Interpretation of upwelling zones and structural controls at Salt Wells and Lee Allen geothermal fields; with two-meter temperature surveys, soil gas surveys, and analysis of existing geothermal exploration data
AdvisorCashman, Patricia H
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Development of an electricity grade geothermal resource requires a geothermal reservoir but these can be difficult to assess since they occur at depth. Near-surface manifestations such as thermal fluid upwelling zones are commonly connected to the geothermal reservoir at depth. Therefore the structural controls on an upwelling zone can be used to better target geothermal reservoirs. In this study two-meter temperature and soil gas surveys were completed and analyzed in relation to existing geothermal exploration data at Lee Allen and Salt Wells geothermal fields, Nevada, to better define upwelling zones and their structural controls, and better target associated geothermal reservoirs at depth. Over 100 two-meter temperatures were measured at the Lee Allen and Salt Wells geothermal areas in western Nevada, USA. The data were corrected for seasonal temperature drift. Analysis of remotely sensed imagery indicated that albedo effects on two-meter temperatures were secondary to geothermal influences. At Salt Wells two-meter temperature anomalies were discovered at Simpson pass and along the eastern flank of the Bunejug Mountains. Soil gas anomalies were found in these same areas. This data are consistent with and expand upon thirty-centimeter temperature data and mapped hydrothermal features. At Lee Allen, two-meter temperature anomalies were discovered at Lee Hot Spring and two other areas lying respectively northeast of Lee Hot Spring, and north of Allen Ridge. Soil gas anomalies were discovered at and near Lee Hot Spring, and north of Allen Ridge. The two-meter temperature and soil gas anomalies near Lee Hot Spring are consistent with elevated temperatures in wells and mapped hydrothermal features. The two-meter temperature and soil gas anomalies north of Allen Ridge correlate with recently mapped hydrothermal features, but current thermal activity was previously unknown in this area. From these data, inferences were made on the structural controls of fluid flow and the location of upwelling zones. At Salt Wells, at least two upwelling zones are suggested. Both of these upwelling zones occur within an accommodation zone between oppositely dipping normal fault sets. One of the inferred upwelling zones is associated with several faults at the southern end of the accommodation zone. The second upwelling zone follows another set of faults along the Bunejug Mountains, near the northern margin of the accommodation zone. At Lee Allen, at least two upwelling zones are indicated. One is in the area of Lee Hot Spring at the southern end of Allen Ridge and one is north of Allen Ridge. Both of these upwelling zones occur near a northeast-striking normal fault, which suggests that this fault exercises a significant control on the geothermal system and is likely connected to the main geothermal reservoir at depth.