiawan and Huanghuachang, Yichang, W Hubei; 9.

Guantangyuan, Wuning, N Jiangxi; 10. Beigong, Jingxian, S Anhui. 11. Yankou, Lin’an, N Zhejiang; 12. Wanyaoshu, Luxi, W Yunnan; 13. Zhiwazuogu, Shenza, south-central Tibet; 14. Yewdale Beck, N England; 15. Satun Province, S Thailand; 16. Kosov, the Prague area, Czech; 17. Precordillera de San Juan, Argentina; 18. Holy Cross Mountains, S Poland

was expressed differently in the sedimentary record on the various paleo-plates around the world. Indeed, even different stratigraphic sections of the same plate record the sea level fall differently. Different sedimentary systems (with separate palaeogeography, palaeogeomorphology, and deposition rates) may respond differently to global sea level changes. In addition, relative sea level changes vary significantly with regional tectonics. According to the records of the Gondwana tillites and global sea level changes at low latitudes, it is thought that glaciation began at the Katian–Hirnantian transition, which largely corresponded to the base of the Kuanyinchiao Bed, when the dominant fauna changed from graptolites to cold-water brachiopods in the Upper and Lower Yangtze regions. During the Hirnantian glaciation, the sea level fall, enhanced ocean thermohaline circulation, and seafloor oxygenation led to the resurgence of benthic organisms, such as brachiopods, trilobites and a small number of corals (Rong et al. 2002; Wang et al. 2018). Global sea level fall during the deposition of the Kuanyinchiao Bed resulted in gaps of various lengths in the underlying strata. For example, in the Taiyanghe section in Enshi, the M. extraordinarius Biozone (WF4) of the Wufeng Formation and the Kuanyinchiao Bed is missing (Fig. 6.20); in the core areas of Yichang Uplift, such as in Wufeng County, the entire Wufeng Formation is missing (Chen et al. 2018b). Sedimentation did not resume in certain place until the late Rhuddanian to Aeronian. This suggests that these areas were sedimentary highlands, where sea level fall led to either non-deposition and/or erosion of existing deposits. The subsequent sea level rise resulted in resumption of sedimentation (Fig. 6.20).

6.4.3 Deposition of Lungmachi Formation The global occurrence of black shale in the Rhuddanian (early Silurian) is attributed to global warming, sea level rise, and the explosion of marine primary productivity during deglaciation at the end of the Ordovician. Chen et al. (2005), and Rong and Zhan (2006) thought that climatic cooling and the Gondwanan glaciation at the end of the Ordovician led to the mass extinction, and vacancy of a large number of ecological niches. This in turn, led to the radiation of primary producers and disaster species at the end of the Ordovician and beginning of the Silurian. When the climate warmed at the beginning of the Silurian, metazoan faunas gradually recovered, but diversity did not rebound to pre-extinction levels until the Aeronian. The absence of metazoans and abundance of primary producers laid the material foundation for the source rocks in Rhuddanian, and accounts for the high TOC in the lower part of the Lungmachi Formation. The recovery of metazoan faunas in the Aeronian inhibited primary productivity and resulted in the gradual TOC decline in the upper part of the Lungmachi Formation. Therefore, the formation of source rocks in the lower part of the Lungmachi Formation in South China is closely related to biological evolution. Conversely, paleoenvironmental conditions are also an important factor in the preservation of substantial amounts of organic matter. Su et al. (2007) proposed that the major controlling factors for the deposition of the Ordovician– Silurian black shale in South China are two-fold: northwestward migration and foreland-basin subsidence caused by deformational loading related to episodic accretion of the Cathaysia Block onto the Yangtze Block during this period;

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Fig. 6.20 a The Lungmachi Formation (LM5, C. cyphus Biozone) directly overlies the Wufeng Formation (WF3, P. pacificus Biozone) in the Taiyanghe section in Enshi, Hubei, suggesting a substantial unconformity between the Ordovician and Silurian (Wang et al. 2011); b Schematic diagrams of the depositional models during the Ordovician– Silurian transition

the anoxic sediment-starved water column caused by the rapid rise of the sea level during the two successive phases of the 3rd-order sea level rise at the Ordovician–Silurian transition in South China. Wang et al. (2008) considered that photosynthesis played an important role in the production of organic matter in the early Silurian. Organic matter accumulated in a marginal depression of the Yangtze Craton, and its preservation benefited from the regional anoxic environments of northern Gondwana. Yan et al. (2008) stated that the organic-rich black shale in the Lungmachi Formation was related to high production and burial during the rapid rise of sea level after the Hirnantian glaciation, and adsorption of clay minerals during the preservation of organic matter. Melchin et al. (2013) noted that black shales occurred at a wide range of localities, paleolatitudes, and depositional settings in the early Rhuddanian. They proposed that the development of black shale in the high paleolatitude Gondwanan and peri-Gondwanan regions might have resulted from a large influx of nutrients released from sediments by the retreat of the Gondwanan ice sheets, which possibly combined with strong oceanic stratification induced by high rates of freshwater input from melting ice. Additionally, a major mode-shift in the thermohaline circulation regime contributed to the spread of anoxia in the world’s oceans during the Rhuddanian.

6.5

Bioevents and Environmental Change from the Late Ordovician to Early Silurian

6.5.1 Global Cooling and Sea Level Changes The two pulse biological extinction at the end of the Ordovician corresponded to the start and end of the Gondwanan glaciation. Therefore, many researchers believed that the two processes are closely related (Sheehan 2001; Brenchley et al. 2003; Finnegan et al. 2011). The glaciation was precipitated by global cooling. Conodont oxygen isotopes and carbon–oxygen clumped isotopes of brachiopods and corals (Trotter et al. 2008; Finnegan et al. 2011) suggest that global sea surface temperature dropped by about 5 °C (Fig. 6.21), which led to the extinction of marine faunas that were adapted to the greenhouse climate. In addition, at the end of the Ordovician global sea level dropped by *80– 100 m due to glacial expansion and large areas of epicontinental seas disappeared and land was exposed, which resulted in a habitat reduction for marine fauna (Sheehan 2001). These two factors may have jointly led to the first pulse of the extinction. The subsequent, rapid increase in global temperature and sea level after climate rebound and

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Timescale

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Llandovery

Wenlock

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Fig. 6.21 Tropical seawater temperature trend through the Ordovician and Silurian transition (modified from Trotter et al. 2008; Finnegan et al. 2011). Abbreviations: Dap., Dapingian; Rhud, Rhuddanian

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C-O clumped isotope paleothermometry (Finnegan et al., 2011)

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glacial retreat led to the weakening of seawater circulation and ocean stratification, which accounted for the second pulse of the extinction. However, the disappearance of large portions of epicontinental seas and the subsequent erosion or non-deposition of sedimentary strata and fossil records may have led to a serious bias in diversity calculations. Finnegan et al. (2012) examined the Middle Ordovician–early Silurian North American fossil occurrences to quantify rock record effects on a per-taxon basis and assessed the interplay of

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macrostratigraphic and macroecological variables in determining extinction risk. Their research suggested that the extinction cannot be entirely attributed to rock record failure, indicating that glacio-eustatic fall and tropical oceanic cooling played important roles in the first pulse of the end-Ordovician mass extinction. Additionally, previous studies also disagreed about the duration of the late Ordovician glacial events. Based on the carbon and oxygen isotopic data from brachiopod shells in a few sections in Europe and North America, Brenchley et al. (1994)

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Paleogeography and Paleoenvironment Across the Ordovician–Silurian Transition in the Yangtze Region

considered that the Late Ordovician glaciation mainly occurred in the Hirnantian and lasted only 0.5–1 Myr. The correlation between the glaciation and mass extinction needs further study.

6.5.2 Ocean Redox Changes Mass extinction events in geological history when accompanied by ocean anoxic events, such as the end-Permian and end of the Triassic events, are mostly related to global warming or hyperthermal events (e.g., Jenkyns 2010; Jost et al. 2017; Clarkson et al. 2018; Zhang et al. 2018). The Hirnantian ocean anoxic event (HOAE) was also thought to occur in the global warming period after the Hirnantian glaciation, and was caused by an increase in temperature and sea level (Melchin et al. 2013; Harper et al. 2014). Global warming enhanced continental weathering, which promoted

Fig. 6.22 Stratigraphic, glacio-eustatic, d238U, d13Ccarb, and d18Oapatite records from the western Anticosti Island, Canada (modified from Bartlett et al. 2018). Gray shading represents the two Late Ordovician mass extinction pulses: LOME 1 (Lower) and LOME 2 (Upper). Abbreviations: LOGC, Late Ordovician glacial cycles; HOAE, Hirnantian ocean anoxic event; HICE, Hirnantian isotope carbon excursion; SC, Schmitt Creek; Prin., Prinsta; LFB, Laframboise; PDB, PeeDee belemnite; V–SMOW, Vienna standard mean ocean water

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the importation of sediments and nutrients into the ocean. It led to increased primary productivity and oxygen consumption, which expanded the oxygen minimum zones. At the same time, the rise of sea level expanded the anoxic water onto the shelf, eventually leading to mass extinction (Brenchley et al. 1994; Melchin et al. 2013). Bartlett et al. (2018) examined d238U and d18Oapatite from marine limestone on Anticosti Island, Canada, and compared their trends with the global sea level and bio-events (Fig. 6.22). The d238U trends suggested that anoxic conditions were coincident with the onset of the second pulse of the end-Ordovician extinction, supporting the interpretations that widespread anoxia was an important aspect in triggering the extinction. However, anoxic conditions lasted longer than the biotic recovery. This study also showed that the Hirnantian anoxic event was initiated during the sea level highstand prior to the peak late Hirnantian glaciation, and continued through the glacio-eustatic lowstand and the

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Away from the ice cap

Close to the ice cap

Anti-Atlas, Morocco

(after Cocks and Fortey, 1986; Bassett et al., 2009; Vandenbroucke et al., 2009)

(after Bourahrouh et al., 2004; Colmenar et al., 2019)

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end-Ordovician extinction (Harper et al. 2014; Wang et al. 2019). The age interpretation of strata on Anticosti Island mainly relies on benthic fossils (e.g., brachiopods and corals) and isotope chemostratigraphy, and not on Ordovician index fossils (i.e., graptolites and conodonts), which makes identification of the two graptolite K-biozones in Hirnantian of Anticosti Island questionable. In addition, a detailed sedimentologic and sequence-stratigraphic study on this section (Ghienne et al. 2014) suggests that it contains several unconformities during the Ordovician–Silurian transition. Therefore, the chronostratigraphy of this section and its correlation with other global sections still require further study. The resolution of the stratigraphic correlation will affect our understanding on the details of the relationship between paleoclimate, sea level changes, anoxic events and biotic events. Wang et al. (2019) reviewed the macro-evolutionary sequence of global biodiversity and benthic assemblages (e.g., corals and brachiopods) at the end of the Ordovician (Fig. 6.23). They integrated graptolite biostratigraphic data, and revised the conodont and chitinozoan biostratigraphy

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ensuing early Silurian deglaciation and sea level rise. Bartlett et al. (2018) concluded that the anoxic event was driven by global cooling and glaciation, which reorganized thermohaline circulation and decreased deep-ocean ventilation. Moreover, it enhanced continental weathering during the glacial period and aeolian-derived nutrient fluxes promoted surface productivity, which expanded oxygen minimum zones. However, Bartlett et al. (2018) inferred the highest peak of the Late Ordovician glaciation was at the end of the Hirnantian (the upper part of the M. persculptus Biozone) based on the relative sea level changes on Anticosti Island. This conclusion is, however, at odds with the glacial records of Gondwana. Based on sedimentary successions with precise biostratigraphic constraints from Gondwana, glacial diamictite and other glacial deposits mainly appeared in early Hirnantian, roughly equivalent to those of the M. extraordinarius Biozone, which suggests that glaciation largely took place at this time. The onset of the glaciation was roughly at the Katian–Hirnantian boundary, and was considered to be the main trigger for the first pulse of the

Fig. 6.23 Three biostratigraphically well-constrained Hirnantian successions at high latitudes of the Gondwana and their correlation with glaciation (simplified from Wang et al. 2019). Abbreviations: SSM, Soom Shale Member; DSM, Disa Siltstone Member

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Paleogeography and Paleoenvironment Across the Ordovician–Silurian Transition in the Yangtze Region

and carbon isotope chemostratigraphy. They then re-interpreted the end-Ordovician extinction as a rapid single-pulse event that was followed by a prolonged initial recovery intermittently impacted by climatic fluctuations through the Hirnantian. Based on a precise biostratigraphic framework, Zou et al. (2018) studied Fe speciation, Mo concentrations, and the chemical index of alteration from four sections of different paleogeographic environments in South China in order to reconstruct ocean redox conditions and climate through the Late Ordovician and early Silurian. The results showed that there were two cycles of expanded euxinia corresponding to the two pulses of the LOME, suggesting a strong causal relationship, and that global cooling played a secondary role in driving the first pulse of extinction for certain low-latitude taxa (Fig. 6.24).

6.5.3 Volcanic Events Volcanic events may cause severe damage to environments and organisms, and are often interpreted as the trigger mechanisms of biotic events (e.g., the end-Permian, end-Triassic and Cretaceous–Paleogene mass extinction events). Hg concentrations and isotopes are a newly developed geochemical proxy for volcanism (e.g., Thibodeau et al. 2016; Jones et al. 2017; Grasby et al. 2019; Shen et al. 2019). Jones et al. (2017) tested the Hg concentrations in marine strata from the Wangjiawan section in Hubei, South China and the Monitor Range section in Nevada, US (Laurentia), and found that there were three Hg enrichment intervals associated with the end-Ordovician extinction. The second interval occurred immediately below the Katian– Hirnantian boundary, which marks the first pulse of extinction (Fig. 6.25). They proposed that the Hg enrichment was the products of multiple phases of large igneous province volcanism, which was a primary driver of the environmental changes that caused mass extinction. Furthermore, the expansion of ice sheets and associated albedo enhancement might be attributed to sulphate aerosol emitted from volcanoes (Jones et al. 2017). Direct evidence of volcanism is from the volcanic ash layers (K-bentonite) in the strata (Buggisch et al. 2010).

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Yang et al. (2019) discovered about 100 K-bentonite layers across the Ordovician–Silurian transition from the Lunshan section in the Lower Yangtze region of South China. They found that the U–Pb dates from the K-bentonite layers agreed with other age calculations of the end-Ordovician mass extinction, which supported the theory that volcanism may have been one of the important drivers for climate changes and mass extinction.

6.5.4 Paleogeography Saupe et al. (2020) combined climate models with species trait simulations to explore the degree to which different palaeogeographic boundary conditions and magnitudes of cooling and glaciation can explain the relative intensity of marine extinction during greenhouse–icehouse transitions in the Late Ordovician and the Cenozoic. The identical changes in sea surface temperature caused higher extinction rates with a Late Ordovician continental configuration than with a Cenozoic continental configuration. This result was due to the relative paucity of long north–south oriented coastlines spanning large latitudinal ranges and the relative abundance of small islands and terrains in the Late Ordovician as compared to the Cenozoic paleogeography (Fig. 6.26). As the climate cooled in the Late Ordovician, it was difficult for marine organisms (especially benthic faunas) to migrate to warm low-latitude areas along continental margins without crossing large areas of open ocean, which increased the risk of extinction. The mass extinction event and environmental perturbations during the Ordovician–Silurian transition is one of the most studied geological events in the Phanerozoic. Nevertheless, there are still many controversies about the mode of mass extinction and its trigger mechanisms. Only by carrying out comprehensive and interdisciplinary studies within a more precise stratigraphic framework can we better understand the relationship between environmental changes and biotic evolution in deep-time Earth history. Understanding the relationship between physical and biologic events in Earth history will then help us predict the impact of current global changes on ecosystems from the perspective of a long-term geological perspective.

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Fig. 6.24 Climate changes and redox conditions across the Yangtze Shelf Sea during the Ordovician and Silurian transition in South China (after Zou et al. 2018). a Simplified paleogeographic map of the Yangtze Shelf Sea showing section localities (scale bar = 100 km). Red circle represents the Shuanghe (SH) inner shelf section; green circle represents the Qiliao (QL) mid-shelf section; pink circle represents the Tianba (TB) outer shelf-slope section; black circle represents the Nanbazi (NBZ) shallow inner-shelf section. b Schematic cross section of the Yangtze Shelf Sea showing estimated paleo-depths.

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c Regional d13Corg excursions from Shuanghe (SH), South China, Dob’s Linn (DL), Scotland, Blacktone River (BR), Canada in Laurentia, and Bellegrav (BL), Denmark in Baltica. d Summary of climate and redox changes across the Yangtze Shelf Sea. e Schematic illustrating redox dynamics across the Yangtze Shelf Sea. Abbreviations: D. cn., Dicellograptus complanatus; D. cx., Dicellograptus complexus; M. e., Metabolograptus extraordinarius; M. p., Metabolograptus persculptus; A. a., Akidograptus ascensus; LOME, Late Ordovician mass extinction

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Fig. 6.25 d13Corg and Hg concentrations across the Ordovician–Silurian transition in the Wangjiawan section, Hubei, South China (after Jones et al. 2017). Abbreviations: SIL, Silurian; Rhu., Rhuddanian; asc., ascensus; VPBD, Vienna PeeDee belemnite; ppb, parts per billion

Fig. 6.26 Influence of palaeogeography on organisms in the greenhouse–icehouse transition when the sea level and temperature changed (after Saupe et al. 2020). a Hypothetical models, as climate transitions from the greenhouse (left column) to icehouse (right column) conditions, the species’thermal niche shifts toward the equator. The shallow-marine-restricted species along the north–south oriented

continent would be able to track its thermal niche. However, the shallow-marinerestricted species along the east–west oriented continent would have no such option and would be driven to extinction. b Because of different paleogeographic settings, the extinction rate was higher in the Late Ordovician than that in Eocene–Oligocene when the sea level and temperature changed

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Gamma Log Responses Through the Ordovician–Silurian Black Shale Graptolite Zonal Succession in the Middle and Upper Yangtze Regions Qun Zhao, Chao Li, Shasha Sun, and Wei Guo

Abstract

The response characteristics between graptolite biozones and gamma logs have a strong correspondence. Four high GR peaks as GR1 to GR4 occur in ascending order: GR1 is located at the boundary of the Pagoda Formation and the Wufeng Formation; GR2 is located at LM1; GR3 is located within LM3–LM4 or is absent locally; and GR4 is located near the boundary of LM5 and LM6. Keywords

Gamma logs




High GR peaks

Well logging, also called geophysical logging or field geophysics, uses geophysical rock properties like electrochemical properties, electric conductivity, acoustics and radioactivity to measure geophysical parameters. It is widely applied in petroleum exploration and development (Asquith et al. 2004; Bond et al. 2010). In recent years, We participated in the zonation and correlation research of graptolite faunas from a large number of explorative shale gas wells in the Middle and Upper Yangtze regions. The graptolite biozones demonstrate a relatively good relationship with the GR logs. Logging gamma is accomplished by receiving radioactive intensity generated by the decay process of uranium, thorium, potassium and other radionuclides. Radioactivity in different forms from various minerals is enriched and concentrated in organic matter during geo-events in Earth history. Therefore, the response characteristics between graptolite biozones and gamma logs have Q. Zhao (&)
W. Guo PetroChina Research Institute of Petroleum Exploration and Development, Beijing, 100083, China e-mail: [email protected] C. Li
S. S. Sun State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology and Center for Excellence in Life and Paleoenvironment, Chinese Academy of Sciences, Nanjing, 210008, China

strong correspondence. In the present book, based on the graptolite biostratigraphy of cores taken from 37 wells and the GR log analysis of 63 wells, a relationship between graptolite biozones and GR log response is established. This provides an easy, stable, reliable, and convenient method for correlating between black graptolite-bearing shale sections and drill cores in the Middle and Upper Yangtze regions. The gamma curves logged from drill cores are distributed in four areas of the Yangtze region, as shown in Fig. 7.1.

7.1

Theoretical Basis

Graptolites have a rapid rate of morphologic evolution and are sensitive to variations in the physical environment. Changes in Late Ordovician and early Silurian graptolite diversity and abundance are recorded in the black shales of the Wufeng and Lungmachi formations. The graptolite evolutionary events that happened across this interval may coincide with those of simultaneous variation in GR logs. Studying a combination of these biologic and geologic events may help explain the marine environmental change through this critical time interval in Earth history.

7.1.1 Primary Formations The Wufeng Formation is approximately 3–5 m thick in the Upper and Middle Yangtze regions. It consists of muddy limestone and black shale in the Weiyuan area, and black organic-rich shale in the Changning–Zhaotong, Wulong– Wuxi and Yichang–Jingmen areas. The overlying Kuanyinchiao Bed is composed of muddy limestone, marl or calcilutite, and is about 0.1–2 m thick (Mu et al. 1978; Rong 1979). The Lungmachi Formation consists of dark or dark-gray organic-rich shale in its lower part with a thickness of 10–40 m, and 50–80 m of gray shale in its upper

© Zhejiang University Press and Springer Nature Singapore Pte Ltd. 2023 X. Chen et al. (eds.), Latest Ordovician to Early Silurian Shale Gas Strata of the Yangtze Region, China, https://doi.org/10.1007/978-981-99-3134-7_7

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Fig. 7.1 Location map of four areas based on correlation between graptolite biozones and GR log response in the Middle and Upper Yangtze regions

part. The main part of the Lungmachi Formation black shale is of Rhuddanian to Aeronian in age (Mu et al. 1978, 1983; Rong 1979; Chen et al. 2003). However, in a narrow belt of the Upper Yangtze such as the Chengkou–Wuxi belt, the Lungmachi Formation black shale may extend up into the Telychian (Hao and Rao 1997; Gu and Liu 1997).

7.1.2 Geologic Information Recorded in GR Logs The radioactivity in sedimentary deposits originates from the decay of uranium (U), thorium (Th) and potassium in minerals. It recorded the simultaneous deposition in well logging (Wei et al. 2016). The amount of radionuclides in clay and organic matter is related to the sedimentary environment. For example, in borehole logs, shale exhibits high GR, while sandstone and limestone exhibit low GR. Gamma rays are primarily emitted by the decay of uranium, thorium and potassium (K) in rocks (Chen 2017). Gamma spectrometry logging is employed to determine the mineral composition, which is based on different radionuclide combinations. The

organic matter in shale has stronger selective adsorption to radionuclides in the course of deposition (Nie et al. 2016), and the TOC interpreted from GR logs is in good agreement with the analytical value of core samples in the lab (Schmoker 1981; Wei et al. 2016; Nie et al. 2016). Lüning and Kolonic (2003) compared the shale GR logs in several areas of North Africa, and discovered that TOC was strongly and positively correlated with the API value of gamma logs. They then suggested that the gamma value should be used to characterize the TOC of the shale. The changes in GR logs reflect the differences in mineral composition and TOC, and indicate changes in the sedimentary environment. Usually, organic-rich shale with relatively more clay and high TOC exhibits high GR, whereas sandstone and limestone exhibit low GR. The well logging instrument used in the present study is ECS (Element Capture Energy Spectrum logging tool). Data is collected, processed and interpreted by Schlumberger and China National Logging Corporation (CNLC). The GR logs in the well logging series were used to determine parameters like mineral composition and TOC based on the differences

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Gamma Log Responses Through the Ordovician–Silurian Black Shale …

in the radioactive energy spectrum in the strata (Wei et al. 2016). The K-bentonite in the altered volcanic ash deposits from volcanic activities contains more radionuclides, which exhibits an abnormally high GR value in GR logs and records significant geological events. The number of K-bentonites in the study interval indicates that Ordovician to Silurian volcanic eruptions were frequent. More radioactive matter from the deep crust was ejected in the form of volcanic ash, and formed the present K-bentonite beds after deposition. Such laminar K-bentonite beds may be instantaneously (in geological time) and widely distributed, providing a unique and precise event-stratigraphic marker bed that is independent of paleontologic and paleo-magnetic data (Hu et al. 2009a). The volcanic rock formed from erupted deep layers is the main source of radionuclides, and the volcanic ash contains a great deal of radioactive materials (Hu et al. 2009a; Wang et al. 2015). Volcanic ash also provided nutrients for biological growth, promoted the growth and evolution of organic matter (Langmann et al. 2010), and strengthened the corresponding relationship between the graptolite zones and GR logs. Iron is a key nutritive element affecting the productivity of marine organisms. The study on the Kasatochi area by Langmann et al. (2010) showed that volcanic ash was the main source of iron ions in the surface water of the ocean. Volcanic activity enriched the iron content in ocean water, and thus provided sufficient nutrition for the explosive growth of algae (Frogner et al. 2001). Our observations on the Shuanghe profile in the Changning area, the Kuanyinchiao profile in the Qijiang area, and the Huangying and Jiangkouzhen profiles in the Wulong area show that the highest graptolite abundance is associated with the greatest number of volcanic ash beds. Ordovician to Silurian volcanic activity occurred in the Middle and Upper Yangtze regions, formed multiple K-bentonite layers (8–20 layers). These K-bentonites contain a great number of radioactive materials such as uranium, thorium and kalium (Hu et al. 2008, 2009b; Luo et al. 2016; Xie et al. 2016; Xiong et al. 2017). The peak period of Ordovician–Silurian volcanic activity was the Latest Ordovician Hirnantian Stage, which coincides with the second largest biologic extinction in geologic history.

7.2

Curve Characteristics of GR Logs in Black Shale Graptolite Zones

Affected by sedimentation, the GR log curve characteristics of the black shale in the Middle and Upper Yangtze regions vary by region. The present study focuses on the GR log curve characteristics from the Wulong–Wuxi, Weiyuan–Yongchuan, Changning–Zhaotong and Yichang–

185

Laifeng areas. The GR log characteristics in these areas have a consistent relationship with the graptolite biostratigraphy. Four high GR peaks, GR1 to GR4, occur in ascending order. GR1 is located at the boundary of the Pagoda and Wufeng formations; GR2 is located at LM1; GR3 is located within LM3–LM4 or is locally absent; and GR4 is located near the boundary of LM5 and LM6. In general, GR2 is regionally and temporally consistent, but GR1, GR3 and GR4 are inconsistent or locally and temporally absent.

7.2.1 Wulong–Wuxi Area The Wulong–Wuxi area mainly covers Nanchuan, Wulong and Wanzhou (Fig. 7.1). There are four obvious peaks on the GR log of the organic-rich black shales in the Wufeng to Lungmachi formations. They are GR1 (Baota Formation– Wufeng Formation), GR2 (LM1), GR3 (LM4–LM5) and GR4 (LM5–LM6) in ascending order. The log of Well JY1 is a good example, as described below. (1) The first peak of GR1 occurs near the boundary of the Chientsaokou and Wufeng formations (Fig. 7.2). In the Well JY1, the GR value of the Wufeng Formation (WF1–WF4) is 2–3 times higher than that of the Chientsaokou Formation. The Wufeng Formation shale is 4.8 m thick, with TOC of 4.0–5.9% (4.6% on average). (2) The second prominent GR peak (GR2) is near the LM1 graptolite zone (Fig. 7.2) with the highest GR value in the well. The shale above GR1 from a depth of 2391.9 to 2410.6 m has a relative high GR value with a TOC of 1.2–5.6%, and an average value of 3.6%. Three intervals from 2391.9 to 2392.9 m, 2393.5 to 2395.5 m and 2400.1 to 2410.6 m have more than 3% of TOC. The former two intervals have 2–3% of TOC. (3) The third conspicuous GR peak (GR3) is near the boundary of zones LM4 and LM5 (Fig. 7.2). From 2368.1 to 2391.9 m (LM5), the layers have a GR value lower than the strata below GR3 with a TOC of 1.8– 3.8%, an average of 2.4%. Among these intervals, the depths from 2381.5 to 2382.4 m and 2384.0 to 2391.9 m have more than 3% of TOC. Depths from 2379.3 to 2381.5 m have 2–3% of TOC. A TOC value lower than 2% occurs from 2368.1 to 2379.3 m. (4) The fourth observable GR peak (GR4) is near the boundary between LM5 and LM6 (Fig. 7.2). Above the GR4, a lower TOC value of 1.2–2.2% (average 1.8%) exists within the interval of 2348.6 to 2368.1 m. 2–3% of TOC occurs within 2348.6–2353.2 m and 2366.9– 2368.1 m. A TOC value lower than 2% presents in 2353.2–2366.9 m.

186 Fig. 7.2 Log responses through successive graptolite zones in Well JY1 (Logging curve is from He et al. 2016; Graptolite identification and graptolite biozones are based on Chen et al. (2015) internal report)

Q. Zhao et al.

Biozones Formation Depth GR(AP) 300 (m) 0

Graptolite fossils

TOC(wt%) 0.1

10

2320

2330

Oktavites circularis LM8

2340

Stimulograptus halli Stimulograptus sedgwickii LM7

2350

LM6

2360

Lungmachi

Petalolithus minor

Lituigraptus convolutus Pseudoplegmatograptus perlatus Petalolithus folium Monograptus arciformis

Demirastrites raitzhainiensis Glyptograptus cf.tamariscus Rastrites peregrinus Monograptus involutus Demirastrites triangulatus Rastrites peregrinus Rastrites longispinus

2370

GR4

Coronograptus cyphus LM5

2380

Coronograptus cf.cyphus Dimorphograptus confertus “Monograptus” tenellus 2390

Coronograptus cf. cyphus GR3

Pseudorthograptus sp. LM4

2400

LM2–LM3

2410

LM1

Kuanyinchiao

GR2

Wufeng Chientsaokou

WF3–WF4

WF2

Atavograptus gracilis Cystograptus vesiculosus Normalograptus normalis Neodiplograptus modestus Avitograptus avitus Normalograptus mirnyensis Appendispinograptus supernus Paraorthograptus pacificus Anticostia lata Dicellograptus ornatus Dicellograptus complexus Appendispinograptus longispinus

GR1

7.2.2 Weiyuan–Yongchuan Area The Weiyuan–Yongchuan area mainly includes Neijiang and Zigong (Fig. 7.1). There are three strong peaks on the GR log of the organic-rich black shales in the Wufeng and Lungmachi formations. They are GR2 below LM2, GR3 at the boundary of LM4 and LM5, and GR4 near the boundary between LM5 and LM6 in ascending order. The Well W202 (Fig. 7.3) is employed as the reference in this area. (1) The Wufeng Formation consists of limestone and black shale (7.9 m thick) and there is no GR1 peak near the base of the formation (Fig. 7.3). The GR2 peak occurs

at the base of the Silurian strata with a TOC lower than 2%. (2) The first obvious GR peak (GR2) is below graptolite LM2 (Fig. 7.3). The GR value between 2569.0 and 2573.5 m twice as high as in the strata below. The shale in the upper part of LM1–LM4 is 5.9 m thick, with TOC values of 3.5–7.8% (average 5.6%). (3) The second prominent GR peak (GR3) is near the boundary of LM4 and LM5 (Fig. 7.3). The GR value of the shale above the peak (2562.1 to 2569.0 m) is slightly lower than that of the strata below, with a TOC of 2.7–4.9% (3.2% in average). The TOC of the interval from 2565.7 to 2569.0 m is more than 3%, and the

7

Gamma Log Responses Through the Ordovician–Silurian Black Shale …

Fig. 7.3 Log responses through the graptolite zonal succession in Well W202 (Logging curve is from Shi et al. 2017; Graptolite identification and biozones are based on Chen et al. (2015) internal report)

Formation

Depth (m)

GR(API) 500

0

Biozones

187

Graptolite fossils

TOC(w%) 0

10

2490

2500

2510

2520

Lungmachi 2530

2540

Spirograptus guerichi

LM8

Glyptograptus nanjiangensis

Monograptus priodon

2550

LM7 Lituigraptus priodon Cephalograptus cometa

LM6

2560

GR4 2570

Kuanyinchiao Wufeng

GR3

LM5 LM2–LM4

GR2 WF2–WF3 2580

Campograptus cf.communis Rastrites guizhouensis Coronograptus gregarius Monograptus sp . Coronograptus gregarius Pseudorthograptus sp. Korenograptus laciniosus Korenograptus laciniosus Mucronaspis (Songxites) sp. Appendispinograptus leptothecalis Paraplegmatograptus sp. Appendispinograptus supernus

Pagoda

TOC value is of 2–3% from the depths between 2562.1 and 2565.7 m. (4) The third obvious GR peak (GR4) is near the boundary of LM5 and LM6 (Fig. 7.3). The TOC of the shale above the peak (2553.9 to 2562.1 m) is 2.6–5.1%, with 3.3% in average.

7.2.3 Changning–Zhaotong Area The Changning–Zhaotong area mainly includes Yibin and Luzhou in Sichuan, and Zhaotong in Yunnan (Fig. 7.1). There are three GR peaks. GR1 is near the boundary of the Pagoda and Wufeng formations; GR2 corresponds to LM1– lower LM2; and GR3 is diachronous within LM4. The Well YS108 is used as a reference well in the area.

(1) Similar to the Wulong–Wuxi area, the boundary of the Pagoda and Wufeng formations exhibits an abrupt GR1 spike (Fig. 7.4). The TOC value is 3.0–4.2% with an average of 3.8% in the 2510.0–2515.3 m interval. (2) The second obvious GR peak (GR2) is in the lower part of LM1 (Fig. 7.4). The GR value becomes uniform between peaks GR2 and GR3. In the interval from 2496.3 to 2510.8 m, the TOC value is 3.3% in average. A TOC value of more than 3% is between 2497.9– 2504.0 m and 2504.7–2510.8 m. However, the TOC value decreases to 2–3% between 2496.4–2497.9 and 2504.0–2504.7 m. (3) Unlike the regions discussed above, the third strong GR3 peak is located in the middle of LM4 (Fig. 7.4). The GR3 peak in the Well YS108 is of smaller magnitude than it is in other areas. However, the TOC value

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Fig. 7.4 Log responses of across the graptolite zonal succession in Well YS108 (Logging curve is from Chen et al. 2016; Graptolite identification and graptolite biozones are based on Chen et al. (2017) internal report)

Formation Depth GR(API) (m) 0 500

Biozones

Graptolite fossils

TOC(wt%) 0

10

2430

2440

LM6 2450

2460

Lungmachi 2470 Rastrites longispinus

2480

LM5

Coronograptus cyphus

2490

Coronograptus cyphus LM4 2500

GR3' LM3

Cystograptus vesiculosus Cystograptus vesiculosus Korenograptus bicaudatus Parakidograptus acuminatus

LM1–LM2 Kuanyinchiao

2510 GR2

Wufeng GR1

WF2–WF4 WF1

Dicellograptus complexus

Pagoda

suddenly decreases in the interval above GR3. In the Well YS108, the TOC value is 1.6–3.2% (2.1% in average) within the 2473.1–2496.4 m interval. Within the drill core, the intervals from 2474.1–2475.4 m, 2480.9–2486.3 m, and 2487.7–2496.4 m have a TOC value of 2–3%. Intervals from 2473.1–2474.1 m, 2475.4–2480.9 m, and 2486.3–2487.7 m show a TOC value lower than 2%.

7.2.4 Yichang–Laifeng Area The Yichang–Laifeng area mainly includes Yichang, Jingmen and Enshi of Hubei Province, and Sangzhi of Hunan Province (Fig. 7.1). There are three obvious peaks on the GR log of the organic-rich black shales in the Wufeng and Lungmachi formations, GR1, GR2 and GR4. The GR1 peak

is located near the boundary between the Pagoda Formation and the Wufeng Formation; GR2 is between LM1 and the middle of LM3; and GR4 is near the boundary of LM5 and LM6. The GR3 peak is absent, although LM4 and LM5 intervals are present. In the Yichang–Laifeng area, Well J101 provides a reference log for research. (1) The GR value of the Wufeng Formation is 2–3 times higher than that of the Pagoda Formation (Fig. 7.5). The Wufeng Formation shale (WF1–WF4) is 7.6 m thick, with a TOC value of 2.0–8.0%, and an average of 2.4% in the interval between 3134.1 and 3131.0 m. (2) GR2 is located in the graptolite LM1 (Fig. 7.5). There is no GR peak present in LM3 and LM4. The shales in LM1–LM5 is 11.6 m thick, with a TOC of 3.5–8.0% (average 4.1%).

7

Gamma Log Responses Through the Ordovician–Silurian Black Shale …

Fig. 7.5 Log responses of graptolite zones in Well J101 (Logging curve is from Ma 2015; Graptolite identification and graptolite biozones are based on Chen et al. (2015) internal report)

Formation Depth (m)

GR(API) 0

500

Biozones

189

Graptolite fossils

TOC(wt%) 0

10

3050

3060

Monograptus arciformis Normalograptus biformis LM7

3070

3080

Lungmachi

Campograptus communis 3090

Lituigraptus convolutus

Monograptus arciformis

3100 LM6

3110

GR4

3120

LM4–LM5 LM1–LM3

GR2

Kuanyinchiao

Wufeng

3130

WF4 WF2–WF3

Petalolithus minor Rastrites sp. Cystograptus penna Cystograptus vesiculosus Dicellograptus ornatus Tangyagraptus typicus Dicellograptus ex gr.complexus

GR1

Pagoda 3140

(3) An obvious GR4 peak occurs near the boundary between LM5 and LM6 (Fig. 7.5). In Well J101, the GR value of the shale above the peak is higher than that of the shale below the peak. The shale in the upper part of LM6 is 6.6 m thick, with a TOC of 3.0–4.1% (3.2% in average).

7.3

Global Nature of the GR

7.3.1 The Comparison of Black Shale Gamma-Ray Curves from the Middle and Upper Yangtze Regions and North Africa The gamma-ray curve from Ordovician–Silurian black shale in the Middle and Upper Yangtze regions is comparable with that from the North African Hot Shale. GRa, GRb and GRc

are peaks observable in the GR log analysis of Well NC174 in the Murzuq Basin (Lüning et al. 2000, 2005) (Figs. 7.6, 7.7). Within a framework of graptolite biozone correlations, we believe that these three high peaks are approximately equivalent to those of GR2, GR3 and GR4 of the Yangtze region.

7.3.2 The Relationship Between Gamma-Ray Curves and Carbon Isotope Chemostratigraphy of the Ordovician– Silurian Black Shale The Ordovician and Silurian glacial event and subsequent warming event were global, to an extent. The high gamma peak, GR2, correlates with the global warming event, and the low value below GR2 corresponds to the Hirnantian glaciation. When the glaciation occurred at the end of the Ordovician, the value of the organic carbon isotopic ratios

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Q. Zhao et al.

Fig. 7.6 Graptolite biostratigraphy and GR of Well NC174 in the Murzuq Basin (modified from Lüning et al. 2000)

Fig. 7.7 Graptolite biostratigraphy and GR of wells ADO-E-1, SED-1and BRD-4 in the Ghadames and Illizi basins of Algeria (modified from Lüning et al. 2000)

(d13C) became higher in different regions of the world. In Changning of Sichuan, Wangjiawan, Fenxiang and Huanghuachang of Hubei, the value of d13C during the Hirnantian glacial period increased from −31‰, −30.5‰, −29.8‰ and −28.5‰ to −29.3‰, −28.5‰, −26.8‰ and −22.0‰, respectively. They returned to normal values after the glacial period ended (Fig. 7.8). In Arctic Canada, Dob’s Linn, Scotland, Estonia, and Latvia, similar changes in organic

carbon isotopic ratios are observed (Fan et al. 2009; Duan 2011; Wang and Chen 1999). In the Middle and Upper Yangtze regions, the high value of d13C correlates with the low gamma value of GR2. In the Wulong–Wuxi, Weiyuan–Yongchuan and Yichang–Laifeng areas, sea level reached its maximum at the end of LM5. In this interval, the total organic carbon content of the shale is high, and the gamma log records

7

Gamma Log Responses Through the Ordovician–Silurian Black Shale …

Graptolite Zones

Stratigraphy

Shuanghe,Changning, Wangjiawan,Yichang, Sichuan Hubei 13

‰ (Duan,2011)

Rhuddanian

Silurian

-31

-29

-30

13

‰ (Fan et al.,2009)

-31

-30

-29

-28

Fenxiang,Yichang, Hubei 13

‰ (Wang et al.,1997)

-31

-29

-27

191 Huanghuachang, Yichang,Hubei

13

‰ (Wang et al.,1997)

-30

-26

-22

Canada Arctic 13

‰ (Wang et al.,1997)

-30

Estonia&Latovia

Dob’s Linn,Scotland 13

-28 -26 -24

‰ (Wang et al.,1997)

-32

-30

-28

13

0

‰ (Wang et al.,1997)

2

6

4

A.ascensus

Hirnantian

M.extraordinarius

Katian

Ordovician

M.persculptus

P.pacificus

Fig. 7.8 Correlation between organic carbon isotopic curves in different regions through the Ordovician–Silurian boundary

peak GR4. However, in Changning–Zhaotong, the Guangxi Uplift effect led to an early end (LM4) of the marine transgression, and black shale was replaced by the deposition of relatively shallow sediments. Hence, the peak GR4 is absent in this area. According to Melchin and Holmden (2006), the d13C at the Rhuddanian–Aeronian boundary was increasing, which is a globally consistent pattern (Fig. 7.9). In the Changning section of the Middle and Upper Yangtze area, the d13C in the shales also increases. At the end of the Rhuddanian, the d13C increased from −30.0‰ to −28.0‰.

Stratigraphy

Graptolite Zones

Telychian Aeronian

Correlation Between the Hirnantian Isotope Carbon Excursion and Graptolite Biozones in the Ordovician–Silurian Transition of the Xike-1 Drillcore in Guizhou

The Ordovician–Silurian carbon isotope research was firstly carried out by Wang et al. (1997) in the Yangtze region. When the Wangjiawan North section in Yichang, Hubei became the GSSP of the Hirnantian Stage, the organic

Cornwallis Is. Dob’s Linn Shuanghe,Changning,Sichuan (Lehnetr et al.,2007) (Antoshkina,2008) δ13 C org δ13 C org δ13 C org -34

Silurian

7.4

-30

-26

-31

-29

-27

-32

-30

-28

Anticosti Is. Estonia (Jin et al.,1996) (Richardson et al.,2019) δ13 C Carb δ13C carb 0

Rhuddanian

4

-1

1

3

S.guerichi S.sedgwickii M.convolutus M.argenteus M.pectinatus M.t.triangulatus C.cyphus C.vesiculosus P.acuminatus A.ascensus

Ord.

2

S.turriculatus

M. persculptus

Fig. 7.9 Correlation between organic carbon isotope curves in the Rhuddanian–Telychian of different regions

?

hiatus

192

Parakidograptus acuminatus

Fm. Biozone

Lungmachi Formation

Fig. 7.10 TOC and d13Corg curves of the Xike-1 drillcore in the Wufeng Formation and the basal Lungmachi Formation (after Fig. 2 of Li et al. 2019)

Q. Zhao et al. Depth(m) 130

131

132

133

M. persculptus – A. ascensus

XQ-217

XQ-215

135

?

136

137

138

XQ-214 XQ-213 XQ-212 XQ-211 XQ-210 XQ-209c XQ-209b XQ-209a XQ-209 XQ-208 XQ-207 XQ-206 XQ-205 XQ-204 XQ-203 XQ-202 XQ-201

?

P. pacificus

XQ-218

XQ-216

134

Wufeng Formation Kuanyinchiao Bed

XQ-219

139

0.00

2.00

4.00

TOC (%)

Hirnantian isotope carbon excursion (HICE) in the Wangjiawan Riverside section was determined by Peng Ping’an (in Chen et al. 2006) and correlated with the Hirnantian glaciation. Since these samples were collected from outcrops, these analyses were inevitably influenced by the outcrop weathering. Recently, the Xike-1 drillcore was drilled in the Meixihe, Liangcun Town, Xishui County, Guizhou Province. Using 22 samples through the Ordovician–Silurian boundary in the Xike-1 drillcore, one of the authors (Li Chao) produced comparable TOC and d13Corg curves in the Wufeng Formation and the basal Lungmachi Formation (Fig. 7.10). As seen in the Fig. 7.10, according to the graptolite biozone division (Wu et al. 2020), the d13Corg positive excursion can be approximately recognized in the boundary of the Paraorthograptus pacificus (WF3) and Metabolograptus extraordinarius (WF4) graptolite biozones, which is the basal boundary of the Hirnantian Stage. This positive excursion ends in the middle of the Metabolograptus persculptus–Akidograptus ascensus (LM1–LM2) graptolite

6.00

31.00

30.00

29.00

28.00

δ13Corg (‰)

biozones. This positive excursion can be precisely correlated in South China and worldwide, and can be recognized as the HICE event. The level of the HICE event is stable in the Yangtze region (Li et al. 2019), commonly begins in the upper part of the Paraorthograptus pacificus Biozone (WF3) and ends in the upper part of the Metabolograptus persculptus Biozone (LM1) in the Yangtze region (Li et al. 2019). Due to the possible dissolution of the organic carbon in the seafloor sediments during the Hirnantian, the shifts of the HICE event recorded in the Yangtze region are all smaller than those in other regions of the world. In South China, the rocks recording the HICE event contain relatively lower TOC. But in the rocks before the HICE (WF3) in Wufeng Formation and after the HICE (LM2) in the transgressive Lungmachi Formation, TOCs are much higher than in rocks recording the HICE event, and are favorable for organic accumulation and shale gas reservoirs (Fig. 7.11). The WF3 and LM2 before and after the HICE event are respectively very close to the GR1 and GR2 peaks mentioned in this chapter.

Silurian

Formation Biozone

Stage

Rhuddanian Lungmachi Kuanyinchiao WF4 LM1 LM2

Ordovician

Hirnantian

System

Gamma Log Responses Through the Ordovician–Silurian Black Shale …

Katian Wufeng WF3

7

favorable layers

δ13Corg HICE event

favorable layers

KYC: Kuanyinchiao

Fig. 7.11 Ordovician–Silurian transition layers containing the d13Corg HICE event (after Fig. 5 of Li et al. 2019)

References Antoshkina AI (2008) Late Ordovician-Early Silurian facies development and environmental changes in the Subpolar Urals. Lethais 41:163–171 Asquith G, Krygowski D, Henderson S, Hurley N (2004) Basic relationships of well log interpretation. In: Asquith G, Krygowski D, Henderson S, Hurley N (eds) Basic well log analysis. AAPG methods in exploration, vol 16, pp 1–20 Bond LJ, Harris RV, Denslow KM, Moran TL, Griffin JW, Sheen DM, Dale GE, Schenkel T (2010) Evaluation of non-nuclear techniques for well logging: technology evaluation. Pacific Northwest National Laboratory, pp 10–20 Chen JY (2017) It is difficult to identify The stratigraphic features and difficulties of Wufeng Formation-Lungmachi Formation by logging for Pingqiao area in Fuling shale gas field. World Well Logging Technol 38(6):41–45 (in Chinese) Chen X, Melchin MJ, Fan JX, Mitchell CE (2003) Ashgillian Graptolite fauna of the Yangtze region and the biogeographical distribution of diversity in the latest Ordovician. Bulletin De La Société Géologique De France 174(2):141–148 Chen X, Rong JY, Fan JX, Zhan RB, Mitchell CE, Harper DAT, Melchin MJ, Peng PA, Finney SC, Wang XF (2006) The Global Boundary Stratotype Section and Point (GSSP) for the base of the Hirnantian Stage (the uppermost of the Ordovician System). Episodes 29:183–196 Chen X, Fan JX, Zhang YD, Wang HY, Chen Q, Wang WH, Liang F, Guo W, Zhao Q, Nie HK, Wen ZD, Sun ZY (2015) Subdivision and delineation of the Wufeng and Lungmachi black shales in the subsurface areas of the Yangtze platform. J Stratigr 39(4):351–358 (in Chinese with English abstract) Chen ZP, Liang X, Zhang JH, Wang GC, Liu C, Li ZF, Zou C (2016) Genesis analysis of shale reservoir over-pressure of Longmaxi

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194 Ma YN (2015) Analysis of Shale reservoir characteristics and gas-bearing control factors of Longmaxi Formation in Jingmen Area. Southwest Petroleum University, Chengdu Melchin MJ, Holmden C (2006) Carbon isotope chemostratigraphy in Arctic Canada: sea-level forcing of carbonate platform weathering and implications for Hirnantian global correlation. Palaeogeogr Palaeoclimatol Palaeoecol 234(2–4):186–200 Mu EZ, Zhu ZL, Chen JY, Rong JY (1978) The Ordovician strata near the Shuanghe of Changning in Sichuan. J Stratigr 2(2):105–121 (in Chinese) Mu EZ, Zhu ZL, Rong JY (1983) The Silurian near the Shuanghe of Changning in Sichuan. J Stratigr 7(3):209–215 (in Chinese) Nie X, Wan Y, Zou CC, Zhang R (2016) Comparative study of w (TOC) logging in shale-gas reservoirs. J Oil Gas Technol 38 (2):19–27 (in Chinese with English abstract) Richardson JA, Keting C, Lepland A, Hints O, Bradley AS, Fike DA (2019) Silurian records of carbon and sulfur cycling from Estonia: The importance of depositional environment on isotopic trends. Earth Planet Sci Lett 512:71–82 Rong JY (1979) Hirnantia fauna in China and discussion of Ordovician/Silurian boundary. J Stratigr 3(1):1–28 (in Chinese) Schmoker JW (1981) Determination of organic-matter content of appalachian Devonian shales from gamma-ray logs. AAPG Bull 65:1285–1298 Shi Q, Chen P, Wang XQ, Liu FX (2017) A method for identifying high-productivity intervals in a horizontal shale gas well and its application: a case study of the Lower Silurian Longmaxi Fm in Weiyuan shale gas demonstration area, Sichuan Basin. Nat Gas Ind 37(1):60–65

Q. Zhao et al. Wang K, Chatterton BDE, Wang Y (1997) An organic carbon isotope record of Late Ordovician to Early Silurian, Yangtze Sea, South China Implications for CO2 changes during the Hirnantian glaciation. Palaeogeogr Palaeoclimatol Palaeoecol 132:147–159 Wang LW, Zhang JF, Chen JH, Zhang YD, Chen XY, Zhu ZH, Liu J, Hu YH, Ma X (2015) Characteristics of Katian (Late Ordovician ) K-bentonites from Anji, Zhejiang Province. J Stratigr 39(2):156– 168 (in Chinese with English abstract) Wang XF, Chen XH (1999) Paleogeography and paleoclimatology of Ordovician in China. Prof Papers Stratigr Palaeontol 27:1–27 Wei B, Zou CC, Li J, Wang LC (2016) Shale gas logging method and evaluation. East China Science & Technology Press, Shanghai, pp 3–33 (in Chinese) Wu XJ, Chen Q, Li GF, Fan JX, Li C, Zhang YD, Wang Y, Yang J, Sun ZY (2020) Stratigraphic subdivision and correlation of the Wufeng and Lungmachi black shales from Xike-1 drillcore in northern Guizhou Province, China. J Stratigr 44(1):1–11 (in Chinese with English abstract) Xie SK, Wang ZJ, Wang J, Zhuo JW (2016) LA-ICP-MS zircon U-Pb dating of the bentonites from the uppermost part of the Ordovician Wufeng Formation in the Haoping section, Taoyuan, Hunan. Sediment Geol Tethyan Geol 30(4):65–69 Xiong GQ, Wang J, Li YY, Yu Q, Meng YP, Zhou XL, Xiong XH, Zhou YX, Yang X (2017) Zircon U-Pb dating and geological significance of the bentonites from the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation in western Daba Mountains. Sediment Geol Tethyan Geol 37(2):46–58 (in Chinese with English abstract)

8

Volcanic Ash Deposition and Organic Matter Enrichment in the Black Shales of the Wufeng–Lungmachi Formations in the Yangtze Region Zhen Qiu and Xiangying Ge

Abstract

In the geologic history, volcanic ash was commonly associated with organic-rich deposits. Volcanic ash layers (K-bentonites) were deposited extensively within the shale of Wufeng–Lungmachi formations in the Sichuan Basin and its periphery. The complex process of the influence of the related volcanic activity on the depositional environment is discussed in the present chapter. Keywords




Volcanic ash K-bentonites Sichuan Basin

8.1




Organic-rich deposits




Study Background

Volcanic ash refers to pyroclastic particles with a diameter of less than 2 mm that are generated during a volcanic eruption, and is usually carried over long distances by gas flow before deposition (Robock 2000; Chen et al. 2014). During the geologic history, large-scale volcanic activities were commonly associated with major biological extinction events, and were often considered as the trigger for mass extinctions of marine life (Wignall 2001). Large-scale (global) volcanic eruptions are often accompanied by the emission of large amounts of SO2 gas and suspended sulfate particles, resulting in acid rain, climatic cooling, environmental deterioration and ecological disruption (Self et al. 2006). Interestingly, volcanic ash is commonly associated with Z. Qiu (&) PetroChina Research Institute of Petroleum Exploration and Development, Beijing, 100083, China e-mail: [email protected] X. Y. Ge Chengdu Center of China Geological Survey, Chengdu, 610081, China

organic-rich deposits (Gaibor et al. 2008; Su et al. 2009; Yang et al. 2010). TOC of tuffaceous (volcanic ash-bearing) mudstone can reach up to 4% or more (Wang et al. 2013). Numerous studies demonstrate that throughout geologic history, volcanic ash produced by volcanic eruptions and their associated hydrothermal activities have added nutrients to aqueous systems and promoted plankton blooms, which leads to the enrichment of organic matter (Gao et al. 2009; Zhang et al. 2009). The influence of volcanic ash deposition on the formation and enrichment of organic matter is essentially a result of improving the biological productivity. Numerous studies demonstrate that volcanic ash originating from modern volcanic eruptions can release nutrient-rich elements (e.g., Fe, P, N, Si and Mn) into sea water, promoting an increase of marine biological productivity in sea surface waters (Frogner et al. 2001; Olgun et al. 2011, 2013; Achterberg et al. 2013). For example, in the modern Pacific Ocean, the content of Fe is relatively low, which limits the productivity of plankton in some areas of the ocean (Boyd et al. 2000). However, an increase in the content of Fe, even on a nanomole scale, is sufficient to promote the bloom of diatoms (Boyd et al. 2000; Wells 2003). Numerous studies have indicated that modern volcanic eruptions play an important role in promoting organic productivity in an adjacent ocean. A typical example is the eruption of the Anatahan volcano in the Mariana Islands in 2003, which improved primary productivity in the nutrient-poor areas of the Northwest Pacific Ocean (Uematsu et al. 2004; Lin et al. 2011), and expanded the area of algae growth to 4.8  103 km2. Other examples include the eruption of the Kasatochi volcano in Alaska, the United States, in 2008, which promoted plankton blooms along the northeast coast of the Pacific Ocean (Hamme et al. 2010; Langmann et al. 2010). The volcanic eruption of Eyjafjallajökull in Iceland, which occurred in 2010, improved the biological productivity of the Central Iceland Basin in the North Atlantic Ocean (Achterberg et al. 2013). Chen et al. (2018) and Zhang et al. (2017) considered that

© Zhejiang University Press and Springer Nature Singapore Pte Ltd. 2023 X. Chen et al. (eds.), Latest Ordovician to Early Silurian Shale Gas Strata of the Yangtze Region, China, https://doi.org/10.1007/978-981-99-3134-7_8

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the fall out and dissolution of volcanic ash would promote the growth of heterotrophic fungus phytoplankton, which subsequently would induce the productivity of other phytoplankton (including microeukaryotes and large eukaryotes). Additionally, volcanic ash experiments have shown that a layer of 1 mm thick volcanic ash with 1 dm2 area (about 20 g in weight) settling through a 50 m thick (equaling the thickness of the euphotic zone) water column with the same area can significantly increase the concentration of nutrients. Within the water column, Fe concentration can be increased from 0.4 to 2.4 nmol/L, and Zn concentration can be increased from 0.1 to 1 nmol/L, concentrations of which are usually less than 0.5 nmol/L in normal seawater. If the volcanic ash layer is 1 cm thick, the concentrations of all elements increase by about 10 times. However, some studies indicate that volcanic ash usually stays in the atmosphere for 1 to 3 years (Robock 2000) before falling into seawater, and its influence on organic productivity in the ocean lasts only several years to decades (Olgun et al. 2011; Achterberg et al. 2013). More information is still needed regarding the influence of volcanic activity on marine productivity through geological time. Several recent studies come to significantly different conclusions regarding the influence of volcanic activity on marine productivity. Shen et al. (2012) discovered that a negative trend in d13C corresponded well with the deposition of a volcanic ash layer, and that the scale of the negative trend correlated well with the thickness of the volcanic ash layer. Their study is based on the value of d13C for the rocks in the proximity of the volcanic ash layers in sections spanning the Permian and Triassic strata in South China (i.e., the Xiakou and Xinmin sections). They believed that volcanic activity had no close relation with the paleoproductivity of biological life in the ocean. Algeo et al. (2013) discovered that the paleo-productivity of biological life in the ocean, whose diversity decreased rapidly during the transition from the Permian to the Triassic in South China, may have been a result of volcanic activity during that period. However, Yan et al. (2015) considered that high marine productivity during the depositional period of the Ordovician Wufeng Formation was related to volcanic activity at that time in the northern part of Guizhou, South China. Ran et al. (2015) also believed that the abundance of radiolarians in the siliceous rocks intercalated in the shales of the Wufeng and Lungmachi formations in the Yangtze region was related to the volcanic activity that occurred during that period. Wu et al. (2018) noticed that there are TOC differences in the K-bentonite-rich and K-bentonitelean sections within the Wufeng and Lungmachi formations in Chongqing, South China. They considered that the frequent volcanism promoted the enrichment of organic matter by providing nutrient substances for improving the productivity of marine life. Marine productivity can trigger an

Z. Qiu and X. Ge

extremely oxygen-deficient environment, which is favorable for the preservation of organic matter. However, Wang et al. (2019) believed that the richness of volcanic ash in the Wufeng and Lungmachi formations has no close relationship with the organic carbon concentration of the shales. They thought that the enrichment of organic matter was related to the regional depression of the Chongqing region. Despite these controversies noted above, the depositional events related with volcanic activity are one of the most important topics of study in the unconventional petroleum sedimentology (Qiu and Zou 2020a, b). Determining the relationship between physical and biological factors with the enrichment of organic matter in shale is of great importance to the exploration and development of unconventional oil and gas resources in China (such as shale oil and shale gas).

8.2

Overview of the Geologic Setting and Deposition of Volcanic Ash

During the Ordovician–Silurian transition, global environmental events caused a rapid extinction of 85% of marine animal species (Sheehan 2001; Chen et al. 2006). A rapid sea level fall of 100 m (Haq and Schutter 2008) and rapid global cooling occurred in less than a 1 Ma interval (Trotter et al. 2008; Finnegan et al. 2012). The causal relationship and the complex interaction of these events present some of the most important research questions in geology and paleobiology (Chen et al. 2006; Harper et al. 2014; Zou et al. 2018). During that critical transition interval, the black, organic-rich shales were extensively deposited across North America, Europe and North Africa (Lüning et al. 2000; Sharma et al. 2005; Saberi et al. 2016). This set of black shale is one of the most important global source rocks for Paleozoic oil and gas resources. In South China, influenced by the convergence and suturing of the Yangtze Block and the Cathaysian Block, the depositional environment in the Sichuan Basin evolved from the Early–Middle Ordovician shallow-water carbonate platform into a latest Ordovician and early Silurian clastic continental shelf. A series of organic-rich shales were deposited in the Sichuan Basin and along its periphery in the Yangtze region. These are the Wufeng Formation and Lungmachi Formation shales (Chen et al. 2004, 2015) (Fig. 8.1a). Numerous layers of altered volcanic ash (K-bentonites) occurred in Ordovician and Silurian shales, indicating frequent volcanism in South China at that time (Figs. 8.1b, c, 8.2). In North America, K-bentonites are well known from Middle and Late Ordovician strata in the Midcontinent, the Appalachian Basin, and the Ouachita Mountains. There are over 100 K-bentonite layers (one to several centimeters each), including two very widespread K-bentonite layers (the Millbrig and Deicke), which are 1–2 m thick in some

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Volcanic Ash Deposition and Organic Matter Enrichment in the Black Shales of the Wufeng–Lungmachi …

Fig. 8.1 a Distribution of the Wufeng Formation and Lungmachi Formation black shales in the Yangtze region; b volcanic ash layers of the Wufeng and Lungmachi formations at the Tianba section, Wuxi, Chongqing; c volcanic ash layers of the Lungmachi Formation shale; d volcanic ash layers of the Wufeng Formation shale at the Tianba

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section. Locations of sections: 1. Tianba section, Wuxi; 2. Qiliao section, Shizhu; 3. Shuanghe section, Changning; 4. Huangying section, Wulong; 5. Tianjiawan section, Daozhen; 6. Fenghuang section, Youyang; 7. Nanbazi section, Tongzi; 8. Wuxingcun section, Hanzhong; 9. Liangba section, Hanzhong; 10. Huanghuachang section, Yichang

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Fig. 8.2 The Wufeng Formation and Lungmachi Formation K-bentonites in the Sichuan Basin and its periphery. a, b the Qiliao section in Shizhu; c the Tianba section in Wuxi; d the Shuanghe section in Changning

localities (Bergström et al. 2004; Huff 2008). The Millbrig and Deicke are recognized as the largest volcanic ash layers deposited during the Phanerozoic (Bergström et al. 2004). More than twenty volcanic ash layers were deposited in the Wufeng Formation and Lungmachi Formation shales in the Upper Yangtze region (Su et al. 2009; Lu et al. 2017; Qiu et al. 2019; Wang et al. 2019), and over one hundred volcanic ash layers were deposited in the Lower Yangtze region (Yang et al. 2019). However, individual volcanic ash layers are usually less than 5 cm thick, and most of them in the Yangtze region are less than 1 cm thick. These volcanic ash layers are the thickest and most abundant in the Wufeng Formation. For example, in the Upper Yangtze region there are over 20 volcanic ash layers in the Wufeng Formation, with most beds having a thickness of over 1 cm on average and a maximum thickness of 10 cm (Qiu et al. 2019). In the Lower Yangtze region, there are over 60 volcanic ash layers in the Wufeng Formation, and the maximum bed thickness is higher than 3 cm (Yang et al. 2019). Silurian volcanic ash layers are mainly distributed in the upper Long-1 Member of the Lungmachi Formation (Rhuddanian to the Aeronian). In Shizhu and Wuxi of Chongqing, more than 10 volcanic ash

layers, each about 5 cm thick, have been found (Qiu et al. 2019; Wang et al. 2019). Thus, there are two episodes of volcanism in South China during the Ordovician–Silurian transition: one is recorded in the latest Katian Wufeng Formation and the other one in the Rhuddanian to Aerolian Lungmachi Formation (Qiu and Zou 2020b).

8.3

Study Method and Sample Collection

Investigating the relationship between volcanic ash deposits and enrichment of organic matter requires the correct identification of K-bentonite beds. Some thin beds of clay within sedimentary strata are essentially crushed, finely ground sediment resulting from intra-strata sliding induced by tectonic activity. These are similar to the fault gouge. The Wufeng and Lungmachi formations are relatively thick-bedded units, consisting of a wide variety of rocks, such as siliceous shale, calcareous shale, thin-bedded siliceous rock, argillaceous siltstone and shelly limestone, and are underlain by limestones of the Pagoda or Linhsiang formations. Inter-strata sliding commonly occurs under the

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Volcanic Ash Deposition and Organic Matter Enrichment in the Black Shales of the Wufeng–Lungmachi …

influence of tectonic activity due to differences in hardness and competence in these rocks. Thus, it is necessary to conduct elemental analyses to distinguish K-bentonites from the non-volcanic clay layers. High-resolution sampling can then be undertaken in sections with well-developed volcanic ash deposits. High-resolution comprehensive data reflecting the paleo-productivity have been collected in integrated sedimentological, elemental geochemical, and isotopic geochemical studies. In order to investigate the influence of the volcanic ash deposits on the enrichment of organic matter, we combined redox conditions with the variation in multiple proxies of paleo-productivity across the strata encompassing the K-bentonites or within beds that are rich in volcanic ash lamina. In conformable sections with multiple well-preserved K-bentonite beds, the Wufeng and Lungmachi formations are excellent stratigraphic units for studying the relationship between volcanic ash deposits and the enrichment of organic matter. We collected samples from five sections: the Shuanghe section (28°23′53″N, 104°52′27″E), Changning, Sichuan; the Tianba section (31°24′41″N, 108°52′37″E), Wuxi, Chongqing; the Qiliao section (29°52′44″N, 108°17′ 06″E), Shizhu, Chongqing; the Guanyinqiao section (28°38′ 2″N, 106°47′47″E), Qijiang, Chongqing; and the Dashan section (28°04′10″N, 106°51′40″E), Tongzi, Guizhou (Fig. 8.1a). The Wufeng and Lungmachi formations in these sections consist mainly of black siliceous, calcareous and carbonaceous shales with numerous K-bentonite beds (Fig. 8.2) range from several millimeters to several centimeters thick. The K-bentonites are light gray, gray, light yellow or blue gray in color. Other samples were also collected from the Huangying Sect. (29°12′48″N, 107°41′36″E) in Wulong, the Tianjiawan Sect. (28°48′21″N, 107°30′37″E) in Daozhen, the Wuxingcun Sect. (32°32′45″N, 107°58′58″ E) and the Liangbai Sect. (32°20′17″N, 107°59′42″E) in Zhenba, the Fenghuang Sect. (28°55′13″N, 108°31′18″E) in Youyang, and the Fucheng Sect. (32°28′39″N, 107°14′11″ E) in Nanzheng (Fig. 8.3).

8.4

Characteristics and Source of Volcanic Ash (K-Bentonite)

The composition of K-bentonites in thin section is clay minerals and phenocrysts, which are dominantly quartz and feldspar, usually less than 10% in total mineral abundance. Other minerals include apatite, garnet and zircon, as well as volcanic glass. Phenocrysts and volcanic glass are embedded in the clay mineral matrix (Fig. 8.4). Clay minerals are also arranged along the margins of the phenocryst particles. X-ray diffraction (XRD) analysis of K-bentonites from the study area indicates that the K-bentonite beds consist mainly of illite, plagioclase and quartz, followed by small amounts

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of kaolinite, K-feldspar, gypsum and dolomite. The mineral composition of K-bentonites in the Wufeng Formation is the same as those in the Lungmachi Formation. Illite is the most common mineral contained in the K-bentonites, ranging from 30.4 to 80.5%, with an average of 63.67%, followed by plagioclase, ranging from 0 to 27.6%, with an average of 14.63% (over 20% in most cases). In addition, the quartz content is also relatively high, ranging from 4.7 to 23.4%, with an average of 15.71% (Table 8.1). Thus, the mineral composition of samples from the study area possesses typical K-bentonite mineral abundances. K-bentonite samples, which were collected from the Wufeng Formation and Lungmachi Formation K-bentonites in the Changning and Wuxi regions of the Sichuan Basin, have been analyzed for major mineral abundances. The results indicate that the major mineral contents in these K-bentonites are similar (Table 8.2). The dominant oxide is SiO2, ranging from 33.19 to 62.13%, with an average of 52.90%. The second is Al2O3, ranging from 15.81 to 29.77%, with an average of 25.50%. Fe2O3 concentration ranges from 1.74 to 13.40%, with an average of 5.50%, similar to that of K2O (3.64–8.10%, with an average of 6.39%). MgO concentration ranges from 2.19 to 4.65%, with an average of 3.48%, similar to or higher than that of normal igneous rocks (3.5%). The least abundant minerals include CaO, 0.16–9.95%, with an average of 1.54%, and TiO2, 0.142–1.336%, with an average of 0.544%. Zhou et al. (2007) noted that the K2O concentration in the majority of K-bentonite samples collected from the study area is over 3.5%, and the K2O/Na2O ratio ranges from 15.54 to 270. This indicates that the K2O concentration is much higher than that of Na2O, and is consistent with the typical characteristics of K-bentonites, which are generally high in K2O concentration and low in Na2O concentration. The stable trace elements and rare earth elements of K-bentonites can serve as an indicator of the geotectonic background of the primitive volcanoes (Teale and Spears 1986; Roberts and Merriman 1990). Pearce and Cann (1973) first proposed to identify the original tectonic environment of magma using geochemical methods, and subsequently built a plot for identification of granite (Pearce 1982; Pearce et al. 1984). Huff et al. (1997) then used this plot to identify the tectonic environment of the original source magma of K-bentonites. In addition, several plots for identifying the geotectonic environment of magma source were proposed by Wood (1980) and Mullen (1983). The Th-Hf/3-Ta tectonic– environmental diagram was applied to the present samples from the study area. Most samples fall into the volcanic arc basalt zone (Zone D) (Fig. 8.5a). Some samples from the Shuanghe section of Changning, Tianba section of Wuxi and Huanghuachang section of Yichang fall into the alkaline intraplate basalt zone (Zone C). On the TiO2-Zr diagram (Pearce 1982; Pearce et al. 1984) the samples from the

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Fig. 8.3 The Wufeng Formation and Lungmachi Formation K-bentonites in the eastern Sichuan Basin. a Wufeng Formation K-bentonites in the Qiliao section of Shizhu; b Wufeng Formation K-bentonites in the Huangying section of Wulong; c Lower Lungmachi Formation K-bentonites in the Liangbai section of Zhenba; d Wufeng

Formation K-bentonites in the Wuxingcun section of Zhenba; e Lower Wufeng Formation K-bentonites in the Huangcao section of Wulong; f Wufeng Formation K-bentonites in the Fenghuang section of Youyang

Dashan section of Tongzi and the Tianba section of Wuxi generally fall into the intraplate magma zone (Fig. 8.5b), although some fall into the arc magma zone. Similar diagrams from different authors (Su et al. 2006; Lu et al. 2017; Hu et al. 2019a, b) have also been used to examine the provenance of the volcanic ash samples collected from the eastern part of Sichuan Basin (Figs. 8.6a, b). The source

rocks of the Wufeng Formation and Lungmachi Formation K-bentonites in the Sichuan Basin and its periphery were formed mainly in the island arc or the intraplate environments. Su et al. (2006) concluded that the Wufeng Formation and Lungmachi Formation K-bentonites in South China originated from an active plate margin, or the eruption of large-scale intermediate- to acid-volcanoes caused by the

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Volcanic Ash Deposition and Organic Matter Enrichment in the Black Shales of the Wufeng–Lungmachi …

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Fig. 8.4 The microscopic features of the Wufeng Formation and Lungmachi Formation K-bentonites in the eastern Sichuan Basin. a Residual pore in pyrite crystals in K-bentonite from the Fenghuang section of Youyang; b black volcanic glass in K-bentonite from the Nanbazi section of Tongzi; c zircon particles in K-bentonite from the

Huangcao section of Wulong; d CL image of zircon in K-bentonite from the Wuxingcun section of Zhenba; e framboidal pyrite in K-bentonite from the Wuxingcun section of Zhenba; f illite/smectite formation in K-bentonite from the Wuxingcun section of Zhenba

collision and convergence of island arcs. Hu et al. (2009a, 2009b) believed that these K-bentonites were generated in the intraplate, island arc, syn-collision and oceanic ridge environments. However, the feasibility of using trace elements and rare earth element composition of K-bentonites to identify the tectonic environment of the K-bentonite source rock was questioned by Nesbitt et al. (1996), Ma et al. (2007), Zhou et al. (2007) and Hu et al. (2009a, 2009b).

They argued that a wide variety of complex physical and chemical reactions that occur during both transportation and deposition might cause some stable trace elements and rare earth elements to become unstable, and thus have a confounding influence on the identification results. Hu et al. (2009a, 2009b) believed that there are two main explanations for this phenomenon. First, the studied samples formed under different tectonic environments. Second, the diagram

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Table 8.1 Whole-rock mineral contents of K-bentonites in the Wufeng and Lungmachi formations in the Changning and Wuxi regions of the Sichuan Basin (%) Sample no.

Formation

V1

Wufeng

4.7

71.9

0

0

23.4

0

0

0

0

V3

Lungmachi

14.2

51.8

5.1

0

27.1

0

0

1.8

0

V6

Wufeng

16.3

64.0

0

0

19.7

0

0

0

0

V7

Wufeng

19.3

80.7

0

0

0

0

0

0

0

V10

Lungmachi

23.4

30.4

0

17.1

27.6

0

0

1.5

0

V12

Wufeng

15.3

73.9

4.5

0

0

1.1

4

0

1.2

V13

Lungmachi

21.0

46.0

0

0

27.1

2.4

0

3.5

0

V15

Wufeng

14.6

78.6

0

0

6.8

0

0

0

0

V18

Lungmachi

12.6

75.7

0

11.7

0

0

0

0

0

15.71

63.67

1.07

3.20

14.63

0.39

0.44

0.76

0.13

Average

Table 8.2 Major oxides contents of K-bentonites in the Wufeng and Lungmachi formations in Changning and Wuxi areas, Sichuan Basin (%)

Quartz

Illite

Kaolinite

K-feldspar

Plagioclase

Gypsum

Calcite

Dolomite

Pyrite

Sample no.

Formation

SiO2

A12O3

TiO2

MgO

Fe2O3

K2O

Na2O

CaO

V1

Wufeng

33.19

15.81

0.171

2.29

13.40

3.64

0.08

8.45

V2

Wufeng

50.89

25.57

0.142

3.81

3.29

6.07

0.06

9.95

V5

Lungmachi

54.14

21.24

0.388

3.04

11.08

4.83

0.08

0.64

V6

Wufeng

48.21

22.97

0.200

2.56

10.92

6.78

0.43

0.36

V7

Wufeng

54.11

27.68

1.089

3.78

3.23

7.86

0.09

0.35

V8

Wufeng

55.97

28.25

0.196

3.72

2.56

7.94

0.08

0.49

V10

Lungmachi

56.18

27.66

0.308

3.95

3.03

7.36

0.33

0.42

V11

Wufeng

50.45

25.38

1.228

3.44

5.23

7.01

0.23

0.98

V12

Wufeng

55.68

27.64

0.474

4.08

3.43

7.57

0

0.16

V13

Lungmachi

50.85

29.77

0.339

2.19

8.21

3.80

0.17

0.18

V14

Wufeng

56.12

26.88

1.336

4.17

3.16

7.83

0.05

0.21

V15

Wufeng

55.6

27.66

0.575

4.15

2.65

8.10

0.03

0.21

V16

Wufeng

62.13

21.35

1.312

2.63

3.28

5.41

0.23

0.26

V17

Lungmachi

57.08

27.99

0.199

3.81

1.74

5.75

0.37

0.27

V18

Lungmachi

52.77

26.67

0.205

4.65

7.24

5.95

0.14

0.17

52.90

25.50

0.544

3.48

5.50

6.39

0.158

1.54

Average

proposed by Pearce (1982), Pearce et al. (1984) is not applicable in identifying the environment of the primal magma of K-bentonites. Although trace elements and rare earth elements may provide important information on the original tectonic environment, the accuracy of the inference is limited by the influence of physical and chemical reactions such as weathering on the K-bentonite samples. Therefore, alternate possibilities for geotectonic settings should be considered. K-bentonites are widespread in Ordovician and Silurian strata along the periphery of the Yangtze Block (Wu 2003), although the origin of volcanic ash in the Yangtze region remains disputed. Su et al. (2006, 2007, 2009) believed that the origin of volcanism is related to the convergence of the

Yangtze and Cathaysia blocks. Volcanism and ash deposition resulted from the convergence zone of these two blocks. Conversely, volcanic activity related to Qinling region tectonics, north of the Yangtze Plate, occurred as well. Thus, Xue et al. (1996), Sun et al. (2002), Yang (2011), and Wang et al. (2015) considered that the K-bentonite beds may also have a source in the Qinling region. Based on the analyses mentioned above, we believe that the K-bentonites within the Wufeng Formation and the Lungmachi Formation are from subduction-related volcanism on the northern margin of the Yangtze Plate. From a sedimentological perspective, there are main two reasons for the view that the source area is the northern margin of the Yangtze Plate. First, the thickness and number

8

Volcanic Ash Deposition and Organic Matter Enrichment in the Black Shales of the Wufeng–Lungmachi …

203

Fig. 8.5 Diagram showing the geotectonic environment of K-bentonites from the Wufeng and Lungmachi formations in the Sichuan Basin and its periphery. a After Wood (1980), Zone A is Type N MORB, Zone B is Type E MORB and intraplate tholeiite, Zone C is alkaline intraplate basalt, and Zone D is volcanic ash basalt; b Modified from Pearce (1982) to Hu et al. (2009a)

Fig. 8.6 Diagram of primal magma and tectonic environment of K-bentonites in the Wufeng and Lungmachi formations deposited in the eastern part of the Sichuan Basin. a After Wood (1980), Zone A is Type N MORB, Zone B is Type E MORB and intraplate tholeiite, Zone C is alkaline intraplate basalt, and Zone D is volcanic ash basalt; b Modified from Pearce (1982) and Hu et al. (2009a)

of K-bentonites in the Wufeng and Lungmachi formations decrease from north to south. The K-bentonites become quite thin in sections in Hunan and Jiangxi provinces. In outcrops near the northern margin of the Yangtze Platform, there are more than 60 K-bentonite beds in the Wuxingcun and Liangbai sections in Zhenba, with a maximum thickness of up to half a meter (Fig. 8.7). Towards the south, in the Huangcao and Huangying sections of Wulong, the central part of the Sichuan Basin, the thickest K-bentonite is between 10 and 20 cm. The maximum thickness decreases to 5 cm in the Tongzi and Nanbazi areas of Guizhou. The number of K-bentonite beds also decreases. These evidences indicate that the source of volcanic ash was from the north, as the K-bentonites thinned and decreased in number with the increasing distance from the eruption center (Fig. 8.7).

Second, the South China Ocean was closed during the Late Ordovician to early Silurian, and the Yangtze and Cathaysia blocks were sutured completely (Zhang et al. 2013; Shu 2012; Chen et al. 2010). The evidences are as follows: (1) there is no record of residual oceanic crust between the Yangtze Plate and the Cathaysia Plate in the Early Paleozoic, and no record of ophiolite and related volcano-magmatic activity; (2) the boundary between the Yangtze Plate and the Cathaysia Plate is located on both sides of the border of Jiangshao and Pingxiang–Chenzhou, which indicates a rapid facies change (Chen et al. 2011); (3) in the eastern part of South China, the Early Paleozoic magmatic activity has a planar distribution, not the zonal distribution that resulted from plate subduction and collision (Shu 2012). Essentially, the Yangtze Plate and the Cathaysia

204

Z. Qiu and X. Ge

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 8.7 K-bentonites of the Wufeng and Lungmachi formations in the eastern part of the Sichuan Basin

Plate were already one unified continental block during the Late Ordovician to early Silurian. The collisional and compressional deformation was actually intraplate crumpling and compression, with no record of volcanic island arc activity. Accordingly, it is believed that the K-bentonites deposited on the Yangtze Plate and the Cathaysia Plate during the Late Ordovician–early Silurian likely originated from island arcs formed by the collision between the northern margin of the Yangtze Plate and the Qinling Ocean, resulting the closing of the ocean.

8.5

Contribution of Volcanic Ash Deposits to the Enrichment of Organic Matter

The present study focused on two sections of Chongqing, the Tianba section of Wuxi and the Qiliao section of Shizhu. The K-bentonites in the Wufeng and Lungmachi formations are numerous in the Tianba section of Wuxi, and 75 shale samples were collected in the Wufeng Formation (Fig. 8.8). However, in the Qiliao section of Shizhu, only 35 samples

8

Volcanic Ash Deposition and Organic Matter Enrichment in the Black Shales of the Wufeng–Lungmachi …

205

Fig. 8.8 Indices of organic matter enrichment of volcanic ash layers through the Wufeng Formation and the Lungmachi Formation in the Tianba section of Wuxi, Chongqing (from Qiu et al. 2019)

were collected from the base of the Wufeng Formation and the Lungmachi Formation (with a thickness of 8 m) (Fig. 8.9). Preliminary studies on anoxic conditions, paleo-productivity, depositional response to global climate

change, enrichment of organic matter, and the origin of siliceous rocks have been previously carried out (Qiu et al. 2017, 2018; Zou et al. 2018; Lu et al. 2019). Based on the analyzed results of these shale samples, a restudy on the

206

Z. Qiu and X. Ge

Fig. 8.9 Indices of organic matter enrichment of volcanic ash layers through the Wufeng Formation and the Lungmachi Formation in the Qiliao Section of Shizhu, Chongqing (from Qiu et al. 2019)

differential enrichment characteristics of organic matter (within 20 cm samples) was carried out by comparing the volcanic-ash bearing shale (VRS) with the normally deposited shale (NS), deposited between periods of active volcanism. The major controlling factors for the enrichment of organic matter and the relationship between the volcanic ash deposits and the enrichment of organic matter are clearly indicated.

8.5.1 Differential Enrichment of Organic Matter in Volcanic-Ash Bearing Shale and Normally Deposited Shale In the Tianba section of Wuxi, there are a total of 25 K-bentonites in the Wufeng Formation, each of which is over 0.5 cm thick. There are 13 K-bentonites at the bottom of the Lungmachi Formation (Fig. 8.8). In the Qiliao section

8

Volcanic Ash Deposition and Organic Matter Enrichment in the Black Shales of the Wufeng–Lungmachi …

Fig. 8.10 Distribution of TOC in VRS and NS of the Wufeng Formation and the Lungmachi Formation in the Tianba section of Wuxi and the Qiliao section of Shizhu

of Shizhu, there are 19 volcanic ash layers in the Wufeng Formation, and only 2 volcanic ash layers at the bottom of the Lungmachi Formation (Fig. 8.9). The TOC value of shale varies greatly across both sections, ranging from 0.6 to 16% (on average 4.1%) in the Tianba section, and 2.6–14% (in average 6.0%) in the Qiliao section. In ascending order, the trend of TOC values is quite similar in both sections; TOC first increases gradually and then decreases slowly. The peak of TOC values occurs at the top of the Wufeng Formation and the base of the Lungmachi Formation (in proximity to the Kuanyinchiao Bed) (Figs. 8.8, 8.9). If we consider only the 20 cm of VRS, and the others are almost only NS, Figs. 8.8 and 8.9 may indicate the characters of intermittent eruptions of a volcano (volcanic group). Statistical data indicates that the TOC in intervals of VRS is noticeably lower than that of NS in the Tianba and Qiliao sections (Fig. 8.10). In the Tianba section, the average value of TOC in the intervals of VRS and NS is 2.1 and 5.1% respectively. In the Qiliao section, the average value of TOC in the intervals of VRS and NS is 3.6 and 7.0% respectively.

8.5.2 Influence of Volcanic Ash Deposits on Enrichment of Organic Matter There has been considerable dispute regarding the controlling factors of formation and enrichment of organic matter, especially in marine deposits (Demaison and Moore 1980; Tyson 2005; Katz 2005). Much of the controversy focused on whether the environmental conditions of preservation (the reducing conditions in water) or the high productivity of sea surface waters is the major controlling factor for the enrichment of organic matter in sediments. Previous studies commonly held that anoxia was the major controlling factor (Demaison and Moore 1980; Rimmer 2004). However, later studies recognized that in some upwelling areas organic-rich

207

deposits were heavily dependent on the relatively high (initial) productivity of sea surface waters (Sageman et al. 2003; Gallego-Torres et al. 2007). During the deposition of organic matter at the sea floor, decomposition occurs, consuming oxygen and creating an anoxic environment at the site of deposition (Pedersen and Calvert 1990). Conversely, a variety of other factors may influence the enrichment of organic matter, such as deposition rate (Tyson 2005; Algeo et al. 2013), clay concentration (Cai et al. 2007; Blair and Aller 2012) and sea level fluctuation (Hofmann et al. 2001; Sageman et al. 2003). Hence, high productivity of sea water provided a foundation for the formation and enrichment of organic matter, with anoxic reducing conditions and high deposition rate as critical factors for such enrichment. As noted above, volcanic ash can release large number of nutrients into sea water (elements such as Fe, P, N, Si and Mn), promoting primary productivity at the sea surface (Frogner et al. 2001; Achterberg et al. 2013; Olgun et al. 2013), which in turn is favorable for the formation and enrichment of organic matter. In recent years, numerous studies have indicated that biogenetic barium (Babio) is often considered as an indicator for modern and ancient marine productivity (Weldeab et al. 2003; Paytan and Griffith 2007). Qiu et al. (2017) provided a method for calculating Babio in sedimentary deposits. Figure 8.11 illustrates that the Babio concentration in the intervals of VRS is slightly higher than that of NS, in the Tianba section of Wuxi. In the Qiliao section of Shizhu, Babio concentration in the intervals of VRS is slightly lower than that of NS. The volcanic ash deposits had a relatively weak influence on productivity during the deposition of the Wufeng and Lungmachi formations with no apparent influence on the enrichment of organic matter in either the Tianba or Qiliao section. The relatively high productivity during this interval may instead be responsible for the enrichment of organic matter (Qiu et al. 2017). Previous studies have indicated that, the nutrients carried by modern volcanic ash (e.g., Fe, P, N, Si and Mn) can effectively promote algae blooms in nutrient-poor marine regions, and improve the paleo-productivity (Wells 2003; Lin et al. 2011) with no significant influence in the high-productivity marine regions. In the high-productivity regions near the modern Equatorial Pacific, Babio concentration is over 1000 ppm (Murray and Leinen 1993), while in the ancient organic-rich deposits Babio concentration is over 500 ppm (Algeo et al. 2011; Liu et al. 2018). Babio concentration in the intervals of VRS and NS is over 500 ppm in the Tianba section of Wuxi and the Qiliao section of Shizhu (Fig. 8.11a, c), indicating a relatively high productivity of the sea surface during that period. Numerous studies have demonstrated that the concentration of the trace elements (U, V, Mo, Ni and Cu), which are sensitive to redox conditions in a water body, can serve as a proxy for those conditions (Calvert and Perdersen 1993;

208

Z. Qiu and X. Ge

Fig. 8.11 Crossplot of TOC, Biogenic Babio concentration and U/Th ratio for the VRS and NS of the Wufeng Formation and the Lungmachi Formation (from Qiu et al. 2019). a, b the Tianba section of Wuxi; c, d the Qiliao section of Shizhu

Jones and Manning 1994). The most common method is to use the ratio of trace elements, such as U/Th and V/Cr (Jones and Manning 1994). Statistical data (Fig. 8.11b, d) shows that the U/Th ratio is positively correlated with TOCs in the Tianba and Qiliao sections, indicating the redox conditions (the variation from oxic to dysoxic and then to anoxic) are closely related to the enrichment of organic matter. It is noteworthy that in some siliceous rock samples collected from the Wufeng Formation in the Tianba section (indicated in Fig. 8.11b by dotted lines), radiolarians are abundant (Fig. 8.12a). The bloom of these siliceous organisms increased the deposition rate and diluted both the organic matter and the terrigenous sediments (Tyson 2005; Schoepfer et al. 2015), leading to significantly low TOC and

Fig. 8.12 a Radiolarian-rich siliceous rock of Wufeng Formation in the Tianba section; b distribution of TOC across different lithologies in the Tianba and Qiliao sections (from Qiu et al. 2019)

high U/Th ratio. Additionally, the impact of the deposition rate on organic matter enrichment is mainly reflected in the difference in lithology. In both the Tianba and Qiliao sections, the lithology of the Wufeng and Lungmachi formations consists mainly of gray-black siliceous shale, thin-bedded siliceous rock and massive marl (with varying SiO2 contents). In general, apart from the radiolarian-rich samples, the TOC of the samples is not strongly affected by the differences in lithology (indicated in Fig. 8.12b by dotted lines) in the Tianba section. As stated above, the increase of the marine productivity resulting from the deposition of volcanic ash is not significant, and does not significantly promote the enrichment of organic matter during the Ordovician to Silurian transition.

8

Volcanic Ash Deposition and Organic Matter Enrichment in the Black Shales of the Wufeng–Lungmachi …

Redox conditions were closely related to organic matter concentrations and seemed to be the major controlling factor for the enrichment of organic matter at that time.

8.6

Prospecting

Volcanic ash usually stays in the atmosphere for only a short time, quickly settling into seawater, and its influence on biological productivity in the ocean also seems limited to only a few years or decades. Thus, the influence of volcanic activity on marine productivity is often considered to be a geologically instantaneous event. Due to the differences in volcanic eruption scale and its type, volcanic ash deposits vary greatly in thickness, ranging from several millimeters to several meters. Some ashes might be directly dissolved in sea water or washed away by bottom currents, which leads to non-preservation. Therefore, it is difficult to study the unbiased influence of volcanic ash deposits on biological productivity and organic matter enrichment. Although attempts have been made by many scholars to analyze the influence of volcanic ash deposits by comparing the volcanic ash-rich zones with the volcanic ash-lean zones, there is still significant controversy. Thus, it is necessary to conduct further comparative studies on volcanic ash deposits with higher-resolution sampling (e.g., centimeter-level and millimeter-level). The latest study by Blattmann et al. (2019) indicated that the type of clay minerals in sedimentary deposits has a significant controlling role on the preservation of marine organic matter. Marine organic matter is readily adsorbed by smectite, and terrigenous organic matter and some minerals (e.g., chlorite) can bind together in seawater to become stable. Volcanic ash is readily decomposed into smectite in seawater, and thus creates favorable conditions for the enrichment of marine organic matter. However, the contribution of volcanic ash deposits to the enrichment of organic matter in shale still requires further study. For example, the volcanic ash-rich zone (i.e., the interval with K-bentonite beds) has a low organic concentration. However, in the volcanic ash-lean zone, the volcanic ashes are usually decomposed or altered, and thus it is difficult to determine their contribution. The ways in which volcanic activity influences the depositional environment are complex. Volcanic ash from volcanic eruptions and their associated hydrothermal activities can not only provide the nearby water mass with abundant nutrients, but may also have an important influence on the chemical properties of the water mass. It is necessary to conduct an integrated study of the overall geochemical characteristics with multiple indices, such as depositional characteristics, elemental abundances and isotopic ratios, in order to determine the influence of volcanic activities on the formation and enrichment of organic matter.

209

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Appendix Plates and Explanation

Xu Chen, Weijuan Wang, and Changmu Lin

Plate 1 A, D. Dicellograptus complexus Davies, WF2, Donggongsi of Zunyi, Guizhou, Collection number: AAE380. B. Tangyagraptus gracilis Mu et Chen, WF3, Fenxiang of Yichang, Hubei, Collection number: AFA139. C. Diceratograptus mirus Mu, WF3, Fenxiang of Yichang, Hubei, Collection number: AFA140.

E. Dicellograptus ornatus Elles et Wood, WF3, Fenxiang of Yichang, Hubei, Collection number: AFA133. F. Appendispinograptus venustus (Hsü), WF3, Fenxiang of Yichang, Hubei, Collection number: AFA129a. Scale bar: 1 mm.

X. Chen State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology and Center for Excellence in Life and Paleoenvironment, Chinese Academy of Sciences, Nanjing, 210008, China W. J. Wang Qingdao Institute of Marine Geology, Qingdao, 266237, China C. M. Lin No. 262 Geological Team of Zhejiang Nuclear Industry Corp., Huzhou, 313000, China © Zhejiang University Press and Springer Nature Singapore Pte Ltd. 2023 X. Chen et al. (eds.), Latest Ordovician to Early Silurian Shale Gas Strata of the Yangtze Region, China, https://doi.org/10.1007/978-981-99-3134-7

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Appendix: Plates and Explanation

Appendix: Plates and Explanation

Plate 2 A. Metabolograptus extraordinarius (Sobolevskaya), WF4, Honghuayuan of Tongzi, Guizhou, Collection number: AFA290. B. Dicellograptus tumidus Chen, WF2, Ludiping of Songtao, Guizhou, Collection number: AAE601. C. D, Styracograptus chiai (Mu), WF2, Daheba of Yongshun, Hunan Collection numbers: CSU 10046, 10047. E. Dicellograptus turgidus Mu and Paraorthograptus pacificus (Ruedemann), WF3, Ludiping of Songtao, Guizhou, Collection number: AAE604. F. Paraorthograptus pacificus (Ruedemann), WF3, Ludiping of Songtao, Guizhou, Collection number: AAE604. G. Dicellograptus cf. complanatus (Lapworth), WF2, Ludiping of Songtao, Guizhou, Collection number: AAE601.

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H. Anticostia lata (Elles et Wood), WF3, Shuibatang of Tongzi, Guizhou, Collection number: SBT2 ph119. I. Rectograptus abbreviatus (Elles et Wood), WF3, Shuibatang of Tongzi, Guizzhou, Collection number: SBT2 ph119. J. Rectograptus uniformis Mu et Lee, WF3, Fenxiang of Yichang, Hubei, Collection number: AFA128a. K. Parareteograptus sinensis Mu, WF3, Fenxing of Yichang, Hubei, Collection number: AFA124a. L. Metabolograptus ojsuensis (Koren’ et Mikhailova), WF4, Wangjiawan of Yichang, Hubei, Collection number: AFA97. Scale bar: 1 mm.

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Appendix: Plates and Explanation

Plate 2

E F D C

B

G

A

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Plate 3 A–E. Hirsutograptus sinitzini (Chaletzkaya), LM3, Wangjiawan of Yichang, Hubei, Collection numbers: WL91-100. F. Korenograptus lungmaensis (Sun), LM3, Wangjiawan of Yichang, Hubei, Collection number: WL56-73. G. Korenograptus laciniosus (Churkin et Carter), LM3, Wangjiawan of Yichang, Hubei, Collection number: WL56-73. H, L. Normalograptus mirnyensis (Obut et Sobolevskaya), LM1, LM3, Wangjiawan of Yichang, Hubei, Collection numbers: WL35-43, WL91-100. I. Parakidograptus acuminatus (Nicholson), LM3, Wangjiawan of Yichang, Hubei, Collection numbers: WL91-100.

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J. Normalograptus ajjeri (Legrand), LM2, Bajiaomiao, Shennongjia, Hubei, Collection number: AFU552. K. Appendispinograptus supernus (Elles et Wood), WF2, Fenxiang of Yichang, Hubei, Collection number: AFA120-9. M. Akidograptus ascensus Davies, LM3, Wangjiawan of Yichang, Hubei, Collection numbers: WL91-100. N. Metabolograptus persculptus (Elles et Wood), LM1, Gaozhiwan of Zhenxiong, Yunnan, Collection number: YZH8b. O, P. Korenograptus bicaudatus (Chen et Lin), LM2, Wangjiawan of Yichang, Hubei, Collection numbers: WL35-43. Scale bar: 1 mm.

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Appendix: Plates and Explanation

Plate 3

B A

H

E C

I

J

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K L

G

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Plate 4 A. Korenograptus guantangyuanensis (Fang et al.), LM2, Bajiaomiao of Shennongjia, Hubei, Collection number: AFU563. B. Avitograptus avitus (Davies), LM2, Bajiaomiao of Shennongjia, Hubei, Collection number: AFU560. C. Neodiplograptus shanchongensis (Li), LM2, Bajiaomiao of Shennongjia, Hubei, Collection number: AFU555. D. Normalograptus anjiensis (Yang), LM2, Bajiaomiao of Shennongjia, Hubei, Collection number: AFU563. E. Neodiplograptus parajanus (Štorch), LM3, Bajiaomiao of Shennongjia, Hubei, Collection number: AFU566.

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F. Neodiplograptus modestus (Lapworth), LM2, Shizishan of Changning, Sichuan, Collection number: AGH121. G. Neodiplograptus anhuiensis (Li), LM4, Shizishan of Changning, Sichuan, Collection number: AGH121. H. Korenograptus angustifolius (Chen et Lin), LM2, Bajiaomiao of Shennongjia, Hubei, Collection number: AFU555. I. Normalograptus lubricus (Chen et Lin), LM3, Bajiaomiao of Shennongjia, Hubei, Collection number: AFU570. J. Paraclimacograptus innotatus (Nicholson), LM3, Bajiaomiao of Shennongjia, Hubei, Collection number: AFU571. Scale bar: 1 mm.

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Plate 4

C

A B E

D

F

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Plate 5 A, C. Cystograptus vesiculosus (Nicholson), LM4, Bajiaomiao of Shennongjia, Hubei, Collection number: AFU574, 573. B. Paramplexograptus paucipsinus (Li), LM2, Shichang of Renhuai, Hubei: Rh391. D. Normalograptus rectangularis (M’Coy), LM4, Miaolinwan of Qianjiang, Chongqing, Collection number: AGJ168.

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E. Normalograptus brenansi (Legrand), LM4, Bajiaomiao of Shennongjia, Hubei, Collection number: AFU579. F. Cystograptus penna (Hopkinson), LM5, Bajiaomiao of Shennongjia, Hubei, Collection number: AFU583. G. Normalograptus normalis (Lapworth), LM2, Bajiaomiao of Shennongjia, Hubei, Collection number: AFU561. H. Atavograptus priminus (Li), LM2, Bajiaomiao of Shennongjia, Hubei, Collection number: AFU565. Scale bar: 1 mm.

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Plate 5

Appendix: Plates and Explanation

Appendix: Plates and Explanation

Plate 6 A. Huttagraptus billegravensis Koren’ et Bjerreskov, LM4, Bajiaomiao of Shennongjia, Hubei, Collection number: AFU574. B. Coronograptus leei (Hsü), LM5, Bajiaomiao of Shennongjia, Hubei, Collection number: AFU591. C. Coronograptus cyphus (Lapworth), LM5, Shizishan of Changning, Sichuan, Collection number: AGH1503. D. Coronograptus annellus (Li), LM5, Bajiaomiao of Shennongjia, Hubei, Collection number: AFU585. E. Demirastrites triangulatus (Harkness), LM6, Shizishan of Changning, Sichuan, Collection number: AGH141.

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F. Normalograptus biformis (G. Wang), LM6, Shizishan of Changning, Sichuan, Collection number: AGH150. G. Coronograptus minusculus Obut et Sobolevskaya, LM5, Bajiaomiao of Shennongjia, Hubei, Collection number: AFU581. H. Monograptus cf. capillaris (Carruthers), LM8, Bajiaomiao of Shennongjia, Hubei, Collection number: AFU607. I. Cephalograptus tubulariformis (Nicholson), LM7, Bajiaomiao of Shennongjia, Hubei, Collection number: AFU599. Scale bar: 1 mm.

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Appendix: Plates and Explanation

Plate 6

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Appendix: Plates and Explanation

Plate 7 A. Petalolithus ovatoelongatus (Kurch), LM6, Bajiaomiao of Shennongjia, Hubei, Collection number: AFU592. B. Penerograptus difformis (Törnquist), LM6, Hanjiadian of Tongzi, Guizhou, Collection number: AAE82. C. Campograptus communis (Lapworth), LM6, Bajiaomiao of Shennongjia, Hubei, Collection number: AFU594.

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D. Pseudorthograptus mutabilis (Elles et Wood), LM5, Bajiaomiao of Shennongjia, Hubei, Collection number: AFU591. E. Demirastrites triangulatus (Harkness), LM6, Shizishan of Changning, Sichuan, Collection number: AGH144. Scale bar: 1 mm.

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Plate 7

Appendix: Plates and Explanation

Appendix: Plates and Explanation

Plate 8 A. Rastrites hybridus (Lapworth), LM7, Shizishan Changning, Sichuan, Collection number: AGH168. B. Rastrites approximatus Perner, LM6, Shizishan Changning, Sichuan, Collection number: AGH141. C. Rastrites guizhouensis Chen et Lin, LM6, Shizishan Changning, Sichuan, Collection number: AGH152. D. Rastrites peregrinus Barrande, LM6, Shizishan Changning, Sichuan, Collection number: AGH153.

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of of of of

E. Pristiograptus regularis (Törnquist), LM7, Bajiaomiao of Shennongjia, Hubei, Collection number: AFU604. F. Normalograptus scalaris (Hisinger), LM7, Shizishan of Changning, Sichuan, Collection number: AGH168. G. Pseudoretiolites perlatus (Nicholson), LM8, Bajiaomiao of Shennongjia, Hubei, Collection number: AFU613. H. Rastrites longispinus (Perner), LM7, Shizishan of Changning, Sichuan, Collection number: AGH167. Scale bar: 1 mm.

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Appendix: Plates and Explanation

Plate 8

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Appendix: Plates and Explanation

Plate 9 A. Pseudoretiolites daironi (Lapworth), LM8, Shizishan of Changning, Sichuan, Collection number: AFU613. B. Normalograptus medius (Törnquist), LM5, Banqiao of Zunyi, Guizhou, Collection number: ZF1. C. Metaclimacograptus hughesi (Nicholson), LM6, Shizishan of Changning, Sichuan, Collection number: AGH149. D. Stimulograptus sedgwickii (Portlock) and Lituigraptus sp., LM8, Shizishan of Changning, Sichuan, Collection number: AGH167.

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E. Lituigraptus richteri (Perner), LM7, Shizishan of Changning, Sichuan, Collection number: AGH142. F. Pristiograptus regularis (Tornqist), LM8, Shizishan of Changning, Sichuan, Collection number: AGH197. G. Pristiograptus pristinus Píbyl, LM9, Bajiaomiao of Shennongjia, Hubei, Collection number: AFU625. H. Petalolithus praecursor Bouek and Píbyl, LM6, Bajiaomiao of Shennongjia, Hubei, Collection number: AFU593. I. Spirograptus andrewsi (Sherwin), LM8, Bajiaomiao of Shennongjia, Hubei, Collection number: AFU613. Scale bar: 1 mm.

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Appendix: Plates and Explanation

Plate 9

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Appendix: Plates and Explanation

Plate 10 A. Stimulograptus sedgwickii (Portlock), LM8, Bajiaomiao of Shennongjia, Hubei, Collection number: AFU613. B. Lituigraptus convolutus (Hisinger), LM7, Xiaohecun of Wufeng, Hubei, Collection number: GHH242. C. Parapetalolithus palmeus (Barrande), LM8, Bajiaomiao of Shennongjia, Hubei, Collection number: AFU613. D. Paramonoclimacis falcatus (Chen et Lin), LM7, Shizishan of Changning, Sichuan, Collection number: AGH171.

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E. Parapetalolithus clavatus (Boucek et Píbyl), LM8, Shizishan of Changning, Sichuan, Collection number: CN230. F. Paramonoclimacis chengkouensis (Ge), LM7, Bajiaomiao of Shennongjia, Hubei, Collection number: AFU604. G. Paramonoclimacis sidjachenkoi (Obut et Sobolevskaya), LM7, Bajiaomiao of Shennongjia, Hubei, Collection number: AFU604. H. Rastrites lenzi Loydell et al., LM8, Shizishan of Changning, Sichuan, Collection number: AGH197. Scale bar: 1 mm.

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Plate 10

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Appendix: Plates and Explanation

Plate 11 A. Spirograptus guerichi Loydell et al., LM9, Sanbaiti of Huaying, Sichuan, Collection numbers: AGH277. B. Pseudoretiolites perlatus (Nicholson), LM8, Bajiaomiao of Shennongjia, Hubei, Collection number: AFU613. C. Monograptus bjerreskovae Loydell et al., LM9, Bajiaomiao of Shennongjia, Hubei, Collection number: AFU625.

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D. Torquigraptus decipiens (Torquist), LM8, Bajiaomiao of Shennongjia, Hubei, Collection number: AFU610. E. Torquigraptus obtusus (Schauer), LM9, Bajiaomiao of Shennongjia, Hubei, Collection number: AFU625. F. Monograptus marri Perner, LM9, Bajiaomiao of Shennongjia, Hubei, Collection number: AFU625. Scale bar: 1 mm.

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Plate 11

Appendix: Plates and Explanation

Appendix: Plates and Explanation

Plate 12 Graptolite from the DDO fauna (A–S. latest Katian) and the N fauna (T–W. Hirnantian to Rhuddanian) (after Fig. 3 of Chen et al. 2003). A. Nymphograptus sichuanensis Mu, WF2,
1.7. B. Diceratograptus mirus Mu, WF3,
5. C. Climacograptus hastatus (T. S. Hall), WF3–WF4,
5. D. Dicellograptus extremus (Mu et Zhang), WF2,
2.6. E. Tangyagraptus typicus Mu, WF3,
5. F. Paraorthograptus pacificus (Ruedemann), WF3–WF4,
5.5. G. Paraplegmatograptus connectus Mu, WF2–WF4,
4.7. H. Sinoretiograptus mirabilis Mu, WF2,
6.8. I. Appendispinograptus Venustus (Hsü), WF2–WF3,
4.4. J. Parareteograptus sinensis Mu, WF2–WF3,
5.5. K. Styracograptus tatianae (Keller), WF3–WF4,
4.3.

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L. Orthoretiograptus denticulatus Mu, WF2,
4.8. M. Dicellograptus ornatus Elles et Wood, WF2–WF4,
5.8. N. Appendispinograptus supernus (Elles et Wood), WF1– WF4,
4. O. Anticostia maximus (Mu), WF2–WF3,
5.5. P. Arachniograptus connectus (Wang), WF2,
2.1. Q. Amplexograptus latus Elles et Wood, WF2–WF4,
8.5. R. Yinograptus disjunctus (Yin et Mu), WF3,
5.5. S. Rectograptus uniformis (Mu et Li), WF2–WF4,
3.7. T. Metabolograptus ojsuensis (Koren’ et Mikhilova), WF4– LM1,
5. U. Neodiplograptus charis (Mu et Ni), WF4–LM1,
5. V. Korenograptus angustifolius (Chen et Lin), LM1–LM2,
5. W. Normalograptus sp. aff. N. indivisus (Davies), LM1,
17.

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Appendix: Plates and Explanation

Plate 12

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M

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Reference Chen X, Melchin MJ, Fan JX, Mitchell CE (2003) Ashgillian graptolite fauna of the Yangtze region and the biogeographical distribution of diversity in the latest

Ordovician. Bull De La Société Géologique De France 175:141–148