Talk Title: Petroleum System of the Upper Cretaceous Lewis Shale, Eastern Greater Green River Basin, Wyoming and Colorado
Speaker: Ira Pasternack, Department of Geology and Geological Engineering, Colorado School of Mines
Talk Date: April 15, 2005
Publication: The Outcrop, April 2005, p. 5
Sandstones within the Upper Cretaceous Lewis Shale are important gas reservoirs in the eastern Greater Green River Basin of southern Wyoming and northwestern Colorado. Total cumulative production from Lewis Shale fields is about 850 billion cubic feet of gas and 8.8 million barrels of oil or condensate.
A stratigraphic framework for the Lewis Shale was developed using laterally continuous bentonitic and organic-rich pelagic shale intervals that had been identified in earlier studies. It is proposed that the name “Weimer marker bentonite” be adopted for the most widespread bentonitic interval, after R. J. Weimer who published the first interpretation using this interval as a datum in 1965. An isopach map of the interval between the Weimer marker bentonite and a deeper bentonite shows significant thickness variations in the lower Lewis Shale.
Core studies of the most widespread and thickest condensed interval within the lower Lewis Shale indicate that total organic carbon content ranges from 2 to 4 weight percent. Spot-sampling procedures did not provide representative source rock characterization of this interval due to high-frequency vertical variations in total organic carbon content. A continuous sampling methodology resulted in data that more accurately represented source rock characteristics.
Hay Reservoir is the second largest Lewis Shale field in the eastern Greater Green River Basin with cumulative production from overpressured, tight-gas sandstones totaling more than 140 BCFG. The excellent core control available at Hay Reservoir provides a unique opportunity among Lewis Shale fields for reservoir characterization. Production at Hay Reservoir is primarily from a single lobe-shaped sandstone interval, interpreted as a series of amalgamated sediment gravity flows deposited within a deep-water setting. Natural fractures were identified in all of the Hay Reservoir cores and may have had a role in controlling secondary porosity development or controlling significant production variations evident at the field.