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ABSTRACT: Eustatic and Tectonic Control on Localization of Porosity and Permeability, Mid-Permian, Bighorn Basin, Wyoming

Scott P. Simmons, Peter A. Scholle

The Goose Egg Formation of the northeastern Bighorn basin was deposited in an arid shoreline (sabkha) environment during a time of global cyclic sea level variations and local tectonic uplift. Eustatic sea level lows are represented by terrestrial red beds (seals), whereas highs resulted in the deposition of supratidal to shallow subtidal carbonates (reservoirs).

Pennsylvanian and Permian differential uplift along the present basin margin localized a broken chain of barrier islands and shoals during deposition of the Ervay and earlier carbonate members, as recognized in outcrop at Sheep and Little Sheep Mountain anticlines. The Ervay Member on these paleohighs is typified by fenestral dolomite, containing abundant tepees and pisoids. This fabric is interpreted to have formed in the highest intertidal to supratidal sabkha environment which developed on the leeward shores of these islands. The fenestral carbonates grade basinward (westward) into narrow bioclastic grainstone beach deposits and then to open-shelf fossiliferous packstones and wackestones. To the east lie laminated lagoonal micritic limestones and dolomites.

Outcrop and core study has shown the fenestral facies to be limited to areas coincident with present-day basin margin anticlines. Not only are these the locations of the most porous facies, but tight Laramide folding of the Goose Egg carbonates resulted in pervasive fracturing and thus very high permeabilities in the same structures. The close association of Laramide folds and productive Permian carbonate horizons in the northeast Bighorn basin could well be characteristic for other yet to be explored structures along the basin-margin trend.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990