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USGS2015

Coupling geophysical investigation with hydrothermal modeling to constrain the enthalpy classification of a potential geothermal resource.

An appreciable challenge in volcanology and geothermal resource development is to understand the relationships between volcanic systems and low-enthalpy geothermal resources. The enthalpy of an undeveloped geothermal resource in the Karckar region of Armenia is investigated by coupling geophysical and hydrothermal modeling. The results of 3-dimensional inversion of gravity data provide key inputs into a hydrothermal circulation model of the system and associated hot springs, which is used to evaluate possible geothermal system configurations. Hydraulic and thermal properties are specified using maximum a priori estimates. Limited constraints provided by temperature data collected from an existing down-gradient borehole indicate that the geothermal system can most likely be classified as low-enthalpy and liquid dominated. We find the heat source for the system is likely cooling quartz monzonite intrusions in the shallow subsurface and that meteoric recharge in the pull-apart basin circulates to depth, rises along basin-bounding faults and discharges at the hot springs. While other combinations of subsurface properties and geothermal system configurations may fit the temperature distribution equally well, we demonstrate that the low-enthalpy system is reasonably explained based largely on interpretation of surface geophysical data and relatively simple models.

Karckar Region↗

USGS2016

The chemistry and isotopic composition of waters in the low-enthalpy geothermal system of Cimino-Vico Volcanic District, Italy

Geothermal energy exploration is based in part on interpretation of the chemistry, temperature, and discharge rate of thermal springs. Here we present the major element chemistry and the δD, δ 18 O, 87 Sr/ 86 Sr and δ 11 B isotopic ratio of groundwater from the low-enthalpy geothermal system near the city of Viterbo in the Cimino-Vico volcanic district of west-Central Italy. The geothermal system hosts many thermal springs and gas vents, but the resource is still unexploited. Water chemistry is controlled by mixing between low salinity,HCO 3 -rich fresh waters (<24.2°C) flowing in shallow volcanic rocks and SO 4 -rich thermal waters (25.3°C to 62.2°C) ascending from deep, high permeability Mesozoic limestones. The (equivalent) SO 4 /Cl (0.01–0.02), Na/Cl (2.82–5.83) and B/Cl ratios (0.02–0.38) of thermal waters differs from the ratios in other geothermal systems from Central Italy, probably implying a lack of hydraulic continuity across the region. The δ 18 O (−6.6‰ to −5.9‰) and δD (−40.60‰ to −36.30‰) isotopic composition of spring water suggest that the recharge area for the geothermal system is the summit region of Mount Cimino. The strontium isotope ratios ( 87 Sr/ 86 Sr) of thermal waters (0.70797–0.70805) are consistent with dissolution of the Mesozoic evaporite-carbonate units that constitute the reservoir, and the ratios of cold fresh waters mainly reflect shallow circulation through the volcanic cover and some minor admixture (<10%) of thermal water as well. The boron isotopic composition (δ 11 B) of fresh waters (−5.00 and 6.12‰) is similar to that of the volcanic cover, but the δ 11 B of thermal waters (−8.37‰ to −4.12‰) is a mismatch for the Mesozoic reservoir rocks and instead reflects dissolution of secondary boron minerals during fluid ascent through flysch units that overlie the reservoir. A slow and tortuous ascent enhances extraction of boron but also promotes conductive cooling, partially masking the heat present in the reservoir. Overall data from this study is consistent with previous studies that concluded that the geothermal system has a large energy potential.

Journal of Volcanology and Geothermal Research↗

USGS2018

Which geologic factors control permeability development in geothermal systems? The geologic structure of Dixie Valley

Geothermal systems occur where subsurface permeability and temperature are sufficiently high to drive fluid circulation. In the Great Basin region of the United States, which hosts ~20% of domestic geothermal electricity generation capacity and much of the projected undeveloped and undiscovered resource, crustal heat flow is relatively high, so permeability is the dominant factor controlling the occurrence or absence of a geothermal system. In the most general sense, fracture permeability along faults and/or networks of interconnected faults and fractures serves as a pathway for fluids upwelling from depth. Within the Great Basin, Dixie Valley hosts an anomalously high number of geothermal systems. It is unclear whether this relatively dense collection of systems is associated with regional strain or structural patterns, local structural or geological characteristics, basin hydrogeology, or some other factors. The relatively rich data-set available for Dixie Valley, and the well-studied nature of the area affords the opportunity to characterize the geologic and lithologic factors that control permeability development at the local scale. There are at least eleven distinct geothermal systems in Dixie Valley, NV. We utilize a wealth of existing data, which have been collected over several decades, to assess the geologic controls on geothermal fluid upwelling in these systems.

Geothermal Resources Council Transactions↗

USGS2022

A geophysical characterization of structure and geology of the Northern Granite Springs Valley Geothermal System, Northwestern Nevada

The northern Granite Springs Valley in northwestern Nevada is the focus of recent studies for its potential for hosting undiscovered geothermal resources. Although the area lacks definitive surface manifestations of an active hydrothermal system, previous studies identify this region as having potential for hosting a blind geothermal resource, based on elevated subsurface temperatures and a favorable structural framework of the area. As part of the Nevada Play Fairway Project, we conducted high resolution geophysical surveys to better characterize the valley’s geothermal resources. This included ground magnetic, gravity, magnetotelluric, and rock property studies aimed at mapping and modeling subsurface geology and structure. Various derivative and filtering methods were employed to delineate buried faults and contacts from gravity and magnetic data. A depth to basement gravity inversion reveals that the basin is deepest on the west side of the valley. Flanking the basin to the east is a prominent gravity high interpreted as an intra-basin horst. A new high-resolution ground magnetic survey reveals a prominent elongate NW-trending magnetic high, interpreted as an unexposed subsurface dike swarm situated near the boundary between the basin and horst and confined to basement. Geophysical models help constrain basin fill comprised of Cenozoic sediments and volcanic rocks. These overlie Mesozoic crystalline basement that, in the west, consists of Cretaceous granitic intrusives and, to the east, dominantly Mesozoic metasedimentary rocks. The contact between these basement lithologies is not certain but inferred to coincide with the geophysically mapped dike swarm. This is partly supported by the fact that the dikes, as projected along strike to the northwest, intersect the contact between the Cretaceous intrusions and older Mesozoic basement rocks to the north of the study area. Although the age of the inferred dike swarm is not known, the trend of the anomaly is consistent with some of the Tertiary dikes in the nearby Sahwave Range, suggesting emplacement predated or was coeval with early development of the basin. The coincidence of the geothermal system, horst, dike swarm, and terminating normal fault zone suggests that basin tectonics and hydrothermal activity were influenced by both pre-existing basement structure and recent deformation. This relationship may pertain more generally to other hydrothermal settings throughout the Great Basin. If so, future efforts focused on mapping basement geology and structure may prove important to understanding underlying structural controls on geothermal systems. This work is supporting the next phase of research involving additional 3D geophysical and geologic modeling under the U.S. Department of Energy funded INGENIOUS project. The focus of this new work is on the western flank and structural corners of the horst block, based on evidence from detailed geophysical structural mapping, new shallow temperature data, and detailed 3D geologic and geophysical modeling, all aimed at identifying sites for temperature gradient drilling that may intersect zones with sufficient permeability and temperature to support geothermal development.

Geothermal Resources Council Transactions↗

USGS2015

Relationships among seismic velocity, metamorphism, and seismic and aseismic fault slip in the Salton Sea Geothermal Field region

The Salton Sea Geothermal Field is one of the most geothermally and seismically active areas in California and presents an opportunity to study the effect of high-temperature metamorphism on the properties of seismogenic faults. The area includes numerous active tectonic faults that have recently been imaged with active source seismic reflection and refraction. We utilize the active source surveys, along with the abundant microseismicity data from a dense borehole seismic network, to image the 3-D variations in seismic velocity in the upper 5 km of the crust. There are strong velocity variations, up to ~30%, that correlate spatially with the distribution of shallow heat flow patterns. The combination of hydrothermal circulation and high-temperature contact metamorphism has significantly altered the shallow sandstone sedimentary layers within the geothermal field to denser, more feldspathic, rock with higher P wave velocity, as is seen in the numerous exploration wells within the field. This alteration appears to have a first-order effect on the frictional stability of shallow faults. In 2005, a large earthquake swarm and deformation event occurred. Analysis of interferometric synthetic aperture radar data and earthquake relocations indicates that the shallow aseismic fault creep that occurred in 2005 was localized on the Kalin fault system that lies just outside the region of high-temperature metamorphism. In contrast, the earthquake swarm, which includes all of the M > 4 earthquakes to have occurred within the Salton Sea Geothermal Field in the last 15 years, ruptured the Main Central Fault (MCF) system that is localized in the heart of the geothermal anomaly. The background microseismicity induced by the geothermal operations is also concentrated in the high-temperature regions in the vicinity of operational wells. However, while this microseismicity occurs over a few kilometer scale region, much of it is clustered in earthquake swarms that last from hours to a few days and are localized near the MCF system.

Salton Sea↗

USGS2002

A Geochemical Trophic Cascade in Yellowstone's Geothermal Environments

We contrast the geochemistry of the Madison drainage, which has high concentrations of geothermal features, with the Lamar drainage of Yellowstone National Park, USA, and trace the consequences of geochemical differences through abiotic and biotic linkages in the ecosystem. Waters in the geothermal-dominated drainage contained anomalously high levels of fluoride (F) and silica (SiO 2 ). Soils, stream sediments, and surface waters that interact or mix with geothermal waters, in turn, had elevated F and SiO 2 concentrations compared to similar samples from the Lamar drainage. The geochemical differences were reflected in the chemistry of forage plants, with some plants from geothermally influenced areas containing four- to eightfold higher concentrations of F and SiO 2 than similar plants in the Lamar drainage. Geothermal heat reduced snowpack, and we found that elk (Cervus elaphus) concentrated in these refugia as snowpack increased each winter. The consequent high dietary intake of F in animals associated with the geothermal areas was confirmed by the finding that bone samples from elk living in the Madison drainage contained sixfold higher concentrations of F than samples collected from animals wintering in the Lamar drainage. High F exposure resulted in compromised dentition due to fluoride toxicosis, which was undoubtedly exacerbated by the abrasive action of silica. The consequent accelerated and aberrant tooth wear resulted in early onset of senescence, reduced life span, and an abbreviated age structure. We speculate that these altered demographics, combined with spatial heterogeneity of snowpack, will result in increased vulnerability of this large herbivore population to wolf predation and less resiliency to compensate demographically for predation.

Wyoming↗

USGS1993

Subsurface temperatures and geothermal gradients on the north slope of Alaska

On the North Slope of Alaska, geothermal gradient data are available from high-resolution, equilibrated well-bore surveys and from estimates based on well-log identification of the base of ice-bearing permafrost. A total of 46 North Slope wells, considered to be in or near thermal equilibrium, have been surveyed with high-resolution temperatures devices and geothermal gradients can be interpreted directly from these recorded temperature profiles. To augment the limited North Slope temperature data base, a new method of evaluating local geothermal gradients has been developed. In this method, a series of well-log picks for the base of the ice-bearing permafrost from 102 wells have been used, along with regional temperature constants derived from the high-resolution stabilized well-bore temperature surveys, to project geothermal gradients. Geothermal gradients calculated from the high-resolution temperature surveys generally agree with those projected from known ice-bearing permafrost depths over most of the North Slope. Values in the ice-bearing permafrost range from ≈ 1.5° C 100 m "> ≈1.5°C100m in the Prudhoe Bay area to ≈ 4.5δ C 100 m "> ≈4.5 δ C100m in the east-central portion of the National Petroleum Reserve in Alaska. Geothermal gradients below the ice-bearing permafrost sequence range from ≈ 1.6° C 100 m "> ≈1.6°C100m to ≈ 5.2° C 100 m "> ≈5.2°C100m .

Alaska↗

USGS2018

Stress concentrations at structural discontinuities in active fault zones in the western United States: Implications for permeability and fluid flow in geothermal fields

Slip can induce concentration of stresses at discontinuities along fault systems. These structural discontinuities, i.e., fault terminations, fault step-overs, intersections, bends, and other fault interaction areas, are known to host fluid flow in ore deposition systems, oil and gas reservoirs, and geothermal systems. We modeled stress transfer associated with slip on faults with Holocene-to-historic slip histories at the Salt Wells and Bradys geothermal systems in western Nevada, United States. Results show discrete locations of stress perturbation within discontinuities along these fault systems. Well field data, surface geothermal manifestations, and subsurface temperature data, each a proxy for modern fluid circulation in the fields, indicate that geothermal fluid flow is focused in these same areas where stresses are most highly perturbed. These results suggest that submeter- to meter-scale slip on these fault systems generates stress perturbations that are sufficiently large to promote slip on an array of secondary structures spanning the footprint of the modern geothermal activity. Slip on these secondary faults and fractures generates permeability through kinematic deformation and allows for transmission of fluids. Still, mineralization is expected to seal permeability along faults and fractures over time scales that are generally shorter than either earthquake recurrence intervals or the estimated life span of geothermal fields. This suggests that though stress perturbations resulting from fault slip are broadly important for defining the location and spatial extent of enhanced permeability at structural discontinuities, continual generation and maintenance of flow conduits throughout these areas are probably dependent on the deformation mechanism(s) affecting individual structures.

GSA Bulletin↗

USGS1979

Assessment of geothermal resources of the United States: 1978

The geothermal resource assessment presented in this Circular is a refinement and updating of USGS Circular 726. Nonproprietary information available in June 1978 is used to assess geothermal energy in the ground and, when possible, to evaluate the fraction that might be recovered at the surface. Five categories of geothermal energy are discussed: a. Conduction-dominated regimes b. Igneous-related geothermal systems c. High-temperature (>1500C) and intermediate-temperature (90°-150°C) hydrothermal convection systems d. Low-temperature (<900C) geothermal waters e. Geopressured-geothermal energy (both thermal energy and energy from dissolved methane). Assessment data are presented on three colored maps prepared in cooperation with the National Oceanic and Atmospheric Administration.

Circular↗

USGS2008

A Review of Methods Applied by the U.S. Geological Survey in the Assessment of Identified Geothermal Resources

The U. S. Geological Survey (USGS) is conducting an updated assessment of geothermal resources in the United States. The primary method applied in assessments of identified geothermal systems by the USGS and other organizations is the volume method, in which the recoverable heat is estimated from the thermal energy available in a reservoir. An important focus in the assessment project is on the development of geothermal resource models consistent with the production histories and observed characteristics of exploited geothermal fields. The new assessment will incorporate some changes in the models for temperature and depth ranges for electric power production, preferred chemical geothermometers for estimates of reservoir temperatures, estimates of reservoir volumes, and geothermal energy recovery factors. Monte Carlo simulations are used to characterize uncertainties in the estimates of electric power generation. These new models for the recovery of heat from heterogeneous, fractured reservoirs provide a physically realistic basis for evaluating the production potential of natural geothermal reservoirs.

Open-File Report↗

USGS2015

Hydrogeology of the Owego-Apalachin Elementary School geothermal fields, Tioga County, New York

The hydrogeology of the Owego-Apalachin Elementary School geothermal fields, which penetrate saline water and methane in fractured upper Devonian age bedrock in the Owego Creek valley, south-central New York, was characterized through the analysis of drilling and geophysical logs, water-level monitoring data, and specific-depth water samples. Hydrogeologic insights gained during the study proved beneficial for the design of the geothermal drilling program and protection of the overlying aquifer during construction, and may be useful for the development of future geothermal fields and other energy-related activities, such as drilling for oil and natural gas in similar fractured-bedrock settings. The southwest geothermal field consists of 204 closed-loop wells that penetrate a major saline water-bearing zone associated with bedding-plane fractures near the middle of an interbedded sandstone and shale interval at depths of 238 to 263 feet below land surface (ft bls). The northeast geothermal field consists of 80 closed-loop wells that penetrate a major saline water-bearing zone associated with bedding-plane fractures near the base of the interbedded sandstone and shale interval at depths of 303 to 323 ft bls. Transmissivity estimates for the major saline water-bearing fractured zones range from 735 to 3,400 feet squared per day. The saline water-bearing zone in the southwest field is hydraulically connected over a horizontal distance of at least 350 feet. The hydraulic connection between subhorizontal, stacked bedding-plane fractures is limited by the number and transmissivity of interspersed higher angle fractures; locally, greater stratigraphic separation results in reduced connectivity to a greater degree than does horizontal distance. The specific conductance of the saline water from the shallower fractured zone in the southwest field was about 16,000 microsiemens per centimeter at 25 degrees Celsius (μS/cm at 25°C), and that from the fractured zone in the northeast field was about 65,000 μS/cm at 25°C. The saline waters were characterized by a chemical composition similar to that of deep formation brines collected from oil and gas wells in the Appalachian Basin. About 40 percent of the geothermal wells discharged methane gas to land surface during and (or) following drilling. Sandstone beds at depths of 348 to 378 ft bls are the likely source of the methane gas, which was determined to be early thermogenic in origin.

New York↗

USGS1976

Hydrologic reconnaissance of the geothermal area near Klamath Falls, Oregon

Geothermal phenomena observed in the vicinity of Klamath Falls include hot springs with temperatures that approach 204°F (96 o C) (the approximate boiling temperature for the altitude), steam and water wells with temperatures that exceed 212°F (100°C), and hundreds of warm-water wells with temperatures mostly ranging from 68° to 95°F (20° to 35°C). Although warm waters are encountered by wells throughout much of the 350 square miles (900 square kilometers) of the area studied, waters with temperatures exceeding 140°F (60°C) are confined to three relatively restricted areas, the northeast part of the City of Klamath Falls, Olene Gap, and the southwest flank of the Klamath Hills. The hot waters are located near, and are presumably related to, major fault and fracture zones of the Basin and Range type. The displaced crustal blocks are composed of basaltic flow rocks and pyroclastics of Miocene to Pleistocene age, and of sediments and basalt flows of the Yonna Formation of Pliocene age. Dip-slip movement along the high-angle faults may be as much as 6,000 feet (1,800 meters) at places. Shallow ground water of local meteoric origin moves through the upper 1,000 to 1,500 feet (300 to 450 meters) of sediments and volcanic rocks at relatively slow rates. A small amount of ground water, perhaps 100,000 acre feet (1.2 x 10 8 cubic meters) per year, leaves the area in flow toward the southwest, but much of the ground water is discharged as evapotranspiration within the basin. Average annual precipitation on 7,317 square miles (18,951 square kilometers) of land surface near Klamath Falls is estimated to be 18.16 inches (461 millimeters), of which between 12 and 14 inches (305 and 356 millimeters) is estimated to be lost through evapotranspiration. Within the older basaltic rocks of the area, hydraulic conductivities are greater than in the shallow sediments, and ground water may move relatively freely parallel to the northwest-southeast structural trend. Recharge to the geothermal systems probably occurs as water, in the deeper basalt rocks, penetrating downward along the extensive fracture zones that transect the area. Shallow meteoric water that is assumed to be the source of the thermal waters has low dissolved-solids concentrations generally dominated by calcium and bicarbonate. During its passage through the geothermal reservoir, the water gains dissolved solids in amounts up to about 900 milligrams per liter. Sodium and sulfate become the dominant ions. Chloride concentrations remain relatively low, and silica concentrations increase from an average of about 35 milligrams per liter to about 100 milligrams per liter. Both cation ratios and silica concentrations in the hot waters indicate that reservoir temperatures are relatively low. The estimate arrived at in this study for the minimum reservoir temperature is 130°C. Silica concentrations are probably more reliable than cation ratios for estimates of reservoir temperatures for these waters. Other chemical indicators, including oxygen and deuterium isotopes, are consistent in indicating that reservoir temperatures are probably not much greater than the minimum estimate. Temperature distributions and heat flows in the shallow rocks of the area are strongly influenced by convective flow of water. Most observed temperature gradients and estimated heat flows are believed to be unreliable as indicators of conditions in or directly above the thermal reservoir. Some evidence from temperature profiles suggests, however, that heat flow in the Lower Klamath Lake basin is about 1.4 microcalories per square centimeter per second (1.4 HFU), a value that is near the minimum expected for the Basin and Range province. The net thermal flux discharged from springs and wells in the area is estimated to be on the order of 2 x 10 6 calories per second. Discharge by thermal waters into the shallow ground-water system beneath land surface may be many times this amount. Reportedly, at present only about 1,300 calories per second of geothermal heat is being put to beneficial use in the area. A conceptual model of the geothermal system at Klamath Falls suggests that most of the observed phenomena result from transport of heat in a convective hot-water system closely related to the regional fault system. Temperatures at shallow depths are elevated above normal both by convective transport and by blockage of heat flow in sediments of low thermal conductivities. Circulation of meteoric water to depths of 10,000 to 14,000 feet (3,000 to 4,300 meters) could account for the temperatures that probably exist in the thermal reservoir, assuming temperature gradients of 30° to 40°C per kilometer in a crustal zone of normal conductive heat flow. Circulation to shallower depths may be sufficient to warm the water to the required temperatures assuming the more likely conditions of convective transport of heat and the insulating effect of overlying sediments. Heat contents in the shallow hot-water system (<3 kilometers depth) are probably in the range 12 x 10 18 calories to 36 x 10 18 calories. The geothermal resource at Klamath Falls may, therefore, be one of the largest in the United States.

Oregon↗

USGS2020

Play fairway analysis in geothermal exploration: The Snake River plain volcanic province

The Snake River volcanic province (SRP) has long been considered a target for geothermal development. It overlies a thermal anomaly that extends deep into the mantle and represents one of the highest heat flow provinces in North America, but systematic exploration been hindered by lack of a conceptual model. Play Fairway Analysis (PFA) is a methodology adapted from the petroleum industry that integrates data at the regional or basin scale to define favorable plays for exploration in a systematic fashion. The success of play fairway analysis in geothermal exploration depends critically on defining a systematic methodology that is grounded in theory and adapted to the geologic and hydrologic framework of real geothermal systems. This study focused on identifying three critical resource parameters for exploitable hydrothermal systems in the Snake River Plain: heat source, reservoir and recharge permeability, and cap or seal. Data included in the compilation for Heat were heat flow, the distribution and ages of volcanic vents, groundwater temperatures, thermal springs and wells, helium isotope anomalies, and reservoir temperatures estimated using geothermometry. Permeability was derived from stress orientations and magnitudes, post-Miocene faults, and subsurface structural lineaments based on magnetic and gravity data. Data for Seal included the distribution of impermeable lake sediments and clay-seal associated with hydrothermal alteration below the regional aquifer. These data were used to compile Common Risk Segment (CRS) maps for Heat, Permeability and Seal, which were combined to create a Composite Common Risk Segment (CCRS) map for all of southern Idaho that reflects the risk associated with geothermal resource exploration and helps to identify favorable resource tracks. Our data suggests that important undiscovered geothermal resources may be located in several areas of the SRP, including the western SRP (associated with buried lineaments capped by lacustrine sediment), at lineament intersections in the central SRP, and along the margins of the eastern SRP. These blind resources are associated with temperatures sufficient to support electricity production, and may be exploitable with existing deep drilling technology. We are testing our methodology by drilling a geothermal test well in Camas Prairie, ID, confirm our predictions of permeability and reservoir temperature.

Idaho↗

USGS2025

Geothermal potential of orphan oil and gas wells

The United States is estimated to have hundreds of thousands of orphan oil and gas wells. Orphan wells are abandoned wells that are both unremediated and have no responsible operator. While traditionally considered environmental and economic liabilities, orphan oil and gas wells may offer new opportunities in sustainable geothermal energy development. This study evaluates the potential of repurposing orphan wells for geothermal energy production. We analyzed more than 1.4 million bottom-hole temperature (BHT) records from oil and gas well logs to create corrected temperature-depth profiles in a grid across the United States. Total depth values, where available, in documented orphan wells from a U.S. Geological Survey (USGS) database were then correlated to these temperature-depth profiles to estimate a corrected BHT for each orphan well. The orphan wells were then categorized as having low (<90°C), moderate (90–150°C), and high (>150°C) geothermal potential, identifying them as wells in the U.S. that could be used to access geothermal resources. In addition, repurposing these wells could contribute to broader environmental and economic goals, including well remediation, rogue methane emissions reduction, and energy production. This study provides a framework for integrating inactive well inventories with geothermal resource assessments and further highlights the potential for orphan wells to play a transformative role in expanding geothermal energy capacity in the United States.

contiguous United States↗

USGS1979

Regional and local networks of horizontal control, Cerro Prieto geothermal area

The Cerro Prieto geothermal area in the Mexicali Valley 30 km southeast of Mexicali, Baja California, is probably deforming due to (1) the extraction of large volumes of steam and hot water, and (2) active tectonism. Two networks of precise horizontal control were established in Mexicali Valley by the U.S. Geological Survey in 1977 - 1978 to measure both types of movement as they occur. These networks consisted of (1) a regional trilateration net brought into the mountain ranges west of the geothermal area from survey stations on an existing U.S. Geological Survey crustal-strain network north of the international border, and (2) a local net tied to stations in the regional net and encompassing the area of present and planned geothermal production. Survey lines in this net were selected to span areas of probable ground-surface movements in and around the geothermal area. Electronic distance measuring (EDM) instruments, operating with a modulated laser beam, were used to measure the distances between stations in both networks. The regional net was run using a highly precise long-range EDM instrument, helicopters for transportation of men and equipment to inaccessible stations on mountain peaks, and a fixed wing airplane flying along the line of sight. Precision of measurements with this complex long-range system approached 0-2 ppm of line length. The local net was measured with a medium-range EDM instrument requiring minimal ancillary equipment. Precision of measurements with this less complex system approached 3 ppm for the shorter line lengths. The detection and analysis of ground-surface movements resulting from tectonic strains or induced by geothermal fluid withdrawal is dependent on subsequent resurveys of these networks. ?? 1979.

Geothermics↗

USGS1981

Vitrinite reflectance geothermometry and apparent heating duration in the Cerro Prieto geothermal field

Vitrinite reflectance measured in immersion oil ( R o ) on kerogen extracted from hydrothermally altered mudstones in borehole M-84 at the Cerro Prieto geothermal field exhibit an increase in mean reflectance ( R o "> R o ) from 0.12 per cent at 0.24 km depth to 4.1 per cent at 1.7 km depth. Downhole temperatures measured over this interval increase from about 60° to 340°C. These R o "> R o data plotted against temperature fall along an exponential curve with a coefficient of determination of about 0.8. Other boreholes sampled in the field show similar relationships. A regression curve calculated for temperature and R o "> R o in borehole M-105 correctly predicts temperatures in other boreholes within the central portion of the geothermal system. The correlation between the reflectance values and logged temperature, together with consistent temperature estimates from fluid inclusion and oxygen isotope geothermometry, indicates that changes in R o "> R o are an accurate and sensitive recorder of the maximum temperature attained. Therefore, vitrinite reflectance can be used in this geothermal system to predict the undisturbed temperature in a geothermal borehole during drilling before it regains thermal equilibrium. Although existing theoretical functions which relate R o "> R o to temperature and duration of heating are inaccurate, empirical temperature- R o "> R o curves are still useful for geothermometry. A comparison of temperature- R o "> R o regression curves derived from nine boreholes within the Cerro Prieto system suggests that heating across the central portion of the field occurred penecontemporaneously, but varies near margins. Boreholes M-93 and M-94 appear to have cooled from their maximum temperatures, whereas M-3 and Prian-1 have only recently been heated. Comparison of the temperature- R o "> R o data from the Salton Sea, California, geothermal system indicates that the duration of heating has been longer there than at the Cerro Prieto field.

Geothermics↗

USGS2024

Geothermal Play Fairway Analysis, Part 2: GIS methodology

Play Fairway Analysis (PFA) in geothermal exploration originates from a systematic methodology developed within the petroleum industry and is based on a geologic, geophysical, and hydrologic framework of identified geothermal systems. We tailored this methodology to study the geothermal resource potential of the Snake River Plain and surrounding region, but it can be adapted to other geothermal resource settings. We adapted the PFA approach to geothermal resource exploration by cataloging the critical elements controlling exploitable hydrothermal systems, establishing risk matrices that evaluate these elements in terms of both probability of success and level of knowledge, and building a code-based ‘processing model’ to process results.

Geothermics↗

USGS2000

Origin of rainwater acidity near the Los Azufres geothermal field, Mexico

The chemical and isotopic compositions of rainwater were monitored at Los Azufres geothermal field (88 MWe) and its surroundings during May - September 1995, which is the rainy season. Samples were collected from eight sites: three within the field, three in its surroundings and two sufficiently far from the field such that they have no geothermal input. The concentrations of Cl-, SO42- and NO3- were measured in about 350 samples and found to be generally <5 ppm. Chloride concentrations remained constant with time, but sulfate and nitrate concentrations decreased, which suggests a nearby industrial source for the sulfate and nitrate. A mixing model for Cl-, SO42- and ??34S also suggests an industrial source for the rainwater sulfur. The determination of pH was found to be necessary, but is not sufficient to characterize rainwater acidity. The Gran titration method was used to determine alkalinity with respect to equivalence point of H2CO3(*). Values of alkalinity were found to range from 10-4 to 10-6 eq/L, and were negative only for some samples from Vivero and Guadalajara. Thus, SO42- and NO3- are in general not in acidic form (i.e. balanced by Na+, Ca2+, etc. rather than H+). Sulfate ??34S values were about -1.5??? in Los Azufres and its surroundings, and in Morelia, but differed from the value of -0.2??? for Guadalajara. The ??34S values for H2S from the Los Azufres geothermal wells are in the range -3.4 to 0.0???. The ??34S ranges for the natural and anthropogenic sources for environmental sulfur overlap, making it difficult to differentiate between the contribution of different sources. However, a similarity of values of ??34S at Los Azufres and Morelia (85 km distant) suggest a regional source of sulfate that is not associated with geothermal emissions from Los Azufres. (C) 2000 Published by Elsevier Science Ltd on behalf of CNR.The chemical compositions of rainwater were analyzed at Los Azufres geothermal field in Spain from May-September 1995. The concentrations of Cl-, SO42- and NO3- were measured and found to be generally <5 ppm. Chloride concentrations remained constant with time, but sulfate and nitrate concentrations decreased, suggesting a nearby industrial source. A mixing model for Cl-, SO42-, and ??34S also suggested an industrial source for the rainwater sulfur.

Geothermics↗