Shale gas is a very promising natural gas with substantial development potential. In this study, Longmaxi Formation shale samples from the Silurian system in Yibin, Sichuan province of China, were selected to characterize the geological parameters, such as total organic carbon, clay mineral content, and vitrinite reflectance. The pore structure of shale was analyzed by field-emission scanning electron microscopy and a low-temperature N-2 adsorption-desorption method. Isothermal adsorption experiments for CH4 and CO2 single components and mixtures were performed using a volumetric method. The second virial coefficient was introduced to calculate for the compressibility factor of the gas mixture. Then, the influencing factors and selection of CH4 and CO2 adsorption capacity of shale were investigated. According to the experimental results, the selected shale samples are black mud shale with high maturity, and they possessed diversified surface morphologies, complicated structures, and various pore types. Calculated pore parameter results showed that microspores comprised the majority of developed pores in shale samples and play a major role in the adsorption process. Isothermal adsorption experimental results for single CH4 and CO2 show that adsorptions of CH4 and CO2 follow similar rules. The amount of CO2 adsorption was higher than that of CH4 adsorption, and a high pressure was required to reach CO2 adsorption saturation. The Langmuir Freundlich equation can be used to fit the isothermal adsorption experimental results. Isothermal adsorption experimental results for the CH4 and CO2 gas mixture followed trends similar to those of a single-gas component. The adsorption isotherms were all I-type adsorption isotherms, indicating that micropores play a major role. The temperature, pressure, pore structure, organic carbon content, and clay mineral composition of shale are important factors that influence the gas adsorption capacity of shale. During the adsorption of the same gas, the adsorption capacity of shale samples is more sensitive to unit pressure changes under lower temperature and pressure. In competitive adsorption, shale prefers to adsorb CO2. Therefore, CO2 is easier to be adsorbed by shale and causes CH4 to be released from the adsorption site. This occurrence holds guiding significance to CO2 driving CH4.