Fluid geochemistry at the Raft River geothermal field, Idaho, USA: New data and hydrogeological implications

Following a period of exploration and development in the mid-late 1970ʹs, there was little activity at the Raft River geothermal field for the next ~20 years. U.S. Geothermal Inc. acquired the project in 2002, and began commercial power generation in January 2008. Since 2004, U.S. Geothermal Inc. has collected geochemical data from geothermal and monitoring wells in the field, as well as other shallow wells in the area. An additional sampling program was completed in July 2010 to measure a wider range of minor and trace elements and δ18O, δD, and 3H (Tritium). The data indicate that the geothermal fluids are alkali chloride in composition, and that the fluid geochemistry is spatially variable and complex, with two compositionally-distinct deep geothermal fluids characterized by differences in K, Na, Cl, Ca, Li, and F, an intermediate fluid, and a groundwater fluid. Deep geothermal waters from the southeast part of the borefield have higher K, Na, Cl, Ca, Li, and lower F concentrations than those from the northwest, although both fluids record similar measured and geothermometer temperatures, and are produced from the same Precambrian reservoir rocks at similar depths. The higher salinities of the southeastern fluids are interpreted to result from interactions with evaporite deposits that were not encountered by the northwest fluids along their flow paths. The separation of these deep fluids within the borefield and the lack of mixing between them indicate a compartmentalized reservoir, which is attributed to a northeast-trending shear zone that appears to act as a permeability barrier. This shear zone is represented by a resistivity high as defined by magnetotelluric data.