The Illinois Basin (Indiana, Illinois, and western Kentucky) holds substantial Pennsylvanian coal resources of high volatile bituminous rank, but much of this resource is considered to be too deep or too thin for economic mining. Sequestration of CO₂ within the unminable parts of these coalbeds is one of the geological options considered for future isolation of CO₂. The remaining coal resource in the basin is newly estimated for this study at 413 billion t (455 billion tons) of which 142 billion t (157 billion tons) (or 34.5%) meets the minable criteria of being less than 305 m (1000 ft) deep and greater than 1.1 m (3.5 ft) thick. Thus, 271 billion t (298 billion tons) are potentially available as a CO₂ sequestration reservoir. The estimated CO₂ storage capacity of the unminable coals in the Illinois Basin is 3.63 billion t (4 billion tons). In addition to storing CO₂, these coals are also targets for enhanced coalbed methane production, with an estimated volume of 6.68 tcf (189 billion m³) of recoverable methane. For the coals studied, the adsorption capacities for CO₂ are three to six times greater than for methane (CH₄). Experiments demonstrate that swelling and shrinkage of the Illinois Basin coals after injection of CO₂ are considerable, indicating the possibility of permeability damage following CO₂ injection. Key parameters that influence gas adsorption capacities were mapped for the Danville, Hymera, Herrin, Springfield, Survant, Colchester, and Seelyville coal members, including thickness, depth, elevation, moisture and ash, vitrinite reflectance, and temperature and pressure.