Multi-source hydrocarbon accumulation in the faulted and deeply depressed Tangdong area, Qikou Sag, Bohai Bay Basin

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.