Low-temperature geothermal resources in the eastern United States



Optimum sites for low-temperature (<150 °C) geothermal resource development in the technically stable eastern United States will probably be associated with crustal igneous rocks containing relatively high concentrations of radiogenic heat-producing elements concealed beneath insulating blankets of sediments. Granitic rocks crop out over a large area of the central and southern Appalachian Piedmont and Blue Ridge and presumably extend eastward beneath the sedimentary cover of the Atlantic Coastal Plain. Choice of a site in the Atlantic Coastal Plain with a high geothermal resource potential depends on favorable (1) concentration of radiogenic elements in granitic rocks beneath a sedimentary insulator, (2) thermal conductivity of the sedimentary insulator, (3) thickness of the sedimentary insulator, and (4) reservoir conditions in the sedimentary rocks overlying the radioactive heat source. Values of heat generation for granitic rocks in the Piedmont are known to be at least as high as 17 HGU (1 heat generation unit = 10-13 cal/cm3 sec. Using the linear relationship between heat flow and heat generation observed for the southeastern United States, a heat generation of 14 HGU in granitic rocks beneath sediments with an average thermal conductivity of 4.2 x 10−3 cal/sec-cm- °C and a thickness of 2 km would yield a temperature of nearly 100°C at the base of the sedimentary section and a gradient of about 40°C/km in the sediments. We have logged approximately 250 wells in the Atlantic Coastal Plain, with temperature gradients in the range 15–45°C/km. Preliminary results suggest that the higher gradients are associated with negative Bouguer gravity anomalies and therefore probably with concealed granitic plutons.