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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
Abstract
AAPG Bulletin, V.
DOI: 10.1306/07092524067
Preliminary resource conceptual model and power capacity estimates for the Northern Granite Springs Valley blind geothermal prospect, Pershing County, Nevada
Kurt O. Kraal,1 Bridget F. Ayling,2 Nicole Hart-Wagoner,3 James E. Faulds,4 Jonathan M. G. Glen,5 Drew L. Siler,6 and Tait Earney7
1Great Basin Center for Geothermal Energy, Nevada Bureau of Mines and Geology, University of Nevada Reno, Reno, Nevada; present address: Ormat Technologies, Inc., Reno, Nevada; [email protected]
2Great Basin Center for Geothermal Energy, Nevada Bureau of Mines and Geology, University of Nevada Reno, Reno, Nevada; present address: Contact Energy Ltd., Wellington, New Zealand; [email protected]
3Great Basin Center for Geothermal Energy, Nevada Bureau of Mines and Geology, University of Nevada Reno, Reno, Nevada; [email protected]
4Great Basin Center for Geothermal Energy, Nevada Bureau of Mines and Geology, University of Nevada Reno, Reno, Nevada; [email protected]
5US Geological Survey, Moffett Field, California; [email protected]
6Geologica Geothermal Group, Reno, Nevada; [email protected]
7US Geological Survey, Moffett Field, California; [email protected]
ABSTRACT
Northern Granite Springs Valley (also known as Adobe Flat) in the Basin and Range province of Nevada in the western United States hosts a thermal anomaly and has been explored as a blind geothermal prospect since the late 1970s. From 2017 to 2021, it was the subject of geoscientific characterization as part of the Nevada Play Fairway Analysis and Innovative Geothermal Exploration through Novel Investigations Of Undiscovered Systems projects. These projects generated a three-dimensional (3-D) geologic map constrained by the available geological and geophysical data from the site. Here, we develop conceptual models that aim to represent the uncertainty in the hypothesized location of the hydrothermal upflow. This was done through the integration of the results of the 3-D geologic map, temperature data from shallow temperature surveys and temperature gradient holes (TGHs), fluid geochemistry and hydrologic data from wells, and the mapped distribution of paleo-geothermal deposits. The resource is defined by an ∼6 × 2-km approximately north–south-trending thermal anomaly along the eastern edge of the Adobe Flat playa. The maximum measured bottomhole temperature is 95.7°C at 250 m depth. The thermal anomaly is also colocated with paleo-geothermal deposits (opaline sinter, silicified sands, and tufa deposits inferred to be Holocene–late Pleistocene in age) and the southern termination of the faults bounding the western side of the Seven Troughs Range. This termination forms a network of west-dipping normal fault strands that bound the western margin of a concealed horst block. Temperature data from 38 TGHs (ranging from 90 to 546 m depth) show primarily warm (up to 600°C/km) conductive thermal profiles. However, evidence of convection is found in five TGHs with near-isothermal profiles, indicating proximity to geothermal upwelling or crossflow. Based on silica geothermometry, multicomponent equilibrium geothermometry, and the silicified sediments at the surface, it is possible that subsurface temperatures could be between 160°C and 180°C. We present three proposed conceptual model scenarios for the Granite Springs Valley prospect (most optimistic, median [P50], and most conservative confidence levels) to capture our uncertainty in knowing which specific fault strand(s) control hydrothermal fluid flow. We use the power density approach for power capacity estimates of the resource and set the reservoir depth of 1.2 km for these calculations. Based on the calculated areas from the model scenarios and inferred power densities based on comparison with analogue geothermal fields, we estimate power capacity between 3.7 and 35.6 MWe, with a P50 estimate of 11.5 MWe.
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