The Blackfoot Volcanic Field, Southeast Idaho: A Hidden High-Temperature Geothermal Resource in the Idaho Thrust Belt

摘要

For more than thirty years since southeast Idaho's Blackfoot volcanic field (BVF) was listed as a "Known Geothermal Resource Area," exploration based on a conventional Basin and Range volcanic paradigm has failed to confirm the existence of a high-temperature resource. Now, a synthesis of data compiled for the National Geothermal Data System (NGDS) has identified a previously unrecognized high-temperature prospect within the Idaho thrust belt (ITB) adjacent to the BVF (Welhan et al., 2013), explaining the hidden nature of this magmatic geothermal system, and why it has remained hidden to conventional exploration for so long. Volcanologic and geohydrologic evidence suggests that a significant amount of unerupted magma remains at a depth of 12 to 14 km beneath the BVF's 58 ka rhyolite domes at China Hat. The magnitude of the CO_2 flux in the southern BVF, rivalling that of many active quiescent volcanoes, and the presence of magmatic helium in these volatiles confirms that a robust magmatic heat source is still actively outgassing (Lewicki et al., 2012). Thermal data from deep wildcat petroleum wells and new heat flow data compiled for the NGDS were evaluated in the context of the structural and stratigraphic architecture of the western ITB. This analysis suggests that magmatic hydrothermal fluids are channeled from depths of 10 km or more beneath the BVF, moving eastward out of the graben along regional thrust faults and permeable Paleozoic and Mesozoic strata, into reservoirs at depths of 3-5 km in the adjacent ITB. The sodium-chloride nature of high-temperature fluids encountered by wildcat wells and the coincidence of microseismic swarms in the area where Jurassic salt beds are believed to be thickest suggest that these magmatic hydrothermal fluids are actively enhancing the bulk porosity and permeability of potential reservoir rocks via dissolution of massive salt beds, leading to localized roof collapse and seismicity. The thermal data derived from historic wildcat petroleum wells in the ITB defines a band of high heat flow, some 20 to 50 km north and southeast of China Hat, over an area of hundreds of square kilometers. Heat flows of 100 to 220 mW/m~2 coincide with the presence of hot (160-220 °C) sodium-chloride fluids at depths of 3-5 km in limestone and sandstone reservoir rocks of Pennsylvanian to Jurassic age. The presence of sodium-chloride thermal springs in other parts of the western thrust belt (e.g., Heise Hot springs, Maple Grove prospect, Crystal Hot Springs, Renaissance prospect) suggests that thermal reservoirs of this type exist in other parts of the Idaho-Utah thrust belt in similar structural settings.