Pennsylvanian accumulations of gas and casing-head gas in the Four Corners region (junction of Utah, Colorado, New Mexico, and Arizona) occur in carbonates of Desmoinesian age in four lithologic zones of the Paradox member of the Hermosa formation. These zones, from oldest to youngest, have been named Barker Creek, Akah, Desert Creek, and Ismay. They are shelf counterparts of basinal evaporitic sequences.

Reservoir beds are calcirudite, calcarenite, and sparsely to moderately fossiliferous carbonate, which is nearly in place, and was deposited in biostromal and biohermal complexes. Dolomitization and other diagenetic changes have affected these units. Vuggy and intercrystalline porosity is predominant; fracturing is important in some places.

In only a few instances can the type of trap, with fair assurance, be defined from present subsurface control. Structurally, all of the gas fields are on surface or subsurface highs of varying relief and areal extent. Sedimentary compaction has contributed to this relief in some places, and late Pennsylvanian-Permian warping has occurred. Most of the present structural relief of these structures is due to folding during the Laramide orogeny. Stratigraphic variations, from porous reservoir beds to non-porous units, contribute to most accumulations, and are the controlling factor in some.

Eleven gas and five casing-head gas fields have been found, but these are still largely undeveloped with respect to gas. The cumulative gas production to July 1, 1959, in the Aneth Complex was 9,682,004 MCF. To September 1, 1959, the cumulative production from sour zones was 179,825,593 MCF at the Barker Creek field, representing slightly more than half of the calculated original recoverable reserves of 315 billion cubic feet of gas at Barker Creek.

These Pennsylvanian gas accumulations seem to be essentially in situ occurrences. The source beds are believed to be present in each zone; migration was predominantly local, not exceeding a few miles; and entrapment occurred in laterally adjacent areas of bioclastics and sparsely to moderately fossiliferous carbonates. After initial entrapment, some later re-migration probably occurred in places.