Most oil traps on the western flank of the Powder River basin at first glance appear to be the result of Laramide anticlinal flexures. This, however, is not true of the fields producing from the Minnelusa Formation. Several Laramide anticlines have little or no production from the Minnelusa rocks. What, then, are the true methods and conditions of oil entrapment in the Minnelusa Formation?

The Minnelusa Formation in this paper includes those rocks below the Permian Goose Egg Formation and above the Mississippian Madison Formation. The Minnelusa Formation has been subdivided in this area into three members by previous authors. The three members are termed Upper, Middle, and Lower. The Upper Member is considered to be Wolfcampian in age and is probably the most important oil producer in the area. The Upper Member has been further subdivided into three rock units named, for simplicity, A, B, and C. A and B appear to be the most productive and also exhibit the greatest stratigraphic variations. Understanding the stratigraphic variations and their relationships to the Laramide structures can lead to a better development program of existing oil fields and further the explorat ry efforts in little-drilled areas of the western Powder River basin.

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