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
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