Print this pageDetailed Description and Interpretation of Outcrop Cores, Spine One Exposure, Dad Member of the Lewis Formation, Southern Wyoming
The Fox Hills and Lewis Formations deposited in the Washakie Basin, Southern Wyoming, represent a source to sink succession from shelf to shelf edge to slope to basin. The slope clinoform packages of the Lewis are well documented from subsurface studies and contain a series of relatively narrow deepwater channel systems within the Dad Member. The Spine One exposure of the Dad Member has been intensively characterized through outcrop sedimentologic and near surface seismic studies. The exposures show variable sand body geometries from narrow, highly confined channel-forms to less confined, more tabular or sheetform bodies (at the scale of the outcrop).
Detailed description of a long series of cores within a 1100 ft logged well (CSM Strat Well #61) taken behind the outcrop succession was undertaken in order to understand how the vertical stacking and small scale sedimentary structures might reflect these variable geometries. The outcrop cores were taken in two intervals (150-605 ft; 860-940 ft) separated by 150 ft of uncored section. Core gamma ray, and a full suite of borehole logs, including a borehole image log were complemented by core plug and Mini-Perm measurements in order to fully characterize the reservoir properties of these sandstones. The detailed characterization permits direct comparison with potential subsurface analogs (Gulf of Mexico, West Africa, Sakhalin Island).
Previous near surface seismic studies identified the same set of channel sandstones observed in outcrop as well as a "master" sequence boundary separating highly confined channel and channel complex scale turbidites from underlying more weakly confined turbidites. Physical sedimentary structures above and below the sequence boundary are remarkably similar suggesting the larger "container" had limited influence on processes at the bed scale. However, there is an increase in net to gross (NTG) and change in stacking at the bedset and storey scale below the sequence boundary. The channelized section above the sequence boundary also contains substantially more transported shale clasts than below. Geostatistical analyses support the discrimination of the gross packaging, with significant differences above and below the boundary.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009