CHARACTERIZING GEOTHERMAL SYSTEMS THROUGH GEOLOGIC, GEOCHEMICAL, AND GEOPHYSICAL TECHNIQUES: A CASE STUDY FROM PAISLEY, OREGON AND FAIRFIELD, IDAHO

South central Oregon is host to numerous geothermal resources that are the result of Miocene volcanism and Miocene-recent faulting. Geothermal resources in the Snake River Plain region of southern Idaho are most likely attributed to residual heat from the passing of the Yellowstone Hotspot. The two field areas for this project are near Paisley, Oregon, which resides in the northwestern most section of the basin and range province and Fairfield, Idaho, which resides on the Camas Prairie in south-central Idaho. The rocks in these areas consist of ash-flow tuffs, basalt to rhyolite lava flows, and granitic plutons of the Idaho Batholith (Fairfield, Idaho). In both areas, normal faults are the dominant structural feature, with structural relief on the order of meters to hundreds of meters. The stratigraphic and structural framework must be identified to properly assess geothermal potential within these rocks. The objective of this project is to use a multi-disciplinary approach combining geophysical, geochemical, and geologic techniques to help characterize geothermal systems, allowing for comparisons to be made between two different geothermal systems.

Linear features identified by aerial photography were used in conjunction with reconnaissance mapping to gain an understanding of regional structural trends in both field areas. From these analyses, we have interpreted two major structural trends in south central Oregon, NW-SE and NE-SW and mostly E-W and NW-SE for the Camas Prairie, Idaho. Gravity data shows a range front fault in the Paisley geothermal system, this fault is interpreted to be the conduit in which hot fluids are circulating. Geochemistry on fluids from wells in the Paisley area have been analyzed for major and some minor elements for use in Geothermometer calculations. Temperatures range between 160-220 °F for most wells in the area. Water samples have yet to be collected for the Fairfield area. A shallow temperature probe survey was conducted at the Paisley site to characterize the subsurface heat flow. This data shows high heat flow near the surface over a several month survey period, ranging from 55-80 °F.