| Abstract |
Stratigraphic reservoirs with characteristically high permeability and temperature at economically accessible depths are attractive for power generation because of their potentially very large areas (> 100 km2) compared to hydrothermal upflow reservoirs (< 10 km2). A preliminary screening of the geothermal power potential of sedimentary basins in the U.S. assuming present day drilling costs and economic constraints (¡Ü 10c/kWh for the levelized cost of electricity over 30 years), and realistic permeability for the reservoirs, indicates that basins with heat flows of more than about 80 mW/m2 are required. Depending on the thermal resistance of the overlying sediments, reservoir temperatures of at least 150¡ãC at depths of less than 4 km are likely. This puts the focus for future geothermal power generation on high heat flow regions of California (e.g. the Imperial Valley and adjacent to The Geysers), the Rio Grande rift system of New Mexico and Colorado, the Great Basin of the western U.S., and high heat flow parts of Hawaii and the Alaska volcanic arc. By far the largest area of high heat flow is within the Great Basin. Here the highest, regionally extensive temperatures at depths of less than 4 km exist beneath the late Tertiary to Recent basins. Basins with more then 2 km of unconsolidated sediments are the most attractive because of the insulating effects of these sediments. The challenge is to locate the hottest basins with likely reservoirs within underlying bedrock units. Not all basins of the Great Basin have high heat flow, and adequate permeability for geothermal power production does not exist beneath some high heat flow basins. The lower Paleozoic carbonate units beneath the eastern Great Basin are known to be locally very thick (up to 5 km), and frequently have high permeability. A review of permeability measurements at 3 ¨C 5 km depth from petroleum and groundwater databases for the Great Basin and adjacent Rocky Mountains shows carbonates have the highest permeabilities (median value of 75 mDarcy), followed by siliclastic units. These values are sufficient for geothermal reservoirs. There is no permeability data for igneous rocks (intrusives and volcanics) in this depth range, but a trend towards low permeability with increasing depth exists between 1 and 2 km depth. In contrast to many oil and gas producing basins in the U.S., there is no evidence over-pressures at depths of 3 ¨C 4 km within the eastern Great Basin. This may be a consequence of extensional faulting and regionally extensive high permeability within the lower Paleozoic carbonates at depth. It appears the key factor determining whether basins here have high heat flow is whether inter-basin flow has occurred and cooled some basins. More heat flow measurements in basins of the northern Great Basin in particular are needed to identify the hottest basins. Understanding and characterizing the permeability at economically drillable depths beneath these basins is a major challenge. The extent to which characteristically high permeability can be found in certain stratigraphic units, and whether subhorizontal, structurally controlled, high permeability (such as in inactive thrust zones) also exists, is a current research focus. Large-scale power production from these reservoirs is likely to require application of enhanced permeability techniques such as acidization (carbonate reservoirs) and hydrofracturing. Reducing the drilling costs for repetitive drilling in these basins to 3 ¨C 4 km depth may be important for the economics, as is the possibility of solar enhan |