Anatomy of a Dolomitized Carbonate Reservoir--Mission Canyon Formation at Little Knife Field, North Dakota: ABSTRACT

Abstract

The Mission Canyon Formation is a regressive, shoaling-upward carbonate to anhydrite sequence deposited in a slowly shrinking epeiric sea. From its base upsection, the formation is mostly subtidal in origin and emergent at its top, and consists of (1) deeper water carbonates, (2) major cycles of open shallow-marine mudstones grading up into skeletal packstone or grainstone, (3) minor cycles of dolomitized transitional open to restricted marine mudstone grading up into skeletal wackestone, (4) dolomitized restricted marine pelletal wackestone or packstone, (5) partially dolomitized marginal marine skeletal wackestone, (6) slightly skeletal, oolitic-pisolitic wackestone or grainstone barrier-island buildups with storm washover aprons, (7) thin lagoonal limestones, (8) tidal flat anhydrite, and (9) sabkha anhydrite. The oil is structurally trapped on the north, east, and west, within the northward plunging Little Knife anticline. Facies changes entrap the oil southward; the vertical seal is the overlying anhydrite beds. Closure is less than 100 ft (30 m). Porous, hydrocarbon-bearing beds were deposited as transitional open-to-restricted marine, restricted marine, and marginal marine lime muds. These became porous dolomitic reservoir rock by undergoing three diagenetic changes: (1) anhydrite replacement of skeletal fragments, (2) dolomitization of the muddy matrix, and (3) later, leaching of the anhydrite to create moldic porosity. The reservoir's pore system is composed of moldic pores and three types of dolomite intercrystalline pores--polyhedral, tetrahedr l, and interboundary-sheet pores. Pore throats in productive beds are of two general sizes (1.2-1.6 µ and 5.2 µ). End_of_Article - Last_Page 854------------