Microbial Methane Resources in the Eastern Great Plains, Usa
Natural accumulations of microbial methane have been described as geologic landfills and as bioreactors in geologic settings. These phrases emphasize the similarities of methanogenesis in both natural and anthropogenic occurrences. Specific similarities include a substrate to provide carbon, water to maintain a favorable environment, and viable methanogens accompanied by supporting consortia. Microbial methane resources in eastern ND, SD, and NE and in adjacent portions of MN and IA include both natural and anthropogenic occurrences.
Natural microbial methane accumulations on the eastern margin of the Williston basin are similar to commercial production areas in the Antrim Shale on the northern margin of the Michigan basin. Immature, fractured, organic-rich Cretaceous shales subcrop beneath glacial deposits. These shales provide a carbon source for the microbes and a plumbing system for the water. Shallow gas shows are associated with formation waters that are favorable environments for the methanogens, viz. high bicarbonate and low sulfate.
A dozen test wells in five separate prospects distributed through eastern ND and SD have consistent shows of shallow gas. Isotopic evidence for microbial methane is available from at least three of these wells. Water samples from two nearby observation wells demonstrate methanogenic activity in laboratory experiments. This strongly suggests that methanogens are also at work within the formation.
EPA data bases describe projects that harvest microbial methane from landfills and digesters. There are 80 projects in MN and IA and 30 projects in ND, SD, and NE at various stages of planning, construction, or production. These anthropogenic systems share attributes with the natural accumulations. Human and animal populations and annual precipitation are larger in MN and IA than in ND, SD, and IA. Consequently, the garbage and manure that provide carbon substrates and the water required to maintain favorable environments for methanogenesis are more readily available in MN and IA. Landfills and digesters provide insights into production from natural accumulations. For example, shallow well design and gathering systems might have similar applications and local consumption may be more economic than long haul pipelines.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009