--> ABSTRACT: Diagenesis of the Upper Cretaceous Sussex Sandstone in the House Creek and Porcupine Fields, Powder River Basin, Wyoming, by Debra K. Higley; #91003 (1990).

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ABSTRACT: Diagenesis of the Upper Cretaceous Sussex Sandstone in the House Creek and Porcupine Fields, Powder River Basin, Wyoming

Debra K. Higley

Reservoir properties of the Sussex Sandstone were determined by sedimentology and thin-section petrography studies. Oil-productive sandstone was deposited primarily as central-ridge and ridge-margin facies in a marine-ridge depositional environment. The Sussex Sandstone ranges in thickness from 2 to about 40 ft (1 to 12 m). The average net pay thickness is about 15 ft (5 m). Reservoir sandstone is mainly fine to medium-grained litharenite that exhibits medium-scale trough and planar cross-bedding. Porosity in reservoir sandstone is primarily intergranular and ranges from 7 to 18%. Minor amounts of microporosity are present in chert and clays. Calcite dissolution following replacement of feldspar grains results in locally extensive intragranular and secondary porosity. Non roductive reservoir facies are generally completely cemented by calcite and dolomite, and may contain extensive quartz cement.

Sussex Sandstone early diagenesis in the study area is characteristic of marine environments; growth of syndepositional aragonite and radial-fibrous calcite was followed by precipitation of rims of iron-rich chlorite on lithic grains, and by chemical alteration of fecal pellets to glauconite.

Quartz and (later) feldspar overgrowths formed prior to significant sediment compaction, as indicated by average 30% minus-cement porosity in reservoir facies. Overgrowths supported the grain framework, preventing the loss of porosity through soft sediment deformation. Precipitation of calcite and dolomite succeeded overgrowth formation. Later dissolution of carbonates created void spaces into which oil migrated. The final diagenetic stage, resulting from meteoric water influx, was the precipitation of pore-filling kaolinite.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990