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Accurately determining the gas sorption capacity of a specific shale reservoir is critical for further assessment of shale gas reserves. A series of high-pressure methane adsorption measurements were conducted at 60 °C with a pressure of up to 30 MPa for Wufeng-Longmaxi shales from the southern Sichuan Basin, which is considered as the most promising shale-gas target in China, to evaluate the fitting quality of different excess adsorption models and to determine the effect of organic matter content, maturity, mineralogy, and pore structure on the gas adsorption capacity. Both the Langmuir- and supercritical Dubinin–Radushkevich (SDR)-based adsorption models are closely fitted with the measured excess adsorption amount. However, the freely fitted SDR model is considered to be the most reasonable model, in which the adsorbed-phase density is always lower than the liquid methane density at the boiling point (0.424 g/cm3) and the average relative error (ARE) is relatively small. Adsorbed-phase density is a key parameter for calculating absolute adsorption isotherms. For a specific shale, a lower constant adsorbed-phase density applied in the adsorption model would result in higher absolute adsorption capacity. For the Langmuir-based model, the actual absolute adsorption capacity would be underestimated when adsorption experiments were only conducted at the low-pressure range (0–15 MPa). The methane adsorption capacities show a great positive correlation with the total organic carbon (TOC) content. The TOC-normalized adsorption capacities have a negative relationship with maturity at an overmature stage. The clay content shows a positive correlation with the TOC-normalized adsorption capacities, indicating that clays also make some contribution to methane sorption on these organic-rich shales. Furthermore, methane adsorption in overmature shales is mainly controlled by the structure of the pore <20 nm in size, revealing that the adsorbed methane is occupied not only in micropores but also in fine mesopores.