Contrasts in Shallow Biogenic Gas Systems on the Western and Eastern Margins of the Williston Basin

Shurr, George W. ¹ (1) GeoShurr Resources, LLC, Ellsworth, MN

Shallow biogenic gas accumulations are located on both the western and eastern margins of the Williston basin. In the western Dakotas, eastern Montana, and southern Alberta and Saskatchewan, there has been significant production over the past several decades. The self-sourced Cretaceous reservoirs are generally less than 2000 ft deep. In the eastern Dakotas and southern Manitoba, historic production extending back more than a century has been used for local consumption. In addition, gas shows are present in glacial sediments and in the underlying Cretaceous subcrops at depths of less than 500 ft. Accumulations on the western and eastern margins represent two separate and distinct biogenic gas systems.

Early generation biogenic gas is produced on the western margin. This “old” biogenic gas formed by methanogenesis near the sediment-water interface during and shortly after deposition of the Cretaceous host rocks. The gas has remained trapped in unconventional reservoirs with relatively little migration since the time of generation. Production tends to be associated with paleotectonic highs where winnowing improved local reservoir quality.

Late generation biogenic gas or microbial methane dominates historic production and gas shows on the eastern margin. This “new” biogenic gas formed by microbial activity long after deposition of the Cretaceous source rocks. It probably resulted from inoculation of organic-rich bedrock by meltwater during glaciation. The geologic setting and ultrashallow gas occurrences are very similar to the northern margin of the Michigan basin where microbial methane is produced from the Devonian Antrim Shale. Microbial methane on the eastern margin of the Williston basin is an unevaluated and underexplored resource.

AAPG Search and Discovery Article #90055©2006 AAPG Rocky Mountain Section Meeting, Billings, Montana