Title: Depositional Environments, Stratigraphic Framework and Reservoir Characterization of the Mud-Rich Turbidite System in the Lewis Shale, Washakie Basin, Wyoming
Speaker: Steven M. Goolsby
Date: February 15, 2002
Publication: The Outcrop, February 2002, p. 4-5
The Upper Cretaceous (Maastrichtian) Lewis Shale of the Green River Basin, Wyoming, provides an ideal setting in which to study sandstones deposited in mud-rich shelf, slope, and toe-of-slope depositional settings. Several masters’ students and the author have studied the Lewis in the Washakie Basin as part of their graduate work at the Colorado School of Mines. These studies provide a robust data set with which to investigate the Lewis Shale and its productive characteristics.
To study the wellbore attributes and reservoir characteristics of the Dad Sandstone member of the Lewis Shale, the Colorado School of Mines drilled a 1700 ft deep behind-outcrop well in 1999. A full suite of logs has been run in the well, including electrical borehole images and nuclear magnetic resonance (NMR). Approximately 580 ft of the well was cored. Conventional core analyses are available for 170 samples, and minipermeameter results are available for every half foot of core. Capillary pressure studies have been completed for selected core plugs, and core NMR measurements are being completed for these same plugs. A detailed core description has been compared to the electrical borehole image log. Core permeabilities match NMR permeabilities over several orders of magnitude. The integration of open hole logs, core information, and outcrop studies in the Lewis have led to new insights in the interpretation of the reservoir characteristics of similar turbidite deposits.
Outcrop studies demonstrate that a variety of lithofacies types are present in the Lewis Shale. Some of these mud-rich turbidite lithofacies are poorly understood. For example, some turbidite channels contain fill that is comprised of stacked high-density turbidite deposits. Differentiating between these channel deposits and unconfined sheet deposits is difficult using open hole logs alone. Identifying the various lithofacies found in these mud-rich depositional environments using image logs can be critical in understanding reservoir continuity and connectivity in the Dad Sandstone.
The integration of outcrop studies, wellbore properties, and subsurface investigations in the Lewis is leading to an improved understanding of the petrophysical properties and reservoir characteristics of the Lewis Shale. This understanding comes at a time of increased interest in the formation due to recent success in completing the zone in the Washakie Basin. Interest in the Lewis is also high because it is a good analogy to mud-rich turbidites that produce in the deepwater Gulf of Mexico, in the offshore of West Africa, and in other offshore producing areas.