Accumulations of gas and casinghead gas in Pennsylvanian rocks in the Four Corners region (junction of Utah, Colorado, New Mexico, and Arizona) are in carbonate strata of Desmoinesian age in four lithologic zones ("substages") 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 types of porosity are predominant; fracture porosity is important in places.

In only a few instances can the type of trap be defined from present subsurface control. Structurally, all of the gas fields are on surface or subsurface highs of varied relief and area. Sedimentary compaction has contributed to this relief in places, and Late Pennsylvanian-Permian warping has occurred. Most of the present structural relief of these features results from folding during the Laramide orogeny. Stratigraphic variations from porous reservoir beds to nonporous units contribute to trapping of most accumulations, and are the controlling factor of some.

The cumulative gas production during the years 1958-1962 inclusive in the Aneth complex was 100,753,592 Mcf. To January 1, 1964, the cumulative production in the Barker Creek field was 197,820,591 Mcf, or nearly two-thirds of the calculated original recoverable reserve of 315 billion cu ft of gas at Barker Creek. Gas from the Barker Creek and Alkali Gulch fields, and from the Aneth complex, is transmitted to Kirtland, New Mexico, and thence to Topock, Arizona. Buyers in Topock distribute to customers in southern California and southern Nevada.

These Pennsylvanian gas accumulations seem to be essentially in situ. 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, sparsely to moderately fossiliferous carbonate beds. After initial entrapment, later migration probably occurred in places.