Lustre Field, the westernmost commercial Mississippian field in the Williston Basin, produces mainly from dolomites in the Charles Formation. Stratigraphic terminology is still debated, but the major reservoir occurs in the Charles “C” or Ratcliffe Zone just above the Richey Shale.

Depositional facies indicate that the Charles “C” interval represents a generally shallowing-upward depositional sequence. Fossiliferous open shelf facies predominate near the base, and grade upward through restricted shelf mudstones into peloidal, oolitic, and oncolitic grainstones near the middle of the zone. The upper part of the Charles “C” consists of anhydritic, laminated, dolomitic mudstones deposited in a sabkha to hypersaline lagoon environment.

The major shallowing upward cycle can be subdivided into five smaller cycles separated by minor periods of transgression. Each of these subcycles is incomplete, i.e., only a few of the facies occur in each, but isopach maps and facies distribution within these subcycles suggest that a northwest-southeast trending structural nose extended through the Lustre Field area and exerted an influence on deposition.

The depositional facies can be traced far beyond the field’s productive limits, but the degree of dolomitization in the lower part of the Charles “C” correlates directly with production. The best wells in the field contain up to 35 feet (10 m) of dolomite with more than 14% intercrystalline porosity and 0.5 md permeability. In the best reservoir zones, porosity reaches 30% and permeability (in unfractured rocks) is typically 1 to 10 md. Sparse vertical fractures enhance production.

The porous dolomites formed when brines generated in the sabkha to hypersaline lagoonal facies of the upper Charles “C” seeped down into the burrowed mudstones and wackestones of the lower Charles “C”. Subtle paleotopography, permeability of the lower Charles “C” rocks, and original composition of the Charles sediments interacted to determine porosity development.