Tectonic Deformation Intensity Controls Fracture Development and Pore Fractal Characteristics in Deep Shales: Evidence from the Longmaxi Formation, Southern Sichuan Basin

Deep shale reservoirs in the southern Sichuan Basin have undergone strong tectonic reworking, producing pore structure heterogeneity that affects reservoir quality and hydrocarbon preservation. In this study, shales with different deformation intensities are analyzed using field emission scanning electron microscopy (FE-SEM), low-pressure nitrogen gas adsorption (LP-N2GA) and fractal analysis to investigate tectonic control on pore–fracture evolution and fractal characteristics. Weakly deformed shales are dominated by intraparticle pores and regular organic matter (OM) pores, whereas strongly deformed shales develop abundant interparticle pores and microfractures along brittle mineral boundaries with irregular, preferentially oriented OM pores. The large-pore fractal dimension (D2) consistently exceeds the small-pore fractal dimension (D1) in both wells, indicating greater adsorption-related structural complexity of larger pores. Strongly deformed shales exhibit a higher mean box-counting fractal dimension (Db), indicating greater geometric complexity of OM pores. Supervised machine-learning image analysis reveals pore-size differentiation associated with deformation intensity. Intense tectonic compression may cause some 80–350 nm pores to contract, whereas localized pressure release and shear-induced dilation may promote pore expansion and fracture propagation, producing pronounced multiscale heterogeneity. Overall, stable overpressure preserves the primary pore system in weakly deformed zones, whereas compressive deformation and localized pressure release drive the coupled evolution of pore structure and fractal characteristics, with potential implications for shale gas storage and preservation.