Influencing factors and microscopic flow mechanism of shale oil during pressurized spontaneous imbibition in the Lucaogou Formation, Jimusar Sag: An experimental study

Crude oil recovery and residual oil distribution at the microscopic pore scale are governed by pore structure, wettability, and imbibition dynamics. However, the evolution of pore structure and wettability during spontaneous imbibition and their impact on oil displacement efficiency remain poorly understood, particularly in saline lacustrine shale reservoirs with complex pore networks and variable wettability. The lack of a systematic framework linking imbibition to oil displacement efficiency hinders a comprehensive understanding of its controlling mechanisms, necessitating further investigation. Therefore, this study selected four typical lithologies from the Lucaogou Formation in the Jimusar Sag. Through a combination of methods, including high-pressure mercury intrusion (MIP), low-temperature N₂ adsorption (LTN₂A), CO₂ adsorption (CO₂GA), nuclear magnetic resonance (NMR), computer tomography scanning (CT), focused ion beam scanning electron microscopes (FIB–SEM) and QEMSCAN identification, along with multifractal theory and wettability index, the flow mechanisms of shale oil and the distribution of residual oil during pressurized spontaneous imbibition were investigated. The results: (1) All lithology samples exhibited a nonlinear recovery trend of “increase–decrease–increase” during pressurized spontaneous imbibition. (2) The pore connectivity of micritic dolomite is poor, with high heterogeneity. Dolomitic siltstone and arenaceous dolomite exhibit better mesopore connectivity and higher oil migration efficiency. Feldspathic lithic fine sandstone, with high pore connectivity, shows strong oil and gas aggregation and fluid transport capabilities in macropores. (3) Two typical residual oil distribution patterns and their controlling mechanisms were identified in the Lucaogou Formation shale reservoir: a macropore-dominated residual oil aggregation pattern associated with water-wet feldspathic lithic fine sandstones, and a micropore-mesopore dominated residual oil retention pattern commonly occurring in oil-wet micritic dolomite. This study systematically reveals the flow mechanisms of shale oil and the distribution of residual oil during pressurized spontaneous imbibition, based on the synergistic effects of mineral composition, pore structure, wettability, and capillary forces, providing a theoretical foundation and practical reference for improving shale oil recovery.