MINNESOTA GEOTHERMAL POTENTIAL

Geothermal power production in a stable continental region can be developed as an Enhanced Geothermal System (EGS, formerly hot-dry rock) with a binary cycle engine at a minimum temperature of 150 °C. Current literature indicates that surface heat flow in the northern United States east of the Great Plains and west of the Appalachian mountains is low, i.e., 35 mW m^-2 to 45 mW m^-2, which implies that the potential for geothermal power in the region is minimal. Based on this low heat flow, the depth to the 150 °C isotherm in most of the region is at least 10 km. We present several lines of evidence to show that heat flow in the region has been underestimated and that geothermal potential in the region is not insignificant. Surface heat flow, q (mW m^-2), is the sum of heat flow from the mantle, q₀, and radioactive heat production, A (μW m^-3), from U, Th, and K in the crust (q = q₀ + Ab). The parameter, b, has units of length and has been determined to be 10 km for the US east of the Rocky Mountains. Mantle heat flow is approximately 32 mW m^-2 in a stable continental region and the average heat production of the continental crust is approximately 2.2 μW m^-3 which yields an average crustal heat flow of 54 mW m^-2. The radioactive component is critical, particularly in Minnesota where all published heat flow sites (4 on land, and 168 in Lake Superior) are located within the mid-continent rift. The rift is a massive mafic intrusive complex that extends from the surface to the Moho and has characteristically low radioactive heat production, i.e., 0.1 μW m^-3. Thus there is a sampling problem in that heat flow has not been measured in the granitic terranes in Minnesota. Heat production of 3.4 μW m^-3 has been reported for samples of granite from the Giants Range batholith. Thus it is possible that heat flow there could be of the order of 66 mW m^-2. Another critical factor is that heat flow determinations from thermal gradient measurements in Minnesota require re-analysis to account for the effects of micro-climates at the drill holes, modification of the temperature gradient by recent AGW and by post-glacial warming. We present an analysis of heat flow and the geothermal potential of Minnesota based on corrections for climate signals and radioactive heat production in granitic terranes.