PROCEEDINGS, Fourtieth Workshop on Geothermal Reservoir Engineering
Stanford University, Stanford, California, January 26-28, 2015
SGP-TR-204
Re-evaluation of the Pre-Development Thermal Regime of Roosevelt Hydrothermal System,
Utah
Rick Allis1, Mark Gwynn1, Christian Hardwick1, Stefan Kirby1, Joseph Moore2, and David Chapman3
1
Utah Geological Survey, PO Box 146100, Salt Lake City, Utah 84114-6100
2
Energy & Geoscience Institute, University of Utah, Salt Lake City Utah 84108
3
Dept. Geology and Geophysics, University of Utah, Salt Lake City, Utah, 84112
1
[email protected], [email protected],[email protected], [email protected]
2
[email protected]
3
[email protected]
Keywords: Roosevelt Hot Springs, Utah, thermal regime, heat flow, Milford
ABSTRACT
The original estimate of heat loss from the Roosevelt Hydrothermal System (RHS) based on integration of conductive heat flow
measurements at 30 – 60 m depth was 60 – 70 MWth (Ward et al., 1978). This heat flow map outlines the convective upflow zone
coinciding with a 5 km length of the Opal Mound fault, and an inferred outflow zone to the northwest. Assuming a reservoir enthalpy of
1180 kJ/kg (270°C water), the pre-development reservoir upflow was about 60 kg/s. However, contouring of conservative chemical
species such as chloride and boron from groundwater wells in the Milford Valley outlines a much more extensive thermal outflow from
the western flank of the Mineral Mountains (Ross et al., 1982). Re-evaluation of the temperature data between 100 and 200 m depth in
the original shallow thermal gradient wells supports a thermal outflow zone at least 14 km long, and extending 10 km west towards the
center of Milford Valley. The area with a temperature of at least 40°C at 200 m depth is about 100 km 2, and appears similar in area to
seismic velocity and attenuation anomalies deeper in the crust. Recent analyses of groundwater from five wells in the northern Milford
Valley confirm water compositions very similar to the original groundwater compositions from about 1980, before the development of
the Blundell power plant. Consideration of chloride-enthalpy trends of the pre-development reservoir water and the original RHS spring
composition shows predominantly steam loss between the reservoir and the surface spring. Both the chemical and thermal groundwater
trends beneath Milford Valley are consistent with mixing between cold groundwater and the RHS spring water. Inclusion of the effects
of mixing with cool groundwater significantly increases the pre-development heat flow, which is estimated here to be more than 100
MW. A deep geothermal exploration well (Acord-1) near the western limit of the outflow has a temperature of 230°C at its total depth
of 3.8 km, a conductive heat flow of 120 ± 20 mW/m2, and may be representative of the regional heat flow beneath northern Milford
Valley. This heat flow, and the higher estimates of predevelopment heat and mass flow from RHS, may be indicative of a significant
deep geothermal resource hosted in a granite intrusion beneath the region. The challenge for further development will be locating or
creating adequate permeability in the large volume of hot granite adjacent to the RHS.
1. INTRODUCTION
Intensive geothermal exploration of the region around Roosevelt Hot Springs during the 1970s by several companies and by Department
of Energy-sponsored researchers at the University of Utah resulted in an unusually large volume of published information on the
underlying thermal regime (summaries in Ward et al., 1978; Faulder, 1991; Ross et al., 1982; Nielson et al., 1986; Moore and Nielsen,
1994). This culminated in the commissioning of the Blundell Power Plant in 1984 (23 MWe net) as the first liquid-dominated
geothermal plant in the U.S. (Forrest, 1994). The facility was expanded by PacifiCorp Energy to include a 10 MWe (net) binary plant in
1987, and both plants continue to operate at capacity (Allis and Larsen, 2012). The original maximum measured temperature was
268°C (in well 14-2), and two new wells have confirmed temperatures of more than 260°C still exist at depth adjacent to the production
borefield.
Using data from over 40 thermal gradient wells, Wilson and Chapman (1980) delineated the thermal signature of the hot water upflow
zone adjacent to the Opal Mound Fault, and a shallow outflow zone to the northwest (Figure 1). A relatively sharp decrease in heat flow
in the Mineral Mountains to the east was interpreted as a cold water recharge zone to the hydrothermal system. The thermal boundary
on the west side of the system was not defined, with several thermal gradient wells indicating heat flows of 300 to more than 400
mW/m2.
Integration of the conductive heat flow anomaly yields a total heat output of 70 MW (Wilson and Chapman, 1980). Based on the
enthalpy of the deep water, this implies an upflow rate of 60 kg/s, which loses heat near the surface by steam formation (boiling),
conductive heat loss, and mixing with adjacent cool groundwater. In this paper we reassess thermal and chemical data to better
understand the pre-development thermal regime of the Roosevelt Hydrothermal System (RHS). Data from several shallow and deep
wells that were not available at the time of the previous interpretation are used, and we have also recently sampled and analyzed the
chemistry of several groundwater wells in Milford Valley that show geothermal chemical characteristics. The thermal characteristics of
the Blundell production borefield, and the changes that have occurred with development, are discussed by Allis and Larsen, (2012).
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Figure 1: Pattern of shallow heat flow in mW/m2 established during the late 1970s (slightly modified from Ward et al., 1978;
Wilson and Chapman, 1980; based on temperature gradients between 30 - 60 m depth). RHS is the Roosevelt
Hydrothermal System, shown as the red elliptical zone. The four production wells occupy the central portion of this zone.
Stipple highlights the extent of intrusive and extrusive rocks in the Mineral Mountains; small plus symbols are centers of
Quaternary rhyolite domes. Red crosses are deep wells for which data was not available at the time of two publications
referred to above. The main surface thermal feature prior to development was Roosevelt Hot Spring (black triangle),
but this ceased flowing between 1957 and 1970 (Nielson et al., 1986). The Opal Mound near the southern end of the red
ellipse indicates a long history of thermal outflow, but it was inactive by the 1970s.
2. SHALLOW THERMAL REGIME
A review of data from thermal gradient wells drilled during the 1970s showed several wells west of the main thermal anomaly
delineated in Figure 1 that may constrain the RHS thermal anomaly. In addition, there were many wells that either reached 200 m depth
or had gradients that could reasonably be extrapolated to 200 m depth. The temperature pattern at 200 m depth was therefore compiled
(Figure 2). At this depth, for a typical conductive Great Basin heat flow of 90 mW/m2 and an average surface temperature of 11°C, the
temperature should be about 25 °C in unconsolidated sediment (assuming a thermal conductivity of 1.3 W/m°C) and about 17°C in low
porosity crystalline rocks (thermal conductivity of about 3 W/m°C). In the Mineral Mountains where the crystalline rocks occur, the
elevation rises over a kilometer above the valley floor, so the mean annual surface temperature could be as much as 5°C cooler (Minder
et al., 2010). The temperature at 200 m depth, assuming thermal conduction from the surface, should be decreased by the same amount,
implying a background temperature at 200 m depth of about 12°C. Temperatures at 200 m depth greater than about 30°C in the valley
and more than about 20°C in the Mineral Mountains, are therefore anomalously warm.
The three wells labeled D, E, K in Figure 2 show the RHS thermal anomaly extends about 10 km west of the eastern edge of the hightemperature upflow zone adjacent to the Mineral Mountains. Well D is new, and was to be a water supply well for a wind farm
maintenance facility at that site. Chemical analysis of the water shows it to be dilute geothermal water and not potable (see below),
confirming the western extent of the thermal anomaly. The north-south dimension is about 15 km, suggesting a total area approaching
150 km2. The area with temperatures more than 40°C at 200 m depth is close to 100 km2. This large area roughly coincides with a mid2
Allis et al.
and lower crustal seismic velocity anomaly (highlighted on Figure 2). Robinson and Iyer (1981) found that the “intensely anomalous
region of low velocity and high attenuation” extends from about 5 km depth to the base of the crust and they attribute the feature to a
small fraction of partial melt. Anomalous He3/He4 in the RHS geothermal fluids supports the partial melt interpretation (Kennedy and
van Soest, 2007).
Figure 2: Compilation of the temperatures at 200 m depth on shaded relief background. Thermal gradient wells used in Figure
3 are labeled A – N. The red triangle is the location of the original Roosevelt Hot Spring. The anomalously low P-wave
velocity and high attenuation region below 5 km depth identified by Robinson and Iyer (1981) is highlighted as green
dashes. Temperatures above about 30°C in the valley, and above about 20°C in the Mineral Mountains are anomalous.
The systematic trend of decreasing temperature (and heat flow) west from the location of the RHS is shown in Figure 3 for the wells
labeled A – N in Figure 2. The temperature gradients below 100 m depth decrease from about 200 to 60°C/km, whereas the gradients
between the surface and 100 m depth are all much closer to 200°C/km. This characteristic is more obvious when the profiles are plotted
against elevation. Wells E and K, which are near the inferred western boundary of the thermal anomaly, show a three-fold decrease in
gradient occurring at an elevation of 1500 m asl. This sharp decrease in gradient cannot be explained by a reasonable thermal
conductivity increase with increasing depth. In the central portion of the valley, the upper few hundred meters are predominantly
lacustrine sediments; the eastern side of the valley (at higher elevation) is predominantly alluvial fan material (Figure 4). The separation
between these two types of basin-fill sediments is the Bonneville shoreline, a terrace cut about 18,000 years ago when Lake Bonneville
reached its peak elevation. A drillers log from an old groundwater well near to the deep Acord-1 well (located on Figure 2) reported the
water level at 15 m depth and claystone at 60 m depth, which corresponds to about 1500 m asl. We suspect that the dilute geothermal
water in this area is flowing laterally on top of the claystone, and that this has been occurring for tens of thousands of years, enabling the
underlying conductive thermal regime to come to equilibrium with the constant temperature condition at the top of the claystone.
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Figure 3: Temperature profiles in thermal gradient wells on the west side of the Roosevelt thermal anomaly. The profiles are
plotted against depth below the ground surface (upper) and against elevation (meters above sea level; lower graph). The
single triangle (D) is the new groundwater well drilled for the wind farm.
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Allis et al.
Figure
4:
Geologic
map
of
the
region)
derived
from
the
UGS
interactive
geologic
map
(http://geology.utah.gov/maps/geomap/interactive/viewer/index.html). A map boundary exists in the middle of the RHS,
so this map is a modified version extracted from the Richfield 30’ x 60’ map sheet (Hintze et al., 2003) and the Beaver 30’
x 60’ map sheet (Rowley et al., 2005) (modifications by Grant Willis and Kent Brown, UGS, 11/3/2014). The blue line
with B symbols is the high-stand shoreline of Lake Bonneville. This line separates Quaternary alluvium to the east (Qaf)
from Quaternary lake deposits (Qla) to the west (i.e. near/at the surface). Preliminary interpretation of the gravity
anomalies suggests that the Bonneville shoreline coincides with a relatively rapid deepening of the granite surface at
depth, and therefore could be fault-controlled. The faint grid on this map outlines section boundaries (each 1 square
mile). Red contours are the temperatures at 2 km depth (derived from Figure 6); labeled red dots are deep wells referred
to in the next section. GPC15 is the same as well I in Figure 2.
3. DEEP THERMAL REGIME
At the time that intensive exploration of the RHS was occurring, a deep exploration well, Acord-1, was being drilled about 10 km to the
west in the middle of the North Milford Valley. An unusually large amount of data from logging was collected while drilling proceeded
(Figure 5). Granite was encountered at 3.18 km depth, and after no significant permeability was found, the hole was plugged and
suspended at 3.8 km depth. A conductive geotherm has been fitted to the corrected bottom hole temperatures (BHT), which indicate a
temperature of at least 230°C at 4 km depth. Although the corrected BHT at 2.4 km depth is a poor fit, the steady increase in
uncorrected BHT parallel to the geotherm suggests the BHT at 2.4 km depth is under-corrected. The heat flow based on assumed
thermal conductivities for the lithologies given in Hintze and Davis (2003) is 120 ± 20 mW/m2, with the uncertainty largely due to the
uncertainty in thermal conductivity. This geotherm has a near-surface (average over upper 200 m) gradient of 90°C/km, and a deep
gradient in the granite of 40°C/km.
The Acord-1 geotherm is combined with temperature profiles for other deep exploration wells around the RHS (Figure 6). This data is
taken from Faulder (1991), Forrest (1994), and Allis and Larsen (2012). Only one well is shown for the hydrothermal reservoir (14-2),
but the other production wells all typically showed boiling-point-for-depth profiles over the upper 200 – 300 m depth prior to
development. A remarkable feature of the deep well profiles is that with the exception of 12-35 at the northern end of the Opal Mound
fault, all the other wells outside of the hydrothermal upflow zone show apparently conductive gradients of 40 - 80°C below 1 km depth.
The deepest gradients in these wells all point to temperatures of about 270°C by 3 – 4 km depth, similar to the maximum temperature so
far observed in the upflow zone. This temperature is predicted at 5 km depth at Acord-1. Geothermometer calculations predict a deep
reservoir fluid temperature of 288 ± 10°C (Capuano and Cole, 1982).
A map view of the temperatures at 2 km depth is superimposed on Figure 4. Although there is an absence of deep wells constraining the
150°C contour to the south and east of the Roosevelt thermal anomaly, it is clear that the area of the anomaly is at least 150 km 2, and
similar in dimensions to the shallow thermal anomaly in Figure 2.
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Figure 5: Acord-1 on west edge of the 200 m depth thermal anomaly shows temperatures of over 230 C at 4 km depth.
Assumed thermal conductivities suggest a heat flow of 120 (± 20 mW/m2 based on thermal conductivity uncertainties).
The lithologies are based on Hintze and Davis, (2003).
Figure 6: Temperature profiles from exploration wells around RHS (wells located on Figure 4). Apart from 12-35 at the
northern end of the Opal Mound fault, the wells all have apparently conductive thermal profiles below about 1 km depth.
In the hydrothermal upflow zone the shallow temperature profiles are close to boiling point for depth (dashed red line).
4. GROUNDWATER GEOTHERMAL SIGNATURES
The groundwater west of the Mineral Mountains has a strong geothermal signature due to the outflow from RHS (Capuano and Cole,
1982; Ross et al., 1982; Moore and Nielsen 1994). Chloride and boron are distinctive signatures of this outflow. Poor water quality is
most likely the main reason the north Milford Valley has not been fully developed for agricultural water, in contrast to the heavy
groundwater use in the southern Milford Valley. This was recently confirmed by First Wind, who drilled a groundwater supply well at
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their wind farm maintenance facility (site D in Figure 2, and FWW in Table 1 and Figure 9) and found warm water (32°C) of poor
quality. Recent analyses from five groundwater wells in north Milford Valley are shown in (Table 1).
The groundwater analyses confirm a strong geothermal signature with Total Dissolved Solid values ranging from about 2000 to 6000
mg/kg. Well D (FWW) contains 67 μg/kg of arsenic, which is considerably above the 10 μg/kg limit recommended by the EPA for
potable water. A stock well 5 km north of site D (NSW) contains 430 μg/kg of arsenic, although it is cool (17°C). This well is in the
down-gradient drainage area of the Negro Mag Wash, the primary direction of the high-temperature hydrothermal outflow from the
RHS (Negro Mag Wash initially drains west from the original hot spring and then north down Milford Valley – see Figure 4). Both
NSW and a second stock well (SSW), also have data from the early 1980s. The data indicate water compositions largely unchanged
since those measurements prior to development of RHS.
A graph of chloride concentration, and temperature (expressed as enthalpy) can identify major mixing and cooling processes as deep
geothermal water rises to the surface and disperses into the regional groundwater (Figure 7). At Roosevelt, the original hot spring
evolved from the deep reservoir water largely by steam loss (boiling), a process that is common in many high-temperature liquid
systems with strong hot spring activity. Some cooling due to mixing with local cool groundwater and/or conductive heat loss also
occurred. The pattern west of the RHS upflow area shows a trend of cooling by mixing with non-thermal groundwater.
An interesting question is the implication of cooling by mixing for the total heat output of the RHS. The original conductive losses from
the surface, which can capture most of the steam loss if the steam condenses before reaching the surface, as well as a component of heat
loss from cooling groundwater in the outflow zone, were 70 MW (Wilson and Chapman, 1980). If the inferred upflow of hot water of
60 kg/s cools from 100 to 20°C by mixing with cool groundwater, the convective heat loss is an additional 20 MW. However, this is a
minimum because there has to be a significant additional hot water outflow roughly south of a line between thermal gradient wells I and
J (Figure 2), and deep well 25-15 (Figure 4). The groundwater gradient is towards the west and north of the RHS upflow zone, so warm
to hot groundwater flowing from this zone cannot flow towards the southwest. The warm water in thermal gradient wells K, L, and N
(Figure 2) has to have originated from south of the high-temperature portion of the RHS upflow zone. The relatively cool water in 2525 and 52-21 at 200 m depth (about 60°C, Figure 6) suggests some of the cooling of hot water on the inferred southern extension of the
Opal Mound fault may have occurred near-surface within the fault (zone). The amount of outflowing warm water south of the mapped
Opal Mound Fault could match or exceed the outflow from the high temperature portion of the RHS.
Figure 8 depicts the extent of the geothermal water outflow using just chloride and boron concentrations from groundwater wells.
Although the data is sparse, the geothermal water is distinctive because of its relatively low Cl/B ratio (100 – 200). The chloride map
shows that the geothermal outflow extends west to the Beaver River, a distance of over 12 km from the RHS upflow zone. The
northward extent of the geothermal outflow is poorly defined because of a relative lack of samples.
5. CONCEPTUAL MODEL
The Acord-1 well, with its well-determined temperature profile (Figure 5) and stratigraphy (Hintze and Davis, 2003), enables the major
structural and thermal characteristics of the basin beneath North Milford Valley to be determined from the gravity variations (Figure 9).
The known depth of granite at 100 m depth in well 82-33, and at 3.2 km in Acord-1 constrains the shape of the upper surface of the
granite when interpreting the 20 mgal gravity low in the basin. Reasonable assumptions for the decreasing density contrast with depth
in the volcanic and sedimentary fill overlying the granite indicate a convex-upwards shape to the granite surface beneath the eastern
flank of the basin, with the deepest part of the basin (> 2 km of fill) relatively narrow compared to the width of the valley.
Figure 10 shows simplified geology and isotherms along the same cross-section as in Figure 9, which intersects the northern end of the
Opal Mound fault zone. Convective upflow dominates the thermal regime within the Opal Mound fault zone, although the temperature
profile in 12-35 indicates an inversion at 1.5 km depth and some mixing with an inflow of cooler waters. If the profile had been drawn
to intersect the high temperature part of the RHS borefield, the 250°C isotherm would be at 400 m depth, and the cooler isotherms
would be tightly bunched at shallower depths. The outflow of geothermal waters to the west is inferred to occur in the upper 100 m of
valley fill. With increasing distance to the west, the outflow mixes with cross-flowing cool groundwater flowing north beneath the
northern Milford Valley. The thermal regime within the granite and much of the overlying basin fill in the central Milford Valley is
assumed to be dominated by thermal conduction, as seen in the temperature profile of Acord-1 and other deep wells.
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Allis et al.
Table 1. Summary of recently collected groundwater chemistry samples. Highlighted data is preexisting chemistry data with
multiple samples.
Figure 7: Chloride-enthalpy trends of waters from the RHS. Steam loss was the dominant cooling process between the deep
high-temperature upflow zone and the original hot spring. The dominant cooling trend of groundwater west of the RHS
upflow zone is mixing with local non-thermal groundwater.
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Allis et al.
Figure 8: Map and graph showing chloride trends in groundwater adjacent to the Roosevelt Hydrothermal System. The Cl/B
ratio for geothermal water is a distinctive marker. RHS and APW refer to average values from analyses in Capuano and
Cole (1982) for the original hot spring and the original composition of deep production wells (corrected for steam loss).
Other non-geothermal analyses are from Kirby (2012), and this study.
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Allis et al.
granite
Figure 9: Residual isostatic gravity contours (5 mgal interval) adjacent to the north Milford Valley. The Opal Mound fault
marks the approximate position of the RHS upflow zone. The red line labeled X - Y marks the line of section for the 2-D
density model of the basin fill and underlying granitic rock (right figure). The model and assumed density contrasts have
been constrained by the known depth to granite in Acord-1 and 82-33. The cause of the seismic velocity anomaly of
Robinson and Iyer (1981; also shown on Figure 2) may also cause a regional gravity low due to a lower mid-crustal
density (modeled by Becker and Blackwell, 1983). The Paleozoic rocks (labeled P) cropping out in the ranges
surrounding the central Mineral Mountains are mostly carbonates and are suspected of having a higher density than
granite (labeled T-Q).
6. CONCLUSIONS
Although the RHS high temperature zone is closely associated with a 5 km length of the Opal Mound fault zone, there is a much larger
thermal anomaly west, north, and south of this fault zone. Thermal gradient wells indicate the outflow of geothermal water probably
extends for a distance of up to 14 km in a north-south direction, presumably on concealed extensions of the Opal Mound fault zone. In
addition, the shallow thermal gradient and groundwater chemistry data suggest this outflow zone extends as far west as the Beaver River
in the center of northern Milford Valley, a distance of more than 10 km from the source of the outflow at the base of the Mineral
Mountains. The magnitude of the geothermal water outflow is unknown, but simple comparisons with the known conductive heat loss
estimate of 70 MW from the top of the RHS suggest that the total heat loss from the larger Roosevelt geothermal system is more than
100 MW. Although this outflow appears to be at less than 100 m depth in the valley fill, there does not appear to be any temperature
inversion beneath the outflow west and south of the high temperature upflow zone of the RHS. However there are indications of
temperature inversions to the northwest, suggesting this part of the system may be younger than the outflows occurring further south.
The total area of the geothermal system is more than 100 km2. The size and location of the geothermal system coincides with a seismic
P-wave low-velocity anomaly and anomalous attenuation zone between 5 km depth and the base of the crust. These findings confirm
that a substantial geothermal resource lies beneath the region, and the challenge to harnessing this resource will be finding or creating
permeability in the hot granite.
ACKNOWLEDGEMENTS
Elizabeth Firmage and Jay Hill assisted with several of the figures; Grant Willis and Kent Brown merged the geology from the Richfield
and Beaver map quadrangles in Figure 4; First Wind, Murphy-Brown and Michael Yardley are thanked for access to groundwater wells
in Milford Valley; and PacifiCorp Energy contributed data on wells in the Roosevelt Hydrothermal System over the last few years. This
work was partially funded by Geothermal Technologies Office of the Department of Energy (Award DE-EE0005128).
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Allis et al.
Figure 10: Thermal regime along a west-east cross-section at the northern end of the Opal Mound fault. The surface of the
granite is based on 2-D interpretation of gravity in Figure 9. The temperatures in the three deep wells are based on the
profiles shown in Figure 6. The near-vertical red arrow symbolizes predominantly upflow in the Opal Mound fault zone,
although the inversion in well 12-35 at 1.5 km depth indicates mixing with cooler waters in this part of the fault zone. The
horizontal orange and blue arrows indicate an outflow and cooling of geothermal water in the shallow valley fill. The
thermal regime in the granite west of the Opal Mound fault is interpreted to be conductive.
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