An evaluation of the resource potential of the Devonian shales, called the Eastern Gas Shales Project (EGSP) was begun. A study of the stratigraphy, structure, composition, and gas content of the Devonian shale in the Illinois Basin was undertaken by the State Geological Surveys of Illinois, Indiana, and Kentucky, under contract to the U.S. DOE as a part of the EGSP. Certain additional data were also developed by other research organizations (including Monsanto Research Corporation-Mound Facility and Battelle-Columbus Laboratory) on cores taken from the Illinois Basin. This report, an overview of geological data on the Illinois basin and interpretations of this data resulting from the EGSP, highlights areas of potential interest as exploration targets for possible natural gas resources in the Devonian shale of the basin. The information in this report was compiled during the EGSP from open file data available at the three State Geological surveys and from new data developed on cores taken by the DOE from the basin specifically for the EGSP. The organically richest shale is found in southeastern Illinois and in most of the Indiana and Kentucky portions of the Illinois Basin. The organic-rich shales in the New Albany are thickest near the center of the basin in southeastern Illinois, southwestern Indiana, and adjacent parts of Kentucky portions of the Illinois Basin. The organic-rich shales in the New Albany are thickest near the center of the basin in southeastern Illinois, southwestern Indiana, and adjacent parts of Kentucky. Natural fractures in the shale may aid in collecting gas from a large volume of shale. These fractures may be more abundant and interconnected to a greater degree in the vicinity of major faults. Major faults along the Rough Creek Lineament and Wabash Valley Fault System cross the deeper part of the basin.