The Tom O'Connor field, discovered in 1934, is in Refugio County, Texas, on the Gulf of Mexico coastal plain. Large accumulations of oil are present in the Oligocene Frio Formation, with increasing percentages of associated gas in the progressively younger sandstone beds of this regressive sequence; primarily gas reservoirs with minor oil columns are present in the transgressive Oligocene Anahuac Formation; and dry gas is found in the regressive Pliocene-Miocene Fleming sandstone. Ultimate recovery is expected to be more than 500 million bbl of oil and 1 trillion ft³ of gas.

Entrapment is the result of anticlinal folding on the downthrown side of the Vicksburg fault zone. Faulting originated in large-scale gravity slumping of incompetent clay at the continental shelf edge or on the continental slope. The faults' arcuate strike, concave toward the basin, is important to the location of the structure. Deep-seated plastic shale, slumping away from the faults in paths normal to them, converged to form a supportive shale core which makes Tom O'Connor the dominant structure in the area.

Because the massive, permeable, inner- to middle-shelf Frio Formation sandstone beds are structurally uninterrupted for 10-15 mi into the basinward source areas, a large hydrocarbon drainage system was available to yield oil and gas to the trap.

Thus, Tom O'Connor is the dominant structure in the area, the first and largest trap in the path of hydrocarbons migrating updip from deeper basin source areas.