Estimating Fault Permeability in Leach Hot Springs, Basin and Range Province, Nevada

Chayan Lahiri and Jerry P. Fairley
Department of Geological Sciences, University of Idaho, [email protected]

Leach Hot Springs, Nevada, is a hydrothermal area located in the Basin and Range province of the western US. The area comprises around 20 individual spring vents, roughly aligned along two parallel lineaments associated with a north-trending normal fault. Springs discharging along the easternmost lineament are near boiling in temperature, slightly to moderately acidic, and low in bicarbonate, chloride, and silica. By contrast, springs discharging from the western lineament demonstrate sub-boiling temperatures, near neutral pH, and have high concentrations of chloride, silica, and bicarbonate compared to the eastern springs. Boiling is assumed to take place along the flow path of the eastern springs, which have lower pH values. Lower pH results from accumulation of sulphuric acid due to boiling in the subsurface. Boiling conditions develop anywhere along the flow path where the vapor pressure of the liquid equals the pressure of the surroundings. The Nusselt number is a non-dimensional heat transfer coefficient that describes the ratio of heat loss to the surroundings to advective flux of heat along the flow path. The Nusselt number is partly a function of the flow path permeability. This is because the rate at which the fluid rises up to the surface is dependent on the permeability of the flow path: the higher the value of permeability of the flow path the faster the fluid will rise to the surface and less time it has to lose heat and cool down. As a result we can use these modeled Nusselt values to establish bounds on the values of flow path permeability.

Faults and related permeability structures primarily control fluid flow in the upper crust. Even though faults have a strong influence on subsurface fluid flow, they are not well understood. This work helps in understanding the architecture of faults and the associated permeability structures, and thus flow of fluid within them. The study of fault controlled fluid flow is of great importance and has applications in the petroleum industry geothermal reservoir engineering.

AAPG Search and Discover Article #90087 © 2008 AAPG/SEG Student Expo, Houston, Texas