T9 Advancement in the Exploration Practices Used to Characterize Geothermal Resources

Sunday, 9 October 2011
8:00 AM - 12:00 PM
Room 101H-J

PROJECT HOTSPOT: THE SNAKE RIVER GEOTHERMAL DRILLING PROJECT

Paper number: T9-5
Program: Technical Sessions
Time: 8:55 AM – 9:10 AM

Authors

Abstract

The Snake River volcanic province (SRP) overlies a thermal anomaly that extends deep into the mantle; it represents one of the highest heat flow provinces in North America, and an area with the highest calculated geothermal gradients. Project Hotspot examines the geothermal potential of this province in three representative locations within the over SRP province: (1) the axial volcanic high of the central SRP, the locus of Quaternary and Holocene volcanism that lies on the path of the Yellowstone hotspot; (2) the margin of a pre-basalt rhyolite eruptive complex that hosts an existing warm water district; and (3) the western SRP, where deep-seated normal faults tap into higher temperature fluids, and thick Pliocene-Pleistocene sediments trap this heat. The axial volcanic high is characterized by high sub-aquifer geothermal gradients associated with the intrusion of mafic magmas and the release of crustal fluids from the associated wall rocks. The valley-margin setting is characterized by high surface heat flow may be driven by the up-flow of hot fluids along buried caldera ring-fault complexes, while the western SRP graben more closely resembles classic Basin and Range geothermal areas.

The primary technology applied in this project is slimhole drilling with a wireline core retrieval system that allows us to rapidly sample a complete section of the crust penetrated by drilling. Each hole is then logged using slimhole geophysical tools. Additional field studies include vertical seismic profiles in each well, surface seismic surveys, and detailed, ground-based magnetics and gravity. At present two of three planned holes have been completed; the third is in progress at the time of writing planned for 1850 m depth. The axial volcanic zone drill hole penetrated to 1912 m depth, almost entirely basalt with minor sedimentary intercalations. The caldera margin drill hole penetrated 1958 m, penetrating rhyolite welded ash flow tuffs with basalt and sediment interbeds in the upper 600 m. Preliminary findings show that the Snake River aquifer is exceptionally thick in the central plain (to 960 m), which suppresses surface heat flow; beneath the aquifer the geothermal gradient becomes steeply conductive (80-90ºC). On the margin of the plain, a thick warm water aquifer (50-60ºC) extends to depths in excess of 1200 m.