Exploration for Hot Dry Rock Geothermal Resources in the Midcontinent USA. Volume 1. Introduction, Geologic Overview, and Data Acquisition and Evaluation

摘要

The Midcontinent of North America is commonly characterized as a stable cratonic area which has undergone only slow, broad vertical movements over the past several hundreds of millions of years. This tectonically stable crust is an unfertile area for hot dry rock (HDR) exploration. However, recent geophysical and geological studies provide evidence for modest contemporary tectonic activity in limited areas within the continent and, therefore, the possibility of localized thermal anomalies which may serve as sites for HDR exploration. HDR, as an energy resource in the Midcontinent, is particularly appealing because of the high population density and the demand upon conventional energy sources. Five generalized models of exploration targets for possible Midcontinent HDR sites are identified: (1) radiogenic heat sources, (2) conductivity-enhanced normal geothermal gradients, (3) residual magnetic heat, (4) sub-upper crustal sources, and (5) hydrothermal generated thermal gradients. Three potential sources of HDR, each covering approximately a 2 sup 0 x 2 sup 0 area, were identified and subjected to preliminary evaluation. In the Mississippi Embayment test site, lateral thermal conductivity variations and subcrustal heat sources may be involved in producing abnormally high subsurface temperatures. Studies indicate that enhanced temperatures are associated primarily with basement rift features where vertical displacement of aquifers and faults cause the upward migration of hot waters leading to anomalously high local upper crustal temperatures. The Western Nebraska test site is a potential low temperature HDR source also related, at least in part, to groundwater movement. The Southeast Michigan test site was selected for study because of the possible presence of radiogenic plutons overlain by a thickened sedimentary blanket. (ERA citation 11:029622)