Porosity and pore size distribution (PSD) are critical reservoir parameters. Pore surface area, pore volume, PSD, and porosity were measured using subcritical nitrogen (N-2) adsorption, and helium porosimetry. A suite of 17 samples were collected from 4 wells in Pennsylvania and West Virginia to analyze the evolution of porosity with increasing thermal maturity in Middle Devonian shales of the Appalachian Basin. The thermal maturity of the tested samples covers a wide range in the hydrocarbon generation sequence from wet gas/condensate zone (vitrinite reflectance (R-o) = 1.16%) to post-mature zone (R-o = 2.79%). Shale samples from the Marcellus Shale and Mahantango Formation used in this study have total organic carbon contents from 0.41 to 7.88 wt%. Results indicate that total organic carbon (TOC) has the strongest effect on porosity and pore structure. The presence of organic matter in shale strongly enhances the storage capacity by increasing the specific surface area and pore volume, which represents sorption storage capacity and free-gas storage capacity. Differences in porosity and pore structure have a complex relationship to thermal maturity, micro texture, mineralogy, clay content, and TOC.