The marine shale in South China has great gas exploration potential, and exploration in the Sichuan Basin has been successful, but the degree of exploration remains low in the Guizhou Province. We used organic geochemical analyses (total organic carbon content and kerogen type), scanning electron microscopy (SEM), field emission SEM, nuclear magnetic resonance (NMR), X-ray diffraction analysis, and low-temperature and adsorption experimental methods to study the micropore types and pore structures and their effects on the methane adsorption capacity of organic-rich shales found in the Fenggang block in northern Guizhou Province. The results indicate that the microscopic surface porosity of the lower Cambrian Niutitang Formation ranges from 2.88% to 5.34%, with an average value of 3.86%. Based on nitrogen adsorption methods, the range of the average pore size distribution is 4.6–9.491 nm, with an average value of 6.68 nm. All of the samples exhibit significant unimodal distributions. The main pore size is less than 10 nm, and these pores account for most of the mesopore volume, which is generally consistent with the NMR results. The methane adsorption capacity of the shale samples gradually increases in the range of 0–8 MPa at 30°C and reaches a maximum at approximately 10 MPa. Positive correlations were found between the gas content and specific surface area, total pore volume, and micropore volume. These strong correlations indicate that the Niutitang Shale has a high specific surface area, a high pore volume, and narrow-diameter pores, demonstrating that it has a high gas adsorption capacity. The results of this study provide valuable information regarding the adsorption characteristics of marine shales and the factors that affect those characteristics.