Outcrop Characterization of Deep-Water Sandstones in the Lewis Shale, Wyoming

Witton-Barnes, E.M., R.M. Slatt, and N.F. Hurley, Colorado School of Mines, Golden, Colorado

Abstract

The gross geometry of confined, unconfined, and thin-bedded turbidites may be resolved using 3D seismic data; however, small-scale elements that control lateral and vertical reservoir connectivity and compartmentalize reservoirs are below seismic resolution. These elements are exposed within outcrops of turbidite and debris flow sandstones in the Upper Cretaceous (Maastrichtian) Lewis Shale, Sierra Madre Uplift, Wyoming.

Four general facies types were identified in the outcrop study area, and lateral continuity was measured and descriptive characteristics were recorded. These facies types consisted of:

1. Laterally continuous turbidite sheet sandstones, which exhibited multiple depositional events of massive to parallel-laminated sandstone and evidence of rapid dewatering.
2. Laterally discontinuous channel-fill sandstones composed of both turbidity current and debris-flow deposits, characterized in outcrop by multiple, highly erosive fill deposits composed of both massive to parallel-laminated turbidites and non-graded de bris flows, scour-based beds of concentrated rip-up clasts, and abundant soft sediment deformation including ball-and-pillow, injection, load and flame structures.
3. Thin turbidites of variable continuity, characterized by massive to parallel-laminated sandstone and non-scoured bases with the underlying substrate.
4. Fine-grained sediments that separate sheets and channels, which are composed of both suspension and traction deposits. Different deposits may be good proximity indicators of associated sandstone types.

Outcrop photomosaics, measured sections, and rock features are included in the complete article. The opportunity to model the complexity of mixed turbidite reservoirs at the inter-well scale in outcrop will not only benefit exploration within the Lewis Shale but elsewhere where turbidites of variable continuity form complex reservoirs.