Abstract

The Shannon Sandstone outcrops of Upper Cretaceous Campanian age contain two stacked, well-exposed sand ridges, designated upper and lower sandstones, in the Salt Creek Anticline of the Powder River Basin, Wyoming. The ridges in both the upper and lower sandstones at Salt Creek are elongate in a north- south direction, trending slightly oblique to the current flow inferred from the abundant moderate-to high-angle cross bedding that is observable in the Shannon outcrops.

The Shannon Sandstone is productive in many relatively large oil fields in the Powder River Basin including Hartzog Draw which contains more than 200 million barrels of oil in place. High-quality, porous and permeable reservoir facies include: Central-Ridge Sandstone, a clean, cross-bedded, moderately glauconitic sandstone and High-Energy Ridge-Margin Sandstone, a highly glauconitic, cross-bedded sandstone containing relatively abundant shale and limonite (after siderite) rip-up clasts and lenses. The high-angle cross stratification in these facies are interpreted to have been formed by tidal and storm driven currents. The ridges are formed primarily by medium to large-scale sand waves and troughs which prograded across the tops of the sand ridges.

With the advent of sequence stratigraphy a variety of different environments have been suggested for the Shannon including estuarine embayment sandstones, shelf sandstones, valley-fill deposits and lowstand shoreface deposits. The Shannon contains few body fossils except for Baculites (smooth), however trace fossils are moderately abundant. Trace fossils common in the Shannon include Conicnus and Terebellina.

Correlation of closely spaced outcrop measured sections of the Shannon is relatively difficult because there is no obvious time marker horizon. Recognition of the outcrop facies variations in the subsurface is difficult to impossible using logs alone. Where cores are available to calibrate the logs, detailed facies correlations are possible.

Teapot Dome Field, which produces from the Shannon, is only three miles southeast of the most southeasterly Shannon outcrops. Porosity and permeability from cored wells from the Field generally are highest near the top of the sandstones. In the Field the Shannon generally has a coarsening upward sequence of facies and the values of porosity and permeability generally increase upwards in a stair-step fashion with the steps being the facies boundaries. The presence of faulting within the Field and between the Field and the outcrops creates barriers to flow and allows production at very shallow depths. Porosities and permeabilities in both outcrop and Teapot Field are facies controlled. Vertical variograms constructed for outcrops and cores in Teapot Field and from sampled outcrop sections are very informative. The sill for porosity in Teapot Field is about 24 feet and for outcrops 32 feet. In contrast the permeability sill for outcrops is only 5 feet and for the Field 6.5 feet. These numbers indicate that this type of reservoir is very permeability sensitive over comparatively short distances, while porosity variation is not as critical.