Storage Capacity and Influencing Factors Among Different Lithofacies of Mixed Shale: A Case Study of the Es 4 of Shahejie Formation in Northeastern Bohai Bay Basin

ABSTRACT Mixed shale depositional facies change rapidly and exhibit strong lithological heterogeneity, resulting in unclear controlling factors of reservoir development and hindering the identification of favourable reservoirs. Therefore, this study focuses on the fourth member of the Shahejie Formation (Es 4 ) in the northeast Bohai Bay Basin. By integrating major/trace element analysis, X‐ray diffraction, thin section examination, scanning electron microscopy, nano‐CT, high‐pressure mercury intrusion and nuclear magnetic resonance, and through stepwise analysis of ‘depositional environment—lithofacies types—diagenesis—pore structure—storage capacity’. It elucidates controlling effects on reservoir development. The results indicate a distinct vertical zonation in the depositional environment of the Es 4 Member. The lower interval represents a deep to semi‐deep lake stage with salinity fluctuations, where dolomite cementation and calcite/dolomite veins developed under high‐salinity, Mg 2+ ‐rich conditions. The middle interval corresponds to a deep‐lake setting with increased aridity and progressive salinization, characterized by calcite enrichment. The upper interval reflects a semi‐deep lake transitioning from semi‐humid to arid conditions, with increased terrigenous input and enrichment of felsic minerals. Under this environment and mineral assemblages, four lithofacies developed. Storage capacity from high to low is: Laminated Calcareous Mixed Shale > Layered Felsic Shale > Laminated Dolomitic Mixed Shale with Calcite Veinlets > Laminated Dolomitic Mixed Shale. This difference is influenced by depositional and diagenetic factors: depositional environment controls mineral composition and facies, while dissolution, cementation and vein filling further affect storage capacity. Under environmental zoning, different facies are vertically distributed, with laminated calcareous mixed shale concentrated at 3566–3581 m, exhibiting the best properties and representing favourable reservoir sections. This research clarifies key controlling factors for reservoir development and resolves the difficulty of locating favourable reservoirs in mixed shales.