A THREE-DIMENSIONAL GEOLOGIC CONCEPTUAL MODEL BENEATH THE IDAHO NATIONAL LAB’S GEOTHERMAL RESOURCE RESEARCH AREA

The U.S. Department of Energy has initiated a research and development program to make a step change in Geothermal Resource Development called the Frontier Observatory for Research in Geothermal Energy (FORGE). The Snake River Geothermal Consortium (SRGC) and its partners, including the University of Idaho, have won a Phase I grant under the competition, proposing to host FORGE at the Idaho National Laboratory (INL). INL has established a Geothermal Resource Research Area (GRRA) for that purpose that is an approximately 100-km² area along the northern margin of the eastern Snake River Plain (SRP). The initial phase of the FORGE initiative called for the development of a conceptual geologic model that demonstrates the suitability of the site for EGS development. The key features of a suitable EGS site are 1) temperatures of 175 -225° C within 1.5 to 4 km of the surface and 2) low permeability (~1e-16 m²). A 3-dimensional conceptual model is presented that integrates all of the available geologic, hydrogeologic, geophysical, and temperature data for this area. The conceptual model shows a system of four nested calderas within the Picabo volcanic field filled with thick, dense flows of rhyolite tuff. Temperature estimates are based on observations from nearby deep boreholes, which suggest that the 175° C isotherm may be encountered between 2.4 and 3.8 km below the land surface. The variability of heat fluxes across the plain, wide distribution of recent volcanic activity, and areas of apparent low seismic activity in the shallow crust suggest a relatively shallow, magmatic heat source that persists, distinct from the time-progressive caldera eruptions associated with development of the plain.