The Paleo-Sedimentary Environment And Formation Mechanism Of The Source Rocks In Shahejie Formation,Qikou Sag

The source rocks of the Shahejie formation in Qikou sag is the most important source rocks deposited in the Paleogene.However,the investigation on its paleo-sedimentary environment is not indepth.This study starts from the development and evolution of source rocks,and conducts in-depth research on the distribution and formation mechanism of source rocks.The geochemical analysis shows that the contents of organic matter has the characteristics of increasing firstly from Es₃to Es₁^xand then decreasing from Es₁^x to Es₁^s.The types of organic matter（OM）are mainly type II₁ and type II₂,followed by type III organic matter,and there is more type I organic matter（OM）in Es₁^x and Es₁^z,reflecting that the source of organic matter in the sedimentary period of the Shahejie formation was lake primary organic matter mixed with terrigenous organic matter,and the terrigenous organic matter in Es₁^x and Es₁^z is lower.The distribution of effective source rocks in the Shahejie formation on the plane has the characteristics of wide area and large thickness,and high-quality source rocks are more developed in the slope area.The paleoclimate during the sedimentary period of the Shahejie formation was affected by moderate weathering intensity,and the overall warm and humid paleoclimate conditions during this period were conducive to the development of organisms.The paleosalinty ranges from 1.00‰to 18.42‰,with an average value of 7.40‰,which belongs to the brackish water.There is no obvious correlation between paleosalinity and paleoclimate,and the high paleosalinty of water may be influenced by seawater intrusion.The relationship between paleosalinity and the content of organic matter shows that when paleosalinity<9‰,the abundance of organic matter increases with the increase of paleosalinity,and when paleosalinity>9‰,the abundance of organic matter decreases when the the paleosalinity increases.The deposition rate of the Shahejie formation is relatively large,and has the characteristics of decreasing firstly from Es₃to Es₁^xand Es₁^z,then increasing from Es₁^z to Es₁^s.According to the study of deposition rate and organic matter content of source rocks,it is found that in the stage of low deposition rate（deposition rate<10 cm/ka）,the content of organic matter increases with the increase of deposition rate,and in the stage of high deposition rate（deposition rate>10 cm/ka）,the organic matter content decreased with the increase of deposition rate.The low deposition rates in Es₁^x and Es₁^z is favorable for the formation of effective source rocks,and the large deposition rates in Es_3,Es₂and Es₁^s play a destructive role in the formation of effective source rocks.According to the ratio of trace elements and C_org:P molar ratio,an oxidative-suboxidative depositional environment developed during the deposition of the Shahejie formation in Qikou sag,and a strongly reducing environment developed intermittently in Es₁^x,which is conducive to the preservation of organic matter.The evaluation of the paleoproductivity at the period of the Shahejie formation deposited shows that the lake had medium-high nutrient content and primary productivity was medium-high.The relatively high paleoproductivity and the high input of terrigenous organic matter in the Qikou Sag,with the highest value in Es₁ period and the lowest value in Es₂period,provide a rich material basis for the formation of effective source rocks.According to the multiple regression analysis of paleoproductivity index（Pz）,deposition rate index（Ti/Al）and redox index（V/Ni）,the quantitative relationship between TOC and Pz,Ti/Al,V/Ni was established,and the quantitative relationship shows that the formation of source rocks in Qikou sag is generally controlled by paleoproductivity.Due to the dual contribution of terrigenous organic matter and aquatic organic matter as well as suitable deposition rate in the slope area,high-quality source rocks are more developed here.