Shallow Gas Systems in Cretaceous Rocks on the Margins of the Williston Basin

By

George W. Shurr

GeoShurr Resources, LLC, Ellsworth, MN

Three distinct shallow gas systems are present on the margins of the Williston basin: migrated thermogenic, late generation biogenic, and early generation biogenic. The first two systems are represented by historic production on the eastern margin. Near Pierre in central SD, migrated thermogenic gas occurs in the Inyan Kara Group. Late generation gas has been produced from the Dakota artesian aquifer near Pierre and in LaMoure County, ND. It is also in artesian aquifers of the Hell Creek and Fox Hills Formations in Bottineau and Renville Counties, ND.

Early generation biogenic gas systems are well documented on the northwestern margin of the Williston basin. Major accumulations in the Southeast Alberta gas field and on Bowdoin dome and Tiger Ridge in Montana provide an emerging exploration model. The first two fields cover large areas but are clearly associated with specific structural features. Distribution of in-situ early generation gas in tight reservoirs is controlled by large paleotectonic, lineament-bound structural blocks. Sweetspots, such as Tiger Ridge, are smaller and have conventional reservoirs filled with migrated biogenic gas.

On the southwestern margin, early generation biogenic gas has been extensively produced, but the exploration model has only had limited application. Migrated gas is in conventional reservoirs in the Judith River Sandstone on Cedar Creek anticline, MT, and in the Shannon Sandstone at West Short Pine Hills field, SD. In-situ early generation gas is in unconventional reservoirs in the Eagle/Gammon Formation on Cedar Creek anticline, MT, and in fields at Little Missouri, ND, Gaslight, MT, and Cady Creek, SD. In contrast, late generation biogenic gas systems probably characterize coalbed methane prospects in the area.

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