Pyrolysis of an organic-rich shale containing type II kerogen before and after oil generation and expulsion: Implications for the generation of late hydrocarbon and hydrogen gases

Thermal simulation of late gas generation of source rock is critical for evaluating the potential of deep gas and shale gas. Retained oil and its interactions with thermally mature kerogen can have a significant impact on late gas generation from source rocks; however, previous studies commonly used expelled oil or total oil, which have different compositions from retained oil. In addition, molecular hydrogen (H2 gas) is not only a pyrolysis product but may also have significant effects on hydrocarbon formation. The effects of oil generation and expulsion, as well as retained oil, on H2 generation from source rocks are currently poorly understood. Our study involved two-stage pyrolysis under high-pressure conditions: main oil generation and expulsion, followed by late gas generation. The yields of both hydrocarbon gas and H2 gas were determined and examined deeply in order to investigate the influence of key factors on late gas generation, such as oil generation and expulsion and the interactions between retained oil and mature kerogen. Furthermore, the influence of kerogen type and thermal maturity on late hydrocarbon gas potential was discussed with a literature review. Firstly, oil generation and expulsion could significantly reduce the yields of methane, C2–5 and H2 gases from the pyrolysis of shale. After oil expulsion, the mature shale produced both CH4 and H2, the majority of which came from the kerogen cracking. The interactions between mature kerogen and retained oil may have slightly promoted the generation of methane and inhibited the generation of H2, but they had little influence on the potential for total hydrocarbon gas. Secondly, the H2 generation process could be sub-divided into four stages, which correspond to early hydrocarbon gas generation (stage I), main C2–5 gas generation (stage II), main C2–5 gas cracking (stage III) and the latest methane generation (stage IV), respectively. More than 80% of the total H2 yields were produced in stages III and IV, suggesting that most of the H2 gas was generated through the demethylation, aromatization and condensation processes. Finally, this study concluded that the late hydrocarbon gas potential of different types of kerogen is mostly related to thermal maturity and can be represented by a simple power function.