A study of available sample longs of the Mississippian Madison carbonates and chemical sediments of the Williston basin reveals a sequence of limestone types which grade from deep, quiet-water limestones through shallow, open-marine shelf-type limestones into a shallow, agitated-water clastic limestone of a barrier shoal facies. These units grade updip into a depositional environment of restricted circulation characterized by evaporites. The three major stratigraphic units are Lodgepole, Mission Canyon, and Charles. Each of these units can be traced through all or part of this sequence. In the center of the basin the individual units generally display this pattern in the following manner: the Lodgepole formation is representative of a fairly quiet, deep-water environment ndicated by fine-grained, argillaceous carbonates with rare fossil remains, some chert, and here and there some pyrite. The dominant lithologic character of the Mission Canyon formation consists of finely crystalline to chalky matrix enclosing bioclastic remains and carbonate pellets with some true oolites, indicating deposition under moderately shallow conditions of open-marine environment. The Charles formation is composed of evaporites and fine-grained argillaceous limestones with zones of fossil remains, pellets, and oolites, and is representative of lagoonal, shallow and/or restricted water deposition. No formation is wholly barrier bank lithologically, but characteristic barrier lithologic type consisting of texturally mature bioclastics can be seen locally in all three units. Rese voir characteristics of porosity and permeability within the barrier zone are directly related to the degree of textural maturity, in that primary porosity and permeability are greater in sediments which have undergone better rounding, sorting, and winnowing by wave action.

The fine-grained, argillaceous sediments of the basin deposits though locally porous, have only sub-capillary openings and present considerable resistance to fluid flow because of surface fraction between the carbonate particles and the fluid medium. The shelf-type carbonates, though containing abundant clastic material, show a very low degree of textural maturity, and here also, surface fraction of sub-capillary openings inhibits free movement of fluids. The texturally mature clastic limestone within the barrier-bank facies furnishes the best avenues for fluid movement and also the best reservoir rock. The presence of sparry calcite cement in the texturally mature calcarenites of the barrier indicates a high degree of original porosity and permeability. A potential reservoir cemented with sparry calcite or sparry anhydrite could retain its original porosity in an updip direction if a trap is present. The concepts of mineral cements and of textural maturity previously applied to sandstones should be extended to include clastic limestones.

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