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The Ratcliffe interval within the Williston basin in North Dakota is included in the Mississippian Madison Group. It is an informal stratigraphic subsurface unit which includes parts of the upper Mission Canyon and lower Charles Formations. Deposition of the Ratcliffe sediments occurred in an open to progressively restricted marine environment along the eastern margin of the basin. Six facies have been recognized in the study area. These are the: (1) brachiopod-bryozoan-echinoderm packstone/wackestone facies; (2) peloid-oolite packstone/wackestone facies; (3) ostracod-foraminifer wackestone facies; (4) laminated mudstone/wackestone facies; (5) anhydrite-dolomite mudstone facies; and the (6) organic quartz siltstone facies. Oil found within the Ratcliffe interval is usually associated ith the peloid-oolite packstone facies. Some moldic porosity has developed by solutioning. Dolomitization has increased intercrystalline porosity. Dolomitized areas commonly are capped by less porous facies making good potential stratigraphic traps. Formation of traps and reservoir rock is highly dependent on porosity and permeability and also on the amount of diagenesis, especially secondary anhydrite, associated with the sediments.

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