Abstract An oval 12,000-foot, 90,000-cubic-mile Paleozoic basin underlies 58,000 square miles, mostly in central and southern Illinois, southwestern Indiana, and western Kentucky. Its eastern and northern borders were defined in Late Cambrian time, its northeastern by Middle Ordovician, its western during Middle Ordovician, and its southern after Pennsylvanian time. The basin has 63,000 wells and 27,000 dry holes, has produced 2 billion barrels of oil in 50 years, and is maintaining 0.8 billion barrels proved reserves despite annual withdrawal of 85 million barrels. Ultimately the upper half of the stratigraphic column will probably yield 4 billion barrels. The lower half of the basin has not produced oil and is still virtually unknown, with only 12,000 feet of well penetration in about 45,000 cubic miles of Upper Cambrian and Lower Ordovician rocks. Over half the basin's oil has come from two hinge belts, the LaSalle and DuQuoin. A central deep between the hinge belts has produced 40 per cent of the oil, and eastern and western (northwestern) shelves have produced about 9 per cent. Production south of the Shawneetown-Rough Creek fault zone has been minor. Oil is related to the tops of three major carbonate sequences in the upper half of the column, each a few hundred to 2,000 feet thick, and each underlying an essentially clastic sequence. Most oil is either in the top 100–200 feet of the carbonate sections, or in sandstones in the lower part of the clastic sections. One half of one per cent of the basin's oil occurs on the west slope in crinoidal limestone with intergranular porosity, high in the Middle Ordovician carbonate sequence. Seven per cent has come from Lower Silurian to Middle Devonian dolomite, limestone, and sandstone reservoirs in reefs, pinchouts, drapes, and normal structures, having in common only their position near the top of Silurian-Devonian carbonates. More than three fourths of the basin's oil is Mississippian. Less than 1 per cent is from reservoirs beneath or well within the major lower Mississippian limestone sequence. About 18 per cent is from the top 150 feet or so of the limstone, nearly all in oölite. Where the limestone was eroded to the north, on the LaSalle anticlinal belt, successively older formations serve as reservoirs. About 57 per cent of the oil comes from upper Mississippian (Chester) sandstones. The prolific formations are toward the bottom of the sequence. In most formations, oil is more abundant in scattered lenses than where the formation is a blanket sandstone. Middle and upper Chester sandstones are more prolific on the east side of the basin and in faulted areas. The Pennsylvanian has yielded 16 per cent of the basin's total oil, largely along the LaSalle belt. Although some of the Pennsylvanian oil in the Illinois basin appears indigenous, it seems likely that much has migrated upward from the Mississippian strata.