Mechanisms of Shale Oil Occurrence at the Nanoscale: A Molecular Dynamics Study

Summary Shale oil occurrence behavior controls shale oil mobility and recovery. However, the shale oil occurrence mechanism is not clear at the molecular scale because of the intense oil-rock interaction in nanopores. This study used molecular dynamics (MD) simulations to explore the adsorption behaviors of individual compounds and their mixtures (methane, n-octane, toluene, pyridine, and asphaltenes) in shales. Both organic matter (kerogen) and inorganic minerals (montmorillonite and quartz) were selected as substrate. The mass density distribution of fluids and the interactions between fluids and minerals were quantitatively characterized. Results show that fluids demonstrate layer-by-layer adsorption on pore surfaces, and pore types significantly impact oil adsorption. In single-component simulation, the interaction strength between n-octane and minerals follows the order: kerogen > montmorillonite > quartz. In mixed fluid simulations, competitive adsorption occurs among oil components, revealing distinct mineral-dependent characteristics: (1) In kerogen pores, n-octane dominates adsorption due to chemical affinity and molecular penetration; (2) in montmorillonite, pyridine exhibits the strongest adsorption capacity caused by polar interactions; (3) in quartz, pyridine forms an additional adsorption layer via hydrogen bonding with surface hydroxyl groups, significantly enhancing its adsorption. The elevated temperatures weaken the interaction force between fluids and minerals, enhancing shale oil mobility. As the pore size increases, more oil transforms from adsorbed to free state. While multiple factors were examined, the study specifically emphasizes the dominant role of competitive interactions among oil components in determining adsorption behavior and occurrence state.