WITTON, BETH M.
Department of Geology and Geological Engineering, Colorado School of Mines, Golden, CO

Abstract: Borehole Image Analysis of Thin-Bedded and Channelized Turbidites and Sandy Debris Flows, Lewis Shale, Wyoming

The Upper Cretaceous Lewis Shale, Sierra Madre Uplift Wyoming provides an excellent opportunity to observe small scale sedimentary features, facies stacking patterns, and bed geometries of thin-bedded and channelized turbidites and sandy debris flows in outcrop. Vertical and lateral features, patterns and geometries seen in outcrop can also be recognized in a correlative Lewis Shale interval through the interpretation of STAR acoustic and electrical borehole images.

The Lewis Shale consists predominantly of thin interbeds of very fine sandstone, mudstone and siltstone with thick, very fine- to medium-grained sandstone found at different stratigraphic levels throughout the formation. The sequence of sandstone an interbedded mudstones found in the middle to upper part of the Lewis Shale is named the Dad Sandstone Member. Outcrops of the Dad Sandstone indicate that the rocks are deep-water turbidity current and sandy debris flow deposits.

Small-scale sedimentary features documented in outcrop have been identified on STAR images, including erosional surfaces, slumps, rip-up clasts, ripple-form bedding and soft-sediment deformation features such as flame and injection structures. Lewis Shale outcrops near Dad, Wyoming have enough lateral exposure to unveil the inter-well complexity of these type of deposits, making the tie between surface outcrop and subsurface well data very important. Identifying environmental characteristics on well logs and understanding the lateral continuity of thin-bedded and channelized turbidites away from the wellbore will not only benefit Lewis Shale exploration in the Greater Green River Basin, Wyoming, but elsewhere where thin-bedded and channelized turbidites form complex reservoirs.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas