Diagenetic Control on Porosity Differences Between Tensleep and Minnelusa Sandstones, Wyoming: ABSTRACT

Abstract

The Tensleep Sandstone (Pennsylvanian) in the Bighorn basin and the Minnelusa Formation (Pennsylvanian-Permian) in the Powder River basin are two prolific hydrocarbon-producing units in Wyoming. Both formations were deposited in a variety of depositional environments, including open and restricted marine, sabkha, and eolian settings. Sandstones of both formations are compositionally similar; they are dominantly quartzarenites containing variable amounts of cements, including dolomite, anhydrite, silica, and calcite. Petrographic data indicate that maximum porosity of the Tensleep sandstones decreases with depth of burial, whereas that of the Minnelusa sandstones is roughly constant with burial depth. This differences in maximum porosity between the two formations is attributed to differences in their diagenetic histories. Tensleep sands were primarily deposited in a sand-sea setting, and early cementation was only minimal. Subsequent compaction occurred in response to regional burial - the greater the depth, the greater the compaction. After compaction of the strata, anhydrite developed within the remaining pore spaces. Late-stage dissolution of anhydrite has led to increases in porosity, but only up to postcompactional, preanhydrite values. In contrast, Minnelusa sands accumulated in coastal dune and sabkha environments that were conducive to the development of cementation soon after deposition. Therefore, subsequent compaction of the cemented sandstones,more » through burial, was inhibited. Late-stage dissolution of anhydrite in the Minnelusa sandstones, therefore, has resulted in relatively greater porosities that exhibit little variation in relation to depth of burial.« less