Shallow Thermal Anomalies in West-Central Utah

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 Tule Valley, 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.

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