Organic Geochemical Characteristics Of Organic-Rich Chert In The Middle Permian Gufeng Formation In Chaohu,Lower Yangtze Area

The black chert of the Gufeng Formation in Chaohu Area,lower Yangtze,South China,are rich in organic matter,which are potential source rocks for oil and shale gas exploration in China.In this paper,the total organic carbon(TOC)content,organic carbon isotope?¹³C_org and saturated hydrocarbon molecular biomarker characteristics of the siliceous rocks of the Middle Permian Gufeng Formation in Pingdingshan area of Chaohu Lake are analyzed,and the organic geochemical characteristics and organic matter enrichment rules are discussed.The results are as follows:The source of organic matter in the Gufeng Formation is mainly marine phytoplankton and bacteria,and a small amount of organic matter comes from land-based higher plants.The thermal maturity of organic matter is in the early stage of oil generation window,which has exceeded the immature and low mature stages,but has not yet entered the peak of oil generation window.The paleo-sedimentary water mass in the Gufeng Formation is a medium layered(not obvious layered)anoxic sedimentary environment,but has not yet entered the high-salinity anoxic environment.The long-term negative deviation of?¹³C_org in the Gufeng Formation may be related to the regression owing to the formation of supercontinent and the extinction of guadelupian organisms in the same period;the sea level fall caused by the Pangea amalgamation leads to the enrichment of ¹²C of dissolved inorganic carbon(DIC)in ocean through the reoxidation of seafloor organic matter which were exposed by regression.The extinction of benthos caused by anoxic environment leads to the decrease of the absorption of dissolved organic matter through the decrease uptake of“marine snow”by neckton or zooplankton so as to the lower burial effect of carbon.This process held the ¹²C-rich dissolved organic carbon(DOC)and particle organic matter(POM)in ocean water mass and thus lowered the C-isotope of DIC,which may lead to the long-term negative deviation of?¹³C_org in Guadelupian in this study.The Gufeng Formation is rich in organic matter,which may be related to the long-term stratification and anoxia of ancient sedimentary water in hot climate,because the preservation environment refers to the redox conditions and sedimentation rate of sedimentary water.The organic matter produced by phytoplankton,bacteria and other organisms in the upper productive water body through photosynthesis or its own death will be in the process of sedimentation into the sediment.Most of the organic matter exists in the water body in the form of suspended particles or dissolved state,and only a small amount of organic matter will enter the sediment and be preserved.Oxygen deficient or anoxic water body is a reducing environment with less oxygen content,which makes the decomposition rate of organic matter in the environment lower,which is conducive to a large number of organic matter deposition.With the sudden cooling of climate in the early and late deposition of Gufeng formation,high-intensity current upwelling is caused by climate change,and the nutrients carried by the upwelling increase the level of primary productivity,resulting in the enrichment of organic matter on a larger scale.