Controls on Reservoir Quality in the Dad Sandstone, Lewis Shale, Wyoming

By

Geoffrey Thyne, Ira Pasternack, Eddy Escalante, and Sarah D'Agostino

Colorado School of Mines, Golden, CO

The Cretaceous Lewis Shale Formation in the Washakie Basin, Wyoming is a tight gas resource. While recent research has included the development of a high frequency stratigraphic framework for the Lewis, the controls on reservoir quality remain poorly defined. This study investigates the controls on reservoir quality by integrating burial history modeling with petrographic and petrophysical data from productive and nonproductive wells. The study includes samples from outcrop to deep burial (13,000 feet) taken from locations within the Washakie and Great Divide Basins, Wyoming.

Preliminary observations show that sandstone samples from within the Lewis Shale are usually fine-grained, poorly-sorted and immature containing angular grains and lithic fragments. Most samples have significant amounts of microporosity. The lithic grains within the Lewis sandstone members are ductile and one of the principle causes of porosity reduction during burial. Later dissolution of unstable grains creates some secondary porosity. Pore-occluding and grain coating authigenic illite and chlorite reduce permeability in deeper wells. Variable amounts of calcite cement, generally declining with burial depth, and minor amounts of quartz cement are present. There is also some evidence for later dissolution of carbonate cements that enhance porosity.

AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming