Physiochemical characteristics were evaluated for over 700 core and drill cutting samples from Upper Devonian Gas Shales. These samples were selected from seven Illinois Basin and fourteen Appalachian Basin wells. The organic carbon content, thermal maturation and organic matter type primarily determine the gas potential of these shales. The average organic carbon content of Devonian shales is consistently greater than the minimum content required for clastic source rocks to generate a significant volume of hydrocarbons. The potential gas yield of shales predominantly containing propelic (marine sourced) material is significantly greater than for those predominantly containing humic (non-marine) material. In general, Illinois Basin shales are organically rich and contain varying proportions of terrestrial versus marine sourced organic matter. However, these shales are too thermally immature to have generated significant hydrocarbons. An exception to this trend occurs in the southern regions of this basin (Hardin County, Illinois) where the shales are primarily composed of terrestrially derived, organic matter and are more thermally mature. The organic matter of the Appalachian Basin shales trend from a very mature, terrestrially derived, organically leaner type in the regions east of the basin axis to a less mature, mixed marine/non-marine, organically richer type in the western regions. The Upper Devonian interval of this latter region contains gases with a higher gas-producing potential and Btu values. The primary source of these wet gases is within shale intervals associated with an organically and algal-rich (restricted marine) facies. Determining shale hydrocarbon gas or organic carbon content by current methods of log analysis appears to be of limited effectiveness. However, alternative methods of remote sensing have the potential to improve the determination of these key reservoir parameters.