GRC Transactions, Vol. 41, 2017

Shallow Thermal Anomalies in West-Central Utah

Mark Gwynn, Rick Allis, and Stefan Kirby Utah Geological Survey

Keywords geothermal, convective, conductive, heat flow, thermal conductivity, thermal gradient

ABSTRACT

Work in the Roosevelt Hot Springs and Cove Fort areas over the last two years forms the basis for an expanded examination of the shallow thermal regime in a large portion of west-central Utah. This area was extensively explored for geothermal resources in the 1970s and 1980s, but limited drilling has taken place since then. The previous research used data from over 300 wells. Most were temperature-gradient wells, but deep geothermal exploration or production wells, oil exploration wells, and water wells with thermal data were also used. This new study characterizes the shallow thermal regime over an area of 150 by 180 km, using thermal data from over 180 shallow (typically less than 150 m) wells with measured temperature depth profiles. The majority were temperature-gradient wells, although some were groundwater monitoring wells or mining-related drill holes. Reported temperatures from an additional 85 water supply wells were also used in this study. In addition, deep well data from 25 plugged and abandoned oil exploration wells and the 3.8-km-deep Acord 1 well in Milford Valley were used to characterize the deep thermal regime across the study area. These deep wells include all those in the study area that have any temperature data available. Previous work showed that temperatures greater than 20°C at a depth of 100 m are anomalous, and this study delineated three shallow (less than 700 m) hydrothermal anomalies in addition to those around Roosevelt Hot Springs and Cove Fort. These newly-defined anomalies cover an area of over 1900 km². These systems also have smaller zones where temperatures exceed 30°C at 100 m depth, one of which has an additional zone that exceeds 40°C. Two of the hydrothermal anomalies, near the San Francisco Mountains and in , appear to be derived from deep circulation and heating of meteoric waters in range front faults, while the mechanism controlling the much larger Drum Mountains-Whirlwind Valley anomaly is less clear. An additional shallow anomaly covering about 600 km² and having temperatures of 20– 30°C at 100 m depth was delineated in the Black Rock Desert. This anomaly differs from the others because the overall thermal regime is conductive in nature and hosts proven temperatures in excess of 230°C at 3.3 km depth. The contrast between the subtle shallow thermal regime and Gwynn, et al.

the elevated deep thermal regime is a function of conductive, rather than convective heat transport, coupled with over 3 km of low-thermal-conductivity basin fill. Bottom-hole temperature data in the western half of the study area, after being corrected for drilling-induced perturbations, show that the background thermal regime at depth in the study area is cooler than in the Pavant Butte area of the Black Rock Desert, and probably has an average heat flow typical of the Basin and Range. The shallow thermal regime over much of the corresponding area is at background levels of less than 20°C at 100 m.

1. Introduction The 1970s and 1980s were a time of extensive geothermal exploration and thermal-gradient drilling in west-central Utah. Around 400 wells were drilled, mostly by Amax Geothermal, Phillips Geothermal, Union Oil Company, the University of Utah, Hunt Energy Corporation, and Mother Earth Industries. Two notable deep exploration wells in the area are McCullough Oil Company’s Acord 1 well (1979, 3.8 km depth) west of the Mineral Mountains and Arco’s Pavant Butte 1 well (1981, 3.3 km depth) drilled south of Delta (figure 1). The temperature at the bottom of Acord 1 is nearly 230°C and has led to renewed geothermal research in the area. The Pavant Butte 1 well was originally drilled as an oil exploration well, but was plugged and abandoned shortly after drilling. Phillips Geothermal re-entered the well for geothermal purposes in 1984 and measured equilibrium temperatures up to 200°C at 2.2 km, but were unable to measure temperatures below that depth due to well conditions (Allis et al., 2015b). Extrapolated temperatures at the original total depth of this well are predicted to be 250°C. These early exploration programs led to the commissioning of the 23 MWe Blundell Power Plant on the western flank of the Mineral Mountains in 1984 (PacifiCorp added a 10 MWe binary plant to the facility in 2007; Allis and Larsen, 2012) and the Cove Fort Plant near the western side of the Tushar Mountains in 1985. The original Cove Fort plant later closed, but was rebuilt and returned to production (25 MWe gross, 19 MWe net) in 2013 by ENEL (Allis et al., 2016). A third geothermal plant (Thermo) was completed southwest of Milford in 2009, but is outside of this study area. Aside from a few notable exceptions, very little geothermal exploration drilling has taken place since the 1980s. Limited drilling has occurred around the geothermal plant sites to develop the plants and better define the local resources. The Utah Geological Survey (UGS) drilled 10 gradient wells in the Black Rock Desert between 2011 and 2012 to explore the geothermal potential highlighted by the Pavant Butte 1 well (Gwynn et al., 2013; Allis et al., 2015b). There were also some test wells drilled in several parts of the study area as part of the site selection process for the U.S. Department of Defense’s cancelled MX missile program as well as some core holes drilled for mining operations that have been logged for temperature. The UGS also drilled a number of wells in the greater Snake Valley area along the Utah-Nevada border as part of an ongoing groundwater monitoring program that began in 2004 (Hurlow et al., 2014). Blackett (2011) notes that between 2007 and 2009, 68 monitoring wells were drilled at 27 sites (some sites had multiple nested wells to monitor water levels in different aquifers). Blackett (2011) was able to log temperatures in 23 of these wells, and these data constitute the majority of the shallow thermal data in far western Utah. Gwynn, et al.

Figure 1. Shallow thermal anomalies on shaded relief with colored elevation levels. The anomalies in the Roosevelt Hydrothermal System (RHS) and Cove Fort (CF) areas in the southwest corner were previously mapped (Allis et al., 2015a, 2016, 2017a, 2017b; Gwynn et al., 2016), and wells used in those studies are shown as white circles. The contours shown at this scale represent 20°C at 100 m depth; higher-temperature contours are shown on area-specific figures. Gwynn, et al.

This study focuses on an area 150 by 180 km and encloses a large part of the Sevier Thermal Anomaly delineated by Mabey and Budding (1987). Most of Juab and Millard Counties, about half of Beaver County, and small portions of Tooele, Sevier, and Piute Counties are covered by this assessment (figure 1). Aside from a small section near the southeast corner, the entire study area fits within the Great Basin Carbonate and Alluvial Aquifer System (GBCAAS) defined by Heilweil et al. (2011). Mapped rock units include thick accumulations of Paleozoic carbonate rocks, siliciclastic rocks ranging in age from Paleozoic to Tertiary, Jurassic and Tertiary intrusives, Tertiary-Quaternary volcanics, and some Precambrian sedimentary and metasedimentary rocks. Quaternary surficial units include Lake Bonneville lacustrine deposits and various categories of alluvial deposits. Recent work on the Utah Frontier Observatory for Research in Geothermal Energy (FORGE) site, a U.S. Department of Energy effort to engineer technology and techniques needed to develop enhanced geothermal systems, included mapping temperatures at 200 m depth (Allis et al., 2015a, 2016; Gwynn et al., 2016). Thermal data were gleaned from 82 shallow or intermediate-depth (<500 m) gradient wells and 22 deep (500–4000 m) exploration or production/injection wells around the Roosevelt Hydrothermal System (RHS; Gwynn et al., 2016). A similar effort was undertaken for the Cove Fort geothermal system that included 160 gradient wells, 20 water wells, and 8 deep wells around Cove Fort (CF) and extending west to RHS and north to the Meadow-Hatton area (Allis et al., 2017a, 2017b). These earlier efforts form the southeastern corner of the study area detailed in this paper. Allis et al. (2017a) suggest that temperatures greater than 20°C at 100 m depth are anomalous in this region. Over 180 newly-reviewed wells with temperature-depth profiles (mostly thermal-gradient wells with some groundwater monitoring wells and mining-related coreholes) were combined with data from the CF and the RHS areas. Temperature data from over 85 water wells were also used to study the shallow thermal regime. Corrected bottom-hole temperature (BHT) data from 24 plugged and abandoned oil exploration wells, along with the Acord 1 and Pavant Butte 1 wells, were used to help investigate the deep thermal regime across the study area because of the potential influence on shallow temperature anomalies.

2. Methods Edwards (2013) compiled temperature-depth data from most of the wells used in this study (gleaned from multiple sources) into an MS Excel database, which can be easily accessed to determine measured or extrapolated temperatures at 100 m. The gradient wells with measured temperature-depth profiles were classified as low or high confidence depending primarily on depth and whether the profile was fairly linear and could be reasonably extrapolated to 100 m. Wells deeper than 100 m often had temperatures measured at the correct depth and could be taken directly (or extrapolated from relatively small distances above and below 100 m depending on sample intervals), so these were classified as high confidence. Wells 70 to 100 m deep with reasonably linear gradients were extrapolated to 100 m and were also considered high confidence. Shallower (<70 m) wells with reasonable gradients were also extrapolated, but were considered low confidence. In some cases, only a BHT and depth, either above or below 100 m, were available. A gradient using the BHT and a mean annual surface temperature (MAST) of 13°C, derived from regional weather station data and temperature profile surface intercepts of nearby wells, were then used to estimate the temperature at 100 m. These data were considered Gwynn, et al.

low confidence (except for a few that were within 5 m of the 100 m depth datum). For the water supply wells, temperatures were assumed to have been measured from flowing water coming from the deepest portion of the well. For water wells >80 m deep, the temperature data were processed in the same manner as for the BHT-only gradient wells, and the wells were likewise considered to be low confidence. Most of the water supply wells are located some distance from anomalous areas, but were useful in assessing background temperatures (typically about 14– 18°C) and refining some of the 20°C contours. Geotherm models based on estimated thermal conductivity for reported rock types and corrected BHTs have previously been developed for 12 of the deeper (about 1400–5300 m) wells in the study area (Gwynn et al., 2013; Allis et al., 2015b). These models allow an estimated temperature-depth profile to be plotted. Data from these models were used to map temperatures at 100 m, but due to uncertainties inherent in the models, these were considered low confidence. Although spatially sparse, data from these models and the other oil wells were also used to investigate the deep thermal regime, which can be important to understanding shallow thermal anomalies. Corrections were applied to the BHTs to account for drilling-induced perturbations to the in situ thermal field using mainly the methods of Henrikson and Chapman (2002) and other methods detailed in Gwynn et al. (2014). Bottom-hole temperature data are notoriously low quality, primarily due to being recorded shortly after drilling while the thermal field is disturbed and because temperatures are typically of secondary interest to oil and logging companies. Numerous methods to compensate for these problems have been developed. Welhan and Gwynn (2014) and Gwynn (2015) discuss how some of the more commonly used methods compare to the corrections used in this study. Temperatures at a depth of 100 m were mapped so that temperature data from all well locations can be viewed on a relatively consistent datum to help interpret the shallow thermal regime. Original work at RHS used a depth of 200 m and made some corrections for high relief between the valley floor and the Mineral Mountains (Allis et al., 2015a, 2016; Gwynn et al., 2016), however, this was not done for the Cove Fort area (Allis et al., 2017a, 2017b) or this study. The 200 m RHS data were later re-evaluated to create 100 m depth contours for consistency with the Cove Fort work. Conductive gradients are generally well established in a well at depths of about 20–50 m, below the influence of seasonal climate cycles, so using a 100 m depth baseline is adequate. Additionally, a 100 m baseline allows for a larger number of shallow wells (depths of about 50–100 m) to be used with less extensive extrapolation to reach the investigation datum. This can be an important factor in areas where wells are scarce. Temperature data were plotted in ArcGIS and hand-contoured using the confidence levels as an additional guide (i.e., preference was given to higher-quality data in any area with conflicting temperature data).

3. Results 3.1 Shallow Thermal Regime In addition to the previously defined CF and RHS anomalies, four new shallow temperature anomalies were delineated in this study. The first newly recognized anomaly is located near the southern end of the San Francisco Mountains (SFM), and is west of the established RHS anomaly. The second anomaly is in the Black Rock Desert (BRD), north of the CF anomalies. The deep thermal regime in the BRD was studied previously (Gwynn et al., 2013; Allis et al., 2015b), but the shallow thermal regime was not examined in detail. The third and fourth Gwynn, et al. anomalies are located in the Drum Mountain-Whirlwind Valley (DM-WV) area and Tule Valley (TV), both in the north-central part of the study area. Additional shallow anomalies may exist in the Crypto area of the Fish Springs Range and in the greater Snake Valley area, but more data are needed to define their possible extents. 3.1.1 San Francisco Mountains The SFM anomaly is located around the southern end of the San Francisco and Beaver Lake Mountains, and is only about 8 km across Milford Valley from the RHS area (figure 2). Although well data between the two areas are sparse, they do not seem to be related to the same shallow outflow plume based on temperature and hydrologic considerations. The SFM 20°C contour encloses an area of about 120 km2, but the boundaries are somewhat speculative due to sparse data (especially to the west in ). The 30°C contour covers about 10 km2 on the west flank of the San Francisco Mountains and is better defined. It encloses four wells relatively close to each other with temperatures of 30 to 36°C at 100 m, two of which are high confidence (figure 3). The southern San Francisco Mountains consist of Proterozoic sedimentary and metasedimentary units, Paleozoic carbonate rocks, and Tertiary intrusive and extrusive igneous rocks. There are no thermal springs in the vicinity of the SFM anomaly. The hottest part of the anomaly is near the San Francisco Mountains (west side) fault.

Figure 2. San Francisco Mountains hydrothermal anomaly on a generalized geologic map over a shaded relief base. The western boundary of the Roosevelt Hot Springs anomaly is visible to the east. Small well symbols reflect low-confidence well data while large well symbols reflect high-confidence well data. The number associated with each well symbol shows the temperature (in °C) at 100 m depth. Letters in parentheses correlate to the well profiles and well names in figure 3. Gwynn, et al.

Figure 3. Temperature-depth profiles for wells defining the San Francisco Mountains shallow thermal anomaly. Letters in parentheses next to the well names correlate to the well locations in figure 2. Bold font indicates high-confidence data. 3.1.2 Black Rock Desert The BRD anomaly is south of Delta and is within an area that has been the focus of geothermal investigations for several years (figure 4; Allis et al., 2011, 2012, 2015b; Gwynn et al., 2013). The anomaly is subtle, having temperatures at 100 m depth of less than 25°C, which initially seems contradictory to the high-temperature findings delineated in previous studies until the differences in heat transport (conductive rather than convective) are considered. There are also systematic differences in the extrapolated zero-depth temperature intercept implying differences in mean annual temperature of up to 5°C, presumably due to differences in ground conditions. Well data are shown in figure 5. The 20°C contour covers an area of about 600 km2. Like the SFM anomaly, data are sparsely distributed. The anomaly is located within a large basin, and the only significant relief is from the northern end of the Cricket Mountains and Pavant Butte volcano a few kilometers east of the Pavant Butte 1 oil well. The basin is filled with thick accumulations of valley fill and significant quantities of Quaternary basalt. There are no thermal springs in the vicinity of the BRD anomaly. The extensive Clear Lake fault zone is present near the center of the anomaly, and there are other faulted areas. Gwynn, et al.

Figure 4. Black Rock Desert thermal anomaly on a generalized geologic map over a shaded relief base. The number associated with each well symbol shows the temperature (in °C) at 100 m depth. Letters in parentheses correlate to the well profiles and well names in figure 5. 3.1.3 Drum Mountains-Whirlwind Valley The DM-WV area is well defined because of significant drilling activity (figure 6). Geothermal exploration in the area was conducted in the 1970s and 1980s, mostly by Phillips. The area contains at least 60 gradient wells and 4 MX missile-site test wells. Additionally, several wells were drilled in the northern part of the area as part of the Snake Valley groundwater monitoring program. Temperature and other data for most of these wells were previously published by Sass et al. (1999) and Sass and Walters (1999). Data quality in the immediate DM-WV area is very good. Of the 72 wells, 20 were deeper than 100 m and could be measured directly, 48 required less than 10 m of extrapolation, and only 4 were deemed to have low-confidence data due to larger extrapolations to 100 m. Additionally, most of these wells have very coherent conductive gradients that were easy to extrapolate to 100 m with reasonable accuracy. The 20°C contour covers at least 1400 km2 and is best constrained by well data adjacent to the Drum Mountains, which is the highest-temperature portion of the anomaly. The contour extends some distance to the north through Fish Springs Flat where it crosses the southern boundary of the Utah Test and Training Range where data are nonexistent (outside of the study area). Data are scarce in the northern part of the anomaly, making the contour in that area much more speculative. The estimated southern tip of the anomaly is about 10 km from the mapped BRD anomaly to the southeast (figure 1). Data also become sparse in the southern part of the Gwynn, et al.

Figure 5. Temperature-depth profiles for wells defining the Black Rock Desert shallow thermal anomaly. Temperature data are from temperature-gradient holes (TGH) and geotherm (GEO) models developed for deep oil exploration wells. The Pavant Butte 1 oil well was re-entered for geothermal purposes when it was at thermal equilibrium. Bold font indicates high-confidence data. anomaly. It is possible that the anomaly could merge with the BRD area, but like the SFM and RHS systems, hydrologic and temperature distributions suggest separation. The 30°C contour is centralized, is supported by ample data of high confidence, and covers an area of about 370 km2. The 40°C contour is comparatively small, but encloses six high- confidence wells ranging from 40 to 49°C at 100 m. A single low-confidence well farther to the south could define a very narrow extension of the 40°C contour, but was contoured as a separate area. The combined area of the two 40°C contours is about 30 km2. Due to the number of wells and their wide range of gradients, it is too cumbersome to show all of the temperature profiles in a single figure. So, they were separated into two groups. The groups were further split based on gradient ranges to separate plots and make them easier to see and evaluate. Figure 7 shows wells with temperatures less than 20°C at 100 m and wells that are 30 to 40°C, while figure 8 shows wells with temperatures of 20 to 30°C and over 40°C. The wells with temperatures below 20°C plot on a very consistent thermal gradient of about 40°C/km (figure 7). Thermal gradients for wells in the other temperature ranges are more Gwynn, et al.

Figure 6. Drum Mountains-Whirlwind Valley hydrothermal anomaly on a generalized geologic map over a shaded relief base. Small well symbols reflect low-confidence well data while large well symbols reflect high-confidence well data. The number associated with each well symbol shows the temperature (in °C) at 100 m depth. Letters in parentheses correlate to the well profiles and well names in figures 7 and 8. Gwynn, et al.

Figure 7. Temperature-depth profiles for wells defining the Drum Mountain-Whirlwind Valley shallow thermal anomaly. This figure displays only wells with temperatures less than 20°C at 100 m depth (background temperatures near the anomaly) and those that are 30 to 40°C. Letters in parentheses next to the well names correlate to the well locations in figure 6. Bold font indicates high-confidence data. scattered, but most appear to be completed in a conductive regime. Most of the hottest wells appear to enter a higher thermal conductivity region at about 75 m that causes a gradient deflection (figure 8). Only the T-MX-60A and the PW19C are deep enough to show a transition from a primarily conductive to a clearly convective thermal regime. The T-MX-60A well is the deepest well in the DM-WV area at 370 m, and it becomes near-isothermal (30°C) at about 300 m. Since it is so much deeper than the rest of the wells, it was not plotted on figure 8, but the location is shown on figure 6. The PW19C well penetrates the convective zone at about 100 m (figures 6 and 7). The predominant rock types in the area are Paleozoic carbonates and Tertiary volcanics. There are no thermal springs or mapped faults in the hottest part of the anomaly, however there are warm springs (26–28°C) along the northeastern flank of the Fish Springs Range, and Wilson Health Springs (60°C) flows from several locations north of the range.

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Figure 8. Temperature-depth profiles for wells defining the Drum Mountain-Whirlwind Valley shallow thermal anomaly. This figure displays only wells with temperatures of 20 to 30°C at 100 m depth and those that are greater than 40°C. Letters in parentheses next to the well names correlate to the well locations in figure 6. Bold font indicates high-confidence data. 3.1.4 Tule Valley At least 16 shallow wells were drilled in the TV area in the late 1970s, and two were drilled more recently as part of the Snake Valley monitoring program (figure 9). Most of these wells are very shallow (47-61 m), so data confidence is low. Additionally, temperature-depth profiles in several wells suggest they are beginning to penetrate convective zones near their total depths, which increases the uncertainty of extrapolated temperatures (figure 10). There is evidence of a shallow anomaly in Tule Valley, but the boundaries are uncertain due to the lack and quality of the data. The approximated 20°C contour extends west-northwest into Tule Valley (perpendicular to the west side of the ), but also seems to have a bulge to the north along the axis of the valley. This extension was mapped primarily due to the existence of at least five thermal springs (25-28°C) that stretch semi-linearly to the north. The area enclosed by the 20°C contour is about 360 km2. Two of the higher temperature wells (TV-09 and TV-10) appear to turn isothermal at about 60 to 70 m, just before reaching total depth. The TV-10 well is the hottest, and is near mapped faults that could serve as conduits for upwelling geothermal fluid. The oblong 30°C contour is perpendicular to the range and covers about 50 km2. Local rock types are predominantly Paleozoic carbonates and siliciclastics along with the Jurassic Notch Peak granite. Gwynn, et al.

Figure 9. Tule Valley hydrothermal anomaly on a generalized geologic map over a shaded relief base. Small well symbols reflect low-confidence well data while large well symbols reflect high-confidence well data. The number associated with each well symbol shows the temperature (in °C) at 100 m depth. Letters in parentheses correlate to the well profiles and well names in figure 10. 3.1.5 Crypto Area There could be another anomaly near the northwest flank of the Fish Springs Range. Four companies drilled 85 holes in the area between 1958 and 2008 targeting the Crypto zinc-rich skarn deposit, but temperature data for most are not available (Staargaard, 2009; Tietz et al., 2010; figure 6). Thirteen closely spaced holes, drilled between 2007 and 2008, do have temperature data reported by Puchlick (2009). However, the distribution of the wells is too closely spaced to delineate a possible anomaly. Gwynn, et al.

Figure 10. Temperature-depth profiles for wells defining the Tule Valley shallow thermal anomaly. Letters in parentheses next to the well names correlate to the well locations in figure 9. All wells are low confidence. All of the holes are much deeper than 100 m, so mapped temperatures were taken directly from the temperature-depth plots (figure 11). These data were collected in 2009, inferring that all drill holes were at thermal equilibrium. However, plotted zero-depth intercepts for some vary from the MAST, suggesting they may not have been at thermal equilibrium or were affected by localized fluid flow. Wells with surface intercepts that suggest conductive regimes in the shallower section of the holes have temperatures of 14 to 17°C. Many of the wells appear to penetrate a convective regime between 500 and 700 m (most of these at around 650 m), while Crypto 08-15 does so at about 200 m. Note that Wilson Health Springs (60°C) is only about 6 km to the north-northeast. The dominant rocks here are Paleozoic carbonates. The thermal gradients are generally uniform at about 35°C/km. Thermal conductivity measurements made on 19 depth intervals from the 08-12 and 08-15 wells averaged about 3.0 W/m·K, suggesting that heat flow in the area is about 105 mW/m2. High heat flow and the existence of nearby hot springs suggest another thermal anomaly could be in the Crypto area, but the lack of additional well data prevents it from being defined. 3.1.6 Greater Snake Valley Area Most of the Snake Valley wells are cooler, with temperatures between 13 and 19°C at 100 m depth. While most temperature depth profiles from Blackett (2011) appear to be conductive, Gwynn, et al.

Figure 11. Temperature-depth profiles for drill holes in the Crypto mining area (see figure 6). The best-fit gradient for these wells is about 35°C/km. All wells are high confidence. some wells (particularly PW06D, PW12A, and PW19C; figure 1) appear to enter convective zones with turnover or isothermal profiles at depth. Established gradients in these wells are typically less than about 30°C/km, although the gradient above the water table (about 230 m) in PW-18A is much higher at about 135°C/km. The BHT in PW18A (figure 1) is about 47°C at 300 m, much higher than all other Snake Valley wells, but a lack of surrounding data makes it impossible to define an anomaly. 3.2 Deep Thermal Regime Data from 12 plugged and abandoned oil exploration wells were used by Gwynn et al. (2013) and Allis et al. (2015b) to characterize the deep thermal regime in the Black Rock Desert. These data, along with data from the Acord 1 well (Allis et al., 2015a, 2016; Gwynn et al., 2016), were combined with corrected BHT data for the rest of the deep wells across this study area (mainly in the western half of the study area and not including the CF and RHS hydrothermal systems) to characterize the overall deep thermal regime. The new data are plotted by well in figure 12, where a best-fit gradient is shown to be about 25°C/km. The typical uncertainty in these corrections is considered to be ±10°C. For comparison, figure 13 combines the new data with the BRD and Acord 1 geotherms, and the Pavant Butte 1 equilibrium profile (Allis et al., 2015b; Gwynn et al., 2016). Gwynn, et al.

Figure 12. Corrected BHT data for deep oil and gas exploration wells in the study area that have not previously been published. The best-fit gradient for these wells is about 25°C/km.

4. Discussion 4.1 Data Quality Extrapolating from shallow temperature-depth profiles introduces an element of uncertainty to the mapped temperatures. Wells that have not returned to pre-drilling formation conditions when temperatures are logged can be an additional factor that can have significant effects. Disturbed profiles may cause spatial incoherency of mapped temperatures in certain areas. Unfortunately, shallow well data rarely include the duration between drilling completions, pumping, or other disturbances, and temperature logging. We assume most of the thermal-gradient wells were at, or close to, thermal equilibrium when they were logged. This assumption is supported because the surface-intercept temperature in most wells was close to the estimated MAST. But it has been reported that many of the gradient wells that were part of large-scale reconnaissance drilling programs probably were allowed very minimal recovery periods, so some uncertainty remains. Spatial distribution of data also leads to uncertainty in the mapped temperatures due to scarcity of gradient well data. Water well data were used to help offset this effect, but are lower quality and are often not near the most thermally interesting areas. Gwynn, et al.

Figure 13. Measured equilibrium profile for the Pavant Butte 1 well, calculated geotherms generated for deep oil exploration wells in the BRD and the Acord 1 well in the RHS area, and new corrected BHT data from 14 other oil exploration wells across the study area (modified from Allis et al., 2015b). These factors affect all of the mapped shallow anomalies to some degree, but probably have the greatest effect on the TV, SFM, and northern BRD anomalies. Because exploration was much more focused in the developed CV and RHS areas, data density is much higher and more care was likely taken to ensure wells were at thermal equilibrium.

4.2 Shallow Thermal Anomalies The BRD anomaly is interesting, and differs from the others, in that both shallow and deep well data suggest a primarily conductive thermal regime and very high temperatures (Gwynn et al., 2013; Allis et al., 2015b). The lack of convective heat transport and relatively low thermal conductivity in the basin result in a subtle shallow anomaly despite the fact that the Pavant Butte 1 well is likely about 250°C at a TD of 3.3 km (figure 13). High heat flow of the Basin and Range (mean 85–90 mW/m2; Lachenbruch and Sass, 1978; Blackwell, 1983; Morgan and Gosnold, 1989), coupled with a local heat source at mid-crustal depths (Gwynn et al., 2013; Allis et al., 2015b), are the likely causes of the deep BRD thermal anomaly. It is also possible that deep fluid flow has had some effect on the thermal system (see Allis et al., 2017b for a discussion of deep fluid flow in the southern Black Rock Desert). Gwynn, et al.

There is little doubt the TV, SFM, and DM-WV anomalies are convective in nature. While only a few wells are deep enough to penetrate definitive convective upflow/outflow zones beneath the shallower conductive zones, convective systems must be present. The high conductive gradients (near-surface) in the DM-WV area and, to a lesser extent, in the TV and SFM anomalies, would lead to incredibly high and unrealistic temperatures at deeper depths without underlying convective zones. Indeed, it is the convective heat transport that is responsible for the greatly elevated temperatures at relatively shallow depths. Among the three anomalies caused by geothermal upflow and outflow, the mechanism controlling the DM-WV anomaly is the most complicated. The others appear to be relatively simple fault-controlled systems, particularly where changes in fault orientation (bends or terminations) likely increase vertical permeability. The outflow in the TV anomaly seems to stem from, and flow perpendicular to, fault strand terminations in the north-south-trending House Range (west side) fault. Gradients in several wells suggest that upflow could be as close as 75 m to the surface. A line of 25 to 28°C springs roughly parallel to the fault zone stretches the 20°C contour to the north, and although no faults are mapped or inferred near the springs, a concealed fault siphoning some of the outflow to the north seems to be a reasonable explanation. This extension is also topographically down gradient. Shallow outflow from RHS appears to move mainly to the northwest (Allis et al., 2015a, 2016; Gwynn et al., 2016). Milford Valley, west of the Opal Mound fault, appears to be primarily conductive to at least 3.8 km (the depth of Acord 1; figure 13), so any significant fluid flow to the west would have to be very deep, and it is difficult to envision a mechanism where such fluid would again ascend to the near-surface in the San Francisco Mountains. The focal point of the SFM anomaly is located near a 90° bend in the San Francisco Mountains (west side) fault (figure 2). What is perhaps more interesting about the SFM system is that outflow is to the west, as would be expected, as well as to the east beneath or through Tertiary volcanic rocks. No gradients in the SFM wells show convective characteristics, so the upflow must be at least 200 m deep. This may help explain how fluids migrate both east and west from the center. Overall, the flow directions for the SFM, TV, and DM-WV anomalies are consistent with mapped flow paths from figure 8.2 of Hurlow et al. (2014). The thermal regime of the DM-WV system is more difficult to understand. There are no known or inferred faults near the center of the anomaly, nor are there any thermal springs. Moreover, it appears that the primary upflow is near a topographic groundwater divide, allowing outflow to extend long distances to the south and to the north. The hottest zone is rather sinuous and stretches along the center of the valley, suggesting upflow may occur at several locations in a semi-linear band. The most plausible cause for this pattern is a concealed fault zone that is most permeable near the topographic high and allows warm water to flow north and south after being heated during deep circulation. The Roosevelt system appears to host a local heat source, likely partial melt at depths below 5 km based on a deep, low-velocity, high-attenuation anomaly (Robinson and Iyer, 1981). It is possible that this heat could boost temperatures beneath the San Francisco Mountains. However, the most plausible model for the SFM system, along with the TV and DM-WV systems, simply involves deep circulation and buoyant upflow of heated water within the high-heat-flow Basin and Range Province.

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5. Conclusions Shallow temperature anomalies in west-central Utah appear to be mainly caused by water movement disturbing the deep conductive thermal regime. The RHS and CF systems are by far the hottest, and both support geothermal power plants. The three newly defined hydrothermal anomalies (SFM, TV, and DM-WV) may be lower temperature systems and more work is needed to determine the deeper thermal regimes. The fourth anomaly (BRD) is in a predominantly conductive regime, and due to a nominally 3-km-thick accumulation of low- thermal-conductivity basin fill, near-surface temperatures are more subdued and do not climb much above background temperatures despite the temperature at 3–4 km being around 250°C. So, caution is needed when evaluating the deep geothermal potential of similar systems. The deep thermal regime outside of the Pavant Butte area of the BRD, CF, and in Milford Valley is likely controlled by high regional heat flow that can be sufficient, especially if combined with thick accumulations of basin fill, to generate thermal anomalies such as those found in the San Francisco Mountains, Tule Valley, and the Drum Mountains-Whirlwind Valley area. The relative lack of both deep and shallow well data outside the areas of the known anomalies could be concealing additional systems. Additionally, the lack of thermal springs in the BRD, SFM, and DM-WV, show that many thermal anomalies may not have any surface expression, which compounds the problem of locating potential geothermal systems. The shallow anomalies identified in this study suggest that geothermal systems may also exist in other underexplored areas of west-central Utah.

6. Acknowledgements We wish to thank Michael Vanden Berg, Stephanie Carney, and Mike Hylland for their helpful reviews of this paper.

REFERENCES

Allis, R., Blackett, B., Gwynn, M., Hardwick, C., Moore, J., Morgan, C., Schelling, D., and Sprinkel, D. “Stratigraphic Reservoirs in the Great Basin—the Bridge to Development of Enhanced Geothermal Systems in the U.S.” Transactions: Geothermal Resources Council, 36, (2012), 351–357. Allis, R., Gwynn, M., Hardwick, C., Kirby, S., Bowers, R., Moore, J., Wannamaker, P., Simmons, S. “Characteristics of the Cove Fort-Dog Valley-Twin Peaks Thermal Anomaly, Utah.” Proceedings: 42st Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA. (2017a). Allis, R.G., Gwynn, M., Hardwick, C., Kirby, S., Moore, J., and Chapman, D. “Re-evaluation of the Pre-Development Thermal Regime of Roosevelt Hot Springs Geothermal System, Utah.” Proceedings: 40th Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA. (2015a). Allis, R., Gwynn, M., and Kirby, S. “Pressure Trends at Cove Fort and Roosevelt Hot Springs Geothermal Systems Provide Insight to their Flow Regimes. Transactions: Geothermal Resources Council, 41, (2017b). Gwynn, et al.

Allis, R.G., Hardwick, C., Gwynn, M., and Johnson, S. “Pavant Butte, Utah Geothermal Prospect Revisited.” Transactions: Geothermal Resources Council, 39, (2015b), 379–387. Allis R.G., and Larsen, G. “Roosevelt Hot Springs Geothermal Field, Utah—Reservoir Response After More Than 25 years of Power Production.” Proceedings: 37th Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA. (2012). Allis, R., Moore, J., Blackett, R., Gwynn, M., Kirby, S., and Sprinkel, D.S. “The Potential for Basin-Centered Geothermal Resources in the Great Basin.” Transactions: Geothermal Resources Council, 35, (2011), 683–688. Allis, R.G., Moore, J., Davatzes, N., Gwynn, M., Hardwick, C., Kirby, S., McClennan, J., Pankow, K., Potter, S., and Simmons, S. “EGS Concept Testing and Development at the Milford, Utah FORGE Site.” Proceedings: 41st Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA. (2016). Blackett, R.E. “Temperature Profiles of Monitoring Wells in Snake Valley, Tulle Valley, and Fish Springs Flat, Millard and Juab Counties, Utah.” Utah Geological Survey Open-File Report 578, (2011), 41p. Blackwell, D.D. “Heat flow in the Northern Basin and Range Province.” Geothermal Resources Council Special Report 13, (1983), 81–92. Edwards, M.C. “Geothermal Resource Assessment of the Basin and Range Province in Western Utah.” M.S. Thesis, University of Utah Department of Geology and Geophysics, (2013), 113 p. Gwynn, M. “Geothermal Potential in the Basins of East-Central and Southeastern Nevada.” Transactions: Geothermal Resources Council, 39, (2015), 13–23. Gwynn, M., Allis, R., Hardwick, C., Hill, J., and Moore, J. “A New Look at the Thermal Regime Around Roosevelt Hot Springs, Utah.” Transactions: Geothermal Resources Council, 40, (2016), 551–558. Gwynn, M.L., Allis, R.G., Sprinkel, D., Blackett, R., and Hardwick, C. “Geothermal Potential in the Basins of Northeastern Nevada.” Transactions: Geothermal Resources Council, 38, (2014), 1029–1039. Gwynn, M., Blackett, R., Allis, R. and Hardwick, C.L. “New Geothermal Resource Delineated Beneath Black Rock Desert, Utah.” Proceedings: 38th Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA. (2013), 1511–1519. Heilweil, V.R., Sweetkind, D.S., and Susong, D.D. “Chapter A: Introduction.” In Heilweil, V.M., and Brooks, L.E. (eds.), Conceptual Model of the Great Basin Carbonate and Alluvial Aquifer System, U.S. Geological Survey Scientific Investigations Report 2010-5193, (2011) 191 p. Henrikson, A., and Chapman, D.S. “Terrestrial Heat Flow in Utah.” University of Utah Department of Geology and Geophysics Report, University of Utah, Salt Lake City, UT. (2002) 47 p., https://geology.utah.gov/resources/energy/geothermal/#tab-id-5. Hurlow, H., ed. “Hydrogeologic Studies and Groundwater Monitoring in Snake Valley and Adjacent Hydrographic Areas, West-Central Utah and East-Central Nevada, Utah.” Utah Geological Survey Bulletin 135, (2014), 294 p. Gwynn, et al.

Lachenbruch, A.H., and Sass, J.H. “Models of an Extending Lithosphere and Heat Flow in the Basin and Range Province.” In Smith, R.B., and Eaton G.P. (eds.), Cenozoic Tectonics and Regional Geophysics of the Western Cordillera, Geological Society of America Memoir 152, (1978), 209–250. Mabey, D., and Budding, K. “High Temperature Geothermal Resources of Utah.” Utah Geological Survey Bulletin 123, (1987), 64 p. Morgan, P., and Gosnold, W.D. “Heat Flow and Thermal Regimes in the United States.” In Pakiser, L.C., and Mooney, W.D. (eds.), Geophysical Framework of the Continental United States, Geological Society of America Memoir 172, (1989), 493–522. Puchlick, K., “Temperature-Depth Data for the Crypto Project of Lithic Resources Limited.” Vancouver, Canada, unpublished digital data, (2009). Robinson, R., and Iyer, H.M., “Delineation of a Low-Velocity Body under the Roosevelt Hot Springs Geothermal Area, Utah, Using Teleseismic P-wave Data.” Geophysics, 46, (1981), p. 1456–1466. Sass, J.H., Priest, S.S., Blanton, A.J., Sackett, P.C., Welch, S.L., and Walters, M.A. “Geothermal Industry Temperature Profiles from the Great Basin.” U.S. Geological Survey Open-File Report 99-425, (1999), http://pubs.usgs.gov/of/1999/of99-425/. Sass, J.H and Walters, M.A. “Thermal regime of the Great Basin and its Implications for Enhanced Geothermal Systems and Off-Grid Power.” Transactions: Geothermal Resources Council, 23, (1999), 211–217. Staargaard, C.F. “Geology and Exploration at the Crypto Zinc-Indium-Copper-Molybdenum Skarn Deposit, Fish Springs Mining District, Juab County, Utah.” in Tripp, B.T., Kahulec, K, and Jordan, L.J. (eds.), Geology and Geologic Resources and Issues of Western Utah, Utah Geological Association Publication 38, (2009), 400 p. Tietz, P.G., Ristorcelli, S., and Staargaard, C.F. “Technical Report on the Crypto Zinc-Copper- Indium Project, Juab County, Utah.” Technical report by Mine Development Associates for Lithic Resources, (2010), 158 p. Welhan, J., and Gwynn, M. “High Heat Flow in the Idaho Thrust Belt: A Hot Sedimentary Geothermal Prospect.” Transactions: Geothermal Resources Council, 38, (2014), 1055– 1066.