Effects of Sedimentologic and Petrologic Heterogeneity on Reservoir Properties of the Upper Cretaceous Sussex Sandstone in the House Creek Field, Powder River Basin, Wyoming

Abstract

Petrologic and sedimentologic heterogeneity in the Sussex Sandstone Member of the Upper Cretaceous Cody Shale in the House Creek field affects recovery of oil through a decrease in sandstone porosity and permeability and by causing isolation and compartmentalization of reservoir facies. The Sussex is an upward-coarsening bed of inter-ridge to central-ridge facies of a marine-ridge sandstone sequence. Oil is produced from fine- to medium-grained, porous, and mainly trough cross-bedded central-ridge and ridge-margin sandstones. Underlying and interbedded inter-ridge sandstones have low porosity and permeability and are laterally continuous, thinly-bedded, and generally tabular. Megascopic heterogeneity includes (1) compartmentalization of the lensate reservoir sandstones by interbedding with inter-ridge sandstones, (2) facies-related upward-increasing porosity and permeability in the marine-ridge sequence, and (3) bedform-related permeability boundaries that result from soft-sediment deformation of glauconite concentrated in trough cross-bedding laminae sets. Petrologic heterogeneities result mainly from highly variable distributions and amounts of cements and clays. Nonproductive inter-ridge and ridge-facies sandstones contain generally equivalent amounts of quartz cement (8% average) and greater amounts of calcite cement (12% average) and pore-occluding clays (14% average) than oil-productive sandstones. Reservoir sandstones contain averages of 9% carbonate cement and 10% clays, but the most productive and porous intervals are cemented by quartzmore » (8% average) with negligible calcite. Average thin-section intergranular porosity is 7% in oil-productive and 4% in nonproductive sandstones.« less