Compositional kinetics for hydrocarbon evolution in the pyrolysis of the Chang 7 organic matter: Implications for in-situ conversion of oil shale

Abstract

The Triassic Chang 7 shale in the Ordos Basin with considerable total organic carbon content (TOC) has been considered as an ideal target for in-situ conversion process (ICP) in China. However, the compositional kinetics for oil and gas generation from the Chang 7 organic matter during heating remain unclear, causing difficulty in the resource evaluation of shale oil and the temperature optimization in future ICP. In this study, the non-isothermal pyrolysis of two Chang 7 kerogens at 2 and 20 ºC/h were conducted to investigate the potential of individual components of products as well as their evolution kinetics. By using a comprehensive two-dimensional gas chromatography (GC×GC), the yields of individual components (including C1–5, C6–14 saturates, C6–14 aromatics, C15+ saturates, C15+ aromatics and NSOs) in pyrolysis products were measured. The maximum yields of oils and C1–5 from ZK-2 kerogen with hydrogen index (HI) of 471 mg/g TOC were 421.5 and 240.5 mg/g TOC, much higher than those from ZK-1 with HI of 332 mg/g TOC. Relatively, the temperature for giving maximum yield of C6–14 is significantly higher than those of C15+ and heteroatomic compounds (NSOs), which were decomposed to re-generate light hydrocarbons in the closed system. It showed that activation energies (Ea) for the generation and cracking of C6–14 and C2–5 were distributed in broader ranges compared with NSOs and C15+ components by kinetic fits. Meanwhile, the mass conversions of oil and gas from the Chang 7 kerogen in ICP at 0.5–10 ºC/d were obtained. The temperatures giving maximum HC yield in ICP were predicted to be 330–368 ºC. The oil and gas generation potentials of the Chang 7 shale in the Ordos Basin giving maximum HC yields were estimated as 542 × 108 t and 89.4 × 108 m3, respectively. Finally, it is elucidated that heated temperatures of about 20 ºC higher than those giving maximum oil yield result in GOR higher than 200 m3/m3. These findings are beneficial for the economic production of shale oil in ICP.