Winnipeg Formation (Middle Ordovician), Williston Basin: ABSTRACT

Although the Winnipeg Formation has long been of interest to the oil and gas industry, little has been published on it. This study established a regional correlation of the Winnipeg Formation members by using mechanical well logs, Amstrat lithologic logs, and North Dakota Geological Survey Circulars. A regional cross section was constructed establishing the correlation markers. Data collected were used to divide the Winnipeg into three members: an upper transitional member, the Winnipeg shale, and the basal Winnipeg Sandstone. Isopach maps of the shale and sandstone members and a structure contour map were also made. The Winnipeg Formation is Ordovician in age. It lies unconformably over Lower Ordovician-Cambrian sediments or Precambrian basement rock and is conformably overlain by the Red River Formation. The Winnipeg shale, a greenish gray calcareous shale, develops a sandstone lens in northwestern North Dakota called the Middle Sand Member in the literature. The shale isopach shows the thickness varying from approximately 90 ft (27 m) along the Cedar Creek anticline to over 200 ft (61 m) in the southeastern corner of North Dakota. The Winnipeg Sandstone, a blanket marine sandstone, shows the thickest sandstone along the Nesson anticline and is absent along the southern end of Cedar Creek anticline. The Winnipeg Sandstone's lithology is similar to the Deadwood Sandstone of Cambrian age, suggesting that the Deadwood Sandstone may be a source for the Winnipeg Sandstone. The transition zone represents a transgressive facies change. The consistent distinctive gamma ray kick used as the transition zone correlation marker was used as datum point for the cross sections and structure contour map. Problems encountered included scattered well control, difficulty determining the bottom of the sandstone interval, wells not penetrating the entire section, difficulty determining the top of the shale interval, and difficulty in correlating old electric logs. Recent gas and oil discoveries in the Winnipeg Formation have renewed interest in its economic potential. Several fields in North Dakota produce from the Winnipeg Sandstone. In Montana, good gas shows have been found in the middle sand member. End_of_Article - Last_Page 1330------------