Fallon Naval Air Station is located immediately south of the small town of Fallon, approximately 100 km east of Reno, in the Carson Desert. The main air facility, referred to as Mainside, covers approximately 32 km2 or 3,200 hectares. The area of greatest geothermal interest lies at the southeast corner of the Mainside complex (Combs and others, 1995, p. 1371).
The Carson Desert is a sediment-filled half-graben with a range-bounding fault on its eastern margin. The bulk of those sediments are lacustrine deposits associated with Plio-Pleistocene lakes and deltas. The fill ranges from a few hundred meters thick on the southern margin to more that 3,000 m in the northern part based on data from the Standard Amoco Oil stratigraphic test well (Hastings, 1979). Lithologic units encountered in that well include lacustrine and fluvial clays, sands, and silts, eolian sands, volcanic flows, tuffs, and epiclastic deposits (Combs and others, 1995, p. 1372).
The geothermal system in this area was found after a shallow (50 m; 77°C) artesian water well was drilled approximately 3.2 km due south of the southeast corner of the Mainside facilities (Frank Monastero, pers. comm., April 5, 2004). Following this discovery, the U.S. Navy conducted studies of the geothermal potential of the Naval Air Station south of Fallon beginning in the late 1970s In mid-2001, the Navy expressed an interest in entering into an agreement with a geothermal developer. In 1993 the Navy drilled a 2,119-m observation hole about 11 km southeast of Fallon with a reported bottom-hole temperature of 193°C. The well penetrated 686 m of sediments, 1,372 m of basalt flows and volcaniclastic rocks, and entered basement (Mesozoic metasedimentary and granitic rocks) at 2,057 m (U.S. Navy, unpub. data, 2001). The reservoir appears to be highly fractured Mesozoic rocks (Combs and others, 1995) probably related to north-northwest-striking faults parallel to the west side of the Bunejug Mountains and Grimes Point just east of the geothermal area. Geothermal fluid is known to exist below an area of 10 km2 or more (U.S. Navy, unpub. data, 2001; see Ross and others, 1996). Trexler, Koenig, Flynn, and others (1981; Table E2) reported two geothermal wells near the air station runway (SE¼ SE¼ Sec. 4, T18N, R29E, 55°C ; and NE¼ SW¼ Sec. 23, T18N, R29E, 70°C). Two industrial wells were drilled in Carson Sink by the Naval Weapons Center: Naval Air Weapons Center N.A.S. No. 0 in NW¼ SW¼ Sec. 23, T18N, R29E to a depth of 535 m in 1978, and Naval Air Weapons Center F.O.H. No. 2 in Sec. 36, T18N, R29E to a depth of 1,367 m in 1986 (Barton and Purkey, 1993, p. 9). Chemical analysis of subsurface fluids and fluid from the hot artesian well as well as Na-K-Ca geothermometry indicate a maximum reservoir temperature of 187 to 204ºC (Combs and others, 1995, p. 1371).
Ormat Technologies, Inc. announced that they have signed a 20-year power purchase agreement with Nevada Power Company for the sale of energy to be produced from their Carson Lake geothermal power plant, projected to be online in late 2009 ( www.ormat.com/index_1.htm ).