SHALLOW MICROBIAL GAS--A POSSIBLE TRANSITIONAL RESOURCE

Shallow accumulations of microbial methane in glacial drift and in underlying, organic rich bedrock may represent a transitional resource between traditional nonrenewable and emerging renewable energy sources. Relatively contemporary methanogenesis in these shallow settings is similar to gas generation in landfills and digesters. However, it contrasts sharply with the deeply buried thermogenic systems that guide most hydrocarbon exploration.

Drift gas shows have been recorded in most states in the upper Midwest. Historically, natural gas sources in glacial drift and shallow bedrock have been important local energy resources. This local consumption included residential, commercial, and even municipal usage in North Dakota, South Dakota, and Michigan. There are currently economically viable gas fields in glacial drift on the northeastern margin of the Alberta Basin and in the Antrim Shale (Devonian) on the northern margin of the Michigan Basin.

A glacial aquifer near Dolton in southern McCook County, South Dakota, provides a natural geologic laboratory for a shallow microbial gas system. Historic production and modern gas shows originally occurred in a low area on the potentiometric surface of the aquifer. Recent analyses of water chemistry and headspace gas demonstrate that the area of gas shows is outlined by high bicarbonate and low sulfate values. The nutrient feedstock probably comes from the organic rich Niobrara Chalk (Cretaceous) that underlies the aquifer, but the actual methanogenesis apparently takes place in the glacial aquifer.