The Blackfoot Volcanic Field, Southeast Idaho: A Hidden High-T Geothermal Resource Revealed through Data Mining of the National Geothermal Data Repository

Abstract

Data compiled for the National Geothermal Data System (NGDS) are reviewed and synthesized, and a refined conceptual model of structurally-controlled heat redistribution in the shallow crust is presented that sheds new light on the high-temperature geothermal potential of the Blackfoot volcanic field (BVF). McCurry and Welhan (2012) have argued that volcanologic and geohydrologic evidence indicates that a robust magmatic heat source exists at 12-14 km, and work by Lewicki et al. (2012) on the magnitude and source of the CO2 flux responsible for the area?s effervescing springs and gas-pressured geyser has now provided the first hard evidence that such a magmatic heat source does exist. On that basis, deep-well data and new heat flow drilling data compiled for the National Geothermal Data System data project were evaluated in the context of the structure and stratigraphy of the western Idaho-Wyoming overthrust belt. This analysis strongly suggests that hydrothermal fluids in this geothermal system did not rise vertically from their magmatic heat source but were channeled by deep thrust faults that acted as both regional barriers to vertical migration and lateral permeable pathways to move heat advectively from the vicinity of the magma body into rocks of the overthrust belt at depths of 3-5 km. The composition of high-temperature brines encountered in wildcat wells indicates that migration of the hydrothermal fluids was enhanced by dissolution of Jurassic-age evaporite beds, thereby enhancing the bulk permeability and storage capacity of the reservoir rocks. Thermal gradients and heat flow estimates derived from deep wells are highest in two regional bands beneath the Meade thrust fault. Maximum estimated heat flows in the highest-temperature area range from 100 to 200 mW/m2, reflecting the presence of reservoir fluids of 160 to 220 °C at 3-5 km depth some 10-30 km east of the magmatic heat source.