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The term "Winnipeg Group" includes all strata which underlie the Red River Formation and overlie the Precambrian basement complex. The upper unit, a greenish-gray, noncalcareous marine shale, is called the Icebox Shale or Winnipeg shale in the Williston basin, and it ranges from 0 to more than 100 ft (30 m) in thickness. Outcrops in the Black Hills contain Middle Ordovician fossils. The lower unit consists mainly of quartzose sandstone ranging from 0 to 465 ft (141 m) or more in thickness. In the northern Black Hills, most of the sandstone is included in the Deadwood Formation and is Late Cambrian in age, but the uppermost beds are Early and Middle(?) Ordovician. In the Williston basin, the terms Black Island and Winnipeg Sandstone have been used to denote the sandstones hich underlie the Icebox or Winnipeg shale. The disconformity which separates Black Island and Deadwood sandstones cannot be traced with confidence into the subsurface of South Dakota, where the entire sequence appears to be a blanket of sandstone containing only thin interbeds of dolomite and shale. Therefore, the term Winnipeg Sandstone is used to denote all the sandstone between the Winnipeg shale and the Precambrian basement.
The Winnipeg Sandstone is present throughout most of the Williston basin and extends into the Kennedy basin. At its featheredge (zero isopach), it is overlapped by the Winnipeg shale. The sand appears to have originally extended farther because isolated remnants occur in a few wells on the Pierre arch. An erosional episode is believed to have removed the sand from areas east of its present regional limit except in a few downwarped or downfaulted areas. This episode was followed by a rapid advance of the Middle Ordovician sea, during which no sand was deposited. The Winnipeg shale was deposited immediately after this transgression. The overlap creates a regional seal over the Winnipeg Sandstone reservoir.
The Winnipeg shale is a mature petroleum source bed in the northern part of the Williston basin. Maturation and migration probably occurred by Permian time when the Winnipeg was buried beneath about 6,000 ft (1,800 m) of younger beds. Although some of the petroleum migrated upward into the Red River Formation, the underlying Winnipeg Sandstone was an ideal conduit for lateral migration. Documented Winnipeg shows indicate that oil migrated at least as far as Mellette County, South Dakota. One of the shows appears to be a "fossil oil field," indicating that entrapment and accumulation did occur. Isopachs of the Minnelusa to Winnipeg interval indicate preferential directions of lateral migration. Live oil shows suggest the possibility of commercial accumulations.
The Winnipeg Sandstone is thick, porous, and permeable. Depending on local conditions, a Winnipeg oil well might yield up to 500,000 bbl or more. Anticlines located basinward from the regional pinch-out of the Winnipeg Sandstone are the most logical exploration targets. Subsurface mapping and interpretation of aerial photographs and Landsat images suggest several large structures capable of containing major oil fields.
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