HEAT FLOW IN THE WESTERN UNITED STATES AND EXTENSIONAL GEOTHERMAL SYSTEMS

Abstract

This paper presents the latest surface heat flow map of the continental United States and analyzes the distribution of known extensional geothermal systems in the western US with respect to background heat flow (heat flow outside the area of disturbance associated with a convecting system).

The heat flow map is an updated version of the DNAG Geothermal Map of North America (Blackwell and Steele, 1992). The update adds a number heat flow measurements to the original map for the central and western US. The overall heat flow picture has not changed from the 1992 map, but the additional data have improved contour detail in some areas (particularly the southwest US) as well as increased confidence in the contouring in other areas. The map is sufficiently detailed and accurate to be used as a basic data source for further studies.

The density of high temperature (>150?C), extensional geothermal systems, increases rapidly with heat flow. While not unexpected, the fit of the curve suggests that sampling of known geothermal systems is unbiased to any significant degree in all but the highest heat flow regions. Reservoir temperatures of known geothermal systems fall off rapidly in areas with heat flow less than 80mW/m2. For typical thermal conductivity and assuming reservoir temperature equals geochemical system temperature, the depth of circulation of most Basin and Range systems is less than 5 km. Taken together, the data suggest that there is little value to exploration for extensional geothermal systems in areas with less then 80mW/m2 background heat flow.