Reservoir rocks were studied from the Stony Mountain (Ordovician) and Interlake (Silurian) producing formations in several oil fields on the Cedar Creek anticline, southwestern Williston basin. In early Paleozoic time the basin was covered predominantly by epeiric seas in which were deposited shallow-water, intertidal, and supra-tidal carbonates of distinctive facies and fabric. These deposits now are dolomite in which intercrystalline porosity predominates. However, their delineation and extent are controlled strictly by original facies and subsequent diagenetic structures. Such facies and structures compare favorably with those of modern tidal-flat and supra-tidal deposits of Florida and the Bahama Islands, including the alteration forms which occur shoreward of the mea high-tide line.

A generalized working model of facies relations was derived. This model shows the proposed environment of deposition and some of the kinds of depositional structures in which original porosity distribution was preserved. Most of the important porous structures result from a combination of organic and inorganic processes in zones of low hydrokinetic potential. These are: pelleted, laminated, and burrowed mud and silt; algal mats and stromatolites; flat-pebble conglomerate; endogenic and solution breccia; and a few cut-and-fill structures. Leaching of fossils and anhydrite in certain places has accentuated and improved pore structure.

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