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ABSTRACT

Astudy of a tide-influenced deltaic sandstone investigated geologic variations that affect hydrocarbon production in analogous reservoirs. The Cretaceous-aged Frewens Allomember was deposited by a delta prograding into a narrow shoreline embayment between an older, wave-dominated delta lobe to the south and a basin-floor ridge created by subtle structural uplift to the north. The Frewens Allomember is exposed in outcrops of the Frontier Formation in central Wyoming (United States). It comprises two 5-km-wide by 20-km-long upward-coarsening sandstone bodies. Each body contains basinward-dipping internal beds. Heterolithic beds capped by extensive shale drapes record episodic tidal deposition in the lower portions of the sandstone bodies, whereas sandier cross-stratified beds in the upper parts of bodies record stronger and more uniformly ebb-directed currents. During diagenesis, calcite concretions formed preferentially at the top of the upper sandstone body as water circulated down from overlying shales.

Diagrams of bedding, facies, calcite concretions, and bed-draping shales were compiled from high-resolution photomosaics and field observations. Sedimentologic logs, field permeameter measurements, and thin-section observations describe petrophysical properties of facies in the delta lobes. Variograms quantify the spatial correlation of permeability in lithofacies. The lengths of bed-draping shales were estimated from outcrop data using a termination frequency model. The spatial distribution of concretions was modeled with indicator geostatistics.

Flow models integrated bedding geometry, lithofacies, and petrophysical properties in an appropriate structure for reservoir simulation. These models were used to analyze sensitivity of reservoir behavior to different geologic features and to investigate methods for modeling and upscaling interwell-scale heterogeneity. Intrafacies variability of permeability has negligible effects at the sandstone-body scale but significant effects at the bed scale. Shale lengths increase toward the lateral margins and toward the base of the sandstone bodies. Inclined shales reduce upscaled permeability, recovery efficiency, and breakthrough time. Calcite concretions decrease upscaled permeability. An upscaling method based on flow simulation and response-surface models accurately and efficiently represents the effects of geologic heterogeneity and flow rate on a coarse simulation grid.

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