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 (E d 3 ), the middle and lower sub-members of the first and third member of Shahejie Formation (E s 1M , E s 1L , E s 3 ). Both the salinity of sedimentary water bodies and the contribution of bacteria in the four source rock sequences gradual increase in the order of E s 3 , E s 1M , E s 1L and E d 3 , as indicated by the Ga/C 30 H, ETR , sterane/C 30 hopane, C 24 TeT/C 26 TT, C 23 TT/C 30 H, and the carbon isotope data of group components. Additionally, certain quantities of 4-methylsterane have been detected in source rocks in the E s 3 , E s 1M , and E d 3 , 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 E d 3 originates from source rocks in the E s 1 L and E s 3 . Class B oil existing in the lower part of the E d 3 is derived from source rocks in the Ed 3 . Class C oil found in the E s 1U is sourced from source rocks in the E s 1M . Class D oil present in the E s 1L is contributed indigenously by the source rocks in the E s 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. • Deep-sag E s 3 and E s 1 source rocks identified, expanding traditional source rock models. • Evidence supports E s 3 deep-sag lithologic reservoirs as self-sourced and self-sealed. • Deep-sag crude oils in E s 3 originate from local source rocks, confirming in-situ generation. • Multi-source accumulation patterns in deep sag enhance understanding of charge systems.