<p>The study of shale reservoirs is a crucial aspect of shale gas exploration and development. With China’s continued breakthroughs in deep and ultra-deep shale gas, the traditional view that reservoir quality deteriorates with increasing depth faces significant challenges. Currently, there remains a limited understanding of the pore development characteristics and controlling factors in shallow, deep, and ultra-deep shales, which affects the effectiveness of shale gas production. Taking the Longmaxi Formation shale in southern Sichuan Basin as a case study, this study conducted a series of experimental analyses, including TOC, XRD, low-temperature N₂ adsorption, CO₂ adsorption, SEM, and large-view FE-SEM splicing technology (Maps-SEM). The results reveal that: (1) the porosity of the Longmaxi Formation shale in southern Sichuan ranges from 3.37% to 7.49%. The pore types are primarily organic matter pores and interparticle pores of clay minerals, with the pore volume mainly contributed by mesopores (50.8% contribution) and micropores (32.97% contribution), while macropores contribute the least to pore volume at 16.22%. (2) The differences in porosity of shallow, deep, and ultra-deep marine shales are not significant, but as burial depth increases, the proportion of macropores in the shale reservoir decreases, while the proportions of mesopores and micropores increase. (3) The porosity of the Longmaxi Formation shale is influenced by reservoir pressure, TOC, and mineral composition. The pressure coefficient of shallow shale is at normal to weak overpressure, with pores primarily influenced by normal compaction, while the pressure coefficient of deep to ultra-deep shale reservoirs is generally greater than 1.8, with overpressure offsetting part of the stress from the overlying rock layers, which is a major reason for the relatively high porosity of deep to ultra-deep shales.</p>