Characterization of a Basin and Range Type Geothermal System in Southeast Oregon, the Paisley Geothermal System
Kyle A. Makovsky, Roy Mink, and Walter Snyder
Boise State University
Two types of geothermal systems dominate in the western United States, those related to Pliocene and younger magmatism and those related to Basin and Range extension. Basin and Range type geothermal systems are evidenced by high surface heat flow values, low concentrations of magmatic contaminants (e.g., SO¬4, As, and B), and by their strong dependence on the regional and local structural framework. The Paisley geothermal system shares many aspects similar to Basin and Range geothermal systems. High regional heat flow values averaging 150 mW/m² are evidence of an elevated regional geothermal gradient. Geologic mapping has revealed structures related to the Basin and Range Province and also to the High Lava Plains. Upwelling of thermal waters in Summer Lake Basin is controlled by a structural transfer zone connecting two en echelon normal faults. Thermal water in the Paisley geothermal system has been determined to be Pleistocene age based on stable isotopes signatures. Recharge to the thermal reservoir is driven dominantly by topographic flow, with residence times on the order of 1000's of years; however, another interpretation is that there is no reservoir and the water simply takes 1000's of years to reach the production zone in the thermal wells from its recharge area in the topographically high areas southwest of Paisley (i.e., long hydrologic flow-paths). Aqueous geochemistry demonstrates that the Paisley geothermal system is not magmatic in origin, with low values of magmatic SO4, As, B, and high values of HCO3 and Na. Geothermometers were used to calculate potential reservoir temperatures between 95 °C – 166 °C. These results were independently checked by X-ray diffraction studies of alteration mineral assemblages from the production zones of two wells drilled by Surprise Valley Electrification; revealing similar temperatures for the stability field of mineral assemblages present in the wells.
AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013