Shale Oil Occurrence and CO2 Displacement in Mineral and Organic-Rich Nanopores: Insights from Molecular Dynamics Simulation

Amid the sustained decline of conventional oil and gas resources, shale oil has become an important unconventional resource. Shale reservoirs are characterized by well-developed nanopores and complex mineral compositions, yet the occurrence state of shale oil in different mineral pores and the molecular mechanism of CO2 displacement remain insufficiently understood. This study focuses on the Es34 shale reservoir in the Nanpu Sag of Bohai Bay Basin. Quartz, illite and Type II kerogen were used to represent typical inorganic and organic nanopores. Non-equilibrium MD simulations were performed to investigate the adsorption, diffusion, and CO2 displacement behaviors of n-decane therein. The results show that mineral properties strongly regulate shale oil occurrence and flow. The less water-wet behavior of kerogen promotes n-decane enrichment but hinders desorption, whereas the weak adsorption on quartz is more favorable for competitive CO2 adsorption and oil migration. Illite exhibits behavior between these two end members. Pore size, temperature, and displacement pressure difference jointly control the transition of the oil phase from an adsorbed state to a movable state under nanoconfinement. This work elucidates the differentiated control mechanisms of mineral surfaces on shale oil occurrence and CO2 displacement efficiency, provides molecular-level insight into shale oil occurrence and migration in nanopores, supports the optimization of CO2-enhanced oil recovery schemes, and contributes to the low-carbon and efficient development of shale oil.