This report concludes the work accomplished during the first phase of evaluating the geothermal resource potential of the Umatilla Indian Reservation (UIR). The project was a coordinated effort between the CTUIR, HotRock Energy Research Organization, AltaRock Energy, and the USGS. Reconnaissance and data collection activities included geologic field visits; paleomagnetic and rock property sampling; gravity, magnetic, and magnetotelluric geophysical surveys; and groundwater geochemical analysis. Data was refined and modeled using state of the art methodology and software. This included using the chemical properties of water samples as inputs to multicomponent geothermometry so that temperatures at depth could be estimated. Resource favorability maps were rendered based on integrated data, and they will guide subsequent phases of geothermal exploration. The ultimate goal of this work is to develop resources for direct-use (heating) and/or power production for the benefit of the Cayuse, Umatilla and Walla Walla Tribes. Located in northeastern Oregon on the northwest flank of the Blue Mountains, the UIR is situated near the intersection of the two major regional fault systems: 1) the Klamath-Blue Mountains Lineament (KBL) which extends southwest across the Klamath Mountains in southwestern Oregon, and 2) the Olympic-Wallowa Lineament (OWL) which extends west-northwest to the Puget Lowland in northwestern Washington. Located in the UIR, the Thorn Hollow, Hite, and Wilahatya faults are major linkages between the KBL and the OWL and are considered important structures for controlling fluid flow. Geophysical surveys identified the location and structure of these and numerous smaller-scale structures which may influence fluid flow and geothermal favorability. The geophysical, geologic, and geochemical data were integrated into a geothermal resource potential model using the methodology of the 2017 Washington State Play Fairway Analysis (PFA). Data were grouped into three categories of geothermal resource indicators: 1) heat, 2) permeability, and 3) presence of fluids or a cap. Data were normalized and weighted based on geothermal favorability using a team-based Analytical Hierarchy Process (AHP) and then loaded into a GIS mapping platform to determine the relative position of resource indicators across the UIR. Overall geothermal favorability was mapped across the UIR based on combined AHP weighted data, and eight locations were identified as the most favorable for further resource investigation. A speculative model of the hydrothermal system below the UIR is presented and serves as a hypothesis to evaluate future new field data (e.g., thermal gradient boreholes). The model begins with meteoric water infiltration in the highlands of the Blue Mountains. As that water descends, primarily through deep-seated crustal faults and major fault intersections, the water heats up, becomes buoyant, and migrates northwest away from the mountains into the central area of the UIR – the region identified as having higher geothermal favorability. Zones along the Wilahatya fault system may tap deep thermal reservoir fluids at fault intersections and provide potential access to those fluids at relatively shallow depths. It is those zones that have been identified as the most favorable locations for further exploration. Findings indicate a potentially viable geothermal resource. Further investigation is necessary for absolute determination. As such, proceeding with UIR Geothermal Resources Assessment – Phase 2 is strongly recommended and will include installing exploratory Temperature Gradient Holes ranging in depth from 1.5 to 2.5 km.  

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