Determination of gas adsorption capacity under geological conditions is essential in evaluating shale gas resource potential. A quantitative determination of gas adsorption capacity was proposed through 1) investigating controlling geological factors (including both internal ones and external ones) of gas adsorption capacity in organic-rich marine shale with geochemical analysis, XRD diffraction, field-emission scanning electron microscopy, and methane sorption isotherms; 2) defining the relationship between gas adsorption capacity and single controlling factor; 3) establishing a comprehensive determination model with the consideration of all these controlling factors. The primary controlling factors of the sorption capacity for the studied O₃w-Lower S₁l shale are TOC, illite and quartz, temperature, pressure, R_o, and moisture (water saturation). Specifically, TOC, thermal maturity, illite, and pressure are positively correlated with sorption capacity, whereas, quartz and temperature contribute negatively to the sorption capacity. We present the quantitative model along with application examples from the Wufeng-Lower Longmaxi Shale in the southeast Sichuan Basin, west China, to demonstrate the approach in shale gas evaluation. The result shows that the comprehensive determination model provides a good and unbiased estimate of gas adsorption capacities with a high correlation coefficient (0.96) and bell-shaped residues centered at zero.