Lowstand Shale Gas Reservoirs of the Western Interior Seaway

Steven Schamel¹ and S. Robert Bereskin^2
1 GeoX Consulting Inc, Salt Lake City, UT
² Bereskin and Associates, Inc, Salt Lake City, UT

Encased within thick Middle and Late Cretaceous basinal shale successions of the Western Interior Seaway muddy silt- and sand-rich intervals possess characteristics of detached lowstand deltaic and shoreface deposits. All sediments reflect relatively low-energy deposits situated 30-100 miles basinward of coeval wave-dominated highstand shorelines. The overall sedimentologic and stratigraphic features of these deposits are remarkably consistent, suggesting a common genesis along the length of the entire seaway. The cleaner sand-rich deposits are already well known reservoirs including as examples the Cardium Formation within the Colorado Group of the Alberta Basin and the Shannon Sandstone within the Gammon Shale of the Powder River Basin. Even the silty deposits produce commercial quantities of gas in various parts of the seaway. These “dirty rocks” include the Lewis Shale of the San Juan Basin, the Prairie Canyon and Juana Lopez Members of the Mancos Shale in the Uinta-Piceance Basin, and the Medicine Hat Formation between the Colorado and First White Specks Shales in the Alberta Basin. These silt-rich low-TOC strata are partly self-sourcing in the manner of normal gas shales, and partly charged externally in the manner of tight sandstone reservoirs. Where sufficiently buried to generate and/or to retain natural gas, the muddy silt-rich deposits are likely to be significant “shale” gas reservoirs. To the extent that these deposits are tied principally to global eustacy, they are likely more widespread in the Western Interior Seaway than previously appreciated and present a substantial opportunity for future hydrocarbon endeavors.