Based on organic geochemical analysis data, this study examined the characteristics of multi-source hydrocarbon accumulation in the Tangdong area of Qikou Depression, Bohai Bay Basin. The results indicate that the Tangdong area contains four source rock sequences: the third member of the Dongying Formation (Ed³), the middle and lower sub-members of the first and third member of Shahejie Formation (Es^1M, Es^1L, Es³). Both the salinity of sedimentary water bodies and the contribution of bacteria in the four source rock sequences gradual increase in the order of Es³, Es^1M, Es^1L and Ed³, as indicated by the Ga/C₃₀H, ETR, sterane/C₃₀ hopane, C₂₄TeT/C₂₆TT, C₂₃TT/C₃₀H, and the carbon isotope data of group components. Additionally, certain quantities of 4-methylsterane have been detected in source rocks in the Es³, Es^1M, and Ed³, suggesting the special contribution of dinoflagellates to these source rocks. Based on biomarker parameters and carbon isotope data, the crude oil can be categorized into classes A to D, which show complex multi-source accumulation characterized by different sources in the same well and even in the same layer. Class A crude oil occurring in the higher part of the Ed³ originates from source rocks in the Es^1L and Es³. Class B oil existing in the lower part of the Ed³ is derived from source rocks in the Ed³. Class C oil found in the Es^1U is sourced from source rocks in the Es^1M. Class D oil present in the Es^1L is contributed indigenously by the source rocks in the Es^1L, establishing this unit as a lithologic hydrocarbon reservoir with source rocks and reservoirs in the same layer. Under the guidance of the understanding of multi-source hydrocarbon accumulation, currently, three wells have been drilled, all yielding satisfactory outcomes.

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.