--> --> Stratigraphic reservoirs as geothermal targets in the Great Basin of the United States

AAPG Pacific Section and Rocky Mountain Section Joint Meeting

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Stratigraphic reservoirs as geothermal targets in the Great Basin of the United States


Geothermal power plants in the United States currently have the capacity to produce 3550 MWe. Fields in the Great Basin provide nearly 1000 MWe of the total generating capacity, with the remainder coming from The Geysers and the Salton Trough in California. The United States Geological Survey has concluded that the current capacity is only a small percentage of the 3 GWe of undiscovered geothermal energy that exists within the western United States. Although recent developments in the Basin and Range have been limited to projects of less than 100 MWe and typically a few square kilometers in area, analysis of heat flow data suggests that subhorizontal rocks in hot sedimentary basins could form stratigraphic reservoirs with areas of 100's of square kilometers. Despite their potential, few geothermal wells have been drilled into these basins. However, measurements from oil and gas wells document temperatures and permeabilities suitable for electric generation. Economic modeling indicates that power could be generated at a cost of less than 10c/kWh and that these costs could be reduced by pattern drilling. The most attractive prospects range from 150°C to 200°C at depths of 2 to 4 km. These temperatures correspond to heat flows of 80 to 100 mW/m2, values found where 2 to 3 km of insulating, poorly consolidated sediments overly the stratigraphic reservoirs. The required permeabilities of 10 to 100 millidarcies are common in carbonate and siliciclastic rocks. These reservoirs will require strongly deviated wells to optimize productivity. In some basins, faulting may enhance reservoir permeability. Sustainable power densities of more than 3 MWe/km2 and large-scale power development exceeding 100's of MWe is possible. Thus, these resources have the potential to significantly expand the geothermal resource base using existing oil and gas and geothermal technologies.