Exploration of the Frio Formation in the Upper Gulf Coastal Plain of Texas, from the past to the present, can be divided into three eras: (1) the early piercement salt-dome era, (2) the era of upper Frio exploration, and (3) the present era of lower Frio exploration.

Northeastern Calhoun County, southern Jackson County and southern Matagorda County exhibit similar structural and stratigraphic conditions in the lower Frio and are considered in this report.

Near the southern limit of the Frio trend in this area, the gently dipping coastal monocline is broken by large regional down-to-the-coast strike faults which form an en echelon pattern. The upper Frio in these fault segments dips southeast, except for a high structural ridge in the Palacios-Appling area where the upper Frio dips northwest. The lower Frio in the same fault segments shows northwest dip, with the Frio section shorter on the upthrown side of the regional faults and thickening greatly into the downthrown side of the next fault on the northwest. Well data indicate that the Palacios-Appling high is probably underlain by a salt ridge. This ridge has influenced the structure of the lower Frio in the updip area.

In a part of northeastern Calhoun County and southern Jackson County the lower Frio is unusually thick, the section being predominantly poorly sorted lignitic sands with minor shale breaks. These are probably deltaic deposits laid down by an ancestral Lavaca River. To the east and into Matagorda County, the lower Frio consists of interbedded sands and shales indicating deposition in an area of littoral and lagoonal environments.

The combination of northwest dip of lower Frio beds, numerous major and minor faults, along with lenticular sands caused by thinning of beds, adds up to a variety of traps for the accumulation of hydrocarbons. Discoveries in the lower Frio have been predominantly gas with high yield of distillate.

Because of the complexity of the structures, exploration has been hazardous, particularly when the high cost of drilling is considered. However, discoveries with thick pay sections have been recorded in the area and they are expected to stimulate exploration for lower Frio reservoirs in the future.

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