The Illinois basin is an oval depression covering approximately 60,000 mi² (155,000 km²) in the U.S. Mid-Continent. The basin contains about 100,000 mi³ (450,000 km³) of Cambrian through Permian sedimentary rocks. These rocks consist primarily of marine carbonates and, to a lesser extent, sandstone, shale, and siltstone. Paleozoic rocks disconformably overlie Precambrian granite and rhyolite, dated between 1420 and 1500 Ma.

The basin began as a failed rift or aulacogen (Reelfoot rift and Rough Creek graben) concurrent with the breakup of a supercontinent during Early and Middle Cambrian time. It subsequently evolved into a cratonic embayment that subsided from Late Cambrian into Permian time. Through most of Paleozoic time, the southern end of the proto-Illinois basin was connected to the pericratonic Arkoma and Black Warrior basins. Uplift of the Pascola arch at the southern end of the Illinois basin during post-Late Carboniferous to pre-Late Cretaceous time resulted in structural closure of the basin.

After the rifting episode, the basin began to form, initially by thermal subsidence (Late Cambrian through Middle Ordovician), as a thick succession of sandstone and carbonate rocks was deposited in a depocenter situated over the rift complex. For the remainder of the Paleozoic Era, tectonic subsidence apparently was driven primarily by an isostatically uncompensated mass in the lower crust beneath the Reelfoot rift. During Middle Ordovician to mid-Mississippian time, the mass was largely supported by the strength of the lithosphere. Subsidence was relatively slow and extended well beyond the limits of the rift complex. Compressional stress associated with the Ouachita and Alleghenian orogenies apparently led to a decrease in the viscosity of the lithosphere, allowing the uncompen ated load to subside more rapidly during latest Mississippian through Permian time. No record of post-Paleozoic subsidence is preserved in the Illinois basin proper; however, the area immediately above the Reelfoot rift subsided and filled with Late Cretaceous and early Tertiary sediments of the Mississippi Embayment of the Gulf coastal plain.

Parts of the six primary stratigraphic sequences spanning the Phanerozoic Eon are represented in the basin. From oldest to youngest, these include the Sauk, Tippecanoe, Kaskaskia, Absaroka, Zuni, and Tejas sequences. All are separated by major unconformities, especially on the flanks of the basin, and all represent major tectonic episodes in basin evolution.

Since the initial discovery in 1886, approximately 4 billion bbl of oil and an estimated 4 trillion ft³ of associated dissolved natural gas has been produced in the Illinois basin from Paleozoic rocks. Most of the basin's hydrocarbons have been produced from Late Mississippian (Chesterian Series) and Pennsylvanian sandstones. Mid-Mississippian (Valmeyeran Series) carbonate rocks are also important producers. Lesser amounts of hydrocarbons have been produced from Devonian, Silurian, and Ordovician rocks.

Most petroleum in the basin has been produced from major anticlinal structures with stratigraphic trapping components. Combination stratigraphic-fault traps, diagenetic porosity pinch-outs, and unconformity pinch-outs also occur.

Oil and source rock analyses indicate that the New Albany Shale (Upper Devonian-lowermost Mississippian) was the primary source of hydrocarbons in the Illinois basin; however, mixing of oils from older sources is probable. Long-distance migration of New Albany oil occurred in the basin, probably in response to uplift of the Pascola arch.

Potential exists for increased reserves in the Illinois basin as a result of (1) strategic infill drilling to find bypassed mobile oil, estimated at more than 1 billion bbl; (2) secondary and tertiary methods to recover immobile oil, estimated at more than 4 billion bbl; (3) exploration for subtle traps; and (4) deep drilling into the thick, poorly known Cambro-Ordovician section.