The Cromwell Sandstone Member of the Union Valley Formation (Pennsylvanian) in six townships in Coal and Pontotoc Counties, Oklahoma, consists of three distinct lithofacies types. The types are identified as parallel depositional bands of shaly, transitional, and sandy quality. The sandstone/shale ratios range from limits of 0/1 in the shaly area to 1/0 in the sandy area. The Cromwell Sandstone Member thickens and sand quality improves northward in relation to the increase in sandstone/shale ratios. Compaction of shale accounts for a considerable part of the thinning of the Cromwell Sandstone Member in the shaly and transitional zones.

Proved Cromwell Sandstone primary reserves of 15,302,000 bbl have been developed on 6,190 acres with 309 producing wells. Proved primary reserves correlate with the lithofacies bands. The transitional area has 39 percent of the reserves developed on 77 percent of proved acreage. The sandy area has 61 percent of the reserves developed on 23 percent of proved acreage. There is no production in the shaly area. Proved primary ultimate per-acre oil recoveries average five times as much in the sandy area as in the transitional area.

The gradation to shale of the Cromwell Sandstone provided the barrier for a giant stratigraphic trap which was divided into separate fault blocks by folding and faulting during post-Cromwell Wichita and Arbuckle orogenies (Pennsylvanian).