A core and petrographic study was undertaken to describe depositional setting and development of porosity within the Ratcliffe beds of the Charles Formation (Meramecian) found in northeast Richland County, Montana. Graphical descriptions were made from three slabbed cores and were augmented by petrographic study of thin sections.
The Madison Group in the Williston Basin includes the Lodgepole, Mission Canyon, and Charles formations. Lodgepole (Kinderhookian) deposition occurred along aerially extensive prograding clinoforms which filled the early Mississippian Williston Basin. At or near the toes of the clinoforms, Waulsortian-type mounds occur locally. Mission Canyon (Osage and Meramecian) and Charles (Meramecian) deposition is characterized by complex intertonguing of shallow basinal, shallow shelf, and peritidal carbonates and evaporite beds. Both the Mission Canyon and Ratcliffe intervals of the Charles Formation are carbonate to evaporite, shallowing-upward, regressive sequences. Locally in the Ratcliffe, dolomitic mudstones and wackestones are oil reservoirs. Ratcliffe beds in the study area occur at depths of approximately 2680 m (8800 ft) and have a gross thickness of about 26 m (85 ft). Most completions in the Ratcliffe have been secondary after depletion of deeper oil reservoirs, primarily the Ordovician Red River.