CARBONATE AND EVAPORITE FACIES, DOLOMITIZATION AND RESERVOIR DISTRIBUTION OF THE MISSION CANYON FORMATION, LITTLE KNIFE FIELD, NORTH DAKOTA

ABSTRACT The Mission Canyon Formation is a regressive-upward carbonate to anhydrite sequence deposited in a shoaling epeiric sea which gave way to an advancing sabkha. The formation is mostly subtidal in origin, deposited upsection as: 1) below wave base basin deeper water carbonates, 2) open shallow marine, represented by major carbonate cycles of mudstone grading into skeletal packstone/grainstone, 3) transitional open to restricted marine, featuring minor carbonate cycles of mudstone grading into skeletal wackestone, 4) restricted marine pelletal wackestone/packstone, and 5) marginal marine skeletal wackestone interbedded with intertidal skeletal, ooid-pisolitic wackestone/grainstone. Atop these deposits intertidal buildups with storm washover aprons are interbedded with lagoonal limestones or directly with supratidal anhydrite beds. These, in turn, are overlain by prograding supratidal anhydrite with local thin interbeds of laminated dolostone. The reservoir is structurally trapped, to the north, east and west, within a northward plunging anticlinal nose. Closure is less than 100 feet. Facies changes entrap the reservoir southward, and the seal is the overlying anhydrite beds. Porous hydrocarbon bearing beds are isolated within transitional open to restricted marine, restricted marine and marginal marine lime muds. These became porous calcareous dolostone to dolostone 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. Thin section petrography and scanning electron microscopy (SEM) studies of core samples and relief pore casts reveal four pore types: moldic and three dolomite intercrystalline; namely, polyhedral, tetrahedral and interboundary-sheet pores. Each pore is progressively smaller in size. Pore throats, in productive beds, are of two general sizes, the largest five times the width of the smallest.