STRUCTURAL SETTING OF THE WEEPAH GEOTHERMAL FIELD, NEVADA, USA

Fault complexities play an important role in localizing permeable fluid flow pathways in deep circulation systems. At the Weepah Geothermal Field, a blind greenfield geothermal system in the Basin and Range province of Nevada, a recent exploration campaign has enabled refinement of the structural model, provided constraint on hydrothermal fluid flow pathways, and aided in conceptual model development. Geothermal indications, including a prominent shallow thermal anomaly and a water well with neutral sodium-chloride type fluids and 165°C silica geothermometry, were first recognized at Weepah in the 1970’s and 1980’s. In 2009 and 2010, two exploration wells were drilled to 1000 – 1500 meters depth, and encountered ~130-150°C measured temperatures, but without significant permeability. Newly conducted multidisciplinary exploration activities to refine the structural model include analysis of LiDAR imagery, detailed field mapping of faulting, bedrock geology, and hydrothermal alteration, and petrography of well cuttings and core. This work also reanalyzed both geophysical data (magnetotellurics, gravity and magnetics) and downhole image log data. Integration of these datasets in 2D and 3D supplemented with legacy data show several moderate angle normal faults offsetting two major lithologic units – Tertiary basin-fill sediments unconformably overlying Precambrian to Cretaceous basement metasediments and magmatic intrusions, with a hypothesized low-angle detachment fault forming the contact. An approximately 2 km wide right-lateral step-over in one of these normal faults has been postulated as the most likely structural discontinuity that localizes 150 to > 165°C upwelling fluids. The surface trace and moderate dip (~40 degrees) of this fault is well-constrained by field mapping, image log analysis, downhole lithologic data, and supported by geophysics. A key remaining uncertainty is the subsurface location of upflow within the 2 km wide step-over. This uncertainty leads to a range of possible upflow-outflow geometries in multiple equally permissible conceptual models. Future well targeting will focus on testing these models, and therefore further derisking geothermal resource development at Weepah.