The Wasson San Andres Field is in Gaines and Yoakum Counties, Texas, on the southeastern edge of the North Basin platform. It was discovered in 1936 and, after producing 394 MM bbl of oil, was unitized in November 1964 into seven waterflood projects. The Denver Unit, which is about 45% of the field area, is the subject of the writer's study. The results are based on examination of more than 4,000 ft of slabbed core from 17 wells, supplemented by detailed log correlation.

The Wasson accumulation is controlled structurally by a NW-SE-trending pre-Permian structural axis and by the buried Wichita-Albany shelf margin. An additional control is imposed by a porosity decrease toward the northwest.

The sediments composing the San Andres reservoir were deposited in a far backshelf, restricted, marine environment. The sedimentary sequence was deposited during a regression, and the entire reservoir interval has been dolomitized completely. Porosity is developed most favorably in the restricted marine facies, but also is present in the intertidal facies. The reservoir is capped by a nonporous supratidal facies. Permeable porosity in the marine facies is developed primarily in particulate, generally unsorted, sediments. Destruction of porosity by secondary anhydrite is common. Individual porous beds are very thin and discontinuous, but generally appear to be better developed near the axes of buried structural features.

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