The need for large quantities of energy has created interest in the Fort Union coal region of the Northern Great Plains. Extensive development of this coal, which may include onsite steam-power generation, gasification, liquefaction, and slurry-pipeline transport of the coal from this region, would place a heavy demand on the region's limited streamflow. Paleozoic rocks that underlie the Fort Union coal region and aquifers in these rocks, including the Red River Formation and the Madison Limestone, might supply, at least on a temporary basis, a significant part of the water required for coal development. 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. This report, one of a series in the Madison Limestone study, uses hydrologic and geologic data to outline potentially favorable areas for well construction - that is, areas in which there is a good probability that large-yield wells (more than 500 gallons per minute) can be completed in the Red River Formation and in the Madison Limestone. Potentially favorable areas in terms of aquifer characteristics, for both the Red River Formation and the Madison Limestone, are given a numerical evaluation from 1 to 3 based on the number of the following criteria that are met: (1) the presence of more than 100 feet of relatively porous rock, (2) the presence of more than 100 feet of dolomite, and (3) the presence of known geologic structures which could affect yield. Areas rated 3 are those in which all three criteria are satisfied; areas rated 2 are those in which two criteria are satisfied; and areas rated 1 are those in which only one criterion is satisfied. The criteria selected for this analysis were chosen because they can be recognized and mapped over the entire study area. Local features such as minor structures, solution zones, and rock facies of small extent were not included in this regional evaluation. In addition, water quality was considered in a general way in defining the favorable areas, by excluding areas in which the electrical resistivity of formation water, as calculated from geophysical well logs, was less than 1 ohm-meter. The numerical scales of the Red River Formation and Madison Limestone are summed to show potentially favorable areas for the combined aquifers. Certain additional factors which may be important to a prospective water user were not included in the numerical ranking - these include depths to the two aquifers, calcite saturation, water temperature, total dissolved solids, and piezometric head in relation to land surface. For a complete evaluation, potential users should consider these factors plus local structures, facies, and solution zones in conjunction with the numerical rankings reflecting aquifer characteristics. To facilitate consideration of piezometric head, maps are included in this report showing areas in which the piezometric head falls in certain ranges with respect to land surface.