Sedimentary evolution and favorable reservoir prediction in the middle of the third member of Shahejie Formation in BZ Oilfield, Bohai Sea

The middle of the third member of Shahejie Formation of the Mid-Eocene (E₂s₃) in Bohai BZ Oilfield is an ultra-low permeability reservoir where fan delta deposits are developed. Due to the rapid lateral change of the reservoir and the unclear understanding of the "sweet spot" layer, the level of development of the oilfield is restricted. This paper focuses on the sedimentary evolution and favorable reservoir prediction of the middle of E₂s₃ to guide the development and adjustment of the ultra-low permeability reservoir. Combined with the regional sedimentary characteristics, drilling and seismic data, the residual thickness method and the impression method were used to reconstruct the palaeogeomorphology. Four types of palaeogeomorphic units are developed in the middle of E₂s₃, including ancient depression, ancient slope, ancient valley, and ancient uplift. Through core observation and well logging facies analysis, we found that distributary channel, estuarine bar, turbidity current, interdistributary bay, and front fan delta mud sedimentary microfacies were developed in the middle of E₂s₃. According to the research, the sedimentary microfacies and thickness of small layers show good correlation with the ancient geomorphic morphology. The reservoirs in the ancient depression developed mainly distributary channels and estuarine bars with high-quality microfacies and the largest sedimentary thickness, followed by the ancient slope and ancient valley, and the ancient uplift was the worst in this regard. Based on the analysis of palaeogeomorphology, sedimentary microfacies, and evolution characteristics, favorable reservoirs are mainly controlled by the palaeogeomorphology and sedimentary microfacies, and the distribution of "sweet spot" area in the middle of E₂s₃ was determined comprehensively based on reservoir thickness and physical properties. This study provided a geological basis for oil exploration in ultra-low permeability reservoirs in different development strategies and well areas.