Sarah Lee D'Agostino¹

(1) Colorado School of Mines, Golden, CO

ABSTRACT: A Petrographic Study of the Dad Sandstone, Lewis Shale, Great Divide and Washakie Basins, Wyoming

The Cretaceous Lewis Shale in the Washakie Basin, Wyoming is a tight gas resource. While recent research has included a high frequency stratigraphic framework for the Lewis, the controls on reservoir quality remain poorly defined. This study describes the paragenetic sequence and burial history using methods that include thin section petrology, x-ray diffraction, scanning electron microscopy, and stable isotope analysis of carbonate cements. The study includes samples from outcrop to deep burial (10,000 feet) taken from locations within the Washakie Basin between Rawlins and Baggs, Wyoming. Forty thin sections were point counted at 300 counts per thin section. Samples were then analyzed using x-ray diffraction for bulk mineralogy and SEM to define authigenic clay mineralogy.

All samples are fine-grained, poorly sorted and immature containing numerous angular grains, lithic fragments and clasts. The paragenetic sequence of the Lewis includes early calcite cement that varies in abundance, declining away from the upper contacts within channels. Deeper samples show later dissolution of calcite and later quartz cement. The ductile lithic grains within the Lewis cause significant porosity reduction (<5%) during burial. Core plug porosity and permeability values in shallow samples range from 15-35% and 30 to 2000 MD, respectively. In contrast, thin section porosity of the same samples ranges from 10-25% indicating there may be significant micro-porosity. It appears that reservoir quality (porosity and permeability) is a function of initial composition (%lithics), presence of calcite cement and extent of burial. The results of this research can then be used as a prediction tool for other tight gas reservoirs in the future.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado