The development of unconventional oil and gas has triggered an upsurge of research in fine-grained sedimentary rocks.However

more research on the formation mechanism and depositional environment of lacustrine fine-grained sedimentary rocks is still needed.Through detailed description of core

thin section identification

X-ray diffraction analysis

automated mineral identification and characterization system (AMICS) and geochemical analysis

this paper establishes a classification scheme for lithofacies based on rock types and sedimentary structures.Fine-grained sedimentary rocks in the second member of the Kongdian Formation in the Cangdong Sag of Bohai Bay Basin was divided into five lithofacies: the laminated felsic fine-grained sedimentary rocks

massive mixed fine-grained sedimentary rocks

laminated mixed fine-grained sedimentary rocks

laminated dolomite and massive dolomite.The depositional environment and formation mechanism of each lithofacies type were analyzed.Four types of lithofacies assemblages were identified based on the superimposition patterns of different lithofacies.These assemblages vary in climate conditions

water body characteristics and terrigenous clastic input; and correspond to four depositional modes characterized respectively by intensive terrigenous sediment input

seasonal climate influence

semiarid seasonal influence

and prolonged arid.Analyses of the classification and geochemical indexes of the lithofacies assemblages show that the depositional environment in the second member of the Kongdian Formation can be divided into three stages from bottom to top: the seasonal climate stage from Ek

2(4)

to Ek

2(3)

the humid climate stage for Ek

2(2)

and the arid-humid transitory stage for Ek

2(1)

.