Geology of the Utah Frontier Observatory for Research in Geothermal Energy (FORGE) Enhanced Geothermal System (EGS) Site

The Utah FORGE field-scale laboratory was established to advance and accelerate development of Enhanced Geothermal System (EGS) resources, and this report provides an update on the geological understanding of the EGS reservoir. The succession of rock types intersected by deep wells beneath the site comprise sedimentary basin fill strata and underlying crystalline basement rocks made of: 1) sheared rhyolite; 2) sheared granitoid; 3) granitoid; and 4) interfingered metamorphic and granitoid. Within the intervals dominated by granitoid, igneous rock compositions range from granite to diorite. Below ∼2300 m, metamorphic rocks are primarily made of orthogneisses, with minor marble, quartzite, and schist engulfed by granitoid. Determining the distribution of granitoid and orthogneiss is complicated by similarities in mineralogy (quartz, plagioclase, K-feldspar, biotite, titanite, hornblende) and log responses, as well as the lack of macroscopic textures in cuttings, whereas metasedimentary rocks are more readily recognizable due to their distinctive mineralogies. The uplift and exhumation of the Mineral Mountains batholith induced early plastic deformation represented by subtle foliation of granitoids, the development of penetrative fabrics, and development of narrow mylonite zones. Overprinting by later brittle deformation involved shearing, alteration, and veining, which are most intense at the top of basement and diminish with increasing depth. The distribution of fractures in the EGS reservoir is heterogeneous, and localized intervals of intense fracturing developed along the contacts between granitoids and metamorphic rocks. Secondary minerals (clay minerals and Mg- and Fe-carbonates; minor epidote, actinolite, albite and quartz; trace anhydrite and halite) comprising open-space fillings and replacement of precursor phases tend to be concentrated in fracture zones, with paragenetic relationships reflecting cooling over time.