Effects of Stress Heterogeneity on Pore Structure and Multifractal Characteristics of Deep Shale Reservoirs in Southeastern Sichuan Basin: Insights from CO2/N2 Adsorption, MIP and Mapping Analysis

Deep shale reservoirs in the tectonically complex margin of the southern Sichuan Basin have experienced multistage deformation, resulting in strong spatial heterogeneity of the present-day geostress field. However, the influence of stress heterogeneity on multiscale pore structure evolution and reservoir quality remains poorly constrained. Here, we integrate in-situ stress measurements, overburden porosity and permeability experiments, CO2/N2 adsorption, high-pressure mercury intrusion, SEM-MAPS (Scanning Electron Microscopy-MAPS) pore imaging, stress well profile interpretation, and multifractal analysis to quantify the controls of present-day geostress heterogeneity on pore structure evolution in deep Longmaxi Formation shale. The results show that the present-day stress regime is characterized by a strike-slip pattern (σH > σv > σh), with significant variations among different structural deformation zones. Increasing structural deformation results in enhanced differential stress, increasing by 30–80% from gentle structures to tight folds and fault-affected zones, accompanied by a 60–70° rotation of the maximum principal stress orientation. Differential stress, effective stress, differential stress coefficient, and stress structure index exhibit strong negative correlations with porosity, whereas permeability decreases nonlinearly with increasing stress, indicating progressive pore-throat compression and connectivity degradation under heterogeneous stress conditions. Multifractal analysis reveals that pore-size domains exhibit different sensitivities to stress heterogeneity. The macropore fractal dimension (DN3) shows the strongest response, followed by mesopores (DN2), whereas micropores (DN1) exhibit relatively limited variations. Fault-affected zones and strongly deformed regions display higher DN3 values (>2.8), reflecting enhanced complexity of macropore and fracture networks. In contrast, gentle structural zones characterized by curvature values <0.10 km−1 and distances >500 m from faults exhibit relatively low and stable fractal dimensions (<2.73), indicating more homogeneous pore structures. Increasing stress heterogeneity induces the transformation of organic matter pores from regular subcircular shapes to flattened and slit-like morphologies, accompanied by pore-size migration toward smaller scales (<15 nm) and enhanced pore heterogeneity (Df > 1.35). These findings reveal that present-day geostress heterogeneity governs shale pore fractal evolution and promotes the transition from micropore-dominated to heterogeneous macropore–fracture systems. This study provides quantitative insights into stress-controlled pore evolution and reservoir quality evaluation in deep shale reservoirs under complex tectonic settings.