PROCEEDINGS, 41st Workshop on Geothermal Reservoir Engineering
Stanford University, Stanford, California, February 22-24, 2016
SGP-TR-209
The Geology, Geochemistry, and Geohydrology of the FORGE Deep Well Site, Milford, Utah
Stuart Simmons1,2, Stefan Kirby3, Clay Jones1, Joe Moore1, Rick Allis3, Adam Brandt1 and Greg Nash1
1
EGI, University of Utah, 423 Wakara Way, Salt Lake City, UT 84108
2
Department of Chemical Engineering, University of Utah, 50 S. Central Campus Dr., Salt Lake City, UT 84112
3
Utah Geological Survey, PO Box 146100, Salt Lake City
Keywords: EGS, geology, geochemistry, mineralogy, faulting, hydrology, Basin and Range
ABSTRACT
The Milford FORGE deep well site is located 5 km west of the Roosevelt Hot Springs, on alluvial fan deposits (200-600 m thick) that
overlie a large volume of hot crystalline basement rock. The site occurs within a region that is geologically complex and characterized
by extensional faulting, sporadic magmatism, and zones of anomalously high heat flow, inside the southeast margin of the Great Basin.
The basement rocks at the FORGE deep well site are made up of Precambrian gneiss and Tertiary plutons. These crystalline units are
exposed across the eastern part of the Milford Valley basin, from the Mineral Mountains in the east to the Acord-1 well in the west, and
they are separated by intrusive and fault contacts. Gneiss contains biotite, hornblende, K-feldspar, plagioclase, quartz, and sillimanite,
and isotopic dating indicates Proterozoic metamorphism ~1720 Ma. Plutonic rocks comprise diorite, granodiorite, quartz monzanite,
syenite, and granite, containing variable amounts of biotite, clinopyroxene, hornblende, K-feldspar, magnetite-ilmentite, plagioclase,
and quartz. The oldest intrusion was emplaced ~25 Ma followed by younger intrusion events at ~18 Ma and 11 to 8 Ma.
Hydrothermal alteration is widespread, but weak, and it is made up of quartz, illite, chlorite, mixed-layered clays, epidote, leuxcoene,
hematite, calcite, anhydrite, and K-feldspar, which partly replace precursor minerals or deposited into open spaces. Temperature-
sensitive phases lack well-defined depth zonation, which suggest that most of the alteration formed during earlier periods of
hydrothermal activity associated with Tertiary magmatism. Modern hydrothermal activity is responsible for steam-heated acid alteration
in the vicinity of fumaroles and steaming ground north of the Negro Mag fault, and silica sinter deposition along the Opal Mound fault.
High angle normal faults formed from east-west Basin and Range extension, but listric sliding and block rotation also produced low
angle structures characterized by narrow zones of cataclasis in crystalline rocks. The Opal Mound fault is a prominent high-angle fault
that dips east and offsets surficial deposits of alluvium and silica sinter, forming a hydrological barrier to lateral fluid flow. Additional
north-south trending normal faults, which are blind to the surface, are likely to occur in basement rocks to the west beneath the alluvial
cover. The Negro Mag fault is high-angle too, but it trends east-west, cutting across the Mineral Mountains. Lineament analysis and
field mapping show that the crystalline rocks exposed in the Mineral Mountains host a fault-fracture mesh, characterized by relatively
dense joint spacing, with fractures oriented in many different directions. Such fractures could play an important role in stimulating new
permeability beneath the FORGE deep well site.
The groundwater regime across the FORGE deep well site is controlled by the west sloping potentiometric surface and an unconfined
aquifer hosted in alluvial gravels. Geochemical data trace shallow hydrothermal outflow to the northwest and west, consistent with
temperature profiles in gradient wells. The groundwater is chemically benign, non-potable, and suitable for EGS heat transfer
experiments.
1. INTRODUCTION
The Milford FORGE deep well site (Fig. 1) is located 350 km south of Salt Lake City and 16 km north northeast of Milford, Utah, in an
unpopulated area that is predominantly used for renewable energy (wind, solar, geothermal). It is situated on a west sloping alluvial fan
in the North Milford valley, about halfway between the crest of the Mineral Mountains to the east and the Beaver River to the west. The
deep well site covers about 1.5 km2 and lies 5 km west-northwest of the Blundell geothermal power plant, which produces 35 MWe
from flash and binary units.
This paper represents a review of geoscientific data pertaining to the geology, geochemistry, and hydrology of the FORGE site, which
has been acquired over a period spanning 40 years. Much of this is the product of exploration and development of the Roosevelt Hot
Springs KGRA. However, Roosevelt Hot Springs represents only a small part of a large area associated with anomalous heat flow, and
the area to the west of the Opal Mound fault has long attracted interest in terms of EGS research and development (e.g. East, 1981; Goff
and Decker, 1983). As a result, data are available from numerous field surveys and the drilling of many shallow and deep wells,
including Acord-1, a 3.8 km deep well. These form the foundation for a 3D understanding the rock types, fault-fracture patterns,
thermal structure, hydrology, and fluid types that make up the reservoir of the proposed FORGE laboratory. Companion papers by Allis
et al (2016) and Hardwick et al (2016) summarize the physical attributes, including temperature and pressure regime and the basin
structure based on gravity data.
1
, Simmons et al.
Figure 1: Location of the FORGE deep well site near Milford, Utah, showing infrastructure and physiography (Allis et al.,
2016).
2. GEOLOGIC SETTING
The Milford FORGE site (Fig. 1) is located within a geologically complex zone that lies inside the southeast margin of the Great Basin
and abuts the western edge of the Colorado Plateau (e.g., Wannamaker et al., 2001). Regional features include folded and imbricated
Paleozoic-Mesozoic strata of the late Jurassic through Eocene Sevier orogeny, volcanic and intrusive centers resulting from Tertiary arc
magmatism, detachment faulting associated with regional extension, tilting and exhumation of core complexes, and north-south trending
normal faults resulting from Basin and Range extension (e.g., Dickinson, 2006; Anders et al., 2012). The zone also includes three
producing geothermal fields, Cove Fort-Sulphurdale, Roosevelt Hot Springs, and Thermo Hot Springs, which are associated with young
extensional faults, centers of Quaternary basalt-rhyolite magmatism, and large areas of anomalous heat flow covering >100 km2 (e.g.,
Mabey and Budding, 1987; Blackett, 2007; Kirby, 2012; Simmons et al., 2015; Wannamaker et al., 2015). The FORGE deep well site
lies within an area of anomalous conductive heat flow that extends west from the Opal Mound fault, outside the western boundary of the
Roosevelt hydrothermal system (Allis et al., 2016).
3. LITHOLOGY AND MINERALOGY
The main rock types associated with the FORGE deep well site are crystalline basement rocks made up of Precambrian gneiss and
Tertiary plutons, Tertiary basin-fill composed of volcanic strata, and Quaternary basin fill made of fluvial-lacustrine sedimentary
deposits. The occurrence and distribution of these units is known from field mapping and petrographic studies of cuttings and cores
mainly from four wells, 14-2, 52-2, 9-1, and Acord-1 (Figs. 2, 3 and 4; Glen and Hulen, 1978; Glenn et al., 1980; Sweeny, 1980; Welsh,
1980; Nielson et al., 1986; Coleman and Walker, 1992; Coleman et al., 1997; Hintze and Davis, 2003). Gravity data constrain the west
sloping contact that separates underlying crystalline rocks, gneiss and granite, from overlying volcanic deposits and fluvial-lacustrine
basin fill (Fig. 3; Hardwick et al., 2016). There is no evidence of any Paleozoic-Mesozoic strata in the vicinity of the deep well site,
despite being a major component of the regional stratigraphy and exposed in the southern and northern parts of the Mineral Mountains
(Nielson et al., 1986).
Precambrian gneiss is the oldest rock type, and outcrops occur sporadically at lowest elevations in the western Mineral Mountains, and
it was penetrated in wells (Figs. 2 and 4). Both banded and massive varieties of gneiss exist. Segregations of quartz-K-feldspar form the
light bands, whereas biotite-plagioclase-quartz-hornblende-K-feldspar form the dark bands (Glenn et al., 1980). Sillimanite occurs in
outcrops. These minerals form an interlocking texture, and planes or zones of weakness, appear to be absent. U-Pb dating of accessory
zircons gives an age of 1720 Ma, consistent with a model Rb/Sr whole-rock isochron of 1750 Ma, indicating early Proterozoic
metamorphism (Aleinikoff et al., 1987).
2
Stanford University, Stanford, California, February 22-24, 2016
SGP-TR-209
The Geology, Geochemistry, and Geohydrology of the FORGE Deep Well Site, Milford, Utah
Stuart Simmons1,2, Stefan Kirby3, Clay Jones1, Joe Moore1, Rick Allis3, Adam Brandt1 and Greg Nash1
1
EGI, University of Utah, 423 Wakara Way, Salt Lake City, UT 84108
2
Department of Chemical Engineering, University of Utah, 50 S. Central Campus Dr., Salt Lake City, UT 84112
3
Utah Geological Survey, PO Box 146100, Salt Lake City
Keywords: EGS, geology, geochemistry, mineralogy, faulting, hydrology, Basin and Range
ABSTRACT
The Milford FORGE deep well site is located 5 km west of the Roosevelt Hot Springs, on alluvial fan deposits (200-600 m thick) that
overlie a large volume of hot crystalline basement rock. The site occurs within a region that is geologically complex and characterized
by extensional faulting, sporadic magmatism, and zones of anomalously high heat flow, inside the southeast margin of the Great Basin.
The basement rocks at the FORGE deep well site are made up of Precambrian gneiss and Tertiary plutons. These crystalline units are
exposed across the eastern part of the Milford Valley basin, from the Mineral Mountains in the east to the Acord-1 well in the west, and
they are separated by intrusive and fault contacts. Gneiss contains biotite, hornblende, K-feldspar, plagioclase, quartz, and sillimanite,
and isotopic dating indicates Proterozoic metamorphism ~1720 Ma. Plutonic rocks comprise diorite, granodiorite, quartz monzanite,
syenite, and granite, containing variable amounts of biotite, clinopyroxene, hornblende, K-feldspar, magnetite-ilmentite, plagioclase,
and quartz. The oldest intrusion was emplaced ~25 Ma followed by younger intrusion events at ~18 Ma and 11 to 8 Ma.
Hydrothermal alteration is widespread, but weak, and it is made up of quartz, illite, chlorite, mixed-layered clays, epidote, leuxcoene,
hematite, calcite, anhydrite, and K-feldspar, which partly replace precursor minerals or deposited into open spaces. Temperature-
sensitive phases lack well-defined depth zonation, which suggest that most of the alteration formed during earlier periods of
hydrothermal activity associated with Tertiary magmatism. Modern hydrothermal activity is responsible for steam-heated acid alteration
in the vicinity of fumaroles and steaming ground north of the Negro Mag fault, and silica sinter deposition along the Opal Mound fault.
High angle normal faults formed from east-west Basin and Range extension, but listric sliding and block rotation also produced low
angle structures characterized by narrow zones of cataclasis in crystalline rocks. The Opal Mound fault is a prominent high-angle fault
that dips east and offsets surficial deposits of alluvium and silica sinter, forming a hydrological barrier to lateral fluid flow. Additional
north-south trending normal faults, which are blind to the surface, are likely to occur in basement rocks to the west beneath the alluvial
cover. The Negro Mag fault is high-angle too, but it trends east-west, cutting across the Mineral Mountains. Lineament analysis and
field mapping show that the crystalline rocks exposed in the Mineral Mountains host a fault-fracture mesh, characterized by relatively
dense joint spacing, with fractures oriented in many different directions. Such fractures could play an important role in stimulating new
permeability beneath the FORGE deep well site.
The groundwater regime across the FORGE deep well site is controlled by the west sloping potentiometric surface and an unconfined
aquifer hosted in alluvial gravels. Geochemical data trace shallow hydrothermal outflow to the northwest and west, consistent with
temperature profiles in gradient wells. The groundwater is chemically benign, non-potable, and suitable for EGS heat transfer
experiments.
1. INTRODUCTION
The Milford FORGE deep well site (Fig. 1) is located 350 km south of Salt Lake City and 16 km north northeast of Milford, Utah, in an
unpopulated area that is predominantly used for renewable energy (wind, solar, geothermal). It is situated on a west sloping alluvial fan
in the North Milford valley, about halfway between the crest of the Mineral Mountains to the east and the Beaver River to the west. The
deep well site covers about 1.5 km2 and lies 5 km west-northwest of the Blundell geothermal power plant, which produces 35 MWe
from flash and binary units.
This paper represents a review of geoscientific data pertaining to the geology, geochemistry, and hydrology of the FORGE site, which
has been acquired over a period spanning 40 years. Much of this is the product of exploration and development of the Roosevelt Hot
Springs KGRA. However, Roosevelt Hot Springs represents only a small part of a large area associated with anomalous heat flow, and
the area to the west of the Opal Mound fault has long attracted interest in terms of EGS research and development (e.g. East, 1981; Goff
and Decker, 1983). As a result, data are available from numerous field surveys and the drilling of many shallow and deep wells,
including Acord-1, a 3.8 km deep well. These form the foundation for a 3D understanding the rock types, fault-fracture patterns,
thermal structure, hydrology, and fluid types that make up the reservoir of the proposed FORGE laboratory. Companion papers by Allis
et al (2016) and Hardwick et al (2016) summarize the physical attributes, including temperature and pressure regime and the basin
structure based on gravity data.
1
, Simmons et al.
Figure 1: Location of the FORGE deep well site near Milford, Utah, showing infrastructure and physiography (Allis et al.,
2016).
2. GEOLOGIC SETTING
The Milford FORGE site (Fig. 1) is located within a geologically complex zone that lies inside the southeast margin of the Great Basin
and abuts the western edge of the Colorado Plateau (e.g., Wannamaker et al., 2001). Regional features include folded and imbricated
Paleozoic-Mesozoic strata of the late Jurassic through Eocene Sevier orogeny, volcanic and intrusive centers resulting from Tertiary arc
magmatism, detachment faulting associated with regional extension, tilting and exhumation of core complexes, and north-south trending
normal faults resulting from Basin and Range extension (e.g., Dickinson, 2006; Anders et al., 2012). The zone also includes three
producing geothermal fields, Cove Fort-Sulphurdale, Roosevelt Hot Springs, and Thermo Hot Springs, which are associated with young
extensional faults, centers of Quaternary basalt-rhyolite magmatism, and large areas of anomalous heat flow covering >100 km2 (e.g.,
Mabey and Budding, 1987; Blackett, 2007; Kirby, 2012; Simmons et al., 2015; Wannamaker et al., 2015). The FORGE deep well site
lies within an area of anomalous conductive heat flow that extends west from the Opal Mound fault, outside the western boundary of the
Roosevelt hydrothermal system (Allis et al., 2016).
3. LITHOLOGY AND MINERALOGY
The main rock types associated with the FORGE deep well site are crystalline basement rocks made up of Precambrian gneiss and
Tertiary plutons, Tertiary basin-fill composed of volcanic strata, and Quaternary basin fill made of fluvial-lacustrine sedimentary
deposits. The occurrence and distribution of these units is known from field mapping and petrographic studies of cuttings and cores
mainly from four wells, 14-2, 52-2, 9-1, and Acord-1 (Figs. 2, 3 and 4; Glen and Hulen, 1978; Glenn et al., 1980; Sweeny, 1980; Welsh,
1980; Nielson et al., 1986; Coleman and Walker, 1992; Coleman et al., 1997; Hintze and Davis, 2003). Gravity data constrain the west
sloping contact that separates underlying crystalline rocks, gneiss and granite, from overlying volcanic deposits and fluvial-lacustrine
basin fill (Fig. 3; Hardwick et al., 2016). There is no evidence of any Paleozoic-Mesozoic strata in the vicinity of the deep well site,
despite being a major component of the regional stratigraphy and exposed in the southern and northern parts of the Mineral Mountains
(Nielson et al., 1986).
Precambrian gneiss is the oldest rock type, and outcrops occur sporadically at lowest elevations in the western Mineral Mountains, and
it was penetrated in wells (Figs. 2 and 4). Both banded and massive varieties of gneiss exist. Segregations of quartz-K-feldspar form the
light bands, whereas biotite-plagioclase-quartz-hornblende-K-feldspar form the dark bands (Glenn et al., 1980). Sillimanite occurs in
outcrops. These minerals form an interlocking texture, and planes or zones of weakness, appear to be absent. U-Pb dating of accessory
zircons gives an age of 1720 Ma, consistent with a model Rb/Sr whole-rock isochron of 1750 Ma, indicating early Proterozoic
metamorphism (Aleinikoff et al., 1987).
2
