Kinetic characteristics of secondary hydrocarbon generation from oil shale and coal at different maturation stages: Insights from open-system pyrolysis

Understanding secondary hydrocarbon generation from organic matter is critical for assessing source rock potential in thermally complex basins. This study investigates the hydrocarbon generation kinetics of immature to low-maturity kerogen types (Type I, II, III) using open-system pyrolysis experiments on oil shale and coal samples from the Minhe and Meihe Basins, China. Results show that Type I kerogen exhibits a narrow hydrocarbon generation window and rapid depletion of generative potential, whereas Types II and III display broader, more gradual trends. Activation energy increases with thermal maturity, and interruptions during active generation raise the energy threshold for reactivation. A parallel first-order reaction model accurately simulates generation rates (R² > 0.9998). By integrating kinetic parameters with burial and thermal histories, we reconstructed hydrocarbon evolution pathways in the Minhe Basin. Simulations indicate that in-situ conversion of shallow oil shale at a heating rate of 0.35 °C/day over 881 days yields maximum hydrocarbon output with minimal energy consumption. These insights provide a theoretical foundation for both conventional reservoir prediction and unconventional resource development.