An Updated Geologic Model for FORGE Utah EGS Laboratory

Abstract

The FORGE Utah EGS laboratory site lies on the eastern side of the north Milford valley, 4 km west of the Roosevelt Hot Springs hydrothermal system. An updated geologic model has been developed from synthesis of numerous independent datasets, including regional mapping, new gravity data and interpretation, processing of legacy seismic reflection, and drilling and logging of the new deep vertical well to 7536 feet below the surface. The reservoir is hosted mainly in granodiorite, which correlates with Miocene plutons that are exposed in the Mineral Mountains, and comparison of drill core and image logs indicates scattered subvertical and subhorizontal fractures, some of which are lined with epidote and chlorite. Within the new deep well, these crystalline plutonic rocks extend upward to about 3200 feet depth, where they are unconformably overlain by a lower interval of coarse clastic, partially indurated alluvium, and an upper interval of fine clastic alluvium. The contact between the basement and the basement fill slopes gently westward, forming a strong continuous seismic reflector. Detailed joint-fracture mapping in the Mineral Mountains indicates three well-defined orientations, including joints with steep dips striking north-south and also east-west, and joints with gentle westward dips striking north-south. Joint-fracture spacing is highly variable and where concentrated forms a mesh of subparallel structures that extend for 100-500 m. Most of these features seem to be products of pre-Quaternary uplift and exhumation, and stratigraphic and gravity data suggest major sub-vertical faults (with 50 m or more vertical offset) are absent. This is consistent with the surfaces of alluvial fans, which form gently inclined ramps that extend 3-5 km, more or less continuously without major break from the center of the basin, across the EGS laboratory site, eastward into the escarpments of incised valleys in the west Mineral Mountains.