Effect of an 860-m thick, cold, freshwater aquifer on geothermal potential along the axis of the eastern Snake River Plain, Idaho

Thomas E Lachmar,Dennis L Nielson,Christopher J Sant,Joseph F Batir,Jeffrey R Walker,James P Evans,Thomas G Freeman,John W Shervais,David D Blackwell

doi:10.1186/s40517-017-0086-8

Abstract

A 1912-m exploration corehole was drilled along the axis of the eastern Snake River Plain, Idaho. Two temperature logs run on the corehole display an obvious inflection point at about 960 m. Such behavior is indicative of downward fluid flow in the wellbore. The geothermal gradient above 935 m is 4.5 °C/km, while the gradient is 72–75 °C/km from 980 to 1440 m. Projecting the higher gradients upward to where they intersect the lower gradient on the temperature logs places the bottom of the cold, freshwater Snake River Plain aquifer, which suppresses the geothermal gradient at this location, at least 860 m below the surface. The average heat flow for the corehole between 983 and 1550 m is 132 mW/m2. Although the maximum bottom-hole temperature extrapolated from a measured time–temperature curve was only 59.3 °C, geothermometers suggest an equilibrium temperature on the order of 125–140 °C based on a single fluid sample from 1070 m. Furthermore, below 960 m the basalt core shows obvious signs of alteration, including a distinct color change, the formation of smectite clay, and the presence of secondary minerals filling vesicles and fracture zones. This alteration boundary could act as an effective cap or seal for a hot-water geothermal system.

Highlights

The eastern Snake River Plain (ESRP) in southern Idaho covers an area of approximately 28,000 km2 (Morse and McCurry 2002), and is a prime target for geothermal exploration due to high geothermal gradients (Blackwell 1989)
Geothermal gradients The maximum bottom-hole temperature (BHT) was acquired from the DOSECC temperature tool
Because the DOSECC tool was never in true equilibrium with the ambient conditions, the maximum BHT measurement must be lower than the true equilibrium temperature at that depth

Summary

Introduction

The eastern Snake River Plain (ESRP) in southern Idaho covers an area of approximately 28,000 km (Morse and McCurry 2002), and is a prime target for geothermal exploration due to high geothermal gradients (Blackwell 1989). Heat flow in excess of 100 mW/m2 has been documented in the area (Blackwell and Richards 2004) This high heat flow is associated with the Yellowstone hotspot, which developed from a mantle plume (Smith et al 2009). The ESRP is home to the Snake River Plain aquifer (SRPA), which is hosted primarily in basalt (Welhan et al 2002a, b). The majority of these basalts are olivine, tholeiite pahoehoe flows (Greeley 1982; Leeman 1982; Kuntz et al 1992) with chemical compositions similar to Hawaiian basalts. The bulk of the volcanic vents are clustered around the axis of the ESRP (Kuntz et al 1992; Smith 2004)

Methods

Results

Discussion

Conclusion