Enrichment model of high-abundance organic matter in shales in the 2^nd member of the Paleogene Kongdian Formation， Cangdong Sag， Bohai Bay Basin

Shales in the 2^nd member of the Paleogene Kongdian Formation (also referred to as the Kong 2 Member) in the Cangdong Sag contain high-abundance organic matter, showing great potential for shale oil exploration. However, the mechanisms underlying the organic matter enrichment are yet to be clarified due to the lack of fine geochemical research. Given this, we investigate the factors influencing of organic matter enrichment and the enrichment model in the Kong 2 Member shale using whole-rock X-ray diffraction, maceral identification on polished surfaces of whole-rock shale samples, rock pyrolysis, measurement of the total organic carbon (TOC) content, chromatography-mass spectrometry of saturated hydrocarbons, carbon isotopic analysis of monomer hydrocarbons, and the analyses and tests of major and trace elements. The results indicate that the organic matter enrichment inthe Kong 2 Member shale is influenced by multiple factors including terrigenous clastic input, paleoproductivity, paleoclimate, paleo-water depth,and paleosalinity. These factors govern the growth and development of algae and bacteria in the lacustrine basin of the Cangdong Sag, contributing to the formation of organic matter enrichment horizons with high TOC content during the T-R cycle transition period of the fifth-order sequence. Specifically, the terrigenous clastic input introducing abundant nutrients enhanced the biological productivity of the lacustrine basin. The paleoclimate, paleo-water depth, and paleosalinity largely determined the terrigenous/aquatic ratio (TAR) of the lacustrine basin. Furthermore, bacterial activity transformed organic matter, increasing the H/C atomic ratio and the saprofication degree of the shale. Shales in sublayer ⑧, primarily taking shape during the T-R cycle transition, of the C₃ layer under development are formed under paleoenvironmental conditions of a warm and humid climate, low salinity, and deep water. This sublayer, with an average TOC content of 2.7 %, S₁ of 3.7 mg/g, and producible oil index (POI) of 215 mg/g, boasts high abundance and favorable organic matter types, proving to be favorable landing zone for horizontal wells.