Illinois Basin

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 (640 km) northwest-southeast, and 200 miles (320 km) southwest-northeast.

The basin is bordered on the northeast by the Kankakee Arch, on the southeast by the Cincinnati Arch, on the south by the Pascola Arch, on the southwest by the Ozark Dome, on the northwest by the Mississippi River Arch, and on the north by the Wisconsin Arch. The New Madrid seismic zone and the Wabash Valley seismic zone intersect the southern portion of the basin.

The major structural features within the basin include the La Salle anticlinal belt, the DuQuoin monocline, the Cottage Grove fault system, and the Fairfield Basin.

At its center beneath southern Illinois, the basin contains a thickness of about 15,000 feet of Cambrian through Pennsylvanian sedimentary rocks, predominantly marine, and mostly dolomite, limestone, shale, and sandstone.

The basement rocks deep below the sedimentary rocks of the basin are Proterozoic granites and rhyolite of the Eastern Granite–Rhyolite Province which dates to around 1.55 Ga.

The rocks of the Illinois Basin are sources of coal, petroleum, and other minerals.

Bituminous coal is present in Pennsylvanian rocks in the basin, deposited in freshwater swamp environments. Over the years, the Illinois Basin has produced more than eight billion tons of coal in Southern Illinois, particularly in the Harrisburg Coal Field, and the Western Coal Fields of Kentucky. Coal production was 99 million short tons in 2008, roughly equally divided between Illinois, Indiana, and western Kentucky. The basin provided 8% of US coal production in 2008.

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.