The Geothermal System Near Paisley Oregon: A Tectonomagmatic Framework for Understanding the Geothermal Resource Potential of Southeastern Oregon

摘要

The tectonic and magmatic framework of southeast Oregon provides the conditions necessary for the existence of geothermal energy resources. However, few detailed studies of geothermal systems in this part of the Basin and Range have been conducted. Young bimodal magmatism and faulting associated with the High Lava Plains coupled with the encroachment of the Basin and Range tectonic province and potentially the Walker Lane have created the structural configuration, heat source, and secondary permeability necessary for geothermal systems in southeast Oregon. The relative contribution of these provinces to the overall tectonomagmatic framework is less well understood. In this study, the geothermal system near Paisley, Oregon has been characterized by a detailed regional heat flow study, geologic mapping, aqueous geochemical analysis, a gravity survey, and a X-ray diffraction analysis of secondary alteration minerals.Based on these analyses, the Paisley geothermal system shares many aspects similar to Basin and Range geothermal systems. Geologic mapping has revealed a sequence of rocks with ages spanning from the mid Eocene-Present as well as structures related to the Basin and Range Province. A structural transfer zone connects two en-echelon normal faults in Summer Lake Basin, which controls the upwelling of thermal waters. The fault controlling fluid flow in the Paisley geothermal system was imaged using a gravity survey. Thermal water in the Paisley geothermal system has been determined to be of meteoric source, and is interpreted to be of Pleistocene age based on stable isotopes having average values of -119.59‰ and -14.18‰ for δD and δ18O, respectively. Recharge to the thermal aquifer is driven dominantly by topographic flow, with residence times on the order of 1000s of years. Aqueous geochemistry was used to determine 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 reservoir temperatures between 95 °C – 166 °C. These results were independently checked by X-ray diffraction studies of alteration mineral assemblages in reservoir rocks from two production wells drilled by Surprise Valley Electrification, which revealed similar temperatures for the stability field of mineral assemblages present in the wells.Because it has been determined that the Paisley geothermal fluids are not influenced by recent magmatism, the role of bimodal magmatism associated with the High Lava Plains on geothermal systems in southeastern Oregon appears to be minimal. However, in areas where magmatism is younger than 2 million years, this may not be true. Like the geothermal systems of the western Great Basin, the location of geothermal systems in southeast Oregon is highly dependent on the regional structural architecture. Also similar to some geothermal systems of the Basin and Range, the source of water is not modern meteoric water, but is “fossil” water, which implies that production must be managed carefully to create a sustainable resource.