The upper member of the Minnelusa Formation in the Raven Creek and Reel oil fields of the Powder River basin consists of a sequence of interbedded sandstones, dolomites, and anhydrites of Early Permian age. This sequence represents depositional environments ranging from sabkha type to a coastal eolian dune complex. The sandstones of the Minnelusa Formation are predominantly moderate to well-sorted, fine to very fine-grained quartzarenites.

Silica overgrowth and anhydrite cement represent an early stage of diagenesis. Dissolution of anhydrite cement was the major process in the development of secondary porosity in the Minnelusa reservoir. Pore-filling dolomite and dolomite replacing anhydrite are late stage diagenetic products. The euhedral dolomite rhombs are ferroan in composition. Authigenic illite-smectite mixed-layered clays are formed contemporaneously with the diagenetic dolomite. Dissolution of anhydrites, formation of secondary porosity, and hydrocarbon accumulation may have occurred during Late Jurassic and terminated during Laramide orogeny.

An analysis of the subsurface formation waters from the Powder River basin was made using WATEQF computer programs. Results indicate that waters are supersaturated with respect to anhydrite and dolomite. The saturation indices of anhydrite and dolomite in the subsurface waters are plotted on maps. A

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trend is noted where saturation indices appear to increase away from the producing zones of the Minnelusa in the northeastern part of the Powder River basin. Therefore, saturation indices may be used as an indicator of secondary porosity development and consequently as an exploration tool for hydrocarbon accumulation in the Minnelusa Formation.

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