Abstract

Two frontier areas with great potential for geopressured geothermal energy development are located at the continental margin in the South-Central United States. Both are tectonic rift belts formed during early Mesozoic continental break-up; both were floored with thick deposits of Upper Jurassic salt; and both were sites of rapid sedimentation with contemporaneous salt diapirism. The Mississippi Salt Dome Basin occupies a rift which opened at the time the Gulf Basin was formed, but was superseded by a rift to the south, now occupied by the Gulf Coast geosyncline and the Gulf Coast Salt Dome Basin. Igneous activity in the Mississippi rift basin is widely known; the sedimentary fill indicating intense hydrothermal activity is perhaps less than half as thick as that in the Gulf Coast geosyncline and geopressured geothermal reservoirs occupy secondary porosity in consolidated rock, mainly Smackover carbonates and sandstones. Fluids are concentrated saline waters or diagenetic and metamorphic gases — CH₄, H₂S, and CO₂. Temperatures range from 356° - 482°F (180° - 250°C) at depths of 16,000 to 22,000 feet (4,880 to 6,710 m).

The Gulf Coast Salt Dome Basin occupies the Gulf Coast geosyncline and forms its gulfward margin. Igneous activity at depths is inferred, based upon salt mobilization features and the geotemperature regime of the noncarbonate clastic sediments that have filled the geosyncline since early Miocene time. Geopressured geothermal reservoirs are compartmentalized sand-bed aquifer systems having primary porosity, reduced by authigenic cements where temperatures range above 302°F (150°C). Fluids are saline waters ranging from less than 10,000 to 200,000 mg/l or more; and diagenetic gases, mainly CH₄, but large volumes of CO₂ are known locally.

Exploration techniques are designed to determine and map five main parameters: 1) geotemperature, in terms of isothermal maps; 2) geopressure, in terms of iso-fluid pressure maps; 3) geothermal fluid reservoirs, generally sand-bed aquifers in the Gulf Coast Salt Dome Basin; 4) geologic structure, principally faults and folds; and 5) the salinity of formation waters, as isosalinity maps. Seismic and borehole logging methods are used in mapping fluid pressure; borehole logging methods provide data for isothermal mapping; seismic and borehole logging methods provide the data for structure mapping; borehole methods, mainly electric logging, provide the data for sand-bed reservoir mapping and for isosalinity mapping. Together, these maps provide the information necessary for selection and evaluation of prospects for geopressured geothermal energy development.