Thermal and structural assessment of the Pirouette Mountain geothermal prospect, Southern Dixie Valley, Nevada

Two blind geothermal anomalies were discovered in Southern Dixie Valley, NV in the late 1970s: the Pirouette Mountain anomaly and the Eleven Mile Canyon anomaly. Exploration of these systems was carried out in the late 1970s and early 1980s by Hunt Energy Corporation, including the drilling of three deep wells before the project was abandoned. At present, exploration has been reinitiated by the U.S. Navy Geothermal Program Office. A case study of Hunt's exploration indicates that explorationists did not have a strong conceptual model of Basin and Range geothermal systems, and therefore, the exploration procedure employed was not optimal. Rather than targeting specific structures hosting geothermal fluids, wells were drilled where temperatures and thermal gradients were highest, which commonly results in unsuccessful exploration in the Basin and Range province. Additionally, geochemical analysis of water samples from the system were reported as contaminated, and reservoir temperature estimates were never made to determine whether the system contained temperatures high enough for electricity production and justified further exploration. Heat loss calculations from shallow (∼150m) borehole data indicate that Pirouette Mountain and Eleven Mile Canyon have natural state surface heat losses of 25 MW and 8 MW, respectively, so the areas are certainly of commercial interest if temperatures are high enough.;Because of its higher heat loss and higher temperatures, Pirouette Mountain was chosen as subject of this thermal and structural analysis. A gravity inversion was performed on a residual gravity data set that was obtained by subtracting regional gravity values from a dataset of Complete Bouguer Anomaly gravity measurements. The inversion highlights a piedmont fault associated with the Dixie Valley fault zone that terminates in a "horse-tail" 1-2 km west of the shallow geothermal anomaly as well as a feature interpreted as a horst that contains elevated temperatures within the anomaly. The thermal regime of the system correlates well with the structure in the study area, but in order to better constrain the flow of geothermal fluids, reservoir temperatures must be estimated from geochemical analysis. If high reservoir temperatures exist, the fluids are most likely sourced from the area where the horse-tail fault termination and the intersection of the interpreted horst with the piedmont fault occur, west of the site of the deep exploration well.