The terminology of the Permo-Pennsylvanian rocks varies with different parts of the Powder River basin. The rocks are called the Minnelusa on the east, Hartville on the southeast, Casper on the south, and Tensleep, Amsden, and Darwin on the west. The sequence can be divided into three members, the Lower, Middle, and Upper, except for the Casper, where only the Middle and Upper are present. The Minnelusa is truncated northward from the central Powder River basin to a zero edge in Montana.

Compressive forces from the northeast and southwest produced a major northwest-southeast and a minor north-south trend of folding during late Minnelusa time. The north-south folds seem to be located over basement fault zones. Subsequent subaerial erosion breached the folds and truncated the upper Minnelusa beds. Following this the Opeche shale was deposited filling in the breaches and covering the Minnelusa beds.

The first commercial Minnelusa production was encountered in 1930 at the Mule Creek field from the Leo sands. In 1936 production was encountered in the Converse sand of the upper Minnelusa. Until recent years Minnelusa oil exploration was restricted to anticlinal structures with the Converse and Leo sands as objectives. The current active upper Minnelusa sand play began in 1957 at Donkey Creek and accelerated to its present activity in 1960 with the discovery of Raven Creek. Nine Minnelusa fields were discovered in 1961, ten in 1962, and five in the first two months of 1963, all from the Upper sandstone beds.

Five known types of traps are possible: (1) primarily structural; (2) structural and stratigraphic combined; (3) stratigraphic by buried topography; (4) stratigraphic by facies change; and (5) stratigraphic by regional truncation.

The primarily structural-type trap is evidenced at the Robinson Ranch field where 75 feet of closure exists on the Minnekahta.

The structural and stratigraphic combined-type trap is evidenced at Donkey Creek where there is 60 feet of closure on the Minnekahta. The "B" sand is a structural reservoir and the "A" sand is truncated through the center of the field, causing a stratigraphic pinch-out.

The buried topography type trap is evidenced at the Raven Creek field where the oil is located in the downdip ridge flanking a Minnelusa age structure, long since

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altered into a gentle nose and sealed by a covering of Opeche shale.

The evidence for entrapment by regional truncation and facies change from sandstone into dolomite and anhydrite is not available for publication.

The recent approach to Minnelusa oil exploration is directed through seismic, magnetic, and subsurface structural mapping with Minnelusa age structures in mind. Isopaching the Opeche gives clues to buried topography. Detailed core and sample studies assist in locating facies changes and truncation as well as depositional character and trends of the reservoir rock. Holes based on little or no geology, drilled by independents and supported by majors, are effective in exploration. The discovery ratio of the eastern Powder River basin is 1:10.4 on wildcats and 1:3.49 for all wells. Average cost of a dry hole is $44,000, and $77,000 for a producer. Forty acres is the basic spacing and the average pay thickness is 25 feet. The gravity of the oil ranges from 12.5° to 37° API and is classed as a heavy, black, sour, intermediate base crude.

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