The solution to America's energy needs might come from a new type of geothermal energy reservoir identified in sedimentary basins of Utah and Nevada. In summer 2012, a team of geoscientists from the Utah Geological Survey (UGS) in cooperation with the U.S. Geological Survey (USGS) drilled seven geothermal gradient holes in Utah's Black Rock Desert basin to test a new concept of high temperature geothermal resources in sedimentary basins. Seven drill holes were funded by the U.S. Department of Energy as part of a National Geothermal Data System project, managed by the Arizona Geological Survey. Based on drilling results, researchers conservatively estimate a basin-wide power density of about 5 to 10 MWe/km2, (megawatt equivalent per square kilometer) which, given the large resource potential, is easily comparable to the energy output at The Geysers field in California, the U.S.'s most productive geothermal system.
Preliminary results show a near-surface gradient of about 60°C with temperatures reaching 170°C to 200°C at 3- to 4-km (1.9 to 2.5 miles) deep. A drill hole near Pavant Butte in the north part of the basin yielded exceptionally high temperatures of 200°C at a depth of just 3 km. The three additional research holes were drilled in the area over the past few years; these were funded by the Utah State Energy Program and the Utah Division of Wildlife. The 1,000 square kilometer Black Rock Desert basin is filled with unconsolidated sediments to a depth of 2 km, while the basin floor comprises porous and permeable Paleozoic-aged carbonates (limestones and dolomites) that host the geothermal reservoir. Earlier geothermal exploration in the Basin and Range focused on narrow, hydrothermal upwelling zones along bounding faults of mountain ranges. High temperatures encountered during oil exploration in the central Black Rock Desert basin motivated the UGS team to test whether deep sedimentary basins floored by older crystalline rock were capable of high heat flow.
Dr. Rick Allis, Director of the Utah Geological Survey and lead scientist of the sedimentary basin geothermal research project, said that existing heat flow maps don't identify the geothermal energy potential in the area. "There are other potentially hot basins across the Basin and Range province that need to be explored using this exploration model. There may be basins across the country that have similar unrecognized geothermal energy potential." The project findings were being presented on Monday, October 1, at the annual meeting of the Geothermal Resources Council in Reno, Nevada.
The National Geothermal Data System is in operational test mode, integrating large amounts of information from all 50 states to enhance the nation's ability to discover and develop geothermal energy. The Black Rock Desert basin – a stratigraphic reservoir, with moderate permeability and temperatures of 200°C, a production-injection 5-point well pattern with 500 m spacing, a pump rate of 1,000 gallons per minute (60 kg/s), and an injection temperature of 75°C – would yield a reservoir power density of 5-10 MWe/km2 for the first thirty years. Preliminary economic models point to air-cooled binary plants for generating electricity. An existing 500kV DC line with 1920 MWe capacity could deliver that electricity to California. The Basin and Range Province contains dozens of basins whose geometry and stratigraphy mimics that of Black Rock Desert basin. The carbonate aquifer unit that hosts the Black Rock Desert geothermal reservoir underlies much of eastern Nevada and western Utah, portending sedimentary basins of the Basin and Range Province as a major new geothermal resource. Because the U.S. Bureau of Land Management manages many of those basins, accessibility for geothermal exploration and future energy production is promising. The UGS – USGS team next plans to drill in the Nth Steptoe and Mary’s River Basins in Nevada to further test this stratigraphic reservoir model for geothermal resources in the Basin and Range Province.