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At least 343 records · Page 19Linked to original sources
USGS2012

The Galapagos Spreading Centre at 86° W: A detailed geothermal field study

We report here measurements of the heat flow field of the Galapagos Spreading Center on crust of age less than 1.0 m.y. The 443 measurements in an area of about 570 km 2 reveal the general planform of the geothermal flux and permit the first truly areal estimate of the near-axis conductive heat flux. The intrusion process and associated hydrothermal circulation dominate the surface heat flow pattern, with circulation apparently continuing beyond the limits of our survey. The areal average of the conductive heat flux is 7.1 ± 0.8 HFU (295 ± 33 mW/m 2 ), about one-third the heat flux predicted by plate models. The remaining heat is apparently removed by venting of hydrothermal waters at the spreading axis and through basalt outcrops and hydrothermal mounds off axis. The pattern of surface heat flux is lineated parallel to the axis and the strongly lineated topography. Sharp lateral gradients in the heat flow, greater than 10 HFU/km near escarpments and commonly expressed as high heat flow at the tops of the scarps and lower heat flow in the valleys, may indicate a local concentration of the circulation by surface fault systems and/or variable sediment thickness.

Journal of Geophysical Research Solid Earth↗
USGS2012

Permafrost, heat flow, and the geothermal regime at Prudhoe Bay, Alaska

Temperature measurements through permafrost in the oil field at Prudhoe Bay, Alaska, combined with laboratory measurements of the thermal conductivity of drill cuttings permit an evaluation of in situ thermal properties and an understanding of the general factors that control the geothermal regime. A sharp contrast in temperature gradient at ∼600 m represents a contrast in thermal conductivity caused by the downward change from interstitial ice to interstitial water at the base of permafrost under near steady state conditions. Interpretation of the gradient contrast in terms of a simple model for the conductivity of an aggregate yields the mean ice content (∼39%), and thermal conductivities for the frozen and thawed sections (8.1 and 4.7 mcal/cm s °C, respectively). These results yield a heat flow of ∼1.3 HFU, which is similar to other values on the Alaskan Arctic Coast; the anomalously deep permafrost is a result of the anomalously high conductivity of the siliceous ice-rich sediments. Curvature in the upper 160 m of the temperature profiles represents a warming of ∼1.8°C of the mean surface temperature and a net accumulation of 5–6 kcal/cm 2 by the solid earth surface during the last 100 years or so. Rising sea level and thawing of ice-rich sea cliffs probably caused the shoreline to retreat tens of kilometers in the last 20,000 years, inundating a portion of the continental shelf that is presently the target of intensive oil exploration. A simple conduction model suggests that this recently inundated region is underlain by near-melting ice-rich permafrost to depths of 300–500 m; its presence is important to seismic interpretations in oil exploration and to engineering considerations in oil production. With confirmation of the permafrost configuration by offshore drilling, heat conduction models can yield reliable new information on the chronology of arctic shorelines.

Journal of Geophysical Research Solid Earth↗
USGS2012

Reservoir processes and fluid origins in the Baca geothermal system, Valles Caldera, New Mexico

At the Baca geothermal field in the Valles caldera, New Mexico, 19 deep wells were drilled in an attempt to develop a 50-MW e (megawatts electric) power plant. The chemical and isotopic compositions of steam and water samples have been used to indicate uniquely the origin of reservoir fluids and natural reservoir processes. Two distinct reservoir fluids exist at Baca. These fluids originate from the same deep, high-temperature (335°C), saline (2500 mg/kg Cl) parent water but have had different histories during upflow. One fluid (from wells 4 and 13) is isotopically light, high in radiogenic noble gases, CO 2 and HCO 3 , and low in Ca. It has a temperature of 290°–295°C and a reservoir chloride near 1900 mg/kg. This fluid resulted from rapid upward flow through 1.1- to 1.4-m.y.-old Bandelier Tuff reservoir rocks after long residence in pre-Bandelier (>7 m.y.) sediments and Precambrian basement rocks and 25% dilution with high-altitude cold groundwater from Redondo Peak. The other water (from wells 15, 19, and 24) moved slowly through the Bandelier Tuff and cooled conductively (with minor steam loss for well 19) from 335°C to 280°–260°C. Apparently, short residence in old basement rocks has left this water with low radiogenic gases. Conductive cooling without mixing has kept the original chloride and relatively heavy isotope composition of the deep water. The recharge to the deep parent water is not well understood but may be from lower elevation precipitation outside the Valles caldera area. Gases are in equilibrium in all-liquid reservoir fluids at near reservoir temperatures, and the concentrations of atmospheric gases are similar to those of air-saturated water, indicating little boiling and steam loss. All water, solutes, and gases in the reservoir fluids originate from air-saturated meteoric recharge water, watermineral reactions, and rock leaching, with the possible exception of excess 3He that must have an ultimate mantle source. This gas could originate directly from magma or from leaching of intrusive volcanic rocks beneath the Bandelier.

Journal of Geophysical Research Solid Earth↗
USGS2012

Geothermal gradients in the conterminous United States

Geothermal gradients from published temperature/depth measurements in drill holes generally deeper than 600 m are used to construct a temperature gradient map of the conterminous United States. The broadly contoured map displays 284 temperature gradients that are applicable to a depth of 2 km. In terms of the number of contoured areas and the fraction of data points having a value not within a contour interval (outliers), the temperature gradient data and associated deep heat flow data have similar measures of contourability. Areally, most of the United States is contoured from 15° to 35°C/km. The eastern United States is generally cooler (average 25°C/km) than the western United States (average 34°C/km), in accordance with broad heat flow trends. Differences between the temperature gradient and heat flow maps are caused by areal differences in rock thermal conductivities. The effect of conductivity on gradients is particularly apparent in the eastern United States where heat flow is relatively constant over large areas. Gradients are elevated where thick, low-conductivity, sedimentary deposits occur such as in the Atlantic Coastal Plain province and in basins in the Allegheny Plateau and the Great Plains provinces. No clear gradient pattern emerges where both heat flow and conductivity vary widely, such as in the northern Basin and Range and Rocky Mountain provinces. Using the temperature gradients determined in this study and associated heat flow values, derived thermal conductivities are calculated for the depth range of a few hundred meters to 2 km; the average conductivity is 2.5±0.8 W/m °C. Some areas show little variation in derived thermal conductivity, while others show a wide range. The Atlantic Coastal Plain, Appalachian Plateaus, Superior Upland, Gulf Coastal Plain, Columbia Plateaus and Cascade Mountains, and Sierra Nevada provinces have restricted ranges of conductivities, which supports the concept of regional conductivities.

Journal of Geophysical Research Solid Earth↗
USGS2012

Overview of the structure and geothermal potential of Newberry Volcano, Oregon

Evidence from a broad range of geological and geophysical investigations of Newberry Volcano in central Oregon suggests the presence of a small magma body beneath the summit caldera. Drilling has encountered temperatures indicative of a hydrothermal system above the hypothesized magma body. On the basis of hydrothermal mineralogy the temperature of the hydrothermal system is at or near its maximum value, indicating that the hydrothermal system is young. Basaltic underplating is postulated to replenish the magma body. The results of interdisciplinary studies at Newberry Volcano document that young, silicic volcanism in the Cascade Range can have geothermal potential. These results should be applied with caution to the rest of the Cascade Range because of the strong influence of Basin and Range extensional tectonics on Newberry Volcano's style of volcanism.

Journal of Geophysical Research Solid Earth↗
USGS2012

Analysis of geophysical well logs obtained in the State 2-14 borehole, Salton Sea geothermal area, California

A complete suite of conventional geophysical well logs was obtained in the upper part of a 3220-m- deep borehole drilled into geothermally altered alluvial sediments on the southeastern edge of the Salton Sea. In situ temperatures greater than 300°C and an inability to cool parts of the borehole by circulation limited the suite of logs run below 2000 m in depth to deep induction, spontaneous potential, un-calibrated neutron, natural gamma, and temperature. Bottom-hole temperature trends given by repeat temperature logs were extrapolated to undisturbed temperatures approaching 355°C at a depth of 3220 m. Geophysical logs obtained in the State 2–14 borehole indicate that neutron porosity, gamma-gamma, and deep-induction logs provide useful information on lithologic trends with depth. The natural gamma log contains almost continuous, high-frequency fluctuations that obscure lithologic trends and that may be related to recent radioisotope redistribution and departure from radiometric equilibrium. Acoustic transit time logs give unrealistically low in situ compressional velocities ranging from 1.8 to 3.0 km/s, whereas acoustic waveform logs indicate that sediment compressional velocities range from less than 3.0 km/s shallower than 1000 m in depth to almost 5.0 km/s at depths greater than 2000 m. Analyses indicate that most log values lie between two lithologic end points: an electrically conductive claystone with moderate neutron porosity, but no effective porosity, and an electrically nonconductive, fully cemented siltstone that has small but finite porosity. A limited number of clean sandstones depart from this trend; geophysical logs from these sandstones indicate an effective porosity ranging from 5 to 14%, and saturation with brines having equivalent NaCl concentrations greater than 100,000 mg/L. Depth- averaged trends in neutron porosity and deep-induction logs, along with trends in acoustic velocity determined from acoustic waveform logs, demonstrate that major changes in the properties of alluvial sediments occur within the depth range from 1200 to 1800 m. Although caliper logs were not obtained deeper than 2000 m, resistivity values less than 10 ohm m at those depths probably correspond to borehole enlargements in production zones rather than local increases in effective porosity. The transition in sediment properties indicated by the geophysical logs in the depth interval from 1200 to 2000 m apparently represents a detailed vertical profile of the transition from relatively unaltered clay minerals in alluvial sediments to electrically nonconductive alteration products such as epidote and feldspar.

Journal of Geophysical Research Solid Earth↗
USGS2017

Potentially exploitable supercritical geothermal resources in the ductile crust

The hypothesis that the brittle–ductile transition (BDT) drastically reduces permeability implies that potentially exploitable geothermal resources (permeability >10 −16 m 2 ) consisting of supercritical water could occur only in rocks with unusually high transition temperatures such as basalt. However, tensile fracturing is possible even in ductile rocks, and some permeability–depth relations proposed for the continental crust show no drastic permeability reduction at the BDT. Here we present experimental results suggesting that the BDT is not the first-order control on rock permeability, and that potentially exploitable resources may occur in rocks with much lower BDT temperatures, such as the granitic rocks that comprise the bulk of the continental crust. We find that permeability behaviour for fractured granite samples at 350–500 °C under effective confining stress is characterized by a transition from a weakly stress-dependent and reversible behaviour to a strongly stress-dependent and irreversible behaviour at a specific, temperature-dependent effective confining stress level. This transition is induced by onset of plastic normal deformation of the fracture surface (elastic–plastic transition) and, importantly, causes no ‘jump’ in the permeability. Empirical equations for this permeability behaviour suggest that potentially exploitable resources exceeding 450 °C may form at depths of 2–6 km even in the nominally ductile crust.

Nature Geoscience↗
USGS2026

Rapid hydrothermal triggering of induced seismicity at the Coso Geothermal Field

The long-term producing Coso Geothermal Field (CGF) in California operates over 100 wells tapping into a reservoir characterized by an extensive fracture network, complex fluid pathways, and regular seismic activity. Understanding the interaction between seismicity and injection can shed important light on the hydrothermal characteristics of the field. Here, we analyze 15 years of local seismic and daily operational data from the CGF, identifying a strong correlation between short-term increase in seismicity rate and seasonal volumetric and temperature variations in the reinjected fluid. Furthermore, the seismic footprint during peak injection of colder fluids reveals a near-instantaneous response up to 2 km away from the injection well, too rapid for pore pressure diffusion alone. This short-term and distant response is observed to have directional preference, indicating structural or permeability anisotropy within the reservoir. Additionally, the seismic response correlates with the initial volumetric increase of colder fluids, but also with temperature decrease during stable injection periods, suggesting thermal effects alone can play an important role in triggering distant seismicity.

California↗
USGS2016

Compartmentalization of the Coso East Flank geothermal field imaged by 3-D full-tensor MT inversion

Previous magnetotelluric (MT) studies of the high-temperature Coso geothermal system in California identified a subvertical feature of low resistivity (2–5 Ohm m) and appreciable lateral extent (>1 km) in the producing zone of the East Flank field. However, these models could not reproduce gross 3-D effects in the recorded data. We perform 3-D full-tensor inversion and retrieve a resistivity model that out-performs previous 2-D and 3-D off-diagonal models in terms of its fit to the complete 3-D MT data set as well as the degree of modelling bias. Inclusion of secondary Z xx and Z yy data components leads to a robust east-dip (60†) to the previously identified conductive East Flank reservoir feature, which correlates strongly with recently mapped surface faults, downhole well temperatures, 3-D seismic reflection data, and local microseismicity. We perform synthetic forward modelling to test the best-fit dip of this conductor using the response at a nearby MT station. We interpret the dipping conductor as a fractured and fluidized compartment, which is structurally controlled by an unmapped blind East Flank fault zone.

Coso Range↗
USGS2002

A landscape ecology approach to assessing development impacts in the tropics: A geothermal energy example in Hawaii

Geographic information systems (GIS) are increasingly being used in environmental impact assessments (EIA) because GIS is useful for analysing spatial impacts of various development scenarios. Spatially representing these impacts provides another tool for landscape ecology in environmental and geographical investigations by facilitating analysis of the effects of landscape patterns on ecological processes and examining change over time. Landscape ecological principles are applied in this study to a hypothetical geothermal development project on the Island of Hawaii. Some common landscape pattern metrics were used to analyse dispersed versus condensed development scenarios and their effect on landscape pattern. Indices of fragmentation and patch shape did not appreciably change with additional development. The amount of forest to open edge, however, greatly increased with the dispersed development scenario. In addition, landscape metrics showed that a human disturbance had a greater simplifying effect on patch shape and also increased fragmentation than a natural disturbance. The use of these landscape pattern metrics can advance the methodology of applying GIS to EIA.

Singapore Journal of Tropical Geography↗
USGS1963

Geothermal brine well: Mile-deep drill hole may tap ore-bearing magmatic water and rocks undergoing metamorphism

A deep geothermal well in California has tapped a very saline brine extraordinarily high in heavy metals and other rare elements; copper and silver are precipitated during brine production. Preliminary evidence suggests that the brine may be pure magmatic water and an active ore-forming solution. Metamorphism of relatively young rocks may also be occurring within accessible depths.

California↗
USGS1975

Earthquakes, active faults, and geothermal areas in the Imperial Valley, California

A dense seismograph network in the Imperial Valley recorded a series of earthquake swarms along the Imperial and Brawley faults and a diffuse pattern of earthquakes along the San Jacinto fault. Two known geothermal areas are closely associated with these earthquake swarms. This seismicity pattern demonstrates that seismic slip is occurring along both the Imperial-Brawley and San Jacinto fault systems.

California↗
USGS1983

Distribution of differentiated tholeiitic basalts on the lower east rift zone of Kilauea Volcano, Hawaii: A possible guide to geothermal exploration

Geologic mapping of the lower east rift zone of Kilauea Volcano, Hawaii, indicates that more than 100 eruptions have extruded an estimated 10 km 3 of basalt during the past 2,000 yr. Six eruptions in the past 200 yr have extruded about 1 km 3 . The eruptive recurrence interval has ranged from 1 to 115 yr since the middle 18th century and has averaged 20 yr or less over the past 2,000 yr. One hundred new chemical analyses indicate that the erupted tholeiites commonly are differentiated beyond olivine control or are hybrid mixtures of differentiates with more mafic (olivine-controlled) summit magmas. The distribution of vents for differentiated lavas indicates that several large magma chambers underlie the lower east rift zone. Several workers have recognized that a chamber underlies the area near a producing well, HGP-A; petrologic and 14 C data indicate that it has existed for at least 1,300 yr. Stratigraphy, petrology, and surface deformation patterns suggest that two other areas, Heiheiahulu and Kaliu, also overlie large magma chambers and appear to be favorable geothermal prospects.

Geology↗
USGS2011

Origin of a rhyolite that intruded a geothermal well while drilling at the Krafla volcano, Iceland

Magma flowed into an exploratory geothermal well at 2.1 km depth being drilled in the Krafla central volcano in Iceland, creating a unique opportunity to study rhyolite magma in situ in a basaltic environment. The quenched magma is a partly vesicular, sparsely phyric, glass containing ∼1.8% of dissolved volatiles. Based on calculated H 2 O-CO 2 saturation pressures, it degassed at a pressure intermediate between hydrostatic and lithostatic, and geothermometry indicates that the crystals in the melt formed at ∼900 °C. The glass shows no signs of hydrothermal alteration, but its hydrogen and oxygen isotopic ratios are much lower than those of typical mantle-derived magmas, indicating that this rhyolite originated by anhydrous mantle-derived magma assimilating partially melted hydrothermally altered basalts.

Krafla volcano↗
USGS2010

Stress, fracture, and fluid-flow analysis using acoustic and electrical image logs in hot fractured granites of the Coso geothermal field, California, U.S.A.

Acoustic and electrical image logs in fractured granitic rocks penetrated by U.S. Navy well 58A-10, Coso Wash, in the eastern margin of the Coso geothermal field, California, were compared to evaluate their relative ability to characterize fractures and fault rock textures and to measure stress orientations from borehole failure. Electrical image logs are sensitive to variations in mineralogy or porosity, which affect conductivity. Thus, they capture both open and healed natural fractures as well as rock foliation.

California↗
USGS1967

Sulfides associated with the Salton Sea geothermal brine

Concentrated saline brine tapped by a deep well drilled for geothermal power near the Salton Sea, California, deposited metal-rich siliceous scale at the rate of 2 to 3 tons per month. The iron-rich opaline scale contains an average of 20% Cu and up to 6% Ag present in bornite, digenite, chalcopyrite, a new dense polymorph of chalcocite, stromeyerite, and native silver.The brines are in equilibrium with an assemblage of sulfide minerals in the reservoir rocks. Sphalerite containing 16.6 mol % FeS is in equilibrium with the brine in which the activity of sulfur (a (sub s 2 ) ) is estimated at 10 (super -10.2) at 325 degrees C. This is in good agreement with the projected high-temperature data of Barton and Toulmin (2) for the system Fe-Zn-S. Other sulfides identified in the reservoir rocks are pyrite, pyrrhotite, chalcopyrite, and galena.The heavy metals in solution are apparently derived from the sediments of the brine reservoir, being released from the silicate minerals in which they occur in trace amounts, as metamorphism of the sediment proceeds. Although the total of the heavy metals in solution greatly exceeds the sulfur, on a molal basis, the brine is saturated with respect to sulfide components and addition of more sulfide ion would only cause precipitation of sulfides.

California↗
USGS1998

Numerical analysis of seawater circulation in carbonate platforms: I. Geothermal convection

Differences in fluid density between cold ocean water and warm ground water can drive the circulation of seawater through carbonate platforms. The circulating water can be the major source of dissolved constituents for diagenetic reactions such as dolomitization. This study was undertaken to investigate the conditions under which such circulation can occur and to determine which factors control both the flux and the patterns of fluid circulation and temperature distribution, given the expected ranges of those factors in nature. Results indicate that the magnitude and distribution of permeability within a carbonate platform are the most important parameters. Depending on the values of horizontal and vertical permeability, heat transport within a platform can occur by one of three mechanisms: conduction, forced convection, or free convection. Depth-dependent relations for porosity and permeability in carbonate platforms suggest circulation may decrease rapidly with depth. The fluid properties of density and viscosity are controlled primarily by their dependency on temperature. The bulk thermal conductivity of the rocks within the platform affects the conductive regime to some extent, especially if evaporite minerals are present within the section. Platform geometry has only a second-order effect on circulation. The relative position of sealevel can create surface conditions that range from exposed (with a fresh-water lens present) to shallow water (with hypersaline conditions created by evaporation in constricted flow conditions) to submerged or drowned (with free surface water circulation), but these boundary conditions and associated ocean temperature profiles have only a second-order effect on fluid circulation. Deep, convective circulation can be caused by horizon tal temperature gradients and can occur even at depths below the ocean bottom. Temperature data from deep holes in the Florida and Bahama platforms suggest that geothermal circulation is actively occurring today to depths as great as several kilometers.

American Journal of Science↗
USGS1975

Assessment of geothermal resources of the United States, 1975

This assessment of geothermal resources of the United States consists of two major parts: (1) estimates of total heat in the ground to a depth of 10 km and (2) estimates of the part of this total heat that is recoverable with present technology, regardless of price. No attempt has been made to consider most aspects of the legal, environmental, and institutional limitations in exploiting these resouces. In general, the average heat content of rocks is considerably higher in the Western United States than in the East. This also helps to explain why the most favorable hydrothermal convection systems and the hot young igneous systems occur in the West. Resources of the most attractive identified convection systems (excluding national parks) with predicted reservoir temperatures above 150 deg C have an estimated electrical production potential of about 8,000 megawatt century, or about 26,000 megawatt for 30 years. Assumptions in this conversion are: (1) one-half of the volume of the heat reservoirs is porous and permeable, (2) one-half of the heat of the porous, permeable parts is recoverable in fluids at the wellheads, and (3) the conversion efficiency of heat in wellhead fluids to electricity ranges from about 8 to 20 percent , depending on temperature and kind of fluid (hot water or steam). The estimated overall efficiency of conversion of heat in the ground to electrical energy generally ranges from less than 2 to 5 percent, depending on type of system and reservoir temperature. (See also W77-07477)

Circular↗