Application of Isotopic Approaches for Identifying Hidden Geothermal Systems in Southern Idaho

Abstract

Southern Idaho is an area of high heat flow with significant potential geothermal resources. However, shallow cold groundwater effectively masks thermal signatures of deep-seated geothermal systems in the area. In order to attempt to see through the shallow groundwater, we are applying a combination of geochemical and isotopic tools relying on dissolved gas and chemical species that have low concentrations in the dilute groundwater to prospect for high-temperature systems in the deep subsurface. For the first phase of the project, our efforts were focused in and around the eastern Snake River Plain (ESRP). We have collected and analyzed the isotopic compositions of more than 40 samples from thermal springs and wells from the region. Of potential isotope geothermometers, the sulfate-water oxygen isotope geothermometer has given the most promising results, yielding calculated temperatures similar to multi-component chemical geothermometers. Other isotopic tools that have proven useful are shifts in the isotopic compositions (δD and δ18O) of groundwater away from the local meteoric water line indicating high-temperature interaction with reservoir rocks or mixing with a magmatically derived fluid. In addition, the δD and δ13C of dissolved methane in several of the samples indicate that the methane formed in a high temperature magmatic system. Taken together with the analyses of multi-component chemical geothermometry and a separate study of the 3He/4He from the same features, the results have identified two promising areas warranting more concentrated study in the Twin Falls area and the Camas Prairie region between the ESRP and the Idaho batholith.