Reef Trends of Mississippian Ratcliffe Zone, Northeast Montana and Northwest North Dakota

Algal pelletoid reef deposits comprise the main oil-productive lithologic type in the Mississippian Ratcliffe zone of the Charles Formation, northeast Montana and northwest North Dakota. These rocks were formed in a moderate- to high-energy environment near the central part of the Williston basin. The reef deposits appear to have formed in slightly shallower parts of the shallow Ratcliffe sea. These low-relief submarine highs were controlled partly by relatively small movements along fault trends and lines of structural weakness that were active intermittently during Paleozoic time.