The shale gas reservoir in the Zhaotong national shale gas demonstration area in the southern margin of the Sichuan Basin is mainly developed in the lower member of the Ordovician Wufeng Formation to the Silurian Longmaxi Formation. Shale pore characteristics are the main indicators for evaluating the quality of shale reservoirs and largely determine shale gas productivity. We have qualitatively and quantitatively analyzed pore types and structures through scanning electron microscopy, CO2/N2 adsorption and high-pressure mercury injection experiments, and nuclear magnetic resonance. The pore types of marine shale gas reservoirs in the study area mainly included organic-matter pores and clay mineral interlamellar pores. The pore diameter of the shale reservoir ranged from 0.5 to 35 nm and was concentrated in the range of 2–10 nm, and the main pores were mesopores. Total organic carbon (TOC) content was positively correlated with fractal dimension, which could be used to quantitatively evaluate the complexity and heterogeneity of reservoir pores. The contents of TOC, clay, and siliceous minerals were the main factors controlling the development of shale micropores. The contents of clay minerals and siliceous minerals determined the storage space of shale gas, whereas micropores and mesopores controlled the adsorption capacity of shale.