Depositional Environment of Mission Canyon (Mississippian) Oil Fields in North-Central North Dakota: ABSTRACT

Abstract

Three fourths of the oil fields in the Mission Canyon carbonates of north-central North Dakota are stratigraphic traps. The hydrocarbons in the Mission Canyon carbonates occur in 6 marker-defined beds, each representing a generally regressive off-lap carbonate cycle which is capped with the supratidal evaporite facies. Two types of depositional cycles are recognized: those formed when the shelf was stable and those which developed when the shelf was structurally deformed. The vertical patterns of the facies changes have been analyzed in the context of these 2 types. The deepest part of the Williston basin during Mission Canyon deposition was in the northwestern corner of North Dakota. A broad open shelf extended eastward, where marine water circulation was sufficient to support bottom-dwelling and skeletal-producing organic communities. A trend of oolitic bars separated the open shelf from a restricted lagoon farther east. The lagoon was an area of nonskeletal carbonate sedimentation which changed facies eastward to tidal flats and supratidal sabkhas where evaporite deposition occurred. The stratigraphic traps are related to the tidal flat facies. The reservoir rocks are covered by evaporites and are best developed where lobes of shelf carbonates extend eastward into the predominantly evaporite areas. Locally, traps also were formed within the structurally controlled mud islands. The preferential dolomitization of tidal-flat and mud-island facies resulted in good reservoirs. Prolific growth of blue-green algae on the tidal flats probably provided the organic source material for petroleum within the stratigraphic traps. End_of_Article - Last_Page 351------------