Thermomechanically Constrained Fractal Evolution of Pore-Fracture Networks in Low-Maturity Shale under Conventional Triaxial In Situ Conditions.

During the in situ thermal upgrading of low-maturity shale, the reorganization of the pore-fracture system directly controls reservoir connectivity and hydrocarbon migration efficiency. However, under true triaxial stress constraints, the thermo-mechanical coupling mechanisms and the overall structural complexity evolution remain insufficiently understood. In this study, low-maturity organic-rich shale from the Jitunjun Formation in the Fushun Basin was subjected to in situ heating experiments under true triaxial conditions. Multiscale characterization methods, including scanning electron microscopy (SEM), low-temperature gas adsorption, and high-pressure mercury intrusion, were employed to investigate the pore-structure evolution at different temperature stages. Fractal parameters across multiple pore-size ranges were derived by using the FHH model and mercury intrusion fractal models. A pore-volume-weighted comprehensive fractal dimension (D comp) was further constructed to achieve unified quantification of pore-fracture network complexity over the full pore-size spectrum. The results show that micropore variation is limited, mesopores develop significantly at 350-450 °C, and macropores continuously increase with temperature, playing a dominant role in connectivity enhancement. The pore system exhibits stable bifractal behavior. D comp is positively correlated with the specific surface area and total pore volume and negatively correlated with average pore size, indicating that structural complexity is primarily controlled by the development of mesopores to macropores and increasing diversity of connectivity pathways. Integrated analysis identifies 400-500 °C as the critical temperature interval during which the pore-fracture network transitions from local connectivity to a multiscale interconnected system. This study confirms the applicability of fractal theory under true triaxial in situ thermomechanical conditions and provides a quantitative framework for evaluating thermal upgrading effectiveness and optimizing reservoir structure.