Synergistic role of supercritical CO2 in hydrocarbon generation during oil shale pyrolysis

ABSTRACT Supercritical fluids, particularly supercritical CO 2 (SC-CO 2 ), have been explored as heat-transfer and extraction media for enhancing oil shale pyrolysis, but their effects on hydrocarbon generation, pore structure, and migration remain underexplored. While most in situ upgrading studies have focused on electrical or steam-based heating, SC-CO 2 ’s role in both carbon capture, utilization, and enhanced hydrocarbon recovery has received less attention. This study investigates SC-CO 2 -assisted pyrolysis of low-maturity, mixed type II 2 oil shale from the Jiufotang Formation. Core-derived blocks were pyrolyzed under confining pressure at 300–500 °C and compared with conventional conductive pyrolysis. At 450 °C, SC-CO 2 increases oil yield by 6.43 mg/g and gas yield by 5.43 mL/g, indicating moderate enhancement in organic matter conversion. SC-CO 2 shifts expelled oil towards lighter fractions, raising saturated hydrocarbons to 81.2 wt% while maintaining low asphaltene content, resulting in higher-quality oil compared to conventional pyrolysis. Rock-Eval and vitrinite reflectance show a faster decline in S 1 +S 2 and hydrogen index under SC-CO 2 , suggesting accelerated thermal evolution. High-pressure mercury intrusion and confocal microscopy reveal increased porosity and better pore connectivity, with less residual oil, which is more finely dispersed and preferentially hosted in well-connected pores. This core-scale, pore-resolved study links SC-CO 2 -induced pore modifications and residual oil architecture to hydrocarbon yields and oil composition, demonstrating SC-CO 2 as a transport and extraction medium that enhances recovery, preserves pore connectivity, and improves oil quality, providing insights for designing SC-CO 2 -based in situ conversion strategies.