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Gravity contour map of Mountain Home, Idaho, region (western SRP), with gravity stations shown, along with seismic traverse lines. Proposed drill site sits on southern edge of inferred horst block in subsurface.
Source publication
The Snake River volcanic province (SRP) overlies a thermal anomaly that extends deep into the mantle; it represents one of the highest heat flow provinces in North America, and an area with the highest calculated geothermal gradients. This makes the SRP one of the potentially highest producing geothermal districts in the United States. Elevated hea...
Contexts in source publication
Context 1
... Home AFB sits on the southern edge of this prominent gravity anomaly, as can be seen in the more detailed Bouguer gravity map in Figure 6. This gravity high that has been interpreted by Shervais et al (2002) to represent a buried horst block with the larger western SRP graben. ...
Context 2
... this interpretation requires that we run new seismic reflection surveys in the area adjacent to MH-AFB; these surveys are planned for September 2010. Figure 6 shows the positive gravity anomaly that lies just north of the base, along with planned seismic reflection profiles. These reflection profiles should delineate basement structure in some detail and lead to a clearer understanding of the nature of the prominent positive gravity anomaly. ...
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Conducting groundwater dye tracer tests have proven to be an effective and efficient method for obtaining essential data to characterize the East Snake Plain Aquifer. A dye trace was conducted from a well (Strickland) located in Gooding County, Idaho, at Township 8 South, Range 14 East, Section 36AAA. The injection was video recorded on November 14...
Citations
... Our calculations suggest that some internal subsidence mechanism is needed (i.e., mafic intrusions and/or diking, intrabasin faulting) to accommodate about half of the observed extension and subsidence. High seismic velocities indicate that the WSRP has been heavily intruded by dense material such as basalts, and a large positive Bouguer anomaly in the axis is consistent with an extensive, 20-30-km-wide central basin horst (Hill and Pakiser, 1967;Mabey, 1976;Wood and Clemens, 2002;Shervais et al., 2011;Kessler et al., 2017). Thus, our data, combined with other geologic and geophysical observations, show that intrabasinal faulting and volcanic diking are important mechanisms accommodating Miocene extension in the WSRP. ...
The western Snake River Plain (WSRP) in southwest Idaho has been characterized as an intracontinental rift basin but differs markedly in topography and style from other Cordilleran extensional structures and structurally from the down-warped lava plain of the eastern Snake River Plain. To investigate mechanisms driving extension and topographic evolution, we sampled granitoid bedrock from Cretaceous and Eocene-aged plutons from the mountainous flanks of the WSRP to detail their exhumation history with apatite (U-Th)/He (AHe) thermochronometry. AHe cooling dates from seventeen samples range from 7.9 ± 1.4 Ma to 55 ± 10 Ma. Most cooling dates from Cretaceous plutons adjacent to the WSRP are Eocene, while Eocene intrusions from within the Middle Fork Boise River canyon ~35 km NE of the WSRP yield Miocene cooling dates. The AHe dates provide evidence of exhumation of the Idaho batholith during the Eocene, supporting a high relief landscape at that time, followed by decreasing relief. The Miocene AHe dates show rapid cooling along the Middle Fork Boise River that we take to indicate focused river incision due to base level fall in the WSRP. Eocene AHe dates limit magnitudes of exhumation and extension on the flanks of the WSRP during Miocene rift formation. This suggests extension was accommodated by magmatic intrusions and intrabasin faults rather than basin-bounding faults. We favor a model where WSRP extension was related to Columbia River Flood Basalt eruption and enhanced by later eruption of the Bruneau-Jarbidge and Twin Falls volcanic fields, explaining the apparent difference with other Cordilleran extensional structures.
... Finally, Potter et al. (2018) showcase a time-integrated petrological and geochemical investigation of basalts from the Kimama borehole in the Snake River Plain of central Idaho, USA (Shervais et al., 2011(Shervais et al., , 2012(Shervais et al., , 2013. This rock core links eruptive processes to the construction of mafic intrusions and highlights the cyclic variations in basalt composition caused by temporal chemical heterogeneity related to fractional crystallization and assimilation of previously-intruded mafic sills over 5.5-millionyear history. ...
... An exploration corehole, designated MH-2B, was drilled to a total depth of 1821.5 m on the Mountain Home AFB in the western SRP near Mountain Home, Idaho (Fig. 1). This corehole was one of the three deep (up to two km) holes on the SRP drilled as part of Project Hotspot: the Snake River geothermal drilling project Shervais et al. 2011;Delahunty et al. 2012;Lachmar et al. 2017). ...
Abstract A geothermal exploration corehole was drilled to a total depth of 1821.5 m on the Mountain Home Air Force Base near Mountain Home, Idaho. The corehole was used to collect an unusually large amount of data, including uniaxial compressive stress (UCS) experiments on core samples, to evaluate the geothermal potential of the western Snake River Plain. In addition, unlike many exploration holes in this region, a fluid entry was encountered at 1745.3 m and flowed artesian to the surface. A maximum temperature of 149.4 °C was calculated for the entry. A temperature log run on the corehole from 3 to 1675 m is nearly linear with little variation. The average geothermal gradient is 73 °C/km, and the average heat flow between 200 and 1500 m is 102 ± 15 mW/m2. Chemical analyses of a sample from the fluid entry suggest that a significant proportion of the water is not meteoric. Five geothermometers show equilibrium temperature in the range of 133–157 °C. Furthermore, based on the unconfined UCS experiments on basalt core samples, a brittle unit was found to comprise the fractured reservoir that the geothermal water flows from, while an overlying ductile unit acts as a hydrothermal caprock. This implies that the reservoir/caprock pair may be a target for future exploration wells drilled to delineate the extent of the potential resource and the boundaries of the connected fracture network.
... We would expect some of that offset to be expressed on the faults in the MH-2 borehole. The original interpretation of the potential field geophysics for the MH-2 borehole region was that the drillhole was located on the southwest flank of a 300°-trending gravity high (Shervais et al., 2011(Shervais et al., , 2014a, slightly oblique to the 310°-315°-trending faults at the surface. Reinterpretation of the gravity and aeromagnetic data and new Structure of Western Snake River Plain, Idaho | RESEARCH data (Glen and Bouligand, 2014) indicate the presence of a well-defined 290°-trending gravity high, the crest of which might be offset by north-to northwest-trending features, and the aeromagnetic signature indicates the presence of short-wavelength north-trending anomalies (Glen and Bouligand, 2014). ...
Project HOTSPOT, the Snake River Scientific Drilling Project (International Continental Scientific Drilling Program), tested for deep geothermal resources and examined the petrology of volcanic rocks with three drillholes in the central and western Snake River Plain (western USA). The MH-2 drillhole targeted fractured crystalline and hydrothermally altered basalt in the area of the Mountain Home Air Force Base (Idaho) to a total depth of 1821 m. At 1745 m depth the drillhole encountered flowing artesian hydrothermal fluids of at least 150 °C. We integrate geological analyses of core, image log, and borehole geophysical data, and in situ stress analyses to describe the structural environment that produces permeability for artesian flow. The rocks in the lower 540 m of the drillhole consist of basalt flows as much as 30 m thick, altered basalt, and thin sedimentary horizons. The mechanical stratigraphy is defined by nine mechanical horizons that are in three ranges of rock strength on the basis of experimentally determined strength data, core logging, and geophysical log signatures. Hydrothermal alteration products and mineralization in the core are associated with three highly faulted sections; the lowermost section is associated with the zone of flowing thermal water. Shear slip indicators on faults observed in core indicate slip ranging from pure strike slip to normal failure mechanisms in the stronger horizons. The borehole breakouts indicate that the maximum horizontal stress, SH, is oriented 047° ± 7°, and drilling-induced tensile fractures indicate that SH is oriented at 67° ± 21°.The in situ stress orientations exhibit little variation over the depth of the measured interval, but the SH magnitude varies with depth, and is best explained by an oblique normal fault stress regime.The geomechanical model indicates that if pore pressures at depth are elevated above the normal hydrostatic gradient, as observed here, the system has the potential to deform by mixed normal and strike-slip failure. Our observations and interpretations suggest that the MH-2 borehole was drilled into oblique normal faults that intersect a buried 300°-trending fault block masked by the basaltic volcanic complex. These data indicate that the transition from the central to western Snake River Plain is characterized by complex structures developed in response to a transitional stress state related to Snake River Plain and western Basin and Range stress regimes. The western Basin and Range stress and tectonic regime may extend from northern Nevada into western Idaho and may enhance the potential for geothermal resources by creating interconnected fracture and fault-related permeability at depth.
... The borehole and surface seismic profiles are located along the west flank of Hansen Butte, immediately south of the Snake River (Fig. 2). The site lithology includes basalt and sediment interbeds above 0.4 km and a continuous rhyolite sequence from 0.4 km to a total drilled depth of 1.95 km (Shervais et al. 2011;Fig. 2). ...
... The calculated velocity values for the thin sedimentary interbeds are likely overestimated due to the influence of the high rhyolite seismic velocities that lie above and below the interbeds, and a smoothing filter used to calculate interval velocity values. However, thick sedimentary interbeds observed at depths of 0.4 km and 0.6 km record interval velocities of 2000-3000 m/s and represent fine-grained unconsolidated sediments that were logged within these zones (Shervais et al. 2011). In summary, we observe a general increase in seismic velocity of approximately 1000 m/s per km within the rhyolite sequence and slow seismic velocities that correlate with sedimentary interbeds at the Kimberly site. ...
Hotspot: The Snake River Geothermal Drilling Project was undertaken to better understand geothermal systems across the Snake River Plain volcanic province. A series of surface and borehole seismic profiles were obtained to provide insights into volcanic stratigraphy and test the capabilities of engineering-scale seismic imaging in such terranes. The Kimberly site drilled through 1.9 km of mostly rhyolite, with thin sedimentary interbeds in the upper part of the section. The Kimama site drilled through 1.9 km of mostly basalt with sedimentary interbeds at ∼200 m depth and 1700 m depth. The Mountain Home site contained numerous sediment and volcanic rock layers. Downhole and surface vibroseis seismic results suggest sedimentary interbeds at depth correspond with low-velocity, high-temperature zones that relate to reflections on seismic profiles. Our results suggest that eruption flow volumes can be estimated and flow boundaries can be imaged with surface seismic methods using relatively high-fold and wide-angle coverage. High-frequency attenuation is observed at all sites, and this deficit may be countered by acquisition design and a focus on signal processing steps. Separation of surface and body waves was obtained by muting, and the potential for large static effects was identified and addressed in processing. An accurate velocity model and lithology contacts derived from borehole information improved the confidence of our seismic interpretations.
... Individual fields were active for 2-4 m.y., producing rhyolite accumulations of 1-5 km thick, mostly within nested caldera systems (e.g., Morgan and McIntosh, 2005;Bonnichsen et al., 2008). Limited borehole data at INL (INEL-1 and WO2; Doherty et al., 1979;McCurry and Rodgers, 2008) and Kimberley (Project Hotspot, Shervais et al., 2013;2011) indicate that the rhyolite is relatively dense, and weakly to strongly hydrothermally altered. Additionally, recent petrologic and geochemical work suggests that greater intensity of hydrothermal of rhyolite occurred within many nested caldera systems, further reducing porosity and permeability (Bindeman et al., 2007;Watts et al., 2011). ...
The geologic characteristics and evolution of the Eastern Snake River Plain are consistent with transfer of large amounts of magma advected heat at mid- to upper-crustal levels. We speculate that some of this heat is stored below surficial basalt lavas, within underlying dense, hydrothermally altered rhyolites that infill large nested calderas. Cryptic hydrothermal systems may occur within caldera-related fracture systems, or megabreccias near caldera collapse scarps. Blind hot zones are also possible below young chemically evolved volcanoes on the ESRP. Other factors in addition to high temperatures and high heat flow that support EGS development are also prevalent on the ESRP. A number of deep exploration wells exist, and hundreds groundwater wells provide additional information on the character and makeup of the subsurface. The abundant cold groundwater resource can provide a significant heat sink for power generation, and also provide working fluid and makeup water as necessary. Regional stress and earthquake data suggest that the Plain is extensional to isotropic and "quiet". The seismic risks of the eastern Snake River Plain have been extensively investigated because of the nuclear facilities at Idaho National Laboratory. The epicenter of the 1983 (magnitude 7.3) Borah earthquake was located about 100km (70 miles) from INL's nuclear facilities, but no significant damage occurred. This is attributed to the alternating layers of hard basalt and soft sediment that lie beneath the Snake River Plain. This attenuation reduces the seismic risk from natural and induced seismic occurrences. Additional studies are currently underway to better understand the EGS potential for the eastern SRP. These include GIS and VSRP analysis of existing data, new geotechnical and geomechanical analysis of core samples, geochemical studies of water-rock interaction, and numerical modeling studies of fracturing, fluid flow, and heat recovery.
... The borehole and surface seismic profiles are located along the west flank of Hansen Butte, immediately south of the Snake River (Fig. 2). The site lithology includes basalt and sediment interbeds above 0.4 km and a continuous rhyolite sequence from 0.4 km to a total drilled depth of 1.95 km (Shervais et al. 2011;Fig. 2). ...
... The calculated velocity values for the thin sedimentary interbeds are likely overestimated due to the influence of the high rhyolite seismic velocities that lie above and below the interbeds, and a smoothing filter used to calculate interval velocity values. However, thick sedimentary interbeds observed at depths of 0.4 km and 0.6 km record interval velocities of 2000-3000 m/s and represent fine-grained unconsolidated sediments that were logged within these zones (Shervais et al. 2011). In summary, we observe a general increase in seismic velocity of approximately 1000 m/s per km within the rhyolite sequence and slow seismic velocities that correlate with sedimentary interbeds at the Kimberly site. ...
New high-resolution downhole and surface seismic reflection data tied to
drill holes related to the Snake River Geothermal Drilling Project (ICDP
Project Hotspot) provide insights into seismic imaging in volcanic
terranes. The downhole data at the Kimberly and Kimama drill sites in
southern Idaho show low seismic attenuation and large seismic velocity
contrasts at volcanic flow boundaries. These lithologic and seismic
boundaries tie to reflections in both borehole and surface seismic
images. The Kimberly site drilled through 1,958 m of mostly rhyolite,
with thin sedimentary interbeds throughout the section. Sedimentary
interbeds at depth correspond with slow velocity zones that relate to
reflections on surface seismic profiles. The reflection observed on
360-channel vibroseis seismic profiles that relates to a flow boundary
at 300-400 m depth increases in depth with increasing elevation away
from the Kimberly drill site, suggesting flow volumes may be estimated
with surface seismic methods. The Kimama site drilled through 1,912 m of
mostly basalt with sedimentary interbeds at depth. Downhole and surface
vibroseis seismic results here also suggest seismic reflection methods
are useful to image flow boundaries. Ongoing drilling at a third site in
Mountain Home, Idaho will tie lithologies and measured physical
properties to surface seismic data. These seismic data show key
lithologic boundaries related to Quaternary basalts, lake sediments
related to paleo Lake Idaho, and underlying Tertiary basalts. Ongoing
analysis should help clarify the limits and capabilities of surface
seismic imaging in volcanic terranes.
Basalts erupted in the Snake River Plain of central Idaho and sampled in the Kimama drill core link eruptive processes to the construction of mafic intrusions over 5.5 Ma. Cyclic variations in basalt composition reveal temporal chemical heterogeneity related to fractional crystallization and the assimilation of previously-intruded mafic sills. A range of compositional types are identified within 1,912 m of continuous drill core: Snake River olivine tholeiite (SROT), low K SROT, high Fe-Ti, and evolved and high K-Fe lavas similar to those erupted at Craters of the Moon National Monument. Detailed lithologic and geophysical logs document 432 flow units comprising 183 distinct lava flows and 78 flow groups. Each lava flow represents a single eruptive episode, while flow groups document chemically and temporally related flows that formed over extended periods of time. Temporal chemical variation demonstrates the importance of source heterogeneity and magma processing in basalt petrogenesis. Low-K SROT and high Fe-Ti basalts are genetically related to SROT as, respectively, hydrothermally-altered and fractionated daughters. Cyclic variations in the chemical composition of Kimama flow groups are apparent as 21 upward fractionation cycles, six recharge cycles, eight recharge-fractionation cycles, and five fractionation-recharge cycles. We propose that most Kimama basalt flows represent typical fractionation and recharge patterns, consistent with the repeated influx of primitive SROT parental magmas and extensive fractional crystallization coupled with varying degrees of assimilation of gabbroic to ferrodioritic sills at shallow to intermediate depths over short durations. Trace element models show that parental SROT basalts were generated by 5–10% partial melting of enriched mantle at shallow depths above the garnet-spinel lherzolite transition. The distinctive evolved and high K-Fe lavas are rare. Found at four depths, 319, 1045, 1,078, and 1,189 m, evolved and high K-Fe flows are compositionally unrelated to SROT magmas and represent highly fractionated basalt, probably accompanied by crustal assimilation. These evolved lavas may be sourced from the Craters of the Moon/Great Rift system to the northeast. The Kimama drill core is the longest record of geochemical variation in the central Snake River Plain and reinforces the concept of magma processing in a layered complex.
The U.S. Air Force is facing a number of challenges as it moves into the future, one of the biggest being how to provide safe and secure energy to support base operations. A team of scientists and engineers met at Mountain Home Air Force Base near Boise, Idaho, to discuss the possibility of exploring for geothermal resources under the base. The team identified that there was a reasonable potential for geothermal resources based on data from an existing well. In addition, a regional gravity map helped identify several possible locations for drilling a new well. The team identified several possible sources of funding for this well - the most logical being to use U.S. Department of Energy funds to drill the upper half of the well and U.S. Air Force funds to drill the bottom half of the well. The well was designed as a slimhole well in accordance with State of Idaho Department of Water Resources rules and regulations. Drilling operations commenced at the Mountain Home site in July of 2011 and were completed in January of 2012. Temperatures increased gradually, especially below a depth of 2000 ft. Temperatures increased more rapidly below a depth of 5500 ft. The bottom of the well is at 5976 ft, where a temperature of about 140°C was recorded. The well flowed artesian from a depth below 5600 ft, until it was plugged off with drilling mud. Core samples were collected from the well and are being analyzed to help understand permeability at depth. Additional tests using a televiewer system will be run to evaluate orientation and directions at fractures, especially in the production zone. A final report on the well exploitation will be forthcoming later this year. The Air Force will use it to evaluate the geothermal resource potential for future private development options at Mountain Home AFB.
