In this study, a compartment is defined as a body of rock that is surrounded by eolian bounding surfaces. These bounding surfaces act as low-permeability baffles to fluid flow, and they occur at different scales. To identify the geometry and volumetric size of eolian compartments, we have constructed a three-dimensional (3-D) computer model of the Tensleep Sandstone based on outcrop exposures. Field data were collected using traditional surveying techniques and a precise global positioning system receiver system at Alkali Creek, Bighorn Basin, Wyoming. The data include coordinates and elevations of 3500 data points in a 2.0 1.5-km (1.5 1-mi) area that are tied to marine-to-eolian (0.0), intraset (0.1), first-order (1.0), and second-order (2.0) bounding surfaces.

First-order, or 1.0-bounding surfaces, display undulatory geometry both in the foreset dip and strike direction. They climb from 0.0-bounding surfaces in the general direction of foreset dip (to the south-southwest), with a calculated angle typically less than 1, are laterally extensive across the study area and display variable thickness in the range of 0–26 m (0–85 ft). These 1.0-bounded compartments are subdivided by 2.0-bounding surfaces into smaller compartments. Perpendicular to strike, 2.0-bounding surfaces have an average spacing of 33 m (108 ft). They display variations in their strike and dip orientation to form laterally discontinuous bounded compartments.

The 3-D model was built from correlative bounding surfaces observed in the walls of parallel canyons that cut down into the Tensleep Sandstone. Present-day topography, when superimposed on the 3-D model, allowed verification by comparisons of model cross sections and photomosaics. After topography was removed, wells were simulated by 0.04-, 0.08-, 0.16-, 0.32-, and 0.65-km² (10-, 20-, 40-, 80-, and 160-ac) templates in the 3-D model. The simulation also included horizontal wells oriented parallel, perpendicular, and oblique to foreset dip direction. For each well, the volume of intersected reservoir compartments was calculated. For the purpose of volumetric calculations, no-flow boundaries were arbitrarily assigned to the bounding surfaces that surround each compartment. Comparison of these volumes with the ideal drainage volume of each well identified the most efficient drilling strategy for Tensleep reservoirs. In summary, horizontal wells drilled parallel to foreset dip direction drain the maximum number and volume of reservoir compartments.