Sedimentary Environment and Controlling Factors of Organic Matter Accumulation in Wufeng Formation-Longmaxi Formation:A case study of Jielong section in eastern Chongqing
[Objective]The Jielong section in the Wulong district of Chongqing was selected for field observations in this study of the sedimentary environment and factors influencing the accumulation of organic matter in the black shale of the Wufeng Formation-Longmaxi Formation in eastern Chongqing.[Methods]Continuous field sampling was carried out to obtain material for geochemical analysis.The variation in element content of the shale was compared with that from well QQ1 in eastern Chongqing.The paleoenvironmental evolution and TOC enrichment factors affect-ing the Wufeng Formation-Longmaxi Formation were analyzed in eastern Sichuan.[Results]The results show that ter-rigenous input indexes Al,Zr,and Ti increased in Wufeng Formation,and then it decreased in Guanyinqiao Forma-tion,and finally it increased rapidly in lower Longmaxi Formation.The V/Cr and V/(V+Ni)redox indices show that the Wufeng Formation developed in an anoxic environment,Guanyinqiao developed in a weak oxygen environment,and the Longmaxi Formation developed in a reducing environment.The P/Al and Ba(XS)content analyses indicated highest productivity in the Longmaxi Formation,and lowest in Guanyinqiao.Analysis of the correlation between TOC and P/Al-Ba(XS),and between TOC and V/Cr-V/(V+Ni),then comparison with well QQ1,indicated that the main in-fluence on TOC content in the Wufeng Formation-Longmaxi Formation was the redox reducibility of the body of water.In addition,the lessening of oxidation depended on water depth:specifically,in the Jielong section located in the deepwater shelf area,in the center of subsidence the reducibility was significantly greater than at the well QQ1 site lo-cated in the shallow-water shelf area surrounding the subsidence.This resulted in a significantly higher TOC content in the Jielong section than at well QQ1.[Conclusions]Although the organic-rich strata of the Wufeng Formation-Longmaxi Formation shales were both formed in anoxic conditions,their sedimentary environments differed consider-ably.The Wufeng Formation shale was formed in a strongly retained water environment with extremely poor organic fluidity;the sedimentary environment of the Longmaxi Formation was mainly anoxic.
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