There are good opportunities for finding stratigraphic traps in the Permian Phosphoria Formation of central Wyoming. Ideally, hydrocarbons may be trapped in an updip reentrant of porous carbonate rock extending into redbed-anhydrite facies or in a porous algal leaf and oocastic mound enclosed in impermeable carbonate rock.

Three facies are found as bands of dolomitized carbonate rocks on the Wyoming shelf between the marine limestones of the Phosphoria Formation and redbeds of the Goose Egg Formation. Westward, the shelf deposits grade into basinal phosphatic and cherty beds.

Two transgressive-regressive marine cycles are present in which limestone grades through dolomite into a redbed-anhydrite facies. Stratigraphic work on the carbonate members (Ervay, Franson, and Grandeur) has defined three mappable facies exhibiting porosity and permeability due to secondary dolomitization. These facies consist of (1) algal oolitic-pellet mounds, (2) algal pellet mounds, and (3) oocastic carbonate rocks.

Traps formed where the algal oolitic-pellet mounds of the Ervay Member grade into impermeable redbed-anhydrite and dense carbonate rocks have proved the most important economically. Cottonwood Creek field, containing 45 million bbl of reserves, is an excellent example. This facies relation can be traced from outcrops in the northwestern Big Horn Mountains, through the Big Horn basin, to wells drilled on the south flank of the Wind River basin, where drilling so far has failed to find a productive trap. The Laramide orogeny further complicated the conditions of entrapment.

The oocastic facies, best developed in the Ervay and Franson Members, is directly west of the algal oolitic-pelletal facies and trends as a band across central Wyoming. This facies has excellent porosity but poor permeability, and has produced mainly from fractured reservoir rock on structures such as Winkleman dome, Circle ridge, and Beaver Creek.

The algal-leaf facies, found as lenses in fine-grained nonporous carbonate rock, parallels the depositional trends of the other facies; it also is interbedded with the algal oolitic and oocastic facies of the Ervay and Franson Members. The algal-leaf facies has excellent capacity and, if oil-saturated, constitutes an excellent reservoir. This facies is the primary pay zone in No Water Creek field in the Big Horn basin. Identification of this facies is difficult in wells, so cores and thin sections are needed.

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