Coso Geothermal Area

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 (Combs, 1978).

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 (Ferguson, 1973). Snow melt patterns were helpful in locating active surface geothermal features and infrared imagery indicated arcuate surface fault traces (Koenig et al., 1972).

Combs (1975) collected thermal conductivity data from nine heat flow boreholes at the Coso sight 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 (Duffield, 1975).

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 (Combs, 1976). 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.

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 (Kaven et al., 2012).

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 (Dilley et al., 2005; McKibben, 1990). 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 6). The gravity anomalies seen in Figure 6 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.

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.

Regional Setting

Figure 1. 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. (Monastero, 2002)

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 1). 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 (Monastero, 2002). 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 (Beanland and Clark, 1994). 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 (Bacon et al., 1982; Hodges et al., 1989; Schweig, 1989; Wernicke et al., 1982). Global positioning system data show approximately 6.5 mm/yr of dextral shearing across the Coso region (McClusky et al., 2001). 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.

Structure

Figure 2: Coso Block Diagram (Unruh et al., 2002)

A silicic magma body is inferred to be present beneath Coso at a depth of approximately 8 km (Bacon et al., 1980). 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 (Lees, 2001). Figure 2 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.

Stratigraphy

Figure 3: Generalized geological map of the principal geothermal area in the Coso geothermal area (Wohletz and Heiken, 1992)

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 3). The domes are offset by numerous late Cenozoic normal faults that provide conduits for the fumaroles and hot springs (Monastero et al., 2005).

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 (Wohletz and Heiken, 1992). 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.

Figure 4: Schematic east west cross section from the Coso Range through Sugarloaf Mountain (Wohletz and Heiken, 1992)

The area has undergone at least three episodes of hydrothermal activity over the last 300,000 years (Adams et al., 2000). 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 4 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 (Lutz et al., 1995). 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 (Fournier and Thompson, 1982; McKibben, 1990).

Figure 5. Temperatures at depths of 5 m (a) and 10 m (b) from shallow heat flow boreholes in the Coso geothermal area (Wohletz and Heiken, 1992)

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 (Monastero and Unruh, 2002). It is estimated that the temperature at the top of the inferred magma chamber range from 425-600°C. Figure 5 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.