FACIES ARCHITECTURE AND RESERVOIR CHARACTERIZATION OF THE D INTERVAL, WILKINS PEAK MEMBER OF THE EOCENE GREEN RIVER FORMATION, WYOMING

Factors such as sandstone architecture, grain size, lithology, and diagenesis can impact reservoir permeability. In the lacustrine Wilkins Peak Member of the Eocene Green River Formation, nine arkosic fluvial intervals were deposited during lowstands. These bedsets are well exposed and can be traced for over 50 km, providing abundant outcrop exposure suitable for lateral and vertical lithofacies description and correlation. Similar to the lacustrine Caspian basin, the WPM contains very fine- to fine-grained sheet-like sandstone, interbedded with siltstone and mudstone. Thus, the WPM may serve as an analog to reservoirs in the Caspian basin and lacustrine basins elsewhere. The D interval is the thickest and most laterally extensive of the WP fluvial intervals and is the focus of this study. To characterize the reservoir potential of the D interval, a combination of sandstone architecture interpretation, point counting, and porosity and permeability measurements were conducted. Key architectural elements include lateral accretion surfaces, scour and fill features, and climbing ripples. Lateral accretion and scour and fill structures are interpreted as channel deposits. These facies contain coarser grain sizes, including fine-lower sandstone, than non-channelized facies, with potentially greater porosity and/or permeability. 24 sandstone samples were point counted at 300 grains per sample. Quartz, feldspar, and mica percentages were plotted on a ternary diagram. Point count data indicate a high percentage of feldspar grains (52-72%); micas range from 3-39% and quartz from 19-40%. Porosity and permeability were measured from four of these samples. Porosity was calculated from weight differences between water-saturated and oven-dried cores; values range from 15.2-24.7%. Permeability was measured by water injection in a pressure vessel, with four or more separate measurements made on each sample at various confining and pore pressures. Measurements range from 0.0157 mD to 0.0678 mD. The extremely low permeability measurements of WPM fluvial facies are partially attributed to the fine-grained nature and poor sorting of these sandstone beds. High feldspar percentages, and presence of micas and clays, suggest that diminished reservoir quality can also be attributed to feldspar diagenesis.