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 concealed
beneath the sediments of the Atlantic Coastal Plain. A conspicuous negative
Bouguer gravity anomaly is generally associated with the granitoid. The
combination of relatively high heat flow from a heat-producing granitoid
concealed beneath sediments of relatively low thermal conductivity was defined
by Costain and others (1980) as the radiogenic model
illustrated below.
The model was confirmed at the
Portsmouth, VA, drill site, 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 C-25 was located near the
center of the circular gravity anomaly at latitude 36o 5l.01' and
longitude 76o 29.83' (see geothermal well
in the illustration
above). 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. Hole C-25
(Portsmouth) was drilled into a late Alleghanian, post-metamorphic,
heat-producing granite that yielded a whole-rock Rb-Sr isochron age of 263 +-
24 m.y. (Russell and Russell, 1980).
Hole C-26 (36o54.5' N, 76o42.2' W) was drilled at Isle of Wight into the non-granitic, non-heat-producing, metamorphosed country rock into which the granite was intruded. C-26 was located 20 km from the center of the Portsmouth gravity anomaly and its purpose was to sample the basement rocks and measure the heat flow at a location not over the buried heat-producing granitoid at Portsmouth. C-26 was drilled to a depth of 325 m by Gruy Federal, Inc., between September 20-26, 1978. From April 1, 1980 to May 6, 1980, the hole was deepened to 512 m by the Virginia Tech drill. Core samples were obtained from depth intervals of 406-413 m and from 424-512 m. These two heat flow locations documented the validity of the radiogenic model.
The temperatures adjacent (C-26) to the granite and over (C-25) the granite body are shown below. Notice the higher temperatures in Hole C-25 at any depth. For example, at a depth of about 500 meters, the temperature in Hole C-25 is about 8º C higher than in C-26. The higher temperatures in C-25 are a direct result of the extra heat produced by the radioactive decay of U, Th, and K (about 80% of the heat comes from U and Th) in the granite beneath C-25. The optimum sites for geothermal resource development are therefore over such granite bodies because higher temperatures are reached at shallower depths; the isotherms are warped upward over the radioactive heat-producing granite. There are numerous such optimum locations in the southeastern United States. Where the granites are concealed beneath Coastal Plain sediments, or where they do not reach the top of crystalline basement, for example at Crisfield, MD, they can be located by geophysical exploration (gravity and magnetics).
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