Integrated Distribution Models of Conventional and Unconventional Oil Reservoirs in Graben Lacustrine Basins: The Zhanhua Depression, Bohai Bay Basin, China

ABSTRACT Integrated characterization of conventional and unconventional reservoirs in graben lacustrine basins is crucial for hydrocarbon exploration. However, progress remains limited due to the complexity of depositional processes and significant lithofacies heterogeneity. This study investigates coarse‐grained and fine‐grained sedimentary rocks of the Lower Third Member of the Shahejie Formation in the Zhanhua Depression, Bohai Bay Basin, with the objective of elucidating the depositional mechanisms and spatial distribution models of coarse‐grained and fine‐grained lithofacies assemblages, and subsequently evaluating the quality of shale oil reservoir. Based on core observation, petrographic analysis, thin section identification, x‐ray diffraction and organic geochemistry data, seven lithofacies types were identified and classified according to provenance characteristics, sedimentary structures, and organic matter abundance. The results indicate a pronounced concentric facies distribution from the steep slope zone in the north to the gentle slope in the south of the depression. Proximal to the sediment source, Massive sandstone (LF1), Terrigenous low‐organic massive siltstone (LF2), and Terrigenous low‐organic massive mudrock (LF3) dominate. In contrast, the deep sag area is characterized by Mixed‐source mid‐organic massive mudrock (LF4), Mixed‐source high‐organic layered mudrock (LF5), and Mixed‐source high‐organic laminated mudrock (LF6). Toward the transitional gentle slope zone, authigenic high‐organic laminated marl (LF7) becomes prevalent. An integrated depositional model for coarse‐grained and fine‐grained sedimentary rocks is proposed, highlighting the combined effects of hydrodynamic condition, sediment supply, and biochemical precipitation. The reservoir quality assessment demonstrates that LF4, LF5, LF6, and LF7 possess higher brittle mineral content, porosity, organic‐matter abundance, and oil saturation, making them the most favourable targets for shale oil exploration. The findings provide a predictive framework for sweet‐spot identification in other graben lacustrine basins.