Integrating High Resolution Gravity, Magnetic and Magnetotelluric Data to Characterize the Structural Framework of a Blind Geothermal System in Southern Gabbs Valley, NV

Southern Gabbs Valley in west-central Nevada, is a recently discovered blind geothermal system identified by employing play fairway analysis (PFA) methodology, as part of a DOE funded multi-phase approach to assess geothermal resource favorability across northern Nevada. The PFA approach adapts petroleum exploration techniques to the identification of previously unknown geothermal resources. Previous studies of the valley identified numerous factors favorable for hosting a hydrothermal system, including complex structural zones, as well as geochemical data and a shallow thermal anomaly that indicated elevated subsurface temperatures. There are no surface thermal features present in southern Gabbs Valley, creating a challenge to pinpoint the resource and necessitating detailed geophysical investigation. High-resolution geophysical surveys (gravity, magnetic, and magnetotelluric (MT)) were conducted to characterize the structural framework of the basin and hydrothermal system. The major results of these geophysical studies include: 1) mapping previously unknown faults using maximum horizontal gradient (MHG) analysis; 2) detailed 2-dimensional (2D) joint gravity and magnetic forward models which delineate subsurface structure; 3) a gravity-derived depth to basement map, and; 4) a 3-dimensional (3D) resistivity model of the basin. These results contributed to a new detailed basin scale geologic map, and supported previous interpretations that the shallow thermal anomaly occurs at the intersection of multiple faults bounding an intra-basinal gravity high. The intra-basinal gravity high is a horst-like structure, and is collocated with multiple geophysical features, including a large subsurface resistive body that is capped by a shallow electrically conductive zone, and a magnetic low. We interpret this to reflect the presence of hydrothermal alteration that has led to demagnetization of the surrounding source rock, and the generation of a conductive clay cap above the reservoir. In 2018, a successful temperature gradient drilling program was conducted in the vicinity of these collocated features. Equilibrated bottom hole temperatures in the two hottest wells measured 114.5° and 124.9° at ~150 m depth, confirming the presence of a blind hydrothermal system in southern Gabbs Valley.