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The Cove Fort-Sulphurdale (CFS) Known Geothermal Resource Area (KGRA) is located in Millard and Beaver Counties in south-central Utah, near the intersection of Interstate Highway 15 and 70.

Cove Fort is an old stone fort built by Mormon settlers in 1867 as a way station for travelers. The presence of sulfur mines, gas seeps that emit hydrogen sulfide and altered ground initially suggested the existence of an extensive and exploitable geothermal resource. The site of the old mine at Sulphurdale is the largest of these sulfur deposits.

In 1893, all of the nation’s native sulfur, about 1200 tons, came form Sulphurdale. The mines produced a total of 30000 long tons between 1885 & 1952, when competition from other sources forced it to close.

Geology

The CFS KGRA lies near the junction of the Pavant Range and Tusher Mountains on the eastern margin of the Basin and Range province. These mountains mark the transition between the Colorado Plateau and the Basin and Range provinces. They are composed primarily of Paleozoic to Mesozoic sedimentary rocks that are covered to the south and east of Cove Fort by Tertiary volcanic rocks. Intrusive rocks related to the volcanic activity are exposed at several places within the area.

Numerous reports and maps have been published on the geology of the CFS area. The sedimentary rocks of the CFS area are part of a broad, north-trending thrust belt deformed during the Late Cretaceous Sevier Orogeny. Rocks penetrated to depths of up to 7,700 feet in Union Oil Company’s deep geothermal wells consist largely of limestone and dolomite that were variably metamorphosed by Tertiary intrusions. Sandstone occurs near the top of the sedimentary sequence.

The Tertiary volcanic rocks erupted between about 30 & 19 mil years ago from widely scattered centers in two distinct volcanic terranes—the Marysvale volcanic field of the High Plateaus to the east of Cove Fort and the Basin and Range to the west. They include lava flows, volcanic breccias and thick sequences of ash flow tuffs derived from local and distant sources. The base of the volcanic sequence near Cove Fort consists mainly of locally derived lava flows and breccias of intermediate composition. The upper parts of the Tertiary volcanic sequence consist predominately of ash-flow tuffs. Some of the units reach thicknesses of several thousand feet. Many of the ash flow tuffs are distinctive and widely distributed, and they are important marker horizons that have allowed detailed mapping of structures within the geothermal field.

Renewed volcanic activity between 1 mil years and 300000 years ago produced a shield volcano in the Cove Fort basalt field. It has been suggested that the heat source of the CFS geothermal system may be related to this basaltic volcanism, but there is no direct geochemical or thermal evidence to support this hypothesis.

In a review of the CFS area, it has been concluded that geologic and geophysical data indicate that the geothermal system is controlled by faults and fractures. The oldest structures are thrust faults that disrupted the sedimentary rocks during the Sevier Orogeny. Thrust faults may be widely distributed at depth in the reservoir rocks of the thermal area. Since Basin and Range tectonism began in the mid-Miocene, rocks of the CFS area have been disrupted by both high- and low-angle northerly and easterly trending normal faults. Continued activity is indicated by fault scraps in the alluvium and lava flows of the Cove Fort basalt field and by a high level of micro-earthquakes in the vicinity of Cove Fort. Here, at Sulphurdale, and along the western margin of the Pavant Range, the trends of the faults are marked locally by the alignment of sulfur deposits, acid-altered alluvium, and gas seeps.

Low-angle faults bound extensive gravitational glide blocks between Sulphurdale and the steeply dipping Cove Creek fault, which parallels Interstate 70. These westerly dipping low angle faults display pronounces arcuate trends in plain view. The gravitational glide blocks form a nearly impermeable cover over the geothermal system that has profoundly influenced the distribution of the surficial alteration and shallow temperatures and thermal gradients along the northwestern flank of the Tushar Mountains.

Early Exploration

The Thermex Company took the first fee geothermal leases in the CFS area in 1972. In 1974, when the Federal geothermal regulations went into effect, a “land rush” began and fee, federal, and state leases were acquired by numerous companies, including AMAX (later Steam Reserve), Phillips Petroleum Company, Chevron Resources Company, Hunt Energy Corporation, and Union Geothermal Division. Also holding federal leases in the CFS area were Earth Power Corporation and the Grace-owned companies of Thermal Resources, Inc and U.S. Geothermal Corporation. Federal leases held by Earth Power and the Grace companies were subsequently farmed out to Hunt Energy.

The companies started a massive exploration effort in 1974. During 1975 and the following three years, numerous rigs were operating throughout the CFS area drilling temperature-gradient holes. Competition was strong and secretive, to the point that gradient holes were being locked to prevent others from logging them. The attempts at locking holes were not always successful and numerous cases of hole “break-in’s” were reported to government agencies.

More than 200 temperature-gradient holes were drilled in an area of more than 100 square miles. Ultimately, most of the companies agreed to trade data. The results showed that the drilling efforts had defined a shallow thermal anomaly that covered more than 70 square miles. It was also discovered that, despite the immense size of the shallow anomaly and all of the surface geothermal manifestations, the deeper and hotter source of the anomaly was still an elusive target.

Between 1975 & 1979, Union Geothermal Division drilled four deep exploration wells to test the geothermal system. The first well recorded temperatures of nearly 350F degrees below depths of 5000 feet. However, the high cost of drilling, high corrosion rates, low reservoir pressures, and the apparent limited extent of the high-temperature reservoir led to a premature conclusion by Union Geothermal Division in 1980 that the field was not economic for large-scale electric power production.

Resource Development

In 1983, Mother Earth Industries, Inc obtained fee leases from Steam Reserve, geothermal leases on the patented mining claims from Forminco and the Union Geothermal Division federal geothermal leases. In October 1983, while drilling theor first well, they penetrated a 100-psi, 350Fdegree dry steam resource at 1165 feet in fractured sandstone below the volcanic rocks. The drillers were unable to contain the steam, and the well discharged uncontrollably for 24 days. Oil field techniques had to be used to cap the well. Although the well was lost, this blow out demonstrated the existence of a shallow steam reservoir. In January and May 1984, they completed 2 more wells within 200 feet of the original well, as dry stem producers. They too penetrated the steam at about 1160 feet.

In 1987, they began a broad scale exploration program that included a soil mercury and several geophysical studies, together with drilling of slim diameter wells offsetting the existing steam production wells. The geophysical work included self potential, ground magnetic, and controlled-source audio magnetotelluric surveys. Based on the results of these surveys, ten new temperature gradient holes were drilled to an average depth of 100 feet at site around the Sulphurdale fee lands.

Following the encouraging results of the 1987 exploration studies and drilling, Mother Earth drill six slim holes and twinned three of them with production-scale wells in 1988 & 1989. All of the production wells and all but one of the slim holes produced steam from the Talisman Quartzite. Flow tests of these wells showed that permeability within the steam cap was very high and that all the wells were hydraulically connected. No wells were drilled in 1990, but in 1991, steam pressure losses due to increased power plant demands dictated the need to explore for the hot-water reservoir long thought to underlie the steam cap. A well was drilled at the northwest corner of the Sulphurdale pit. It encountered 315F degree liquid-dominated geothermal reservoir in Paleozoic carbonate rocks at a depth of about 1800 feet. Small amounts of steam were produced before the well hit the water table at 1050 feet, but these zones were cased off.

Utilization

The CFS geothermal resource was developed in three phases. Construction of the first phase began in 1984 and came online in 1985. It consisted of four ORMAT binary units that used isopentane as a working fluid. These units had a combines generating capacity of about 1.5MWe.

The second phase of development started in March 1988 with a con-condensing topping turbine capable of producing 1.8MWe. Both of these phases utilized steam from two wells on the federal lease. A third phase, consisting of a condensing turbine with a 7.5MWe capacity, was constructed in 1990 and utilized steam from three wells located on the fee land.

From mid-1992 to April 1994, steam production from the federal wells was stopped, but wells on the fee land continued to produce steam for the 7.5MWe condensing turbine. Normal operations resumed in April 1994. In May 1996, the water well was put into production and the non-condensing topping turbine was removed from the system. Subsequently, all five steam wells and the water well supplied steam to the condensing turbine.

In June 2003, the plant was sold. The plant has been shut down since the sale. No plans have been released to restart the plant or for further development of the resource.