Upper Mission Canyon Coated-Grain Producing Facies in Williston Basin

Michael L. Hendricks

The upper Mission Canyon formation, along the northeastern flank of the Williston basin, is a regressive carbonate and evaporite sequence, which 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 dominated by coated grains. Porosity is intergranular and vuggy, and production from these reservoirs locally exceeds 400,000 bbl of oil/well. Upper Mission Canyon beds are also productive in island-shoal reservoirs, which developed basinward of shorelines. These limestone reservoirs are also dominated by coated grains and porosity is intergranular and vuggy. Oil production from these reservoirs is variable, but wells within the Sherwood field along the United States-Canadian border have produced over 2.0 MMbbl of oil/well.

Coated grains from shoreline reservoirs have both radial fibrous and laminar micritic coatings. The radial fibrous coatings were originally aragonite or high-magnesium calcite, which 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 are rarely affected by postdepositional 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 more complex pisoids near the top of sequences. Most upper Mission Canyon pisoids formed in subtidal and intertidal environments and not as grains in soil profiles.

AAPG Search and Discovery Article #91023©1989 AAPG Eastern Section, Sept. 10-13, 1989, Bloomington, Indiana.