Abstract: Exploration for Geopressure Geothermal Resources - Frio Information, Lower and Middle Texas Gulf Coast

By

Don G. Bebout

Exploration methods used in the search for Gulf Coast geothermal reservoirs are identical to those used by the petroleum industry in the search for oil and gas reservoirs. However, the requirements for an economical geothermal reservoir differ considerably from that of an oil and gas reservoir. A geothermal reservoir should be from 50-100 square miles in areal extent and 200-300 feet thick with permeability of at least 20 millidarcies; fluid temperature should be higher than 300°  F. A grid of electrical-log cross sections and net- and sand-percent maps are the primary tools used in delineating potential geothermal reservoirs meeting these specifications.

The Frio Formation, one of several sand/shale wedges, thickens and dips toward the Gulf of Mexico. The Frio is less than 200 feet thick near the outcrop and greater than 9,000 feet thick in the deep subsurface near the present coast where the top of the formation is deeper than 10,000 feet. Growth faults developed during deposition of the Frio resulting in the accumulation of thick sections of sand and mud on the downthrown side of the fault; these growth faults also isolated sand bodies within thick shale sections, thereby inhibiting movement of fluids out of the sands during loading and compaction. In this manner high-pressure reservoirs were developed. Using micropaleontological markers and major shale breaks, the Frio was subdivided into six correlation units. Sand-percentage and net-sand maps of each unit identify five main depositional environments: fluvial, high-constructive delta, prodelta, strand-plain, and shelf.

This knowledge of the regional sand distribution, along with data concerning distribution of geopressure, subsurface fluid temperatures, and porosity and permeability, is essential in order to evaluate potential geothermal reservoirs in the geopressure zone along the Texas Gulf Coast. These regional surveys delineate prospective areas which merit further, more detailed studies. Several such detailed studies have been completed along the Lower and Middle Texas Gulf Coast.

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