Developing a Conceptual Model of One of Three Geothermal Prospects at Hawthorne, Nevada, USA

Abstract

The Hawthorne, Nevada area in the western part of the Basin and Range province in the western USA has been the focus of geothermal investigations for over 40 years, with initial discovery of blind resources via anomalously warm water wells. Subsequent studies and drilling of temperature gradient holes and geothermal wells identified three separate blind geothermal prospects in the Hawthorne part of the Walker Lake basin. In 2017, the US Department of Energy funded a project to investigate the feasibility of installing deep, direct-use geothermal technology at the Hawthorne Army Depot and adjacent community of Hawthorne. For this project, we conducted a detailed review of all existing geoscience data acquired at the site to date to develop a quantitative estimate of geothermal resource potential for one of the Hawthorne geothermal prospects (prospect A – along the southwest side of the basin). This included review of substantial well data from water wells and geothermal exploration wells (downhole temperature logs, lithology, water chemistry, borehole televiewer, and alteration mineralogy), detailed geological and structural mapping information, geophysical data (gravity, magnetic, and seismic reflection), 2 m temperature data, and an existing 3D geological model of the basin. We find that the thermal anomalies associated with prospect A reflect the influence of two, related geothermal fluids in close proximity that are chemically distinct, with different temperatures and spatial extent (lateral and vertical). One fluid represents a deeper resource, hosted in altered, fractured Mesozoic granitic basement along a segment of the Wassuk Range-front fault system, and characterized by mature, alkali-chloride fluids, with ~4000 ppm total dissolved solids (TDS) and a maximum measured temperature of ~115 °C at ~1,500 m depth. A second fluid is hosted in Neogene basinal sediments at less than 400 m depth, with maximum measured temperatures of ~100 °C, TDS of ~1000 ppm, and a sodium-sulfate fluid chemistry. The outflow of this shallow resource can be tracked down gradient into the basin using well temperature data, which map a vertically constrained plume that cools with distance from the inferred upflow location. The data suggest that the deeper resource is conductively transferring heat to the shallow resource, and structural and/or stratigraphic compartmentalization is preventing direct interaction and fluid mixing. Here we integrate the data to develop a new conceptual model of prospect A, including the P10, P50, and P90 scenarios, and compare this with previous conceptual models developed for Hawthorne.