The Thomas oil field, located in the southwest corner of Kay County, Oklahoma, has produced approximately 3,000,000 barrels of oil from five horizons ranging from Pennsylvanian to Ordovician in age; but the major part of the production has come from the Wilcox sand (Ordovician).

Though the Thomas field does not rank as a major pool in point of production, it is of exceptional geological interest because of certain structural features of the pre-Pennsylvanian rocks and because of evidences which it yields as to the geological history of the area.

At the close of Mississippian time a small segment of sedimentary rocks was elevated approximately 1,000 feet above the adjacent area. Subsequent erosion removed about 400 feet of sediments from the top of this eminence, leaving a hill with the Wilcox sand (Ordovician) exposed at the crest and truncated surfaces of the "white lime" (Ordovician) Chattanooga shale (Devonian), and "Mississippi lime" (Mississippian) successively exposed on the flanks.

Later, Pennsylvanian and Permian sediments were deposited over the area, and this hill of resistant rocks impressed a structural likeness of its topographic form upon the successive layers of the younger rocks, with gradually diminishing intensity as the thickness of intervening sediments increased.