This geothermal field is located near the small community of Stillwater, about 20 km east of Fallon and just south of the Stillwater National Wildlife Refuge. The Stillwater area, like Desert Peak and Soda Lake, is a "hidden" geothermal area, its presence being discovered in 1919 when Charles Kent hit hot water in a shallow well (Garside and Schilling, 1979). Shortly thereafter, W. W. Wheeler and a man named Freeman struck hot water in a well drilled for oil near the picnic grove. The well reportedly geysered about once a minute, shooting water into the air to a height of over 9 m. This phenomenon continued until some very brave souls capped off the well (de Braga, 1964, p. 30-31). This well was the first of many hot artesian wells in the area. The well at Greenwood's store in the small community of Stillwater was used to heat the store in 1947. The well is 70 m deep and has a temperature of 87.7°C (Morrison, 1964, p. 117). The community of Stillwater in Sec. 7, Tl9N, R31E is near the center of a thermal groundwater anomaly covering 52-65 km2 (figure). The high water temperatures are believed to be due to flow from much greater depth along faults (figure and figure) and into shallower aquifers (Morrison, 1964). The Stillwater thermal area appears to be in a portion of the Carson Sink that has had recurrent Quaternary faulting (Morrison, 1964) and to lie along the extension of a fault bordering the west side of Rainbow Mountain a few kilometers to the south. Olmsted and others (1975) suggest that the velocity of upflow of the thermal water rising along the fault near Stillwater is sufficient to maintain the water temperature near that of the deep source, which was inferred to be close to 160°C on the basis of geochemical data (Mariner and others, 1974). The source of thermal water probably lies at a depth of several kilometers, well within the pre-Tertiary basement (Olmsted and others, 1975). The geochemistry of Stillwater springs is discussed by Carr and others (1991).
Several geothermal wells have been drilled in the Stillwater area, but data from older wells are available only for the O'Neill Geothermal Inc. (Oliphant) Reynolds No. 1 well, which was drilled to a depth of 1,292 m in 1964. The maximum temperature recorded was 136°C. Three other wells were drilled to about 1,220 m in 1976 and 1977 by Union Oil Co. in several sections about 1.6 km north of Stillwater. A geothermally heated greenhouse was operated in the community of Stillwater in the early 1980s.
There is currently one binary power plant at Stillwater producing from the 158°C reservoir. The production wells are located in NW¼ Sec. 6, T19N, R20E and NE¼ Sec. 1, T19N, R30E. The plant came on-line in 1989, and produces 13 MW of electricity. The rock units at depth consist of unconsolidated Quaternary lacustrine sediments overlying poorly consolidated Tertiary lacustrine sedimentary rocks that are intercalated with discontinuous basalt flows (Forest and others, 1995). Production is mainly developed in a poorly consolidated Quaternary sand and locally, in fractured basalt. Geothermal fluid flow is thought to have moved from north to south in the sand before production (Forest and others, 1995). To the north, higher subsurface temperatures could be related to hot fluid movement upward from considerable depth along a northerly striking fault (Olmsted and others, 1975). A somewhat higher temperature geothermal resource is located about 3 km to the north of the Stillwater plant (Sec. 30, T31N, R20E), where temperatures as high as 182°C have been recorded from 700-1,350 m drill holes (GeothermEx, 2004, Fig. STI00-1).
Stillwater Geothermal Co. sold the plant to Geothermal Management Services in April 2002. The plant was aquired by AMP Resources in 2004. AMP began construction of a 37-MW generating plant adjacent to the existing facility (AMP Resources website, 2006), and in May 2006, the Nevada Public Utilities Commission approved a permit to build a new 26-MW power plant. Upon completion the new power plant will replace the existing Stillwater plant, online since 1989, which will be dismantled. The new plant will be known as Stillwater 2 Geothermal Power Plant.
In March 2007, Enel North America, Inc. purchased AMP Resources, LLC from AMP Capital Partners and a minority investor. Total 2006 electrical production at the Stillwater plant was 84,727 MWh gross with 49,352 MWh net generation.
Photos
Steam plume from blow out of production well during drilling in 1989. The Stillwater geothermal plant is operated by Amp Resources, Stillwater Holding LLC, 4785 Lawrence Lane, Stillwater, NV 89406.
Ormat Stillwater Geothermal Plant. Currently (2008), the Stillwater geothermal plant is operated by Amp Resources, Stillwater Holding LLC, 4785 Lawrence Lane, Stillwater, NV 89406.
Stillwater geothermal plant; Air cooling units (Ormat). Currently (2008), the Stillwater geothermal plant is operated by Amp Resources, Stillwater Holding LLC, 4785 Lawrence Lane, Stillwater, NV 89406.
Cross section of the Stillwater geothermal field. .From Benoit and others (1982). The Stillwater geothermal plant is operated by Amp Resources, Stillwater Holding LLC, 4785 Lawrence Lane, Stillwater, NV 89406.