Ultrashallow Unconventional Gas Accumulations

George W. Shurr¹, Layne D. Schulz², and Richard H. Hammond^3
1 GeoShurr Resources, LLC, Ellsworth, MN
² South Dakota Geological Survey, Vermillion, SD
³ Hammond Wetmore Drilling, Vermillion, SD

Exploration programs for natural gas from coal and shale usually emphasize thermogenic systems in which burial depth and thermal maturity are important. Late generation biogenic gas systems in very shallow, dynamic aquifers have received less attention. There are thousands of ultrashallow gas wells in North America that are less than 500 ft deep. Important commercial examples include coalbed methane in the Powder River and Cherokee basins and fractured shale reservoirs in the Michigan and Illinois basins. Recent published descriptions of economic production from fluvial sediments in China and from glacial deposits in Canada, demonstrate that ultrashallow gas accumulations can occur in very young reservoirs.

A glacial aquifer on the southeastern margin of the Williston basin provides a natural geologic laboratory for ultrashallow gas accumulations. It is described very well by data generated for rural water systems near Dolton, South Dakota. Gas shows and historic production occur in a low area on the potentiometric surface of the aquifer. Recent analyses of headspace gas and water chemistry confirm that the shows and production are outlined by high bicarbonate and low sulfate values. Although methanogenesis probably takes place in the glacial aquifer, the nutrient feedstock may come from the underlying, organic rich Niobrara Chalk (Cretaceous).

Ultrashallow gas accumulations present unique opportunities and challenges. Generally, they are found in under-explored areas where leases are inexpensive. Water well drilling and completion techniques and low compression gathering systems further reduce expenses. However, the dynamic aquifer system can degrade to a habitat unfavorable for the microbes and ultrashallow gas accumulations may easily leak and be depleted.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005