Pennsylvanian oil and gas accumulations in the southern Paradox basin occur in carbonate mounds of

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Des Miones age. Major oil production at Aneth, Ismay, Tohonadla, Gothic Mesa, Anido Creek, and other fields is from algal mounds elongated in a general northwest-southeast direction along the basin shelf. Carbonate reservoir are closely associated with sapropelic black shales and evaporites, occurring in cycles repetition in the shelf area and grading basinward into a predominantly salt section.

As far as can be determined all reservoir are isolated bodies of porous carbonates, mostly limestone. About 30 oil and gas fields in Pennsylvania rocks have been found in the Four Corners area, about half of which are classed as stratigraphic and the other half as either structural or structural-stratigraphic. In almost all cases it can be demonstrated that the accumulation would have developed even if no structural closure were present, although in many places the occurrence is localized by structural growth.

The Ismay and Aneth fields are selected as examples showing both stratigraphic and structural influence on accumulation, with the latter much more strongly influenced by stratigraphic boundaries than the former.

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