The Pennsylvanian and Lower Permian Minnelusa formation is a proved oil reservoir with 81,000,000 barrels produced to January 1, 1958. This formation is present and prospective throughout the Powder River Basin.

Density of Minnelusa tests is low and many large areas remain undrilled. To date, the structural approach has been used almost entirely in prospecting for Minnelusa oil, and present production is from closed anticlines. Many low-relief structures, similar to that at Donkey Creek field, should be present and oil-productive on the eastern flank of the Powder River Basin.

The Minnelusa is entirely marine in origin, having been deposited in a basin whose area coincides closely with the present Powder River Basin except on the south and southeast. Following deposition of the upper Minnelusa, an anticlinal arch formed in the north-central part of the Powder River Basin which was then truncated and buried by younger Permian shales. The oil found in this area appears to be related to the ancient anticline, with future prospects also being very good.

The Minnelusa formation can be divided by two intraformational unconformities into three separate rock units herein designated as the Upper, Middle, and Lower members. Paleontological evidence indicates that each unit is probably the same in age throughout the basin.

The Upper member of the Minnelusa is a sandstone-carbonate-anhydrite sequence of Lower Permian (Wolfcamp) age. Of the three Minnelusa members, the Upper exhibits the greatest thickness and facies variations and is also the most important for past oil production and future potential. The facies changes of porous sandstones to dense dolomites and anhydrites should provide favorable conditions for the generation and trapping of oil in stratigraphic traps.

The middle Minnelusa is a sandstone-carbonate sequence of Middle and Upper Pennsylvanian (Des Moines, Missouri, and Virgil) age. The "Leo" sandstones of the Lance Creek region (southeast Powder River Basin) are the principal oil reservoirs of the Middle member. Future production may be found in stratigraphic traps formed by the pinch-out of reservoir sands on the flanks of individual structures in the southeastern Powder River Basin and in the regional pinchout of the "First Leo" sandstone on the east flank of the basin.

The lower Minnelusa consists of cherty carbonates and red shales, usually with a basal sandstone unit, and is Lower Pennsylvanian (Atokan and Morrowan?) in age. Some oil production has been

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found in a porous limestone unit in the middle of the Lower member. Due to limited control, no precise areas for stratigraphic traps are delineated; but all closed anticlines, especially in the southeastern Powder River Basin, should be tested through the limestone and the basal sandstone units.

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