Rourke Gap Field--A Model of Three-Dimensional Facies Relationship in Minnelusa Formation, Powder River Basin, Wyoming

Christopher L. Reel, John C. Horne, James B. Truman

The vast majority of stratigraphic trapping mechanisms found in the Permian upper Minnelusa Formation are associated with an unconformity at the top of the formation. More recent exploration programs have focused on the identification of stratigraphic traps resulting from lateral facies changes found in these eolian to shallow marine strata, the most common being the juxtaposition of porous oelian sandstones downdip of nonporous interdune sandstones and carbonate/evaporite sequences.

The growing maturity of the Minnelusa play has led geologists to both develop and explore progressively deeper sandstone bodies in frontier areas. Due to the depth (greater than 10,000 ft) and relative thinness of these sandstone bodies (averaging less than 40 ft, 15 m), their identification (with an associated seal rock) is, at best, extremely difficult based on seismic data alone. There exists, however, a predictive tool based on vertical sequences observed in the Rourke Gap field and vicinity that can be utilized in areas where existing well data are shallower than the planned objective.

In Rourke Gap field, the most significant relationship for the explorationist is the en echelon arrangement of reservoir-potential eolian sandstones in succeeding stratigraphic intervals. Moreover, there is an inverse thickness relationship between dune sandstones and the overlying carbonate-evaporite seals. This relationship is the result of syndepositional paleotopography and the influence of syndepositional and postdepositional compaction in marine carbonates and interdune carbonate-evaporite sequences. These observations can provide useful clues to explorationists for predicting underlying unit thickness distribution in areas where drill-hole data do not penetrate the objective interval with sufficient frequency. If the thickness of a carbonate-evaporite unit overlying a targeted eservoir is thin, the underlying reservoir-potential sandstone is most likely thick, and vice versa. Where dune sandstone in one stratigraphic interval thins, the sandstone in the next underlying stratigraphic interval thickens. These relationships are defined most readily in areas with a sufficient density of control, and the concepts are most successful in the evaluation of deeper objectives and development of step-out drilling programs.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.