THERMAL SIGNATURE OF SUBSURFACE FLUID FLOW NEAR THE DIXIE VALLEY GEOTHERMAL FIELD, NEVADA

Abstract

Most of the geothermal development in the United States during the past decade has occurred within the Great Basin. One of the largest developments to date has been the Dixie Valley Geothermal Field (DVGF), one of a number of geothermal fields located along the southern margin of the "Battle Mountain High," a region of high heat flow (> 100 mW/m2) extending over much of the northern Great Basin. The DVGF lies east of the Stillwater Range and is centered on a southwest-northeast trending thermal anomaly generated by hot water moving up the Stillwater fault, a basin-bounding normal fault characterized by pervasive fracturing and hydrothermal alteration. As
part of a multidisciplinary investigation of fracture permeability, in-situ stress, and fluid flow within the Dixie Valley hydrothermal system, we measured
subsurface temperatures in four deep wells bordering the DVGF. Precision temperature logs from two flowing wells (45-14 and 66-21) provide detailed
information on the location and in-flow rates of permeable fractures intersecting the well bores. Temperature logs from two shut-in wells (76-28 and 62-21) provide information on conductive heat flow near the DVGF. Analysis of the combined dataset yields estimates of the rate and resulting thermal effects of fluid flow along the Stillwater fault. In well 66-21, located 6 km southwest of the current producing limits of the DVGF, and in well 45-14,
located 17 km southwest of the DVGF, water flowing up the Stillwater fault may increase the measured heat flow by 20 to 40% over the regional value. In wells 62-21 (4 km southeast of the DVGF) and 76-28 (2 km northeast of the DVGF), heat flow above the fault is close to the regional value, despite the presence of permeable fractures at depth. The limited spatial extent of anomalous heat and fluid flow associated with the DVGF is consistent with deep crustal thermal conditions indicated by the maximum hypocentral depths of nearby earthquakes and suggests that permeability enhancement along the
Stillwater fault is highly localized to regions a few kilometers in extent.