Fractal Characteristics of Organic Nanopores in Naturally Deformed Shales from the Dabashan Thrust-Fold Belt, Northeast Sichuan Basin

Tectonic activity can induce deformation or failure of shale formations, thereby altering their macroscopic and microscopic structures to varying degrees. Naturally deformed shale samples from the Lower Cambrian Qiongzhusi Formation in the Dabashan Thrust-fold Belt, northeast Sichuan Basin, were investigated using scanning electron microscopy (SEM), low-temperature gas adsorption, and fractal analysis to characterize the nanoporous structure of organic matter. The results show the following: (1) SEM observations indicate that the organic nanopores are predominantly subcircular, with shape factors ranging from 0.736 to 0.859 and an average areal porosity of 7.678%. The pore diameters are mainly distributed within 10–50 nm, with micropores and mesopores being dominant, showing typical nanoscale pore characteristics. (2) Gas adsorption results show that the organic mesopores are mainly distributed within 20–30 nm. Their average pore volume and specific surface area are 0.169 cm3/g and 33.318 m2/g, respectively, indicating a relatively small number of mesopores and a low degree of mesopore development. The organic micropores exhibit a trimodal pore-size distribution, with peaks at 0.3–0.4 nm, 0.4–0.6 nm, and 0.75–0.85 nm. Their average pore volume and specific surface area are 0.012 cm3/g and 38.975 m2/g, respectively. (3) The fractal dimensions of the organic nanopores calculated using the box-counting method, the FHH model, and the V-S model are 1.280, 2.524, and 2.448, respectively, indicating relatively low pore-structure complexity and heterogeneity in the organic matter of the shale.