| Authors |
Stuart SIMMONS, Stefan KIRBY, Rick ALLIS, Kris PANKOW, Clay JONES, Phil WANNAMAKER, Rob PODGORNEY, John MILLER, Christian HARDWICK, Joe MOORE |
| Abstract |
The Utah FORGE site is an EGS field laboratory, and the current geoscientific understanding 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; the latter is 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 detachment fault. 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. These are likely responsible for subsidiary faults that have formed outside the EGS reservoir, including the Opal Mound fault, which forms the west boundary of the Roosevelt Hot Springs hydrothermal system, and the Mineral Mountains West fault system, which runs south of the FORGE site. By contrast, the Negro Mag fault runs roughly east west, intersecting the Opal Mound fault and coinciding with the northern boundary of the Roosevelt Hot Springs hydrothermal system. Importantly, no major fault structures transect the FORGE site. The well 58-32 penetrated the basement contact at 3176 ft (968 m), and below this depth the dominant lithology 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 shows the predominance of north-south, east-west, and northeast-southwest fracture orientations, with fractures induced by drilling orientations that northeast-southwest with near vertical dips. This is consistent with the northeast-southwest direction of maximum horizontal stress determined from geologic observations. Permeability measurements range from 6 to 30 microdarcies, and such low values are consistent with very low fluxes of soil CO2 over the FORGE site. Additional information based on drilling results planned for March 2019 and from analysis of recently acquired data, including ongoing seismic monitoring, new processing of 2D-3D seismic reflection data, regional assessment of groundwater chemistry, and water-rock interaction modeling, will be incorporated into the final paper. |