About two-thirds of Illinois is underlain by coal-bearing strata of the Pennsylvanian System. Major coal seams crop out along the margins of the Illinois basin coalfield and are at depths of up to 1,000 ft (300 m) in the deep part of the basin in southeastern Illinois. Identified bituminous coal resources of Illinois are 162 billion tons (147 billion metric tons) which exceeds the bituminous resources of any other state. Over 20 billion tons (18 billion metric tons) of potentially surface-minable coal (< 150 ft or 45 m deep; > 18 in. or 45 cm thick) have been mapped, of which 5 billion tons (4.5 billion metric tons) are estimated to be economically recoverable at present. At 50% recovery, about 50 billion tons (45 billion metric tons) of coal with an average thickne s of 4 ft (122 cm) or greater are recoverable by deep-mining techniques. Most of these resources occur primarily in two seams--the Springfield and Harrisburg (No. 5) and the Herrin (No. 6) Coal Members. Little exploratory drilling has been done on the Jamestown, Seelyville, De Koven, Davis, and other coals that may represent a substantial additional coal resource. Coal seams of Illinois commonly are overlain by black shale or limestone, which provides a relatively stable roof for modern, high-speed underground mining. Illinois coal is a high-volatile C bituminous coal having high heat values. About 4% of the state's identified coal resources are relatively low in sulfur (< 2.5%). Studies of stratigraphy and depositional environments have revealed deposits of low-sulfur coal in areas o erlain by thick gray shale. As deeper parts of the basin are explored, new discoveries of low-sulfur coal are expected. At present in Illinois, approximately 60 million tons (54 million metric tons) of coal are produced annually from about 60 mines. At least 12 new mines are expected to open during the next several years, and annual production may exceed 75 million tons (68 million metric tons) by 1985.

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