Pore structure is the key to understanding the shale gas accumulation mechanism. The effects of organic matter and maturity on pore size distribution and gas storage capacity in high-mature to post-mature shales are analyzed using gas adsorption (CO2, N-2, and CH4), mercury intrusion porosimetry (MIP), and helium ion microscopy (HIM) for the Silurian Longmaxi and Cambrian Niutitang marine shales in southern China. The results show that the pores of high-mature Longmaxi shales with 2.32-2.42% Ro (vitrinite reflectance equivalent) are primarily composed of mesopores and macropores, whereas the pores of post-mature Niutitang shales with 3.49-3.66% Ro are primarily composed of mesopores and micropores. Organic matter clearly has a positive contribution to the micropore and mesopore development. In addition, increasing maturity appears to lower total porosity for post-mature shales, which has a lower total pore volume than that of high-mature shales. The free gas storage capacity of high-mature shales is controlled by the mesopores and macropores, whereas the free gas storage capacity of post-mature shales is controlled by the mesopores and micropores. The adsorbed gas storage capacity for both high-mature and post-mature shales is primarily controlled by the micropores.