Abstract

The Red River Formation in the Williston Basin provides a remarkable record of Upper Ordovician cyclic deposition in a relatively structureless, slowly subsiding, nearly circular, intracratonic basin more than 400 miles in diameter. Red River depositional sequences can be correlated across the entire basin with little lateral variation in facies and only minor changes in thickness.

The formation, which is more than 700 feet in the basin center, consists of three brining-upward depositional sequences. These sequences, labelled “C”, “B”, and “A” in ascending order, have respective thicknesses of a few hundred feet, several tens of feet, and tens of feet. Each sequence has a burrowed, open-marine mudstone to wackestone at the base, a laminated mudstone (generally dolomite) representing restricted hypersaline subtidal deposition in the middle, and, in the basin center, bedded to enterolithic anhydrite at the top. A thin package of open-marine burrowed wackestone overlies the “A” Anhydrite and is conformable with the overlying Stony Mountain Formation, also of Late Ordovician age. Paraconformities are present between each anhydrite and the overlying carbonate unit, but otherwise the section represents nearly continuous deposition from about 450 to 444 Ma in the early Cincinnatian.

Despite the continuity of depositional facies, diagenetic processes, particularly dolomitization, show high lateral variability. Pervasive dolomitization of the entire Red River Formation and overlying Stony Mountain carbonates is common west of the limits of the thickest and oldest of the three basin-centered anhydrite deposits (the “C” Anhydrite). Where this anhydrite is present, however, dolomitization in the “C” laminated and “C” burrowed members is highly variable over distances of less man one mile. Localized downward seepage of magnesium-rich brines generated during subaqueous precipitation of the “C” Anhydrite is responsible for most of the dolomitization in the basin center. These magnesium-rich brines have economic importance because they created porous and permeable dolomites that serve as reservoirs for many millions of barrels of oil in the basin.

Most of the oil in the Red River reservoirs was generated in thin, organic-rich beds knows as kerogenites or kukersites. These beds were not deposited uniformly across the basin, but are laterally continuous over distances of tens of miles. They are best developed in the upper part of the “C” burrowed member, and formed as accumulations of a unicellular organism known as Gloeocapsomorpha prisca. These organic-rich beds are common in the southern and western parts of the basin where there is now an abundance of Red River oil fields, but they are also common in the northern part of the basin where relatively few such fields have been found to date.