Coso Hot Springs

Coso Hot Springs

Painting Date
30th of November -0001
Detailed Image Link

36.046111, -117.770278


Area Overview

Geothermal Area Profile

Location: California

Exploration Region: Walker-Lane Transition Zone

GEA Development Phase: Operational

Coordinates: 36.170876398225°, -117.83246211914°

Resource Estimate

Mean Reservoir Temp: 298°C [1]

Estimated Reservoir Volume: 35.51 km³ [2]

Mean Capacity: 300 MW [3]

USGS Mean Reservoir Temp: 285°C [4]

USGS Estimated Reservoir Volume: 30 km³ [4]

USGS Mean Capacity: 518 MW [4]

Figure 1. Location map of western U.S. showing Coso geothermal field within the boundary of the Naval Air Weapons Station, China Lake, California [5]

In the eastern portion of central California, on the military-owned Naval Air Weapons Station at China Lake, the Coso Geothermal Field has been producing geothermal power continuously since 1987 (Figure 1). The project is fully financed by private investment, and a prime example of industry-military cooperation in power development. The Geothermal Program Office (GPO) manages the military geothermal program at China Lake. The GPO is a part of the U.S. Navy, but has jurisdiction over exploration and development of geothermal resources on all military-owned land. The governing policy states that no development will proceed if the military’s mission is found to be adversely effected. Through the creativity of the GPO, they were able to resolve many hurdles and the power development projects were successfully implemented.

The generating facility at Coso consists of four geothermal power plants that have a total of nine 30 MW turbine-generator sets for a total of 270 MW of rated capacity.[5] The plants were constructed by Mitsubishi and Fuji from 1987 through 1989. The net running capacity is higher than the rated capacity at 302 MW. This increase in capacity is due to the high pressures and temperatures encountered in the field, which allows for the units to operate above their initial rated capacity. Between 80-90 production wells operate at a given time, producing a mass flow rate of more than 14 million pounds per hour. Depending on the volume of fluid that needs to be handled and where pressure support is required the Coso field can use between 30 to 40 injection wells. The power plants utilize double-flash technology for steam extraction due to the high temperature fluids. Wellhead pressures range from 85-500 psig. Produced fluids are moderately saline chloride brines with total dissolved solids from 7,000-18,000 ppm. Non-condensable gases account for 6% of the gas fraction, with 98% of that from CO2. Hydrogen sulfide ranges from <10-85 ppm.

History and Infrastructure

Operating Power Plants: 3

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Developing Power Projects: 0

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Power Production Profile

Gross Production Capacity: 167.7 MW

Net Production Capacity: 229.3 MW

Owners  :
  • Coso Operating Co.

Power Purchasers :

Other Uses:

The thermal surface features at China Lake have been long known, with the Coso Hot Springs held as sacred land to the Paiute and Shoshone Native American tribes who settled in the area. The earliest written account of the hot springs is from M.H. Farley, a miner at nearby Silver Peak, who mentioned “boiling hot springs to the south” in 1860. A government survey of the area in 1881 described the area, often labeled “Hot Sulphur Springs” at the time, as “thousands of hot mud springs of all consistencies and colorsi.” The land was deeded to William T. Grant in 1895, who created a health resort there by 1909. The waters, mud, and steam of the Coso area was claimed to cure a range of ills from venereal disease to constipation. The water and mud of the area was sold at high prices providing “Volcanic Health and Beauty from Nature’s Great Laboratory.” The resort first only attracted visitors from nearby eastern Sierra communities, but the advent of the automobile changed the area, and people came from as far as Los Angeles, San Bernardino, and San Francisco. The resort remained open until 1943, when the U.S. Navy began purchasing land for the China Lake Naval Ordnance Test Station. This is currently called the Naval Air Weapons Station.[6] All the land was obtained by 1947.

The Coso geothermal project was made possible through the determination of Dr. Carl Austin, a research rock-mechanics scientist at China Lake. Recognizing the huge potential of this resource in the early 1960s, he began a campaign to convince the Navy to develop the geothermal resource despite the fact that it was not an explicit part of their mission. Dr. Austin soon ran into a problem of encroachment management, still a barrier to military projects, while trying to convince anyone of the viability of the geothermal resource. The USGS, for example, believed the resource was too small to be economical. He also had to convince industry that the Navy was capable of doing business with them, as an energy project of this nature had never been attempted before. A full scientific and engineering investigation of the resource had begun by 1977. 17 heat-flow holes were drilled, large quantities of geophysical and geological data was collected, and one deep test hole was drilled in 1977. The study was summarized in a special volume of the Journal of Geophysical Research (1980) and supported the existence of a large, viable geothermal resource. The 4,850-foot deep test hole provided commercial temperatures and fluid flow rates. In 1979, the Navy awarded a contract to California Energy Company (CEC) to develop the field and to supply power to the Navy with an initial output of 20 MW.

John F. Lehman, Secretary of Navy of the time, declared December of 1981 was the completion of the first successful production well. Using this well, reservoir testing showed a production capacity greater than 30 MW.[5] No one at the time suspected the capacity would eventually be greater than 270 MW. Navy I, Unit I, the first power generating unit, was completed from 1981 to 1987. By the time Unit I came online on July 15, 1987, all financing, power sales, and revenue sharing concerns were resolved. Further drilling confirmed an even larger resource than expected, allowing two more units with a total rated capacity of 270 MW. The last of the units was brought online in January 1990 and since then the average on-line availability has been 98 percent, with a peak 2,318 GW hours delivered to Southern California Edison in 1995. The field was originally operated by Caithness Energy, LLC., but in 2009 Terra-Gen Power took over operation. It is currently operated by Coso Operating Company, a subsidiary of Terra-Gen Power LLC. Since 1987, the field has produced more than 26,000 gigawatt hours (GWh) of electricity. The US Navy is the Surface Management Entity for four power plants at Coso: Navy I, Navy II, BLM East and BLM West. The development model used for Coso is a public and private venture development and production model. All generated power is sold into the local utility grid under a long-term power sales agreement.

DOE Involvement

Figure 2. Plan view of the northern section of the EGS study area showing the wellhead locations and trajectories of wells 34-9 and 38-9. [7]

In 2002, DOE awarded $4.5 million to a five year project at Coso to use hydraulic fracturing technology, common in oil and gas production, to enhance productivity of the existing reservoir. The DOE award was part of a $12 million total effort with cost share coming from Caithness Energy. The project partners, the University of Utah’s Energy & Geoscience Institute (EGI) and Caithness Energy, were funded to study the feasibility of opening sealed fractures to increase the permeability of the reservoir through analysis and on-site demonstrations. The project planned to pump water under high pressure into injection wells in a less productive region on the margin of the field and measure increases in fluid injectivity and production. Since the Coso geothermal field is not heat-limited there could be large gains in electricity production if the reservoir was made more permeable and more liquid was available for production.

There were two projects undertaken as part of this DOE funding. First, tracer and geologic data from well 34A-9 which was stimulated using funds from Coso Operating Company prior to the beginning of the DOE funded EGS project was analyzed by EGI. As part of this analysis, they determined that the stimulation was successful and that connection was achieved between injection well 34A-9 and a nearby production well 38-9 showing.[7] Wells 34A-9 and 38-9 are located in the northeastern part of the Coso field referred to as the East Flank. Their locations are shown in Figure 2. The second project involved a demonstration of stimulating an injection well to increase injection capacity. First, injection well 34-9RD2 in the East Flank was chosen for stimulation to create an EGS doublet with production well 38C-9 (Figure 2). However, when deepening the well to the zone intended for stimulation, a large natural fracture was found that negated the project objectives. The stimulation project was moved to well 46A-19RD2 in the southwestern part of the Coso Field. However, the recompletion of well 46A-19RD2 for stimulation was unsuccessful because the well liner could not be removed from 2065 feet to the total depth. After this second failure to complete the project objectives, the DOE EGS project at Coso was ended.

Time Line
Historic: Coso Hot Springs held as sacred land to the Paiute and Shoshone Native Americans.
1860: “Boiling hot springs to the south” mentioned by a miner, M. H. Farley.
1881: Government survey of the area described “Hot Sulphur Springs”
1895: Land deeded to William T. Grant.
1909: Health resort developed by Grant.
1943: Health resort closed.
1947: Land obtained by Naval Air Weapons Station.
1960s: Dr. Carl Austin advised the Navy to develop the geothermal resource.
1977: Full-scale scientific and engineering investigations of the geothermal resource; drilling 17 heat flow holes, collecting large quantities of geophysical and geologic data; and drilling one deep test hole.
1979: 4,850 foot deep test hole provided commercial temperature and flow rate, leading to a contract with California Energy Co.
1981: First successful production well completed.
1987: First double flash geothermal power unit on-line (Navy I) of 90 MW Caithness Energy LLC, becomes the operator of the field for the Navy delivering power to Southern California Edison.
1988: 2nd double flash geothermal power unit on-line (Navy II) of 90 MW.
1989: 3rd and 4th double flash geothermal power unit on-line (BLM East and BLM west) for a total of 90 MW.
1993: Implementation of LO-CAT® process for hydrogen sulfide removal and a sulfided, activated carbon media upstream for mercury removal.
2001: Brookhaven National Lab wins R&D 100 award for development of silica removal technology.
2002: DOE award to demonstrate hydraulic fracturing technology at the Coso location for a total of $4.5 million over five years as part of a $12 million dollar effort by the Energy and Geoscience Institute at University of Utah and Caithness Energy.
2009: Terra-Gen Power takes over operation of the field; construction begins on pipeline for increasing reservoir recharge. Pipeline finished in late 2009.


Regulatory and Environmental Issues

The development of the Coso geothermal field was made possible by creative cooperation between the Navy and industry, and its success has been the inspiration behind the current military business model. Based on the concept of “farming-in,” developed by the oil and gas industry more than five decades ago, the approach attempts to limit the front-end, high-risk exploration investment that must be done by one company.[5] By seeking partners to share the initial investment, agreements can be made taking into consideration how much was put into the delineation phase, current market conditions, and current/projected operating expenses. The involvement of more parties can help bring in the necessary investment and improve the economics for everyone involved. This model is particularly useful for the Navy because it lowers their own front-end risk, and helps secure financing without a large initial investment by the geothermal developer. It is also a familiar strategy to industry and helps encourage development of renewable resources (required by U.S. Department of Defense policy) by allowing the Geothermal Program Office (GPO) to provide industry data on the resource before seeking industry investment. The model further eliminates a major concern to the military, the intrusion of speculators who secure development rights but don’t have the capital to prove the resource. Their presence on a military facility must be managed, but provides no value to the mission. The success of the GPO model at Coso has encouraged further development of geothermal resources on military land, identifying more than 25 potential locations in the continental U.S.

Future Plans

Terra-Gen, LLC, the current owner of the Coso Geothermal Facility, has many plans to increase production from the field. Since the field is liquid-limited, one idea is to inject more water into the reservoir. In 2009, Terra-Gen obtained the critical permits from the Bureau of Land Management (BLM) to begin construction of a 9-mile pipeline for recharging the existing reservoir.[8] The intent of the project, referred to as the Hay Ranch Water Project, is to inject supplemental water into the reservoir to stabilize and enhance the field, increasing electricity production to serve an estimated 50,000 more homes, or about 50 MW. The BLM completed an extensive environmental review and concluded there will be no significant negative impacts from the project. Coso also obtained a Conditional Use Permit from Inyo County, after another Environmental Impact Review . A further barrier to the project was overcome by reaching settlement with Little Lake Ranch, Inc. to provide improvements around Little Lake to ensure the availability of water for recreational and habitat conservation purposes. After this delay, Terra-Gen completed the construction of the pipeline in late 2009.

Exploration History

First Discovery Well

Completion Date:

Well Name:



Initial Flow Rate:

Flow Test Comment: 30 MW

Initial Temperature:

Figure 3. Active mud pot at Coso (Andrew Alden, 2008)

Hot springs and other surface thermal features at China Lake were first identified by the Paiute and Shoshone Native American tribes that settled in the area. Miners noted “boiling hot springs” in the Coso region as early as 1860. A government survey in 1881 observed numerous hot mud springs and pots in the area (Figure 3). Dr. Carl Austin, a Navy geologist specializing in rock mechanics, recognized the geothermal potential of the Coso area in the late 1960’s. Austin found the location of the field, planned the project, obtained the funding and negotiated the contracts with the Navy. In pressing his vision of geothermal energy, Austin overcame a host of skeptics, including the US Geological Survey, who doubted the economic viability of the geothermal field. In 1967, Coso #1 Core hole was drilled to 114m into the Coso Springs fault zone associated with fumaroles and hot springs. The hole recorded a maximum temperature of 142°C (288°F).

The Coso area has ample surface evidence of geothermal energy including fumaroles, hot springs, hydrothermally altered rocks, and Late Cenozoic volcanics (as young as Pleistocene 21,000-41,000 YBP) including thirty-seven rhyolite domes. The presence of these surface features and the results from the first exploratory well prompted the USGS to classify Coso as a Known Geothermal Resources Area in 1971.[9]

Austin and others (1971) recognized the young silicic volcanism and associated ring structure and inferred a magmatic heat source providing energy for the geothermal surface manifestations. Chapman and others (1973) mapped negative gravity anomalies in the geothermal area and interpreted a shallow intrusive body. Early exploration activities at Coso included field geological reconnaissance using electrical surveys, petrology, mineralogy, photogeology, gravity and magnetic measurements.[10] Snow melt patterns were helpful in locating active surface geothermal features and infrared imagery indicated arcuate surface fault traces.[11]

Combs (1975) collected thermal conductivity data from nine heat flow boreholes at the Coso site. High heat flow in the rhyolite dome field was found, with values ranging from about 2 HFU to 18 HFU, higher than the world-wide average of about 1.5 HFU. The rhyolite dome field was found to be associated with low electrical resistivity (Furgerson, 1973) and microearthquake epicenters (Combs and Rotstein, 1976). Surface geologic mapping in the rhyolite dome field indicated intensive fracturing, which was ultimately found at depth and provides the permeability for the Coso hydrothermal reservoir.[12]

Exploration work intensified in 1976 with the drilling of 22 shallow boreholes with a maximum depth of 133 m. Temperature data from these shallow boreholes led to estimates of the geothermal gradient between 24°C/km to 450°C/km and demonstrated the presence of a large geothermal resource at Coso.[13] In 1977, the first deep test hole was drilled to 1,477 m. Downhole geophysical data collected from this first deep test hole included an acoustic televiewer log, gamma and neutron logs, static temperature data, as well as a cuttings analysis. This initial deep test hole demonstrated commercial temperatures and flow rates. The first production well was completed in 1981. Initial reservoir testing indicated production capacity of over 30MW. The first power was delivered from the Coso geothermal field in 1987, exactly 20 years after the first exploratory well had been drilled.

Figure 4. Complete Bougeur Gravity Map of the Coso Geothermal Area [14]

Exploration activity at the Coso geothermal field continued after the first production well was drilled and focused on better understanding the resource to maintain sustained production. One of the main techniques deployed to determine the fault structure within the field was microearthquake and other seismic monitoring. Seismic monitoring began in 1975 with 16 stations that created a regional telemetered network that was operated by the U.S. Geological Survey. The U.S. Navy has a permanent seismometer network that has been operating since the 1980s. By 2011, over 600,000 microseismic events had been recorded in the Coso Geothermal Field.[15]

In addition, fluid inclusion analysis was carried out on drill cuttings from the 1990s through 2005 to determine the geology and thermal history of the geothermal field.[16][17] Other activities include numerical modeling and continual improvement in the conceptual model of the Coso Geothermal field using the most recent data. Although identification of the presence of a geothermal resource was greatly facilitated by surface expressions, it has been the continued exploration focus at the field that has sustained the production over the past 25 years. For example, a complete bougeur gravity map of the Coso Geothermal field was taken in 2005 (Figure 4). The gravity anomalies seen in Figure 4 along with geochemical and seismic data led to the conclusion that Coso is a nascent metamorphic core complex. Developing a sound conceptual model can help guide drilling of new production or injection wells necessary to sustain the field.

Well Field Description

Well Field Information

Development Area:

Number of Production Wells:

Number of Injection Wells:

Number of Replacement Wells:

Average Temperature of Geofluid: 275°C [18]

Sanyal Classification (Wellhead): High Temperature

Reservoir Temp (Geothermometry):

Reservoir Temp (Measured):

Sanyal Classification (Reservoir):

Depth to Top of Reservoir: 500 m [19]

Depth to Bottom of Reservoir: 3500 m [19]

Average Depth to Reservoir: 2000 m [19]

The first successful well in the geothermal field was completed at the end of 1981. Reservoir testing after this well was completed showed a resource of more than 30 MW, much less than the eventual 302 MW currently produced. By 1992, the California Energy Company, Inc. had developed about 90 wells in the field. Later development by the Coso Operating Company brought the well count to over 150. The first production well, drilled to a depth near 2,000 m, had a bottom-hole temperature of approximately 340°C and produced dry steam. The boiling interface has remained intact since the beginning of production even with the reinjection of all production fluids through 30-40 wells located around the margin of the field.

Research and Development Activities

Coso plans to use hydraulic fracturing technology, common in oil and gas production, to enhance productivity of the existing reservoir. The project partners, the University of Utah’s Energy and Geoscience Institute (EGI) and Caithness Corporation, were funded by the US Department of Energy to reopen sealed fractures in subsurface rocks and increase the permeability of the reservoir . By pumping water under high pressure into injection wells in a less productive region on the margin of the field, the project hopes to circulate water through the fractured rocks and to the surface to drive steam turbines. As the field is not heat-limited there could be large gains in productivity if the reservoir was made more permeable and more liquid was available for production. DOE awarded a $1.875 million Geothermal Resource and Exploration grant to Caithness for use at Coso over a three-year period.

Technical Problems and Solutions

The Coso geothermal fluid contains a high concentration of non-condensable gases (NCG), consisting of mostly carbon dioxide, as well as dissolved solids, such as silica.[20] If not properly separated and disposed of, these can limit power generation, as well as cause environmental, health and safety problems. The Coso plants have developed technical strategies to overcome these problems, meeting strict California emissions regulations and even creating an alternative source of profit. At first the geofluid condensate, with NCG, was reinjected back into the reservoir after power generation. This practice began to affect the performance of the reservoir and was stopped.[20] It was decided to build treatment facilities to remove the hydrogen sulfide and mercury from the NCG and exhaust the remaining gas, carbon dioxide and water vapor into the atmosphere. Investigation into various hydrogen sulfide and mercury removal systems in 1993 led to the selection of LO-CAT® process for hydrogen sulfide removal and a sulfided, activated carbon media upstream for mercury removal. After using this treatment process for more than 15 years, Terra-Gen Power LLC considers it the “Best Available Control Technology” (BACT) for geothermal power plants. More detailed information on the LO-CAT® process at Coso is available here:[21] The other difficulty obstructing power production was the dissolved solids, mostly silica, in the geofluid. The problem was addressed by a private and governmental collaboration between Caithness Operating Company (who owned Coso at the time, as well as Dixie Valley and Steamboat Springs in Nevada, all three of which served as test centers for the new technology) and Brookhaven National Laboratory (BNL). In order to develop a method for extraction silica, BNL tested reaction parameters such as temperature, pressure, pH, concentration of reagents, and aging to see their impacts on the properties of silica products. After it was shown that the silica could be extracted, they also tested surface modification on the produced silica to increase its marketability. The data was used to predict silica production and associated costs, showing the viability of commercial mineral extraction in these geothermal power plants. BNL won a 2001 R&D 100 Award for developing the technology, but has since stopped further research and development on the project.

Geology of the Area

Geologic Setting

Tectonic Setting: Extensional Tectonics [22]

Controlling Structure: Pull-Apart in Strike-Slip Fault Zone [23]

Topographic Features: Horst and Graben

Brophy Model: Type E: Extensional Tectonic, Fault-Controlled Resource

Moeck-Beardsmore Play Type: CV-3: Extensional Domain

Geologic Features

Modern Geothermal Features: [24]

Relict Geothermal Features: Hydrothermal Alteration [25]

Volcanic Age: Pleistocene [25]

Host Rock Age: Mesozoic [25]

Host Rock Lithology: granitic [25]

Cap Rock Age:

Cap Rock Lithology:

Regional Setting

Figure 5. Shade relief map of the region around the Coso Geothermal Field (star). The field lies in a triangular block between the Walker Lane, the Sierra Nevada, and the Garlock Fault. [5]

The Coso geothermal field is located at the boundary of the Basin and Range and Sierra Nevada tectonic provinces, and is situated at a releasing bend stepover in a dextral strike-slip fault system between the Walker Lane Fault Zone, the Sierra Nevada and the Garlock Fault (Figure 5). A shallow (<2 km) and hot at 200-328°C (393-622°F) resource is a result of crustal thinning, seen in the shallow seismic-aseismic boundary and rock and fluid chemistry.[5] The geothermal field at Coso is classified as a hot water resource compared to a steam dominated system with the system most likely liquid-limited and not heat-limited. The superheated groundwater flashes to steam at less than 2 km depth. The area also shows its youthful character in the abundance of surface thermal features. The hot springs, mud pots, mud volcanoes, and fumaroles of the area indicate an active near-surface resource over nearly 6,400 acres. The many surface features in the area show considerable variability both temporally and spatially.

The Coso Geothermal Field is located in a zone of high seismicity that produced a magnitude 7.5 earthquake in 1872 and large seismic events continue through to the present.[26] The earthquakes in the area near Coso are predominantly dextral strike-slip events, consistent with the minimum of 150-170 km of extension that affected the southwestern Basin and Range region in the late Cenozoic.[27][28][29][30] Global positioning system data show approximately 6.5 mm/yr of dextral shearing across the Coso region.[31] Recent micro-seismicity within the field is related to production and injection of fluids and is diagnostic of fracture permeability. Clusters of seismicity beneath the field correlate with the projection of surface faults and appear to represent permeable pathways for circulation of hydrothermal fluids.


Figure 6: Coso Block Diagram [32]

A silicic magma body is inferred to be present beneath Coso at a depth of approximately 8 km.[33] The magma body may still be partially molten, since basaltic eruptions have occurred as late as a few thousand years ago. The trend of extrusive rock ages and volumes suggest that eruptions will continue in the future. Seismic studies have shown a shallow brittle-ductile transition zone at a depth of 5 km that has been interpreted as either evidence a deep hydrothermal system or the top of the magma chamber.[34] Figure 6 shows a block diagram of the Coso Geothermal Field.

Monastero and others (2005) interpreted Coso to be a nascent metamorphic core complex within an upper plate of fault-bounded blocks resting structurally on a lower plate of highly metamorphosed rocks that have been subjected to ductile deformation. The plates are separated by a mylonitic shear zone, into which the faults in the upper plate terminate. Samples of exhumed metamorphic core complexes exhibit extensive hydrothermal alteration, volcanism, and fracturing.


Figure 7: Generalized geological map of the principal geothermal area in the Coso geothermal area [35]

The Coso geothermal system lies in fractured Mesozoic plutonic basement rock associated with the Sierra Nevada batholith. About 35 km3 of volcanic rocks ranging in age from 4 to 0.04 MY have erupted and overlie the Mesozoic basement. The most prominent volcanic features are Pleistocene rhyolitic domes (Figure 7). The domes are offset by numerous late Cenozoic normal faults that provide conduits for the fumaroles and hot springs.[14]

The first successful production well drilled in the Coso geothermal system in December 1981 encountered intensely fractured Mesozoic plutonic and metamorphic rocks ranging in composition from leucogranite to gabbro. The fracturing has been attributed to several mechanisms, including natural hydraulic fracturing during volcanic eruptions, thermal stresses from elevated heat flow, and extensional tectonics.[35] Permeability in the field is likely created by active normal faults that are accommodating the regional dextral transtension. The reservoir is not confined to a specific rock type. The controlling factor for the presence of hydrothermal fluids is the transtensional fracturing.

Hydrothermal System

Figure 8: Schematic east west cross section from the Coso Range through Sugarloaf Mountain [35]

The area has undergone at least three episodes of hydrothermal activity over the last 300,000 years.[25] Travertine deposits represent the earliest (300,000 YBP) hydrothermal episode, a large low- to moderate-temperature geothermal system. The second phase (238,000 YBP) produced sinter in the southern part of the current geothermal field, with fluid inclusions indicating a large high-temperature system (up to 328°C) with an upflow zone. The current hydrothermal field is partitioned into at least three weakly connected or isolated reservoirs distinguished by differences in temperatures and production fluid chemistry. While steam is locally produced in parts of the field, the geothermal field is principally a liquid-dominated system. Numerous studies, from the GPO and other governmental and academic groups, confirm that the controlling factor of the hot fluids is fracturing caused by modern tectonic forces. Figure 8 shows a schematic east- west cross section of the Coso Range through Sugarloaf Mountain. This cross section depicts the interpretation of geological and geophysical data that indicate that the origin of the high heat flow is from a rhyolite magma chamber that lies under a horst of crystalline basement rocks. It also shows the rhyolite domes that have been extruded through this basement rock.

Geochemical data indicate that production is from a narrow, asymmetric plume of hydrothermal fluid originating in a southern deep reservoir flowing to the north.[36] The outflow plume is partially controlled by the distribution of fractured crystalline intrusive rocks within foliated metamorphic rocks. A smectite clay zone in the overlying metamorphic rock caps the productive zone. The source of fluid recharge for the Coso system is not known with certainty but has been thought to be either the Sierra Nevada or the Coso and Argus Ranges.[37][17]

Heat Source

Figure 9. Temperatures at depths of 5 m (a) and 10 m (b) from shallow heat flow boreholes in the Coso geothermal area [35]

The heat source for the Coso hydrothermal system is interpreted to be a silicic-magma chamber, possibly still partially molten, at a depth of about 5-8 km. Data suggest that there is an ongoing process of mafic magma intruding to relatively shallow depths. Temperature gradients in the geothermal field, determined from down-hole measurements, range between about 85-120°C/km.[38] It is estimated that the temperature at the top of the inferred magma chamber range from 425-600°C. Figure 9 shows the temperature at 5 m and 10 m below the surface measured from shallow boreholes. The areas with high temperatures are near the Devil’s Kitchen area and Sugarloaf Mountain.

Geofluid Geochemistry


Salinity (low): 5300 [39]

Salinity (high): 6500 [39]

Salinity (average): 5900 [39]

Brine Constituents:

Water Resistivity:

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NEPA-Related Analyses (1)

Below is a list of NEPA-related analyses that have been conducted in the area – and logged on OpenEI. To add an additional NEPA-related analysis, see the NEPA

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