Diagenesis and Evolution of Secondary Porosity in Upper Minnelusa Sandstones, Powder River Basin, Wyoming

Abstract Lower Permian sandstones of the Minnelusa Formation have produced a significant number of hydrocarbon reservoirs in the Powder River basin. At the Raven Creek and Reel fields the Upper Minnelusa consists of interbedded sandstones, dolomites, and anhydrites. These sediments represent shoaling-upward cycles consisting of three facies: subtidal, intertidal, and supratidal. Complete cycles are interrupted by well-sorted, fine- to coarse-grained sandstones. Quartzarenites, the dominant sandstone type, have framework constituents of quartz, feldspars, micas, and heavy minerals. Diagenetic minerals include anhydrite, dolomite, mixed-layer illite-smectite, kaolinite, quartz, chert, and pyrite. A progressive sequence of diagenetic events from oldest to youngest evident in Upper Minnelusa sandstones is: (1) precipitation of poikilotopic anhydrite and quartz overgrowths, (2) dissolution of anhydrite cement resulting in the formation of secondary porosity, (3) precipitation of dolomite rhombs, mixed-layer illite-smectite, kaolinite, and chert in both primary and secondary pore space, and (4) precipitation of pyrite and the accumulation of hydrocarbons in the reservoirs, halting further chemical diagenesis. Dissolution of anhydrite is related to tectonism during Jurassic or Early Cretaceous times. Hydrocarbons migrated into the reservoirs in response to the Laramide orogeny. Subsurface formation waters from the Upper Minnelusa were analyzed using the WATEQF computer program. Results indicate that the waters are undersaturated to slightly supersaturated with respect to anhydrite. There is a positive correlation between the zones of low anhydrite saturation and the zones of high porosity. This suggests that the development of secondary porosity is attributable to the dissolution of anhydrite. The WATEQF program may be used in delineating secondary porosity fairways in similar geologic settings.