Multiscale Pore Structure and Fluid Occurrence Mechanisms in the Gulong Lacustrine Shale: Effect of Supercritical CO₂...

The Gulong shale in the first member of the Qingshankou Formation, Songliao Basin, is an important lacustrine shale oil reservoir in China. However, the coupling between multiscale pore structure and fluid occurrence remains insufficiently understood. In this study, four shale samples were investigated using mercury intrusion porosimetry (MIP) and N₂ physisorption (NP) to characterize pore structure from the nano- to microscale. Nuclear magnetic resonance (NMR) experiments, together with solvent extraction under different temperature-pressure conditions and spontaneous imbibition, were further employed to examine the occurrence and redistribution of water and oil in shale pores, with n-dodecane used as the oil phase. In addition, supercritical CO₂ (scCO₂) extraction was conducted to evaluate its influence on pore structure and residual hydrocarbon occurrence. The results show that the Gulong shale is dominated by clay and siliceous minerals, with mesopores contributing most of the pore volume. MIP and NP data indicate that the pore system is characterized by narrow pore throats and relatively poor connectivity. Contrast variation-small angle neutron scattering (CV-SANS) results show that water-accessible pores account for 42.52-48.41% of the total porosity, and pores smaller than 10 nm exhibit stronger water accessibility and affinity. NMR results further indicate a scale-dependent fluid distribution, in which water preferentially occupies smaller pores, whereas oil mainly occurs in relatively larger pores. Solvent extraction under high temperature and pressure broadens the NMR secondary peak and is associated with improved pore accessibility, while scCO₂ extraction modifies the pore-size distribution and suggests enhanced residual oil mobilization. These results provide experimental constraints on pore-fluid coupling in lacustrine shale and offer implications for fracturing-fluid optimization and CO₂-assisted shale oil recovery.