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Paleoenvironment and Organic Matter Accumulation of the Upper Ordovician-Lower Silurian, in Upper Yangtze Region, South China: Constraints from Multiple Geochemical Proxies

1
School of Geoscience and Technology, Southwest Petroleum University, Chengdu 610500, China
2
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, China
3
Research Institute of Petroleum Exploration&Development, PetroChina, Beijing 100083, China
4
College of Geophysics and Petroleum Resources, Yangtze University, Wuhan 430100, China
5
Shale Gas Research Institute, PetroChina Southwest Oil&Gas Field Company, Chengdu 610051, China
*
Author to whom correspondence should be addressed.
Energies 2020, 13(4), 858; https://doi.org/10.3390/en13040858
Received: 18 January 2020 / Revised: 12 February 2020 / Accepted: 14 February 2020 / Published: 16 February 2020
The origin of the organic-rich shale in the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation is complex and controversial. This paper reports the geochemical data of Wufeng-Longmaxi Formations in the Upper Yangtze region to restore the paleoenvironment and explore the accumulation mechanism of organic matter. The total organic carbon (TOC) content of the Wufeng Formation was relatively high, with an average of 2.86%. The Lower Longmaxi Formation showed the highest TOC content, with an average of 3.99%, and the upper part was a continuously low value with an average of 1.22%. The paleoproductivity proxies (Babio, Cu/Al, Ni/Al, Siexcess) showed that in the Katian and Rhuddanian-Aeronian Stages, the Upper Yangtze Sea had high primary productivity, indicating that organic matter accumulation was more affected by terrigenous influx and redox conditions. Al, Zr, and Zr/Al indicated that terrigenous influx was relatively high in the Kaitian-Hirnantian Stages, it was at a constant low in the Rhuddanian Stage, and increased again in the Aeronian Stage. The correlations between redox-sensitive trace elements (Mo, U, V) and TOC indicate that the organic-rich shale of the Wufeng Formation was deposited in the anoxic–euxinic environment. In the Longmaxi Formation, organic-rich shales formed in a more hypoxic environment, and overlying organic-lean shales formed in a suboxic environment. Therefore, the anoxic–euxinic conditions of the Late Ordovician Yangtze Sea were the main reason for the organic matter accumulation, but the high terrigenous influx caused by regression and/or structural controls diluted the organic matter to some extent. For the Early Silurian, a complete transgression–regression cycle changed terrigenous influx and redox conditions, resulting in significant differences in organic matter accumulation. View Full-Text
Keywords: Wufeng and Longmaxi Formations; paleoenvironment; terrigenous influx; paleoredox conditions; paleoproductivity; organic matter accumulation Wufeng and Longmaxi Formations; paleoenvironment; terrigenous influx; paleoredox conditions; paleoproductivity; organic matter accumulation
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Huang, Z.; Wang, X.; Yang, X.; Zhu, R.; Cui, J.; Shi, W.; Zhu, Y. Paleoenvironment and Organic Matter Accumulation of the Upper Ordovician-Lower Silurian, in Upper Yangtze Region, South China: Constraints from Multiple Geochemical Proxies. Energies 2020, 13, 858.

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