Comparative Pyrolysis and Oxidation of Medium–Low-Maturity Shale: Lowered Reaction Barriers, Enhanced Mass Loss, and Exothermic Enthalpy for Autothermic Upgrading

Medium-to-low-maturity shale underwent limited thermal evolution and retained considerable unconverted organic matter, thereby exhibiting substantial potential for hydrocarbon generation and oxidative exothermic release. In this study, the organic geochemical and mineralogical characteristics of the shale were characterized using Rock-Eval pyrolysis, vitrinite reflectance, and X-ray diffraction (XRD), and the mass loss and evolved gas compositions during pyrolysis and oxidation of the medium-to-low-maturity shale were investigated using thermogravimetry–Fourier transform infrared spectroscopy (TG-FTIR). Furthermore, differential scanning calorimetry (DSC) was adopted to examine the exothermic features of the shale during oxidation. The results indicated that shale pyrolysis exhibited three stages: volatilization of light components at 40–358 °C, cracking of organic matter at 358–590 °C, and mineral decomposition-related reactions at 590–800 °C. Shale oxidation also exhibited three stages: low-temperature oxidation (LTO) at 40–305 °C, thermo-oxidative degradation of organic matter and combustion of residual carbon at 305–622 °C, and mineral decomposition accompanied by continued oxidation of thermally stable carbonaceous residues at 622–800 °C. Compared with pyrolysis, shale oxidation exhibited a lower onset temperature for the second stage and a higher total mass loss, indicating that oxygen promoted the thermal conversion of organic matter in the shale. Moreover, the shale oxidation exhibited two exothermic intervals, with heat enthalpies of 1332 and 2154 J/g, respectively. The DSC results further indicated that the heat released during oxidation provided energy for subsequent reactions, thereby promoting organic matter cracking and the oxidation of carbonaceous residues. Overall, these findings provided a fundamental experimental basis for in situ conversion during air injection in medium-to-low-maturity shale oil reservoirs.