Depositional Environments and Organic Matter Enrichment Mechanisms of Shales in the Second Member of the Kongdian Formation, Cangdong Sag, Bohai Bay Basin

The second member of the Kongdian Formation (Ek2; also referred to as the Kong 2 Member) in the Cangdong Sag within the Bohai Bay Basin contains a series of high-quality lacustrine shales characterized by high organic matter abundance and significant hydrocarbon shows. However, the mechanisms governing organic matter enrichment in the deep parts of the sag remain poorly understood, and the impacts of depositional environments on organic matter enrichment are yet to be determined. This study investigated shales in the C1, C3, and C5 sublayers of the Kong 2 Member. Specifically, this study examined the mineralogy and petrology, organic geochemistry, and elemental geochemistry of the shales using whole-rock X-ray diffraction (XRD) analysis, total organic carbon (TOC) analysis, pyrolysis experiments, and analyses of macerals, major and trace elements, and stable carbon and oxygen isotopes. Additionally, numerical analyses were conducted. The results indicate that shales in the Kong 2 Member consist primarily of felsic, dolomitic–calcareous, and mixed shales. These shales exhibit high TOC content (average: 3.07%), and favorable organic matter types dominated by liptinite and interbedded with minor planktonic algae and amorphous sapropelinite. These suggest great potential for hydrocarbon exploitation. During the deposition of shales in the Kong 2 Member, substantial terrigenous clasts were deposited at moderate rates under relatively arid climates characterized by frequently alternating dry and humid conditions. In this period, the anoxic to reducing depositional water bodies showed elevated salinity, resulting in saline-to-brackish water environments and moderate paleoproductivity. The organic matter enrichment of shales in the Kong 2 Member was jointly governed by paleoclimate dynamics, terrigenous input, and redox conditions, as demonstrated by multivariate analyses including the correlation analysis of depositional environmental factors, the univariate analysis of TOC content, gray relational analysis (GRA), and robust regression analysis. Two organic matter enrichment patterns were identified: (1) the preservation-dominated pattern under arid climates, governed by intense reducing environments, and (2) the productivity-driven pattern under humid climates, enhanced by terrestrial input.