Examples are presented of reservoir rocks taken from Stony Mountain (Ordovician) and Interlake (Silurian) producing formations in several oil fields situated on the Cedar Creek anticline, southwestern Williston basin. In lower Paleozoic time the basin was dominantly covered by epeiric seas in which were deposited shallow-water, intertidal, and supratidal carbonates of distinctive facies and fabric. These deposits are now dolomite in which intercrystalline porosity predominates. However, the delineation and extent of the latter is strictly controlled by original facies fabrics, the character of which is favorably comparable with modern tidal-flat and low-profile supratidal deposits reported for Florida and the Bahama Islands.

A generalized working model of facies relationships is presented showing the proposed environment of deposition and some of the kinds of fabrics in which producible oil has been found. Most of the important oil-bearing fabrics suggest combined organic and inorganic processes in zones of low hydrokinetic potential, viz., pelleted and laminated muds and silts; burrowed, bored, and reworked muds and silts; algal mats and stromatolites; flat-pebble conglomerates; and endogenic and (or) solution breccias. Leaching of fossils

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and anhydrite in certain cases has accentuated and improved pore structure.

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