Pyrolysis Kinetics of Kerogen and Bitumen in Shahejie Shale: Implications for In Situ Heating Strategies

Unconventional shale resources remain crucial to energy security. In situ conversion technology (ICP) offers a promising pathway for exploiting low–maturity shale, yet the distinct roles of kerogen and bitumen during thermal conversion are not fully understood. This study investigates the decomposition behavior of kerogen and extracted bitumen from the Shahejie Formation through gold–tube pyrolysis experiments at 50 MPa and heating rates of 2 °C/h and 20 °C/h. The results show that the yield curves of C1, C2–C5, and C6–C14 generated from kerogen and bitumen exhibited similar trends. In contrast to the C15+ fraction from kerogen, which initially increased and then decreased, the yield of C15+ from bitumen began to decline from the onset of cracking. Additionally, the CO2 generated from the kerogen continued to increase until the end of pyrolysis, whereas the CO2 from the bitumen reached its maximum at an EasyRo of approximately 1.8%. The kinetic results show that bitumen has a higher activation energy for gas generation than kerogen, while kerogen has a higher activation energy for oil generation than bitumen. A heating program of 1 °C/day rate, 324 d duration, and a final temperature of 360 °C was applied to predict oil and gas generation during ICP. Below 326 °C, the proportion of C1 and C2–C5 contributed by kerogen increased and exceeded 90%. Although kerogen’s contribution ratio of C6–C14 exhibited fluctuating variation characteristics, it remained above 50% across most of the intervals. The gas–to–oil ratio increased rapidly above 299 °C and reached 375 m3/m3 by the end of heating.