Diagenesis of Sandstones in Permian Upper Minnelusa Formation, West Mellott Field, Powder River Basin, Wyoming: ABSTRACT

Abstract

Dolomite-anhydrite-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 alternation 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 sandstones were then partially cemented with anhydrite (or gypsum). Clay coatings and partial cementation by anhydrite 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 anhydrite 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 partiallymore » dissolved grains. Hydrocarbons then migrated into and occupied the secondary porosity of the upper arts of eolian-dune sandstones and reworked-eolian sandstones at West Mellott.« less