The need for large quantities of energy has increased interest in the Fort Union coal region of the Northern Great Plains. Extensive coal development would place a heavy demand on the region's limited streamflow. Some Paleozoic rocks that underlie the Fort Union coal region might supply, at least on a temporary basis, a significant amount of the water required for coal development. This report provides information on ground-water resistivity, rock characteristics, and ground-water temperature, from which general inferences relating to water quality and flow direction may be drawn. The area of study covers approximately 200,000 square miles, and includes eastern Montana, western North Dakota and South Dakota, northeastern Wyoming, and northwestern Nebraska.
Borehole geophysical data and bottom-hole temperature data were used to determine porosity, apparent electrical resistivity of ground water (Rwa), and temperature of ground water for the Red River Formation (Ordovician), Interlake Formation (Uppermost Ordovician and Silurian), Duperow Formation (Upper Devonian), Birdbear Formation (Upper Devonian), and a chronostratigraphic interval within the Madison Limestone (Mississippian). Rwa indicates the areal distribution of fresh and salty water and the probable direction of water movement. Maps showing areal distribution of Rwa, rock porosity, and ground-water temperature were prepared for each formation.
Rwa values ranged from about 13 ohm-meters to 0.04 ohm-meter. The highest Rwa is in recharge areas, and the lowest, in the areas of dense brine in the Williston basin. The areas of brine are not centered in the deepest part of the basin, but are shifted to the east and south, apparently in response to hydraulic effects associated with the flow of less salty water around the brine and into overlying formations. The distribution of water of different quality, which controls Rwa, is governed by the flow system, which in turn is affected by proximity of geologic structures, by the distribution of rock types, and by porosity trends within the rocks.
Temperatures of ground water ranged from about 80°F to as much as 320°F. Generally, temperatures are lowest nearer the mountains and uplift areas and highest in the deeper parts of the basins. Temperature anomalies may be caused by geologic structures, thermal conductivity of overlying beds, and deeper than expected circulation of water in fractures related to intrusive igneous rocks. Thermal gradients ranged from 1.0°F/100 ft to 4.2°/100 ft.
