Shurr, George W.¹, Derric L. Iles² 
(1) GeoShurr Resources, LLC, Ellsworth, MN 
(2) South Dakota Geological Survey, Vermillion, SD

ABSTRACT: Late Generation Biogenic Gas Systems on Shallow Basin Margins

Accumulations of late generation biogenic gas in a variety of unconventional reservoirs are concentrated on the shallow margins of structural basins. Late generation biogenic components are superimposed as a secondary enrichment on dominantly thermogenic coalbed methane in the northern San Juan and eastern Black Warrior basins. Late generation components are also superimposed on early generation biogenic gas in the Niobrara Formation (Cretaceous) of the eastern Denver basin and in the Medicine Hat Formation (Cretaceous) of the southeastern Alberta basin. Commercial accumulations that are dominantly late generation biogenic gas include CBM in the northeastern Powder River basin, production from glacial drift in the Sousa area of the northeastern Alberta basin, and production from the Antrim Shale (Devonian) in the northern Michigan basin. 
Most “drift gas” produced from glacial deposits is late generation biogenic gas. Shows of drift gas are particularly frequent over subcrops of highly-fractured, organic-rich bedrock. Both the Sousa and Antrim production areas have this geologic framework. Methanogenesis is thought to occur as ground water circulates through the fractured bedrock beneath the glacial drift. Similar conditions are encountered on the eastern margin of the Williston basin in the Souris River area of north central North Dakota, the Jamestown area of eastern North Dakota, and at several locations in eastern South Dakota. 
The Dolton aquifer in southern McCook County, South Dakota, provides a natural laboratory that contains a microcosm of geologic conditions encountered in areas of commercial production. This glacial outwash is contained within a low-permeability till and directly overlies fractured, organic-rich Niobrara Formation in at least one locality. Methane has been produced from the aquifer.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.