Sedimentary environment and organic matter accumulation of Wufeng-Longmaxi shales, southwest Yangtze Plate, China: Insights from geochemical and petrological evidence

Abstract

Upper Ordovician-Lower Silurian Wufeng-Longmaxi Formation is the most developed strata of shale gas in southern China. Due to the complex sedimentary environment adjacent to the Kangdian Uplift, the favorable area for organic-rich shale development is still undetermined. The authors, therefore, focus on the mechanism of accumulation of organic matter and the characterization of the sedimentary environment of the Wufeng-Longmaxi Shales to have a more complete understanding and new discovering of organic matter enrichment and favorable area in the marginal region around Sichuan Basin. Multiple methods were applied in this study, including thin section identification, scanning electron microscopy (SEM) observations and X-ray diffraction (XRD), and elemental analysis on outcrop samples. Five lithofacies have been defined according to the mineralogical and petrological analyses, including mudstone, bioclastic limestone, silty shale, dolomitic shale, and carbonaceous siliceous shale. The paleo-environments have been reconstructed and the organic enrichment mechanism has been identified as a reduced environment and high productivity. The Wufeng period is generally a suboxic environment and the early Longmaxi period is a reducing environment based on geochemical characterization. High dolomite content in the study area is accompanied by high TOC, which may potentially indicate the restricted anoxic environment formed by biological flourishing in shallower water. And for the area close to the Kangdian Uplift, the shale gas generation capability is comparatively favorable. The geochemical parameters implied that new favorable areas for shale gas exploration could be targeted, and more shale gas resources in the mountainbasin transitional zone might be identified in the future.

Keywords

Shales / Shale gas / Sedimentary environment / Graptolite / Upper Ordovician-Lower Silurian / Organic matter accumulation / Radiolarian / Geochemical and petrographic evidence / Wufeng-Longmaxi Formation / Oil-gas exploration engineering / Suboxic environment / Sichuan Basin / Reduced environment / Yangtze Plate

CRediT authorship contribution statement

An-kun Zhao and Zi-hui Lei conceived of the presented idea and wrote the manuscript. Dong Wang and Qian Zhang carried out the experiment and analyzed the geochemical proxies. Di Zhang conducted the graptolite observation and zonation. Qian Yu supervise the project and contribute to the final version of the manuscript. Ye-xin Zhou contributes to the stratigraphic column analysis. All authors discussed the results and contributed to the final manuscript.

Declaration of competing interest

The authors declare no conflicts of interest.

Acknowledgment

This research was jointly funded by the National Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (PLC20210104), China Geological Survey (DD20221661), and China National Science and Technology Major Project "Test and Application of Shale Gas Exploration and Evaluation Technology (2016ZX05034004)". The authors are indebted to Prof. Gang-yi Zhai, Prof. Zhi-qiang Shi, and Prof. Jun Peng for comments and suggestions on this paper. This manuscript has benefited from valuable advice from Dr. You-li Wan and Mr. Wei Sun.

References

[1] Algeo TJ, Tribovillard N. 2009. Environmental analysis of paleoceanographic systems based on molybdenum-uranium covariation. Chemical Geology, 268(3), 211-225. doi: 10.1016/j.chemgeo.2009.09.001.

[2] Bernard S, Horsfield B. 2014. Thermal maturation of gas shale systems. Annual Review of Earth and Planetary Science, 42(1), 635-651. doi: 10.1146/annurev-earth-060313-054850.

[3] Cavelan A, Boussafir M, Rozenbaum O, Laggoun-Défarge F. 2019. Organic petrography and pore structure characterization of lowmature and gas-mature marine organic-rich mudstones: Insights into porosity controls in gas shale systems. Marine and Petroleum Geology, 103, 331-350. doi: 10.1016/j.marpetgeo.2019.02.027.

[4] Caplan ML, Marc Bustin R. 1998. Sedimentology and sequence stratigraphy of Devonian-Carboniferous strata, southern Alberta. BulletinofCanadianPetroleumGeology, 46(4),487-514.https://doi.org/10.1306/212f9255-2b24-11d7-8648000102c1865d.

[5] Chen X, Rong JY, Li Y, Boucot AJ. 2004. Facies patterns and geography of the Yangtze region, South China, through the Ordovician and Silurian transition. Palaeogeography, Palaeoclimatology, Palaeoecology, 204(3-4), 353-372. doi: 10.1016/S0031-0182(03)00736-3.

[6] Chen XJ, Jia LQ, Jia T. 2022. China achieved fruitful results in oil-shale gas-coalbed methane exploration and development in 2021. China Geology, 5(2), 355-356. doi: 10.31035/cg2022031.

[7] Dai JX, Zou CN, Dong DZ, Ni YY, Wu W, Gong DY, Wang YM, Huang SP, Huang JL, Fang CC, Liu D. 2016. Geochemical characteristics of marine and terrestrial shale gas in China. Marine and Petroleum Geology, 76, 444-463. doi: 10.1016/j.marpetgeo.2016.04.027.

[8] Dong DZ, Wang YM, Li XJ, Zou CN, Guan QZ, Zhang CC, Huang JL, Wang SF, Wang HY, Liu HL, Bai WH, Liang F, Lin W, Zhao Q, Liu DX, Qiu Z. 2016. Breakthrough and prospect of shale gas exploration and development in China. Natural Gas Industry B, 3(1), 12-26. doi: 10.1016/j.ngib.2016.02.002.

[9] Fan JX, Chen X. 2007. Preliminary report on the Late Ordovician graptolite extinction in the Yangtze region. Palaeogeography, Palaeoclimatology, Palaeoecology, 245, 82-94. doi: 10.1016/j.palaeo.2006.02.019.

[10] Ge XY, Mou CL, Yu Q, Liu W, Men X, He JL. 2019. The geochemistry of the sedimentary rocks from the Huadi No. 1 well in the WufengLongmaxi formations (Upper Ordovician-Lower Silurian), South China, with implications for paleoweathering, provenance, tectonic setting and paleoclimate. Marine and Petroleum Geology, 103, 646660. doi: 10.1016/j.marpetgeo.2018.12.040.

[11] Guo X, Liu R, Xu S, Feng B, Wen, T, Zhang T. 2022. Structural deformation of shale pores in the fold-thrust belt: The WufengLongmaxi shale in the Anchang Syncline of Central Yangtze Block. Advances in Geo-Energy Research, 6(6), 515-530. doi: 10.46690/ager.2022.06.08.

[12] Hackley PC, Cardott BJ. 2016. Application of organic petrography in North American shale petroleum systems: A review. International Journal of Coal Geology, 163, 8-51. doi: 10.1016/j.coal.2016.06.010.

[13] Hammes U, Eastwood R, McDaid G, Vankov E, Gherabati SA, Smye K, Shultz J, Potter E, Ikonnikova S, Tinker S. 2016. Regional assessment of the Eagle Ford Group of South Texas, USA: Insights from lithology, pore volume, water saturation, organic richness, and productivity correlations. Interpretation, 4(1), C125-C150. doi: 10.1190/INT-2015-0099.1.

[14] Han C, Jiang ZX, Han M, Wu MH, Lin W. 2016. The lithofacies and reservoir characteristics of the Upper Ordovician and Lower Silurian black shale in the Southern Sichuan Basin and its periphery, China. Marine and Petroleum Geology, 75, 181-191. doi: 10.1016/j.marpetgeo.2016.04.014.

[15] Hao F, Zou HY. 2013. Cause of shale gas geochemical anomalies and mechanisms for gas enrichment and depletion in high-maturity shales. Marine and Petroleum Geology, 44, 1-12. doi: 10.1016/j.marpetgeo.2013.03.005.

[16] Jia AL, Wei YS, Jin YQ. 2016. Progress in key technologies for evaluating marine shale gas development in China. Petroleum Exploration and Development, 43(6), 1035-1042. doi: 10.1016/S1876-3804(16)30120-3.

[17] Knapp LJ, McMillan JM, Harris NB. 2017. A depositional model for organic-rich Duvernay Formation mudstones. Sedimentary Geology, 347, 160-182. doi: 10.1016/j.sedgeo.2016.11.012.

[18] Kuypers MMM, Pancost RD, Nijenhuis IA, Sinninghe Damsté JS. 2002. Enhanced productivity led to increased organic carbon burial in the euxinic North Atlantic basin during the late Cenomanian oceanic anoxic event. Paleoceanography, 17(4), 3-1-3-13. https://doi.org/10.1029/2000PA000569.

[19] Lalonde K, Mucci A, Ouellet A, Gélinas Y. 2012. Preservation of organic matter in sediments promoted by iron. Nature, 483(7388), 198-200. doi: 10.1038/nature10855.

[20] Li SZ, Zhou Z, Nie HK, Zhang LF, Song T, Liu WB, Li HH, XU QC, Wei SY, Tao S. 2022. Distribution characteristics, exploration and development, geological theories research progress and exploration directions of shale gas in china. China Geology, 5(1), 110-135. doi: 10.1016/S2096-5192(22)00090-8.

[21] Li YF, Zhang TW, Ellis GS, Shao DY. 2017. Depositional environment and organic matter accumulation of Upper Ordovician-Lower Silurian marine shale in the Upper Yangtze Platform, South China. Palaeogeography, Palaeoclimatology, Palaeoecology, 466, 252-264. doi: 10.1016/j.palaeo.2016.11.037.

[22] Liu ZH, Algeo TJ, Guo XS, Fan JX, Du XB, Lu YC. 2017. Paleoenvironmental cyclicity in the Early Silurian Yangtze Sea (South China): Tectonic or glacio-eustatic control? Palaeogeography, Palaeoclimatology, Palaeoecology, 466, 59-76. doi: 10.1016/j.palaeo.2016.11.007.

[23] Ma YQ, Fan MJ, Lu YC, Guo XS, Hu HY, Chen L, Wang C, Liu XC. 2016. Geochemistry and sedimentology of the Lower Silurian Longmaxi mudstone in southwestern China: Implications for depositional controls on organic matter accumulation. Marine and Petroleum Geology, 75, 291-309. doi: 10.1016/j.marpetgeo.2016.04.024.

[24] Murray RC. 1990. Diagenetic silica stratification in a paleosilcrete, North Texas. Journal of Sedimentary Research, 60(5), 717-720. https://doi.org/10.1306/212F9255-2B24-11D7-8648000102C1865D.

[25] Nitzer SF, Stephenson MH, Davies SJ, Vane CH, Leng MJ. 2016. Significance of sedimentary organic matter input for shale gas generation potential of Mississippian Mudstones, Widmerpool Gulf, UK. Review of Palaeobotany and Palynology, 224, 146-168. doi: 10.1016/j.revpalbo.2015.10.003.

[26] Pan SQ, Zou CN, Yang ZY, Dong DZ, Wang YM, Wang SF, Wu ST, Huang JL, Liu Q, Wang D, Wang ZY. 2015. Methods for shale gas play assessment: A comparison between Silurian Longmaxi shale and Mississippian Barnett shale. Journal of Earth Science, 26(2), 285-294. doi: 10.1007/s12583-015-0524-0.

[27] Tan JQ, Weniger P, Krooss B, Merkel A, Horsfield B, Zhang JC, Boreham CJ, Graas GV, Tocher BA. 2014. Shale gas potential of the major marine shale formations in the Upper Yangtze Platform, South China, Part II: Methane sorption capacity. Fuel, 129, 204-218. doi: 10.1016/j.fuel.2014.03.064.

[28] Tang X, Zhang JC, Liu Y, Yang C, Chen Q, Dang W, Zhao PW. 2017. Geochemistry of organic matter and elements of black shale during weathering in Northern Guizhou, Southwestern China: Their mobilization and inter-connection. Geochemistry, 78(1), 140-151. doi: 10.1016/j.chemer.2017.08.002.

[29] Torsvik T, Cocks L. 2016. Earth History and Palaeogeography. Cambridge, Cambridge University Press. doi: 10.1017/9781316225523.

[30] Tribovillard N, Algeo TJ, Lyons T, Riboulleau A. 2006. Trace metals as paleoredox and paleoproductivity proxies: An update. Chemical Geology, 232(1), 12-32. doi: 10.1016/j.chemgeo.2006.02.012.

[31] Tribovillard N, Algeo T J, Baudin F, Riboulleau AJCG. 2012. Analysis of marine environmental conditions based onmolybdenum-uranium covariation-Applications to Mesozoic paleoceanography. Chemical Geology, 324, 46-58. https://doi.org/10.1016/j.chemgeo.2011.09.009.

[32] Tserolas P, Maravelis AG, Tsochandaris N, Pasadakis N, Zelilidis A. 2019. Organic geochemistry of the Upper Miocene-Lower Pliocene sedimentary rocks in the Hellenic Fold and Thrust Belt, NW Corfu island, Ionian sea, NW Greece. Marine and Petroleum Geology, 106, 17-29. doi: 10.1016/j.marpetgeo.2019.04.033.

[33] Wang S, Dong D, Wang Y, Li X, Huang J, Guan Q. 2016. Sedimentary geochemical proxies for paleoenvironment interpretation of organicrich shale: A case study of the Lower Silurian Longmaxi Formation, Southern Sichuan Basin, China. Journal of Natural Gas Science and Engineering, 28, 691-699. doi: 10.1016/j.jngse.2015.11.045.

[34] Yuan K, Huang WH, Wang T, Li SZ, Sun XC, Fang XX, Xiao JP, Guo J. 2023. Tectonic evolution and accumulation characteristics of Carboniferous shale gas in Yadu-Ziyun-Luodian aulacogen, Guizhou Province, South China. China Geology, 6(4), 646-659. doi: 10.31035/cg2022059.

[35] Zhao AK, Shi ZQ, Wang XF, Peng J, Lei ZH, Yu Q, Hao JY, Zhang D. 2022. Characteristics of organic-rich dolomites from upper Ordovician Wufeng Formation -Lower Silurian Longmaxi Formation in the eastern part of Kangdian ancient land and their geological significances. Bulletin of Mineralogy, Petrology and Geochemistry, 41(2), 307-316. doi: 10.19658/j.issn.1007-2802.2021.40.105. (in Chinese with English abstract)

[36] Zhao AK, Yu Q, Zhou YX, Lei ZH, Yan JF, Men YP, Zhang Q, Zhang Di. 2021. Analysis and quantitative evaluation of the shale gas preservation conditions in the margin areas of Sichuan Basin. 41(3), 376-386. doi: 10.19826/j.cnki.1009-3850.2021.05003.

[37] Zhao L, Mao W, Liu Z, Cheng S. 2023. Research on the differential tectonic-thermal evolution of Longmaxi shale in the southern Sichuan Basin. Advances in Geo-Energy Research, 7(3), 152-163. doi: 10.46690/ager.2023.03.02.

[38] Zhou L, Algeo T J, Shen J, Hu ZF, Gong HM, Xie SC, Huang JH, Gao S. 2015. Changes in marine productivity and redox conditions during the Late Ordovician Hirnantian glaciation. Palaeogeography, Palaeoclimatology, Palaeoecology, 420, 223-234. doi: 10.1016/j.palaeo.2014.12.012.

[39] Zou CN, Du JH, Xu CC, Wang ZC, Zhang BM, Wei GQ, Wang TS, Yao GS, Deng SH, Liu JJ, Zhou H, Xu A, Yang Z, Jiang H, Gu ZD. 2014. Formation, distribution, resource potential, and discovery of Sinian-Cambrian giant gas field, Sichuan Basin, SW China. Petroleum Exploration and Development, 41(3), 306-325. doi: 10.1016/S1876-3804(14)60036-7.

[40] Zou CN, Yang Z, Dai JX, Dong D, Zhang BM, Wang YM, Deng SH, Huang JL, Liu KY, Yang C, Wei GQ, Pan SQ. 2015. The characteristics and significance of conventional and unconventional Sinian-Silurian gas systems in the Sichuan Basin, central China. Marine and Petroleum Geology, 64, 386-402. doi: 10.1016/j.marpetgeo.2015.03.005.

[41] Zou CN, Zhu RK, Chen ZQ, Ogg JG, Wu ST, Dong DZ, Qiu Z, Wang YM, Wang L, Lin SH, Cui JW, Su L, Yang Z. 2019. Organic-matterrich shales of China. Earth-Science Reviews, 189, 51-78. doi: 10.1016/j.earscirev.2018.12.002.