The scanning electron microscope (SEM) image analysis of clay rocks collected from the field demonstrates that pores of clay minerals are distributed among grains and at natural fractures of intragranular plates, where the former has an irregular shape and the latter shows a wedge-shaped or plane gap. The methane adsorption capacity of various clay minerals determined by experiments is consistent with the degree of micropore development measured by SEM, indicating that dominant nano-pores in shales, to a certain extent, determine the surface to volume ratio of shales as well as the storage capacity of shale gas.