STRUCTURAL CONTROLS ON THE BRADYS GEOTHERMAL SYSTEM, WESTERN NEVADA: INSIGHTS FROM INTEGRATED GEOLOGIC, GEOPHYSICAL, AND 3D CHARACTERIZATION

The Bradys geothermal field lies ~80 km NE of Reno in W-central Nevada. It has a reservoir temperature of 180-207°C at 1- 2 km depth and has supported a power plant since 1992 (current 26 MW capacity). Initial exploration occurred in the 1950s. We have conducted detailed geologic mapping (including surficial geothermal features), structural analysis, detailed logging of cuttings and core, and seismic reflection surveys to elucidate the structural controls and generate a comprehensive 3D model of the system.

The area is dominated by NNE-trending gently to moderately tilted fault blocks consisting of Tertiary volcanic and sedimentary rocks resting on Mesozoic metamorphic-granitic basement. The fault blocks are bounded by moderately to steeply dipping, mainly NNE-striking normal faults. The Bradys fault zone (BFZ) controls the hydrothermal activity and consists of a complex system of en echelon, primarily WNW-dipping faults, some with Holocene ruptures. It has accommodated ~500 m of down-to-west throw. The surface expression of the geothermal system is a 4-km-long, NNE-trending zone of extensive sinter, warm ground, fumaroles, and mud pots along the BFZ.

The geothermal system occupies a discrete left step in the BFZ but also lies in a broader accommodation zone consisting of overlapping NW- and SE-dipping normal faults. The main production wells penetrate the down-plunge projection of the left step in the BFZ. Kinematic data indicate normal slip on NNE-striking faults. The BFZ is orthogonal to the regional WNW-trending extension direction and thus favorably oriented for fluid flow. We suggest that multiple intersecting fault strands in the step-over produce a zone of high fracture density that enhances fluid flow and facilitates the rise of a deep-seated thermal plume. The 3D model shows that production is from steeply plunging pipe-like zones of high fault density in the step-over at two stratigraphic levels: 1) Miocene dacite, and 2) Oligocene ash-flows and directly underlying Mesozoic metamorphic basement. Wells outside these pipes are hot but generally lack sufficient permeability for production. This work demonstrates the usefulness of integrated geologic-geophysical studies and 3D modeling in targeting productive wells at known systems and for potentially discovering blind or hidden geothermal fields.