ABSTRACT: Upper Mission Canyon Coated-Grain Producing Facies in the Williston Basin
Michael L. Hendricks
The upper Mission Canyon Formation along the northeast flank of the Williston basin is a regressive carbonate and evaporite sequence that has been informally divided into log defined intervals. Oil production locally occurs at the transition from anhydrite to carbonate for each of the regressive intervals. These carbonate shoreline reservoirs are limestones that are dominated by coated-grains. Porosity is intergranular and vuggy and production from these reservoirs locally exceeds 400,000 BO/well. Upper Mission Canyon beds are also productive in island-shoal reservoirs that developed basinward of shorelines. These limestone reservoirs are also dominated by coated-grains. Porosity in these reservoirs is mostly vuggy. Oil production is variable, but some wells within the Sh rwood field along the United States-Canadian border have produced over 2.0 MMBO/well.
Coated-grains from shorelines and island-shoal reservoirs have both radial fibrous and laminar micritic coatings. The radial fibrous coatings were originally calcite that precipitated in marine phreatic environments. Along shoreline trends, these radial fibrous coatings are preferentially dolomitized producing intraparticle porosity. The laminated micritic coatings are probably algal in origin and were sparsely affected by post-depositional diagenesis.
Coated-grain facies normally occur at the tops of shallowing-upward cycles. Simple ooids near the base of grain sequences are commonly overlain by complex pisoids and intraclasts near the top of sequences. Reworking of carbonate sediment was common as evidenced by the abundance of intraclasts in shoreline and island-shoal depositional environments. Most upper Mission Canyon pisoids formed in subtidal and intertidal environments and not as grains in soil profiles.
AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990