Thermal Gradient Holes At Waunita Hot Springs Geothermal Area (Zacharakis, 1981)

GEOTHERMAL ENERGY

=====

Exploration Activity: Thermal Gradient Holes At Waunita Hot Springs Geothermal Area (Zacharakis, 1981)

Exploration Activity Details
Location	Waunita Hot Springs Geothermal Area
Exploration Technique	Thermal Gradient Holes
Activity Date	1981
Usefulness	could be useful with more improvements
DOE-funding	Unknown

Exploration Basis

Thermal gradient holes were installed to estimate regional heat flux around the hot springs and potentially identify the location of the geothermal reservoir feeding the hot springs

Notes

Eight thermal gradient boreholes were installed by AMAX Exploration, Inc. within a 3.6 km radius of Waunita Hot Springs, at an undetermined time prior to 1981. All boreholes were drilled to 100 m depth except #3 and #7, which were completed to 88 and 82 m, respectively. Boring logs are of limited detail with regard to lithology, but generally indicate sequences of clay, shale, and sandstone, with some rhyolite in Boreholes #5 and #8 and schist in Borehole #6. The water table was encountered between 14 and 55 meters in most boreholes, though it is unknown if these boreholes were allowed to equilibrate first. Bottom hole temperatures for most ranged from 7.1�C (in Borehole #7) to 14�C (in Borehole #6); however, Borehole #3 exhibited an anomalous high bottom hole temperature of 47.5�C, along with the highest calculated thermal gradients for any of the boreholes. Borehole #3 was the closest to the hot springs, located approximately 0.6 km to the southwest of the Upper Springs on the opposite side of County Road 887. Most boreholes exhibited significant anisotropy in thermal gradients, except Boreholes #1 and #6, for which differential thermal gradient data was not available, and Borehole #8, which was the furthest away (~3.6 km) from the hot springs, on the opposite side of Tomichi Dome. Again, Borehole #3 exhibited the greatest anisotropy, with a reported temperature gradient of 1,080�C/km between 34-40 m depth, and gradients of 340�C/km and 247�C/km immediately above and below, respectively [Zacharakis, 1981]. This anisotropy in thermal gradient argues for a hydrothermal system dominated by by fracture or similar preferential flow in these anomalously high zones. As such, the utility of these boreholes in assessing deeper crustal heat flow and potential reservoir temperatures is severely limited. Averaging the temperature gradient from the surface to the bottom of the boreholes may provide a more reasonable estimate of the overall thermal flow for the resource area. These values range from 25 to 62�C/km for all the boreholes, with 25�C/km representing the average thermal gradient of the continental crust. The boreholes which exhibited the lowest average gradient were several kilometers from the hot springs and up-dip. None of them penetrated the Dakota Sandstone .

References

• T. G. Zacharakis (1981) Temperature, heat flow maps and temperature gradient holes