Abstract

In the northern Powder River Basin and around the north flank of the Black Hills, the Shannon Sandstone Member of the Gammon Shale (Upper Cretaceous) consists of several regional sandstone lentils interpreted to represent sandridge fields deposited on a siliciclastic shelf. Elongate lenses embedded within these lentils have the geometry and facies of individual sand ridges and constitute shallow gas reservoirs in the Shannon. Because all of the individual lenses have a very similar geometry, it is possible to use only a few wells to predict the maximum extent of the reservoir with reasonable accuracy.

The geometry of elongate lenses is documented by log correlation and subsequent subsurface mapping from which measurements of maximum thickness, width, and length are extracted. Statistically significant correlations exist between thickness and width and between width squared and length squared measurements from 35 individual lenses. These relationships can be used to derive an equation describing the ellipsoidal geometry of lenses; however, application of the equation for ellipsoids is not practical. Instead, the elliptical map aspect of a single lens is approximated using correlations between geometric measurements extracted from the total population. A computer technique for locating an initial series of three wells results in a calculated outline for the total lens which can subsequently be refined as successive wells are drilled.

The computer technique probably has application in sand-ridge fields mapped in other geographic areas and in other stratigraphic intervals. For best results, however, the geometric summaries used to map an individual lens should be based on data from the sandridge field in which the lens is located.