A ReaxFF molecular dynamics analysis of the pyrolysis evolution and molecular migration in Huadian oil Shale kerogen

Understanding the relationship between reaction characteristics and molecular migration behavior during oil shale pyrolysis is essential for elucidating kerogen decomposition and hydrocarbon transformation mechanisms. In this study, a molecular structure model of Huadian oil shale kerogen was constructed and validated. ReaxFF molecular dynamics simulations were performed under non-isothermal (100 K/ps) and isothermal conditions (1800–3300 K, 300 K intervals) to investigate the temperature-dependent evolution characteristics of kerogen pyrolysis. The results indicate that kerogen pyrolysis can be divided into three stages: initial bond cleavage, major pyrolysis, and deep cracking, with 2500–3150 K identified as the main pyrolysis regime. Kinetic analysis shows that the C14–39 fraction has the lowest apparent activation energy, leading to its preferential formation and accumulation during early pyrolysis. As temperature increases, reaction activity and molecular migration are both enhanced during the main pyrolysis stage. MSD and diffusion analyses reveal that molecular mobility increases with temperature and depends on molecular size. A coupled relationship among product evolution, reaction reactivity, and molecular migration behavior is observed, indicating a continuous transition from primary bond cleavage to secondary transformation. These findings provide a molecular-scale kinetic description of kerogen pyrolysis and insights into hydrocarbon generation and oil shale utilization.