Insights into Pore–Throat Fractal Characteristics and Shale-Oil Mobilization by HTHP Imbibition in Lacustrine Calcareous Shale

Upper Es4 lacustrine calcareous shale in the Dongying Depression is characterized by strong pore–throat heterogeneity that limits shale-oil producibility. This study quantifies multiscale pore–throat complexity using high-pressure mercury intrusion-based fractal analysis (segmented fractal dimensions D1–D3 and a weighted comprehensive fractal dimension, Dc) and evaluates its control on oil occurrence and mobilization using low-field 2D NMR (T1–T2) and confocal microscopy before and after high-temperature, high-pressure spontaneous imbibition. Reservoirs show clear scale segmentation, with micropore fractality governing quality differentiation. Imbibition promotes desorption and redistribution from adsorbed to free oil, but effective mobilization is primarily controlled by pore–fracture connectivity: samples with well-connected networks can mobilize both adsorbed and free oil efficiently, whereas poorly connected systems exhibit desorption without effective production, implying that thermal stimulation alone is insufficient without fracture-assisted flow pathways. Movable-oil saturation decreases systematically with increasing Dc, indicating that higher roughness and tortuosity intensify capillary retention and Jamin trapping. Dc provides an actionable criterion for sweet-spot ranking and for designing stimulation–imbibition coupling and water-based EOR strategies in lacustrine calcareous shale-oil reservoirs.