We have used focused ion beam-helium ion microscopy and low-pressure adsorption to investigate the pore characteristics of Lower Silurian Longmaxi shale. These results, combined with the molecular potential energy theory, reveal the relationship between pore size and the occurrence state of shale gas. Our results reveal that (1) the pore volume and the specific surface area of Lower Silurian Longmaxi shale are mainly contributed by the pores with diameters of less than 10 nm. Nanoscale organic-matter pores are predominant, and the pore surface is not generally smooth and has fractal characteristics. (2) When the distance between the methane molecule and the pore wall in organic-matter pores is limited to 2 nm, there is a strong interaction force between them, and the methane molecule is affected by the interaction force and is in the adsorbed state accordingly. When the distance between them is greater than 2 nm, the interaction force can be ignored, and the methane molecule is not affected by the interaction force and is in the free state accordingly. (3) In the pores having a radius greater than 2 nm, the adsorption capacity is positively correlated with the specific surface area; whereas in the pores having a radius smaller than 2 nm, the average gas concentration is related to the pore radius. (4) First, the adsorption-zone volume increases, and then it decreases with increasing pore diameter. When the pore diameter ranges from 1 to 6 nm, the adsorption-zone volume is significantly larger than the free-zone volume. Within this range, the adsorption volume nearly equals the pore volume. When the pore diameter ranges from 6 to 60 nm, the adsorption volume gradually decreases, whereas the free volume increases, which equals the adsorption volume when the diameter reaches 15 nm.