| Title | THE GEOSCIENTIFIC UNDERSTANDING OF THE UTAH FORGE EGS SITE |
|---|---|
| Authors | S.F. Simmons, S. Kirby, R. Allis, J. Bartley, J. Miller, C. Hardwick, C Jones, P. Wannamaker, R. Podgorney, and J. Moore |
| Year | 2019 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Utah FORGE, EGS, geology, groundwater hydrology, heat flow |
| Abstract | The current geoscientific understanding of the Utah FORGE site has been obtained from synthesis of numerous independent datasets, including new geological, geophysical, and geochemical surveys, plus drilling and logging of three new wells, the deepest being 58-32 which penetrates to 7536 ft (2248 m) depth. The stratigraphy consists of two broad rock types, comprising basin fill sediments and crystalline basement rocks mostly made of Miocene granitoids. The contact between these rock types forms an inclined plane, which dips ~20° west and which likely represents a large-scale normal fault that has been rotated during extension. Anomalous heat flow comprises localized hydrothermal convection east of the Opal Mound fault and regional conduction (~70°C/km, well 58-32) west of the Opal Mound fault. The modern stress regime is extensional, characterized by normal faulting and a maximum horizontal compressive stress oriented approximately N25°E. Well 58-32 penetrated the basement at 3176 ft (968 m), which consists of granitic rock containing plagioclase, K-feldspar, and quartz. Between 1700 and 7536 ft (518-2248 m), the temperature profile increases linearly with a maximum bottom hole temperature of 197°C. The FMI log imaged ~2000 natural fractures, with predominant north-south, east-west, and northeast-southwest orientations that strongly resemble fracture patterns exposed in the Mineral Mountains. Analysis of about 100 thermal gradient and deep exploration holes show how convective and conductive heat transfer are partitioned. Convective heat transfer is a small-scale feature restricted to Roosevelt Hot Springs, whereas conductive heat transfer is a large-scale feature that is regionally developed in the crystalline basement rocks in and around the EGS reservoir. The groundwater hydrology and chemistry reflect the thermal structure with hydrothermal upflow being confined to east of the Opal Mound fault. Where the rising hot water gets to shallow level, it forms a westward outflow zone that disperses down the hydraulic gradient through shallow alluvium. |