Application of Hydrothermal Alteration Mineral Analysis to Geothermal Reservoir Characterization for Three Geothermal Fields in the Western United States.

Geothermal energy is a renewable electricity resource that has the potential to reduce greenhouse gas emissions through continued development of new resources and management of currently produced reservoirs. In order to expand geothermal energy production, it is important to increase our understanding of these resources. One technique for characterizing geothermal resources is through analysis of hydrothermal alteration minerals. In this dissertation, I present detailed hydrothermal alteration analysis of three geothermal fields in the western Unites States: 1) a proposed Enhanced Geothermal System (EGS) site in the Basin and Range province (the Fallon FORGE EGS site), 2) an amagmatic convective fault-controlled intermediate-temperature geothermal resource in the Basin and Range province (Tungsten Mountain geothermal field, operated by Ormat Technologies.), and 3) a high-temperature magmatic-heat-sourced vapor- dominated geothermal system in northern California (The Geysers, operated by Calpine Corp). These three fields represent three distinct endmembers in terms of resource type, distinguished by variability in temperature, permeability, and geologic setting.Hydrothermal alteration minerals and lithologic characteristics of surface rock samples, drill core, and drill cuttings from geothermal wells are analyzed using the following techniques: sample inspection, petrographic microscopy, infrared spectroscopy in the Short Wave (1000-2500 nm) and Long Wave (2.5-25 μm) infrared wavelength ranges, bulk X-Ray Diffraction, and Scanning Electron Microscopy equipped with an energy dispersive X-ray spectroscopy (EDS) system. The hydrothermal alteration data generated in this study are compared with multidisciplinary datasets including geologic maps, downhole geophysical logs, temperature data, and fluid geochemistry in order to understand how the type and distribution of hydrothermal alteration minerals relate to the resource conceptual models of these sites. At the Fallon FORGE EGS site, the analyses supports previous ...