The answer to whether oil exists in Illinois is a definitive yes; it has been commercially produced in the state for over a century. Illinois maintains a long-standing, though modest, presence among U.S. oil-producing states. While not a contemporary energy giant, its geological structure holds significant hydrocarbon reserves that have contributed billions of barrels to the nation’s energy supply. The industry today relies on mature fields and specialized recovery techniques, maintaining a steady, low-volume output.
The Illinois Basin: Geography and Geology
Nearly all of Illinois’s oil production comes from the Illinois Basin, a vast, bowl-shaped geological depression covering most of southern Illinois, southwestern Indiana, and western Kentucky. This intracratonic basin formed over hundreds of millions of years, accumulating a thick stack of Paleozoic sedimentary rocks that can reach depths of nearly 15,000 feet. The basin is structured as a syncline, or downward fold, modified by major features like the La Salle Anticlinal Belt and the Rough Creek Fault Zone.
These structural features created numerous traps for migrating hydrocarbons. The primary source rock is the organic-rich New Albany Shale (Devonian-Mississippian formation), which generated the oil under heat and pressure. The oil then migrated upward into porous reservoir rocks, primarily sandstones and limestones from the Mississippian and Pennsylvanian periods.
The most prolific oil-bearing strata are the Mississippian Chesterian and Valmeyeran series, holding nearly 80% of the basin’s ultimately recoverable oil. Key reservoirs include the Cypress and Bethel sandstones and the oolitic Ste. Genevieve Limestone, often called the “McClosky sand.” Oil accumulation is concentrated in the southern half of the state, particularly across the Fairfield Basin, with major production centered in White, Marion, and Crawford counties.
History and Peak Production Periods
Commercial oil production in Illinois began in 1905, marking the state’s first major boom period. This initial development focused on shallow formations along the La Salle Anticlinal Belt in eastern Illinois. Production peaked in 1908 at approximately 34 million barrels, briefly making the Illinois Basin the third-most oil productive area in the country.
Production declined steadily for over two decades as the shallow fields became depleted. A second, much larger boom began in 1937, driven by the successful application of new exploration techniques, specifically reflection seismology and subsurface geological mapping. These methods allowed companies to identify deeper, previously undiscovered reservoirs in the central part of the basin.
The discovery of the deeper Mississippian strata led to a dramatic production surge that dwarfed the first boom. Annual production reached an all-time peak of 140 million barrels in 1940, establishing Illinois as a significant national producer during the lead-up to World War II. Following this peak, the industry entered a slow, long-term decline, punctuated only by a smaller spike in the 1950s attributable to the widespread use of new recovery methods.
Modern Extraction Methods and Current Output
The Illinois oil industry is sustained by an extensive network of mature fields, necessitating advanced recovery techniques to maintain production. Primary recovery, which uses the reservoir’s natural pressure, has long been exhausted in most fields. Most of the state’s approximately 23,400 active oil and gas wells are classified as “stripper wells,” producing an average of only 1.5 barrels of oil per day.
Secondary recovery methods, chiefly waterflooding, are the industry’s mainstay and have been in use since the 1950s. Waterflooding involves injecting water into the reservoir to sweep the remaining oil toward producing wells. This technique is necessary because over 60% of the original oil in place remains “stranded” in the depleted reservoirs after primary and secondary methods.
To target this remaining resource, operators are exploring Enhanced Oil Recovery (EOR) methods, such as Alkaline-Surfactant-Polymer (ASP) flooding. ASP flooding lowers the interfacial tension between the oil and water, improving sweep efficiency and potentially recovering an additional 130 million barrels in fields like the historic Lawrence Field. The state’s crude oil production totaled approximately 7.2 million barrels in 2024, a modest figure heavily concentrated in southeastern counties, with White County alone contributing over 2.2 million barrels.