Control of sedimentary environment on the lithofacies of lacustrine fine-grained sedimentary rocks in the second member of the Kongdian Formation in the Cangdong sag, Bohai Bay Basin, China

The thick fine-grained sedimentary rocks in the second member of the Kongdian Formation (Ek(2)) in the Cangdong sag, Bohai Bay Basin, eastern China, are typical lacustrine source rocks and have recently been regarded as the main target for shale oil exploration. Compared with marine shales, the strong heterogeneity in the mineral composition, structure feature, and vertical association pattern of the Ek(2) lacustrine fine-grained sedimentary rocks in the Cangdong sag is significantly influenced by frequent variations in the sedimentary environment and depositional process. Based on the detailed core and thin-section description analyses as well as the two-dimensional X-ray fluorescence (2D-XRF) analysis, the Ek(2) fine-grained sedimentary rocks in the Cangdong sag can be classified into six types of lithofacies by employing the content of three main mineral compositions together with structure features, among which five types are mainly deposited, including laminated felsic fine-grained sedimentary rocks, laminated and massive carbonate fine-grained sedimentary rocks, and laminated and massive mixed fine-grained sedimentary rocks. The sedimentary environment during the sedimentary period of the Ek(2) in the Cangdong sag was reconstructed by several indexes (including salinity, water depth, redox condition, climate, and detrital input) using the element geochemical parameters. The variations of detrital input and lake physicochemical characteristics driven by climate were vertically divided into four phases, indicating that the lake gradually changed from the transgression to the highstand period. In addition, the water column stratification led to the overall dysoxic-anoxic condition. The depositional process can be interpreted by correlating the lithofacies and the sedimentary environment. The results indicate that the mineral composition is controlled by the detrital input and carbonate production driven by climate and that structural features are mainly affected by the water column stratification. The Ek(2) fine-grained sedimentary rocks in the Cangdong sag present the vertical distribution of lithofacies that matches with the evolution phases of the sedimentary environment. This study's results enhance the understanding of the formation mechanism of lacustrine fine-grained sedimentary rocks and provide basic geological knowledge for shale oil exploration in the study area.