Optimum sites for low-temperature (< 150°C) geothermal resources in the tectonically stable eastern United States will probably be associated with crustal igneous rocks that contain relatively high concentrations of the heat-producing radioactive isotopes of uranium, thorium, and potassium. Such rocks are concealed beneath the Atlantic Coastal Plain sediments, which, because of their low thermal conductivity, act as a thermal insulator. Granitoids crop out over a large area of the central and southern Appalachian Piedmont and Blue Ridge, and extend eastward in the basement rocks conceale associated with the granitoid. The combination of relatively high heat flow from a heat-producing granitoid cd beneath the sediments of the Atlantic Coastal Plain. A conspicuous negative Bouguer gravity anomaly is generallyoncealed beneath sediments of relatively low thermal conductivity was defined by Costain and others (1980) as the radiogenic model. The model was confirmed at Virginia drill sites C25 and C26 where a -40 mgal Bouguer gravity anomaly near Portsmouth, Virginia was believed to be caused by a granite body beneath the sediments of the Atlantic Coastal Plain. Drill-site CP25 was located near the center of the circular gravity anomaly at latitude 36 degree 5l.01 minute and longitude 76 degree 29.83 minutes. The hole was drilled through the coastal plain sediments to 557 m (1828 ft.) by Gruy Federal, Inc. during December 1978. From January to April 1979, the hole was deepened to 611 m (2005 ft.) and a continuous, 1-1/2 inch diameter core was obtained from 557 to 611 m (1828-2005 ft.). The presence of a heat-producing granite body at this location was confirmed.
The choice of a site on the Atlantic Coastal Plain with a high geothermal resource potential depends upon: 1) the concentration of heat-producing radioactive isotopes in granitoids beneath a sedimentary insulator, 2) the thermal conductivity of the sedimentary insulator, 3) the thickness of the sedimentary insulator, and 4) the reservoir conditions in the sediments that overlie the radioactive heat source.
Although the relatively stable tectonic setting of the eastern United States seems to rule out the possible occurrence of conventional, high-temperature hydrothermal resources, the region does contain geothermal resources that are being developed. These are low- to moderate-temperature fluids that are best suited for direct heat and heat pump applications.
As a result of the Virginia Tech geothermal program, geothermal gradients in the eastern United States are known to be as high as 50° C/km (at Smith Point, VA, hole C-59). Temperatures that are high enough for many applications can be reached at relatively shallow depths. The most significant applications at the present time are for district heating. An evaluation of the competitive standing of low-temperature geothermal industrial applications, including district heating, agricultural uses, air conditioning, and applications other than power generation, requires the development of a reliable geological, geochemical, and geochemical data base. Such a data base is offered here at this World Wide Web site. As low-temperature geothermal energy becomes competitive with conventional energy sources, an understanding of the regional and local variations in the regional and local geologic framework becomes essential for efficient site selection.
The importance of the heat-producing radioactive isotopes of uranium, thorium, and potassium with respect to regional and local variations in heat flow was made more apparent by the discovery in New England and the Adirondacks of the linear relation between near-surface heat flow and heat production. According to present interpretations, deep-seated rocks in New England and the Adirondacks contribute two thirds of the total observed heat flow from the decay of radioactive isotopes. Thus, the distribution of uranium and thorium has important implications for the development of geothermal resources in regions where the heat flow is normal.
Geothermal resources in the eastern United States can be grouped into four types:
The resources are listed in the order in which they are most likely to be exploited in the eastern United States on a large scale in the near future.
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