Geothermal resource characterization and evaluation at Astor Pass, Nevada
A blind geothermal system adjacent to a large tufa tower at Astor Pass was characterized and evaluated according to a comprehensive field investigation consisting of geophysical and shallow temperature surveys, structural geological analysis, and reservoir characterization techniques. The geothermal reservoir system, located in basalts, basaltic andesites and rhyolites of the Lower Pyramid Sequence, likely originated as the result of transfer of dextral shear to normal fault displacement creating a localized region of enhanced extension and fracture permeability. Shallow temperature and magnetotelluric surveys and seismic image interpretation informed the location of two exploration wells. These wells reached down to the underlying granodiorite basement, confirmed fault structures seen in the seismic images, and were used to characterize the reservoir through borehole fracture characterization and a long duration hydraulic reservoir test. High fracture densities (0.7 m average spacing) were encountered in the reservoir rocks, and results from the hydraulic test indicate that the reservoir is well connected with drawdown responses observed in a shallow water well located approximately 1 km from the pumping well. Borehole spinner log surveys with temperature probes and high resolution DTS measurements indicate that fracture inflows are relatively constant with isothermal temperatures ranging between 92 to 95degC along the exploration wells. Analysis of the reservoir drawdown response yields transmissivity values of 4.8x10 4 to 6.7x10 5 m2/s, with fracture transmissivity values ranging between 1.0x10 4 to 7.2x10 7 m2/s. This comprehensive set of data is used to conclude that the Astor Pass geothermal reservoir is a small, low enthalpy system with relatively isothermal temperatures and high fracture connectivity and permeability. Introduction Astor Pass is located approximately 50 km northeast of Reno, Nevada on the Pyramid Lake Paiute Reservation (Figure 1) In the absence of surficial geothermal expressions, such as hot springs, fumaroles and gysers, the Astor Pass area is considered a blind resource. Astor Pass is, however, located approximately 6 km northwest from the 'Needles' a culturally sensitive area named after an impressive series of 90 m high calcium carbonate (tufa) towers along two northwest trending subparallel ridges (Coolbaugh et al. 2006; Vice et al. 2007) Several hot springs and an abandoned geothermal well with artesian conditions, all with boiling water temperatures at the surface, are located at the basal area of the Needles. Tufa towers and mounds, though not as large as the Needles, are also present at Astor Pass. The formation of these tufa features along faults and fault intersections at both Astor Pass and the Needles likely resulted from hot springs discharging into Pluvial Lake Lahontan during the Pleistocene (Vice et al. 2007) Previous geothermal resource investigations on the Pyramid Lake Paiute Reservation include: characterization of the Needles area (Garside and Schilling, 1979; Geothermal Development Associates, 1988) reservation wide investigations utilizing Geothermal Resource Characterization and Evaluation at Astor Pass, Nevada Donald M. Reeves1, Greg Pohll1, Brad Lyles1, Jim Faulds2, John Louie3, Bill Ehni4, Chris Kratt5, Clay Cooper1, Rishi Parashar1, Satish Pullammanappallil6, and Donna Noel7 1Desert Research Institute, Reno, NV, USA 2Nevada Bureau of Mines and Geology, University of Nevada, Reno, NV, USA 3Great Basin Center for Geothermal Energy, University of Nevada, Reno, NV, USA 4Ehni Enterprises, Inc. Carson City, NV, USA 5Zonge Geosciences, Inc. Sparks, NV, USA 6Optim, Reno, NV, USA 7Pyramid Lake Paiute Tribe, Nixon NV, USA