The outstanding structural feature of the Dakotas is the Williston basin. It is bounded on the east and southeast by gentle, rather featureless dips off the Canadian shield and the Sioux uplift and on the west by the abrupt Black Hills uplift and the lesser Baker-Glendive anticline of eastern Montana. More pronounced local structures are present in South Dakota, but the chief present interest in the petroleum possibilities of North Dakota is related to the regional structure.
The combined area of the two states is approximately 100,000 square miles. The thickness of sedimentary rocks ranges from 1,000 to 12,000 feet or more. The average thickness is about 6,000 feet and the sedimentary volume is about 115,000 cubic miles. Subsurface structural information is available from seventy-nine wells, and thirty-two wells have been drilled into pre-Cretaceous beds. Not more than half of these wells were actual tests for oil and gas. The deepest test is The California Company's Kamp No. 1, drilled to 10,281 feet in Williams County, North Dakota. It stopped in either Ordovician or Devonian limestone.
The percentages of different lithologic types in the sedimentary rocks vary widely through the area. Rough estimates for North Dakota are 45 per cent shale, 16 per cent sandstone, 37 per cent limestone and dolomite, and 2 per cent such evaporites as anhydrite and salt. For South Dakota, they are 66 per cent shale, 17 per cent sandstone, 16 per cent limestone and dolomite, and 1 per cent evaporites.
A basal sand and shale