An altered zone up to 30 m wide and 3 km long is present in Secs. 9 and 16, T29N, R23E along the east side of the San Emidio Desert. Cinnabar, sulfur, gypsum, siliceous sinter, opal, chalcedony, quartz, kaolinite and other alteration minerals occur in sands and gravels of Pleistocene age along the north-south zone. These altered deposits are covered by younger, unaltered alluvial and lacustrine deposits (Bonham, 1969). The alteration and mineralization represent the deposits of hot springs which were probably more active in the past.
Both a vegetable dehydration plant (Lund, 1995) and a binary power plant are operational at the Empire (Farms) geothermal area. Following exploration drilling in the early 1980s, the binary power plant produces 4.6 MW of electricity from a 155°C resource; the plant came on-line in 1987. The Empire Energy binary plant shares the geothermal resource with the onion and garlic dehydration plant to the north. The dehydration plant produces about 27 million pounds (about 12 million kg) of dried onions and garlic per year. Wells at the dehydration plant produce fluids of 130-152°C (Bloomquist, 2004; Trexler and others, 1995). The U.S. Department of Energy has provided funds ($1.6 million over 4 years) to help construct a small-scale geothermal power plant (1 MW net) adjacent to the dehydration facility (www.eren.doe.gov/geothermal).
Geothermal potential in the San Emidio Desert was little known until the late 1960s, when hot water was encountered in shallow drill holes exploring for sulfur in alluvial material along the east side of the Desert (valley) over 1 km west of the bedrock forming the Lake Range. H.F. Bonham noted mercury for the first time in about 1966 (NBMG mining district files), and described a ~4.4-km-long zone, presumably the surface expression of a fault, having hydrothermal alteration in addition to mercury and sulfur (Bonham, 1969, p. 95-96). Water in shallow drill holes was found to be 53°C at 1 m below the ground surface (Garside and Schilling, 1979), but no surface springs were identified at that time. However, a spring was present in an area to the west of the altered zone, in S½ SE¼ SW¼ Sec. 9, T29N, R23E (Mariner and others, 1976; Dennis Trexler, written commun., 2003). This spring was used by Empire Farms in the 1970s and photographed from the air by Tom Flynn in 1979; however, it is not shown on topographic maps of the area or described in any geothermal or water resource reports.
Also, a drill hole encountered boiling water at 29.5 m in 1955 in this same section ( figure), according to T.A. Alberg (written commun., 1975). Chevron Oil Co. drilled 1,223-m and 1636-m geothermal test wells to the west of this area (Sec. 8, 9, T29N, R23E) in 1975 and 1978 (Garside and Schilling, 1979, Appendix 2). Temperatures up to 127ºC are reported from these wells (Bloomquist, 2004). Data on gradient wells are available in Sass and others (1999) and Matlick (1995). Peterson and Dansereau (1975) have reported principal facts for gravity stations in the San Emidio Known Geothermal Resource Area, and Mackelprang and others (1980) have described the geology and geophysics. A map of the thermal anomaly is available in GeothermEx (2004). It appears the San Emidio resource was nearly concealed until discovered by drilling for sulfur.
The San Emidio Desert is an east-tilted half graben, with major fault displacement on a fault near the east side of the valley. Wells (about 500 m deep) for the power plant and dehydration facility produce from Miocene volcaniclastic rocks which overlie Mesozoic metasiltstone and quartzite. The productive area is at the intersection of NNW- and NNE-striking faults about 1.5 km west of the mountain-front fault (Matlick, 1995).