The Illinois Basin is an elliptical intercratonic basin that covers approximately 34 million acres in Illinois, southwestern Indiana and western Kentucky. Almost all coals are found in Pennsylvanian strata in which 75 individual seams have been identified and of which 20 have been actively mined since the 1800s. The coals of interest for coalbed methane are in the Middle and Upper Lower Pennsylvanian rocks of the Carbondale and Spoon formations. These coals vary in thickness from a few inches to over 15 ft. The coals vary in rank from high volatile A in the southern part of the basin, grading to high volatile bituminous C on the eastern, northern and western flanks.
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