The Balcones-Ouachita trend of central Texas is an often overlooked zone with great potential for widespread occurrence of low-temperature geothermal resources. The area with potential resources extends from north of Dallas to Del Rio. Thermal input to these systems is from the natural thermal gradient of the earth. Downdip movement of ground water from recharge zones to the west, or updip flow from overpressured zones within the Gulf Coast basin, are 2 possible sources for fluids. Permeability in these systems is typically stratigraphic, but local enhancement by fractures and faults aids fluid flow. Basal Cretaceous sandstones, such as the Hosston Formation and its lateral equivalents, and Cretaceous limestones, such as the Edwards formation, have identified resources. R source models developed to date indicate higher temperatures where updip flow from the basin is enhanced by fault permeability. Recent tests of these resources include a geothermal well in Marlin, where the Hosston Formation produces 67°C (153°F) fluids from a depth of 1,183 m (3,881 ft), and a program conducted in the fall of 1983 to assess the geothermal characteristics of the Hosston Formation in the San Antonio area. Results of this program are still being evaluated. In the San Antonio area, warm waters have been produced from the Edwards formation.

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