Abstract
The differentiation between adsorbed and free oil and its control on shale oil mobility remain critical uncertainties in evaluating lacustrine shale oil sweet spots, particularly under nanoscale confinement where fluid occurrence deviates from conventional reservoir behavior. This study investigates the pore-scale mechanisms governing shale oil occurrence and mobility in the Shahejie Formation shale oil reservoirs of the Dongying Sag by integrating saturation, centrifugal displacement, and low-field nuclear magnetic resonance experiments. A quantitative framework was developed to distinguish adsorbed, free, and movable oil fractions and to establish the coupling relationships among oil occurrence state, pore structure, and mobility. The results demonstrate that organic matter abundance exerts a dual control on shale oil enrichment and producibility by promoting both pore development and oil–rock interfacial adsorption. Organic-rich shales exhibit higher oil-bearing capacity, stronger adsorption-layer development, and enhanced nanoscale confinement effects, resulting in a greater proportion of adsorbed oil while maintaining substantial free-oil reserves due to their multiscale pore systems. Free oil occurrence is strongly governed by pore size, with a critical pore-size threshold controlling the transition from adsorbed to free oil, whereas medium-to-large pores and microfractures represent the dominant domains for free-oil accumulation and migration. Further analysis reveals that shale oil mobility is not determined by resource abundance alone but by the dynamic balance among organic matter enrichment, effective flow space, and oil–rock interaction intensity. Based on these findings, an integrated evaluation framework coupling adsorption–free oil differentiation, pore-scale occurrence, and fluid mobility was proposed, providing new insights into the mechanisms of shale oil enrichment and recoverability and offering a theoretical basis for more accurate sweet-spot identification and efficient development of lacustrine shale oil reservoirs.
