Phosphoria reservoir rocks at Cottonwood Creek field, in the Big Horn basin, exhibit many characteristics of sediments deposited by a confined current, such as those in channels in modern carbonate tidal flats. Such physical properties, observed in thin sections and hand samples, are marked in contrast to the appearance of the nonporous carbonate facies of the Phosphoria. Nonporous strata, updip and laterally adjacent to this large stratigraphic oil accumulation, have properties common to sediments of interchannel and supratidal environments observed in modern tidal flats. These observations, in conjunction with the problem of anhydrite pore-filling and fracture distribution, help to explain the distribution of the reservoir and trap at Cottonwood Creek field, and may hav application in exploration elsewhere in this province.

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