Using a geology-based assessment methodology, the U.S. Geological Survey estimated the following quantities of undiscovered, technically recoverable
oil and gas resources in the Illinois Basin, USA: (1) a mean of 214 million barrels of oil; (2) a mean of 4.65 trillion cubic feet of natural gas; and (3) a mean of 24 million barrels of natural gas liquids.
Additional publication details
USGS Numbered Series
Assessment of Undiscovered Oil and Gas Resources of the Illinois Basin, 2007