HIGH HEAT FLOW BASINS - THE NEXT MAJOR INCREMENT IN GEOTHERMAL POWER DEVELOPMENTS IN THE U.S.?

Most geothermal power production from the Great Basin of the western U.S. is located near extensional faults that allow upflow of geothermal fluids to near-surface. However, improved drilling technologies and the prospect of rising power prices raises the possibility of economically viable large-scale power production from the centers of high heat flow basins like the Great Basin. Oil and gas exploration wells, and water wells in the Great Basin have proven the existence of laterally extensive, high permeability within Paleozoic carbonates. In the southern Great Basin, regional scale ground water flow towards the Colorado River in these carbonates has depressed the heat flow. However, in general the northern Great Basin has not been flushed by ground water, and the heat flow is about 80 - 100 mW/m². This equates to gradients of about 30 - 40 ºC/km in bedrock formations (e.g. beneath the ranges), and about 55 - 75 ºC/km within unconsolidated sediments and shale sequences due to the effects of thermal conductivity. There is the potential for temperatures of 150 - 300 ºC at 3 - 5 km depth in basins with thick basin fill, as supported by several oil exploration wells in the eastern Great Basin where the temperatures are > 200 ºC at 3 km depth. In addition, several shallow wells near one of these deep wells confirm regionally extensive gradients of 65 ºC/km. The critical issue for the geothermal potential is whether there is laterally extensive permeability in the 3 - 5 km depth range. The geologic evidence for near-horizontal Paleozoic formations at depth across much of the Great Basin, some of which are known to have characteristically high permeability, suggests the geothermal resource potential beneath the basins could be significant.