--> ABSTRACT: Diagenesis of Sandstones in Permian Upper Minnelusa Formation, West Mellott Field, Powder River Basin, Wyoming, by Christopher J. Schenk, James W. Schmoker, and Richard M. Pollastro; #91030 (2010)
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Diagenesis of Sandstones in Permian Upper Minnelusa Formation, West Mellott Field, Powder River Basin, Wyoming

Christopher J. Schenk, James W. Schmoker, Richard M. Pollastro

Dolomite-Previous HitanhydriteNext Hit-sandstone cycles of the Permian upper Minnelusa Formation, Wyoming, record multiple progradations of marine, siliciclastic sabkha, and eolian sediments. Heterogeneity in the sandstones, studied in five cores from present-day depths between 7,100 and 7,300 ft, is partly the result of diagenetic alterations. The earliest alteration was the formation of clay coatings on the detrital grains, particularly in siliciclastic sabkha and related eolian sandstones. Quartz cement formed small overgrowths, particularly in the siliciclastic sabkha sandstones. Carbonate clasts in the sandstones were pervasively dolomitized, as were the adjacent carbonate beds, possibly by reflux associated with brines that formed during the evaporitic stage of the next sedimentary cycle. The sands ones were then partially cemented with Previous HitanhydriteNext Hit (or gypsum). Clay coatings and partial cementation by Previous HitanhydriteNext Hit did not completely prevent a progressive loss of intergranular volume by pressure solution with burial. The illitization of early clay coatings was also a progressive process with burial.

Late-stage dissolution of Previous HitanhydriteNext Hit produced irregularly distributed secondary sandstone porosity that ranged to approximately 30%; dissolution was most complete in the upper parts of eolian-dune and reworked-eolian sandstones, resulting in excellent reservoir properties. Late-stage dissolution of potassium feldspars also occurred, as evidenced by the lack of compaction of the partially dissolved grains. Hydrocarbons then migrated into and occupied the secondary porosity of the upper parts of eolian-dune sandstones and reworked-eolian sandstones at West Mellott. Small rhombs of authigenic dolomite grew along most of the secondary pores and partially replaced the remaining Previous HitanhydriteNext Hit. Quartz overgrowth cement formed subsequent to the dolomite, in some cases engulfing the dolomite rhombs in syntaxial overgrowths. Pyrite appears to be a late authigenic phase that automorphically replaced Previous HitanhydriteTop, quartz, and dolomite.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.