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The AAPG/Datapages Combined Publications Database

Earth Science Bulletin (WGA)


Earth Science Bulletin
Vol. 15 (1982), No. 1. (Annual), Pages 139-140

Abstract: Petroleum Potential of the Winnipeg Sandstone in South Dakota

W. Bolyard Dudley,1 George L. Davenport2

Wyoming Geological Association: 1982 Luncheon Meetings Casper, Wyoming: Abstracts of Papers

The term “Winnipeg Group” includes all strata which underlie the Red River Formation and overlie the Precambrian basement complex. The upper unit is greenish-gray, non-calcareous marine shale, called the Icebox Shale or Winnipeg Shale in the Williston Basin, which ranges from zero to more than 100 ft in thickness. Outcrops in the Black Hills contain Middle Ordovician fossils. The lower unit consists mainly of quartzose sandstone ranging from zero to 465 ft 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 which 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 an essentially continuous blanket of sandstone containing only thin (probably discontinuous) interbeds of dolomite and shale. Therefore, the term “Winnipeg Sandstone” is used in this paper to denote all the sandstone between the Winnipeg Shale and the Precambrian basement.

Paleozoic structural elements of western South Dakota include the Pierre Arch which occupies part of Lyman, Jones, Stanley and Haakon Counties. Subsurface data indicate that some of the lineaments seen on Landsat images coincide with faults and that some of the faults had Paleozoic movement. The Williston Basin axis plunges northward from Penningtom County through Meade, Perkins and Harding Counties toward the Paleozoic depocenter in North Dakota. The Kennedy Basin, which is located south of the Pierre Arch and extends into Nebraska, is poorly understood because of sparse well control and a widespread cover of Tertiary beds. However, the Chadron Arch, which flanks the Kennedy Basin on the southwest, was a pre-Pennsylvanian positive element as well as a Laramide uplift.

The Black Hills Uplift is a younger, Laramide feature which exposes Precambrian, Paleozoic and Mesozoic formations. All reservoirs that were breached by Tertiary erosion on this uplift probably have lost whatever hydrocarbons may have been present in pre-Laramide times. Although oil and gas occur in lenticular deposits, they are not to be expected in the Winnipeg Sandstone or other blanket porosity zones for a considerable distance downdip from where these formations crop out near the Black Hills Uplift.

The Winnipeg Sandstone is present throughout most of the Williston Basin and extends into the Kennedy Basin. At its feather edge (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 where the sand was preserved. 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.

Research by others has established that the Winnipeg Shale is a mature source bed of oil and gas in the northern part of the Williston Basin. Maturation and migration is thought to have occurred by Permian time when the Winnipeg was buried beneath about 6,000 ft of younger beds. Although some of the petroleum migrated upward into the Red River and other formations, the underlying Winnipeg Sandstone was an ideal conduit for lateral migration. Documented shows in the Winnipeg Sandstone 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 Minneslua to Winnipeg interval indicate the 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 bbls or more. Anticlines and domes located basinward from the regional pinchout 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. Seismic data confirms the existence of such structures. In one instance, a structure with 150 ft of closure and evidence of early growth has been documented.

Acknowledgments and Associated Footnotes

1 W. Bolyard Dudley: Bolyard Oil and Gas, Ltd.

2 George L. Davenport: Bolyard Oil and Gas, Ltd.

© Wyoming Geological Association, 2015