The El Dorado oil field is the largest in Kansas and one of the largest in the United States. Until its discovery in 1916, little oil had been produced in Kansas. Since that time the state has been one of the leading producers of petroleum. The accumulation of oil and gas is due to the existence of a buried anticline over which the younger rocks have been folded a lesser amount. Oil and gas are found in these younger rocks, but the major production of oil has come from the Ordovician at the unconformity between this system and the Pennsylvanian.

The anticline, with a total structural relief of approximately 1,400 feet, is typical of the "Granite ridge" of which it is a part. Faulting and most of the folding is confined to the pre-Pennsylvanian rocks, although there is closure of 150 feet on the surface beds. Because of the erosion which took place after the major uplift, the lower part of the Ordovician "Siliceous lime" is in contact with the upper part of the Pennsylvanian Cherokee formation, the unconformity being measured by the absence of approximately 1,400 feet of sediments. The stratigraphy of the pre-Pennsylvanian is typical of that of south-central Kansas.