The Tom O'Connor field, discovered in 1934, is in Refugio County, Texas, on the Gulf of Mexico coastal plain. Large accumulations of oil occur in the Oligocene Frio Formation; increasing proportions of associated gas appear in the progressively younger sandstones of this regressive sequence. In the basal, transgressive Oligocene Anahuac Formation, primarily gas reservoirs with small oil columns are present. In the regressive Miocene Fleming sandstones, dry gas occurs. The immense oil and gas accumulation is due

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to the existence of three factors: large structures, favorably located pinchouts of most Frio sandstones, and a very large structurally uninterrupted drainage area extending basinward to the hydrocarbon source.

Entrapment is the result of anticlinal folding on a typical "rollover" feature common to the downthrown side of the Vicksburg flexure. Structural growth probably had its origin in large-scale gravity slumping of the clays of the early Oligocene Vicksburg Formation, either on the continental slope or near the continental shelf margin. A series of structure maps and cross sections show trap configuration at various horizons.

The excellent quality of the reservoir sandstones is discussed in relation to depositional environment. Reservoir and fluid characteristics are given by formation.

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