The McAllen-Pharr field produces gas and gas condensate from the lower, middle, and upper Frio Sandstone between the depths of 5,800 and 13,765 ft. The Frio is a regressive offlap sequence. Consequently, the environment of deposition of the productive sandstone bodies ranges from very shallow marine in the lower Frio through brackish and transitional to continental in the upper Frio. Hydrocarbon entrapment is in a simple faulted anticline in the upper Frio, a large stratigraphic trap in the Hansen sand, and many small complex fault blocks in the lower Frio. Structurally, the McAlllen-Pharr field changes from a simple downthrown fault closure with basinward regional dip to a complexly faulted structure with steep, away-from-the-basin dips.

The great amount of deposition associated with the Rio Grande embayment, and the major contemporaneous McAllen fault system, have created two significant features in the McAllen-Pharr field that are characteristic of this area. They are the stratigraphic nature of the Hansen sand "fairway" and the development of "reverse dip" into the McAllen fault. A knowledge of these phenomena will aid in a clearer understanding of the overall geology and can be the key to future deep exploration in the area.

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