MICROBIAL METHANE SYSTEMS ON SHALLOW BASIN MARGINS IN THE GREAT PLAINS

There are two separate and distinct microbial methane systems on shallow basin margins in the Great Plains. These natural accumulations include both established, commercial reserves and potential, unevaluated energy resources.

Microbial methane that forms at the time of deposition of the host rock is ancient gas trapped in a closed system. This early generation methane is found in dominantly marine rocks on the margins of depositional basins. Examples include gas in Cretaceous units deposited on the shallow-water shelves of the Western Interior Basin in Montana, Colorado, Kansas, and the Dakotas.

Microbial methane that forms in the relatively recent geologic past is young gas that may be locally trapped, but is generally part of an open, dynamic flow system. This late generation methane is found on the margins of structural basins, usually in continental environmental settings. Examples include coalbed methane in Tertiary rocks in the Powder River Basin and in Pennsylvanian rocks in the Forest City Basin. Methane in fractured Cretaceous shales and overlying glacial aquifers in the eastern Williston Basin is an additional example, though unevaluated.

In both microbial methane systems, fractures and total organic carbon sources are important. However, the microbes in early generation systems are no longer present; modern marine environments must be used as analogs. In late generation systems, the microbes are often still alive and viable. Consequently, these natural accumulations provide direct evidence for the environmental conditions needed to support the methanogens. Consortia, community structures, and metabolic pathways may not be the same for the two systems.

Traditionally, isotopic compositions have been used to distinguish the two systems, but data from Great Plains examples suggest that simple bulk composition is also diagnostic. However, exploration and evaluation of this potential domestic energy resource requires concepts from the life sciences, as well as the earth sciences.