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3D Characterization of the Astor Pass Geothermal System, Washoe County, Nevada
AdvisorFaulds, James E.
Geological Sciences and Engineering
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Seismic reflection interpretation, detailed analysis of well cuttings, stress field analysis, and construction of a 3D geologic model have been used in the characterization of the stratigraphic and structural framework of the Astor Pass geothermal area. The Astor Pass geothermal system resides in the northwestern part of the Pyramid Lake Paiute Reservation, on the margins of the Basin and Range and Walker Lane tectonic provinces in northwestern Nevada. The area is primarily comprised of middle Miocene Pyramid sequence volcanic and sedimentary rocks, nonconformably overlying Mesozoicmetamorphic and granitic rocks. Previous investigations of the Astor Pass area focused on the general structural setting, argillic alteration of the Miocene rocks, lineations of Quaternary tufa mounds, and a shallow temperature survey. Wells drilled at Astor Pass show reservoir temperatures of ~95°C. Analyses of well cuttings show a ~1 km thick section of highly transmissive and permeable Miocene volcanic and sedimentary rocksoverlying a relatively impermeable granodiorite and metavolcanic basement. Seismic reflection interpretation confirms a high fault density in the geothermal area, with many possible fluid pathways penetrating into the Mesozoic basement. Stress field analysis using borehole breakout data from the wells reveals a regionally consistent west-northwest-trending least principal stress direction, with stress magnitudes reflecting a complex transtensional faulting regime. A lack of data on the mechanical properties of the reservoir rocks, such as strength parameters, results in someambiguity in the calculated stress field. However, reasonable assumptions regarding these variables produced both a normal faulting and strike-slip faulting regime. Taking into consideration both possible stress regimes, the stress data wereutilized in a slip and dilation tendency analysis of the fault model, which suggests twopromising fault areas that would control the upwelling geothermal fluid. The first is amoderately dipping fault intersection plunging to the northwest away from the prominenttufa mound in the center of Astor Pass. The second subvertical fault intersection isdirectly beneath the tufa mound and closest to the highest temperatures measured in thegeothermal system. Both of these fault intersection areas show positive attributes forcontrolling geothermal fluids, but hydrologic tests show the 1 km thick volcanic sectionis highly transmissive. Thus, focused upwellings along discrete fault conduits may beconfined to the Mesozoic basement before fluids diffuse into the Miocene volcanicreservoir above.Astor Pass contains a moderate temperature geothermal system with a large,diffuse reservoir in the Miocene volcanic rocks, capable of sustaining high pump rates.Understanding this type of system may be helpful in exploring for large, permeablereservoirs in deep sedimentary basins of the eastern Basin and Range and the highlyfractured volcanic geothermal reservoirs in the Snake River Plain and elsewhere.