ENHANCED GEOTHERMAL SYSTEMS IN THE MIDCONTINENT

The midcontinent of North America has promising targets for development of EGS within and below cratonic basins and in an anomalously high heat flow area in southeastern South Dakota and northeastern Nebraska. Low thermal conductivities of thick fine-grained clastic sedimentary rocks of Mesozoic and Cenozoic age in the cratonic basins yield high geothermal gradients in regions of normal (50 mW m^-2 to 60 mW m^-2) continental heat flow. Temperatures exceeding 150 °C occur at depths of 4 km within and below the Williston, Kennedy, and Denver basins. The anomalously high heat flow area in SD & NE (100 mW m^-2 to 140 mW m^-2 ) is due to lateral and vertical heat transport in a regional groundwater flow system. Temperatures above 150˚C occur at depths of 5.5 km in this region over an area of 1500 km². High heat flow (106 mW m^-2) observed in a mining exploration well drilled into carbonatite body in southeastern Nebraska is due to high radioactive heat production in the carbonatite. Ongoing research on thermal conductivity, radioactive heat production, bottom hole temperature data, temperature vs. depth logging and other data being assembled by the Dept. of Energy’s National Geothermal Data System may reveal new areas of target for EGS development.