| Authors |
James E. FAULDS, Nicholas H. HINZ, Mark F. COOLBAUGH, Jason W. CRAIG, Emma G. McCONVILLE, Jonathan M. GLEN, Bridget F. AYLING, Andrew J. SADOWSKI, Drew L. SILER, Stephen DEOREO |
| Abstract |
Most geothermal resources in the Great Basin region of the western USA are blind, and thus the discovery of new commercial-grade systems requires synthesis of favorable characteristics for geothermal activity. The geothermal play fairway concept involves integration of multiple parameters indicative of geothermal activity to identify promising areas for new development. In Phase 1 of the Nevada play fairway project, nine geologic, geochemical, and geophysical parameters were initially synthesized to produce a new detailed geothermal potential map of 96,000 km2 from west-central to eastern Nevada. These parameters were grouped into subsets and individually weighted to delineate rankings for local permeability, intermediate permeability, regional permeability, and thermal potential, which collectively defined geothermal play fairways (i.e., most likely locations for significant geothermal fluid flow). This initial work was aimed at reducing the risks in regional exploration and therefore facilitating discovery of new commercial-grade systems in blind settings, as well as in areas with surface expressions of geothermal activity. Phase 2 of the project involved detailed analysis of some of the most promising areas identified in Phase 1. Twenty-four highly prospective areas, including both known undeveloped systems and previously undiscovered potential blind systems, were identified for further analysis. After reconnaissance of these areas, five of the most promising sites were selected for detailed studies. Multiple techniques were employed in the detailed studies, including geologic mapping, shallow temperature surveys, gravity surveys, LiDAR, geochemical studies, seismic reflection analysis, and 3D modeling. The goal of the detailed studies was to identify areas with the highest likelihood for high permeability and thermal fluids, such that drill sites could be targeted. Phase 3 of the project involved more detailed geophysical analyses and temperature-gradient drilling in southeastern Gabbs Valley and northern Granite Springs Valley, deemed the two most promising sites, with the goal of providing preliminary validation of the play fairway methodology. In southeastern Gabbs Valley, the collocation of a favorable structural setting (displacement transfer zone and fault intersections), Quaternary faults, intersecting and terminating gravity gradients, magnetic low, shallow (2 m) temperature anomaly, low resistivity anomaly, and promising geothermometry from nearby water wells provided evidence for a blind system. Drilling of six temperature-gradient holes (TGH) defines an apparent geothermal system at this locality with temperatures as high as 124°C at 152 m. This system is blind, with no surface hot springs, fumaroles, or paleo-geothermal deposits. For northern Granite Springs Valley, a favorable structural setting (termination of a major Quaternary normal fault), terminating gravity gradient, magnetic gradient, newly discovered sinter deposits, nearby warm water wells, previously drilled TGHs, and promising geothermometry suggest a hidden system. Drilling of four TGHs yields temperatures of ~96°C at ~250 m, suggesting the presence of a geothermal system. Lessons learned in the course of this project include: 1) initially identified sites commonly include multiple favorable structural settings at a finer scale; 2) promising sites in Cenozoic basins cannot be recognized without detailed geophysical surveys; and 3) play fairway analysis should be updated as the exploration program vectors into the most promising sites and finer-scale data are acquired. This requires revision of initial predictions of permeability potential to reflect more detailed analyses and preparation of higher-resolution play fairway maps prior to selecting sites for drilling. |