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Sedimentary Geochemistry and Patterns of Organic Matter Enrichment of Wufeng-Longmaxi Formations in the Southern Margin of Sichuan Basin, China —A Case Study of Tianlin Profile in Xuyong Area(PDF)


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Sedimentary Geochemistry and Patterns of Organic Matter Enrichment of Wufeng-Longmaxi Formations in the Southern Margin of Sichuan Basin, China —A Case Study of Tianlin Profile in Xuyong Area
ZHENG Yu-long12 MOU Chuan-long2* WANG Xiu-ping2
(1. College of Earth Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, Shandong, China; 2. Key Laboratory of Sedimentary Basin and Oil and Gas Resources of Ministry of Land and Resources, Chengdu Center, China Geological Survey, Chengdu 610081, Sichuan, China)
sedimentary geochemistry Wufeng Formation Longmaxi Formation paleoproductivity paleoredox extinction event organic matter enrichment Sichuan Basin
P588.2; P59
Late Ordovician-Early Silurian transition period is an important geological history period. Global plate tectonics, paleoclimate conditions, paleo-ocean environment and biota types occur during this relatively short period(440-453 Ma). With these great changes, there are two sets of organic-rich shales in Wufeng Formation and Longmaxi Formation in Yangtze area. It is a key exploration layer for shale gas exploration and has received extensive attention. A lot of influencing factors on organic matter enrichment are paleo-ocean primary productivity, palaeoredox facies and sedimentation rate, diagenetic degradation, etc. However, the two sets of organic-rich shales in Wufeng Formation and Longmaxi Formation show strong heterogeneity vertically. Therefore, systematic measuring, continuous sampling and detailed petromineralogy and sedimentary geochemistry(major, trace, and rare earth elements)of Tianlin profile in Xuyong area were conducted in order to make better sense of the evolution of the vertical sedimentary environment and the causes of differences in organic matter enrichment of Wufeng-Longmaxi Formations. The results show that the ratio of Al/(Al+Fe+Mn), Ce anomaly and Eu anomaly indicate that non-terrestrial Si is in biogenic origin; the contents of SiO2 and Sixs are significantly negative correlated with the contents of Ca and TiO2, indicating that silica contents are mainly affected by terrigenous flux and paleo-water depth; the ratios of Mo/TOC and Mo/U indicate that the early period of Wufeng Formation has a strong restricted environment, while the organic-rich intervals form in moderately restricted environment; the intervals with dense bentonite layers have higher paleoproductivity; stronger positive correlation between paleoredox proxy coefficient and TOC indicates that paleoredox is the main controlling factor of organic matter enrichment; due to the different biohabitat types and the different influences of terrigenous flux on paleoproductivity and paleoredox, the correlation between paleoproductivity proxy coefficient and TOC is poor in the mass. A comprehensive analysis suggests that organic matter enrichment patterns in different sedimentary stages of Wufeng-Longmaxi Formations are different. Organic matter enrichment of Wufeng-Longmaxi Formations is mainly affected by anoxic environment, which is caused by deep water environment with low terrigenous flux during non-extinction period. It is mainly affected by high sedimentation rate of bio-precursors, which is caused by explosion of bio-precursors and “marine snow” during the first episode extinction period(corresponding to the terminal period of Wufeng Formation), and is affected by high sedimentation rate of bio-precursors and extremely anoxic environment caused by rapid transgression and organic decomposition during the second extinction period(corresponding to the early period of Longmaxi Formation).


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Last Update: 2019-09-19