Abstract

The discovery of Amos Draw Field in 1982 by Davis Oil Company extended the western limit of production of the Lower Cretaceous Muddy Sandstone in the Powder River Basin, Wyoming, bringing about renewed exploration interest in the unit. Regional correlation of geophysical logs throughout the subsurface of the basin, in conjunction with detailed examination of surface outcrops, indicate that the producing sandstone is an upward-coarsening, progradational strandplain deposit, herein called the Rozet member of the Muddy Sandstone. The Rozet member was originally widespread and had a sheet-like geometry. The reservoir at Amos Draw is contained within erosional remnants of that sheetsand, but the actual trap was formed by diagenetic processes.

A major sea-level drop coincided with the progradation of the Rozet member sandstone and resulted in subaerial exposure of the strata. During this period streams incised into and through the Rozet, creating an intraformational unconformity of low topographic relief. Moreover, the upland strandplain surface underwent pedogenic modification. Coincident with the sea-level lowstand, volcanic ash produced from eruptions to the west or northwest accumulated on the Rozet surface, eventually infiltrating into the uppermost portion of the sandstone. Later pedogenic alteration of the ash, in situ, produced a kaolinitic matrix that occluded most of the original porosity in the few feet directly beneath the unconformity. In time the pedogenic surface was covered by impermeable shales and silty mudstones of a valley-fill sequence.

During burial diagenesis, the sandstone was extensively cemented by calcite, which also peripherally replaced the clay matrix in the lower parts of the weathered zone. The calcite later underwent partial to complete dissolution, thereby generating well-interconnected, secondary pores. This zone of variable secondary porosity defines the Amos Draw producing interval.