Abstract: Thick carbonate rocks bearing detrital components are widespread in the 3rd member of Shahejie Formation in Bonan Sag,Bohai Bay Basin,where shale oil resource potential is huge.The causal mechanism of organic matter enrichment in Bonan Sag was investigated drawing on lithofacies description,as well as analyses of terrigenous debris sources,primary productivity and redox environment.The core of Well L69 was systematically sampled,with the samples being mainly classified as massive micrite,lenticular micrite,laminar micrite and laminar sparite in terms of lithofacies features and sedimentary structures.In addition,the terrigenous debris sources were discussed based on analysis of major and trace elements,organic carbon and nitrogen isotopes,and inorganic carbon and oxygen isotopes,and the Al/K and Al/Mg were shown to be stable,indicating that the clastic components of shale in Well L69 are homogeneous.Under the comparison of organic nitrogen isotopes between bulk sample and kerogen,the redox environment of water and sediments was restored; the Ba content,organic carbon isotopes of kerogen,and their coupling relationships with organic nitrogen isotopes were employed to demonstrate the primary productivity variation.On this basis,the generating and preserving mechanisms of hydrocarbon in the study area were investigated.The terrigenous clastic input is supposed to bring nutrients to the study area,promoting the growth of algae,and producing a large amount of organic matter as a result.The sedimentation of organic matter consumed oxygen and resulted in a reducing environment in water,which has in turn facilitated the accumulation and preservation of organic matter.

Key words: organic carbon isotope, organic nitrogen isotope, paleoproductivity, redox environment, lacustrine carbonate rock, the 3rd member of Shahejie Formation, Bonan Sag, Bohai Bay Basin