Storage Capacity and Controlling Factors of Lacustrine Mixed Shale Oil Reservoirs: A Case Study of Shahejie Formation in the Northeastern Bohai Bay Basin

ABSTRACT Lacustrine mixed shale oil reservoirs exhibit strong heterogeneity, making the prediction of favourable reservoirs highly challenging. In this study, the fourth member of the Shahejie Formation (Es 4 ), in the Northeastern Bohai Bay Basin, was taken as an object. A systematic investigation was conducted to quantitatively characterise the differences in reservoir capacity among various lithofacies and their controlling factors, including petrography, organic geochemistry, multi‐scale pore structure, and depositional environment reconstruction. The results showed that four typical lithofacies developed within the Es 4 Member, exhibiting significant variations in reservoir capacity, ranked in the following order: Calcareous Shale > Felsic Shale > Dolomitic Shale > Dolomitic Shale with Calcite Veinlets. Reservoir capacity was primarily controlled by pore types and their abundance. Calcite dissolution pores and felsic‐clay composite pores provide the primary reservoir space, whereas dolomite cementation and calcite veins significantly reduce connectivity. The development of pore types was further governed by mineral composition interplay, depositional environment, and diagenetic evolution. Vertical mineral differentiation was dominantly influenced by the evolution of depositional environments: salinization‐freshening lake cycles and paleoclimate fluctuations dictated the spatiotemporal distribution of calcite, dolomite, and felsic minerals. Early evaporative conditions favoured dolomite precipitation while inhibiting pore development. Mid‐stage saline‐reducing environments facilitated calcite precipitation, followed by organic acid dissolution forming secondary dissolution pores. Late‐stage freshwater conditions promoted the influx of terrigenous felsic minerals, resulting in composite pores with enhanced compaction resistance. This three‐tiered control ultimately led to the superior reservoir properties of calcareous shale developed within the 3566–3598 m interval. This study proposes an integrated evaluation model of ‘depositional environment‐mineral diagenesis‐pore type‐reservoir capacity’, providing a theoretical basis and technical support for predicting sweet spots in lacustrine mixed shale oil reservoirs.