The Illinois Basin is a Paleozoic depositional and structural basin in the United States, centered in and underlying most of the state of Illinois, and extending into southwestern Indiana and western Kentucky. The basin is elongate, extending approximately 400 miles northwest-southeast, and 200 miles southwest-northeast.
The Illinois Basin has produced more than four billion barrels of petroleum. Major oil production began in 1905, and from 1907 through 1912, the basin was the third-most oil productive area in the United States. Oil production peaked in 1908 at 34 million barrels per year, and declined steadily to 5 million barrels in 1933. A new wave of exploration brought oil production to a new high of 140 million barrels in 1940, after which production again declined. Waterflooding of old reservoirs caused a third peak of oil production in the 1950s.
Much recent drilling activity has targeted shale gas in the Devonian-Mississippian New Albany Shale.