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.