FACIES STACKING PATTERN WITHIN THE ORDOVICIAN RED RIVER FORMATION, WILLISTON BASIN: A CASE STUDY FROM EASTERN NORTH DAKOTA

The Ordovician Red River Formation is the second largest conventional reservoir within the intracratonic Williston Basin of USA and Canada. This study focuses on a 126 m (414 ft) thick continuous interval of the Red River Formation that was cored from the subsurface in eastern North Dakota. The core was logged noting depth, porosity, mineral composition, sedimentary structures, texture, grain size and type, and dolomitization. Twelve samples were taken at irregular intervals for standard petrographic and dolomite analysis.

The Red River Formation in the study area is characterized by burrow-mottled texture, with Thalassinoides-like burrow fills within a finely crystalline matrix. Major carbonate lithofacies include: (1) skeletal dolomudstone and dolowackestone (deeper subtidal); (2) skeletal dolowackepackstone (shallow subtidal); (3) crinoid dolopackstone (high-energy shoal); and (4) thrombolite (upper subtidal-lower intertidal). Major skeletal components include disarticulated crinoid stems and fragments of brachiopods and trilobites. The studied succession exhibits an overall shallowing-upward trend, with predominantly deeper subtidal facies in the lower part, and shallow subtidal to lower intertidal in the upper part. The lithofacies are characteristically stacked into meter-scale cycles (parasequences). Muddy parasequences (dolomudstone-dolowackestone) are 1-20 m thick and make up the lower part of the formation. Grainy parasequences (dolowackepackstone and dolopackstone; thickness 0.3-5 m) predominate in the upper part of the sequence where they commonly are capped by thrombolite.

Dolomite crystal size varies between 6 μm and 826 μm (mean 55 μm). Two types of dolomite can be identified based on crystal size and shape: (1) coarse-grained euhedral (planar-e) dolomite with well-developed crystal faces and sharp boundaries, and (2) fine-to-medium-grained anhedral and non-planar dolomite with irregular crystalline boundaries. Coarse euhedral dolomite fills burrows; intercrystalline pores are well connected and porosity is high. Anhedral and non-planar dolomite is more abundant and it makes up the matrix, where tightly interlocking crystals result in very low to no porosity.