Thermal evolution and hydrocarbon generation of organic matter in shales via sequential high-pressure hydrous pyrolysis: Implications for in-situ conversion of unconventional resource

Understanding kerogen transformation under geological conditions is critical for optimizing the in-situ conversion (ISC) process of organic-rich unconventional resources. Sequential high-pressure hydrous pyrolysis was employed to investigate the geological thermal evolution and hydrocarbon generation mechanisms of organic matter in immature Huadian (Type II₁ kerogen) and Fushun (Type I kerogen) shales. Experiments progressed through four thermal stages, that is Stage 1 (350 °C, 6 h), Stage 2 (350 °C, 24 h), Stage 3 (380 °C, 24 h), and Stage 4 (420 °C, 24 h), with comprehensive analysis of hydrocarbon products by gas-chromatography mass-spectrometry and solid residues by vitrinite reflectance (Ro) and Rock-Eval pyrolysis. The results revealed that the hydrocarbon-generation potential of these two shales declined sharply with a Ro of 0.78–1.23 %, correlating with peak oil generation. Type I kerogen (Fushun) exhibited higher reactivity, generating twice the cumulative oil yield (normalized by TOC) compared to Type II₁ (Huadian) and transitioning earlier to oil dominance. Biomarker evolution (OEP decline, sterane/hopane isomerization) in expelled oil and declining gas dryness index (C₁/ΣC₁–C₅) correlated strongly with the maturity of organic matter, enabling non-destructive ISC monitoring. Compared to typical temperatures used in ex-situ retorting (520 °C), the kerogen conversion was completed at lower temperatures of 350–420 °C in this study, validating prolonged heating as a viable low-energy ISC strategy. However, high-pressure conditions in geological formations may impede hydrocarbon expulsion efficiency, leading to the retention of viscous bitumen and thus necessitating engineered solutions for effective oil recovery. This research enriches the understanding of high-pressure pyrolysis mechanisms of immature/low-maturity unconventional resources and establishes a geochemical framework for optimizing ISC in recovering the oil from these source rocks, ultimately contributing to advancing sustainable exploitation of unconventional resources.