Geothermal Exploration Methods and Results

Geothermal Energy in the Eastern and Southeastern United States

Summary and Overview

The Geothermal Program at Virginia Tech was started in 1967 by Professor John K. Costain who began making heat flow determinations in the southeastern United States under a program funded by the National Science Foundation. Primary emphasis was on regional heat flow determinations using methods patterned after those developed by Professor Francis Birch at Harvard University. In 1975 the U.S. Department of Energy supported a geological and geophysical study of the origin of the warm springs in Bath County, VA, under DOE Contract No. EY-76-S-05-4920 to J.K. Costain. Results were published in Final Report TID-28271 to the U.S. Department of Energy by Costain and others (1976), and Perry and others (1979).

The objectives of the Geothermal Program at Virginia Tech from 1976 to 1982 were to develop and apply geological and geophysical targeting procedures for the discovery of low-temperature geothermal resources related to heat-producing granitoids. The different geologic setting associated with the warm springs in Virginia was discussed by Perry and others (1979).

Optimum sites for geothermal development in the tectonically stable eastern United States are associated with areas of high heat flow derived from crustal igneous rocks that contain relatively high concentrations of uranium (U), thorium (Th), and potassium (K). Exploration strategy was directed toward confirmation of the radiogenic model (Costain and others, 1980). In this model, granite plutons with relatively high concentrations of U, Th, and K relative to the surrounding country rock occur in the crystalline basement concealed beneath the wedge of relatively unconsolidated sediments of the Atlantic Costal Plain. Such granites are sources of heat produced by the natural radioactive decay of isotopes of U, Th, and K (Birch, 1954). Because the thermal conductivity of the Coastal Plain sediments is approximately half that of crystalline rocks (Ziagos and others, 1976; Perry, 1979), the sediments act as a thermal insulator, and raise geothermal gradients within the sediments to average values at least as high as 50°C/km over depth intervals of 300 m. In a sedimentary section above a heat-producing granitoid, isotherms are warped upward, and anomalously higher temperatures occur at shallower depths. The objective of the Virginia Tech geothermal program was to locate the highest temperatures at the shallowest depths in Coastal Plain sediments. Similar heat-producing granitoids are exposed in the Piedmont to the northwest where they can be sampled more easily at lower cost. For this reason, the drilling program began in the Piedmont while geophysical logging of available, accessible holes was carried out on the Atlantic Coastal Plain. The radiogenic model was conceptually confirmed by the Portsmouth granite, where the heat flow and the geothermal gradient were considerably higher over the granitoid than in the surrounding country rock.

In order to locate optimum sites systematically, a methodology employing geologic and geophysical methods of investigation was developed and applied. Heat-producing granitoids similar to those in the crystalline basement beneath the Atlantic Coastal Plain are exposed to the northwest in the Piedmont Province. The petrology, radioelement distribution, and bulk chemistry of several of these young (254-420 Ma.) granites was studied in detail (Speer and others, 1980; 1982). These late Paleozoic granitoids in the central and southern Appalachians are the youngest known to date.

A linear relation between surface heat flow, q, and surface heat generation, A, was confirmed for many heat flow sites in granites exposed in the Piedmont of Virginia, North Carolina, South Carolina, and Georgia. This was a significant result of the geothermal program because from this relationship a deep crustal temperature profile can be calculated (Lachenbruch, 1968). An understanding of the linear relation between heat flow and heat generation remains an important unsolved problem wherever it has been observed on other continents, and has important implications for predicting crustal temperatures in areas of potential hot-dry-rock applications.

Following detailed study of selected heat-producing granitoids exposed in the Piedmont, emphasis shifted to granitoids concealed beneath the thermally insulating sediments of the Atlantic Coastal Plain. Thermal conductivity varies with sediment type. Abrupt lateral and vertical facies changes occur within the Coastal Plain sediments (Brown and others, 1972) making prediction of lithology, thermal conductivity, and temperature at the surface of crystalline basement difficult. Generally, the sediments in the deeper sections of the northern Coastal Plain are non-marine deposits that are more quartz-sand rich than the upper 300 m. This tends to increase thermal conductivity in the deeper part of the section relative to the upper part and thus lowers the geothermal gradient by as much as 15 per cent. Thermal conductivity also increases with compaction. Granites beneath such sediments must be located indirectly by means of geophysical data including gravity, heat flow, and reflection seismology. Virginia Tech's Radiogenic Model was later confirmed by the drill (Hole C25). The unmetamorphosed granitoids at locations PT1, PT2, and PT3 are also heat-producing. Other suspected heat-producing granitoids beneath the Atlantic Coastal Plain remain unsampled by the drill including Smith Point, VA (C59), Crisfield, MD (C32, DGT1) and Lumberton, NC (LU1).

We use Hole C25 near Portsmouth, VA, as an example of how to access this geothermal data base. Data from other holes can be accessed by backing up one page using the Web browser (e.g., Netscape) and clicking on "Virginia Data", then on "Wells". A 40 mgal negative Bouguer gravity anomaly near Portsmouth, Virginia was believed to be caused by a granitic body beneath the Atlantic Coastal Plain. In order to sample the basement rock, drill-site C25 was located near the center of the circular anomaly at latitude 36° 5l.01' and longitude 76° 28.83'. 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 obtained from 557 to 611 m (1828-2005 ft.). The basement rock obtained from drillcore C25 is a post-metamorphic granitoid, the Portsmouth granite. Heat production values for the Portsmouth drill core as determined using gamma-ray spectroscopy at Virginia Tech are listed in this table.

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Regional Geophysics Laboratory, Department of Geological Sciences, Virginia Polytechnic Institute and State University / comments to:

last updated: 9/08/95