UNCONVENTIONAL RESOURCES

“Farming” late-generation natural gas shows potential

An ultra-shallow glacial aquifer in South Dakota provides perspectives on microbial methane systems in the eastern Great Plains, USA.

George W. Shurr, GeoShurr Resources, LLC; and Thomas Scheier, Scheier Exploration, LLC.

Environmental concerns are a main driving force behind high demand for natural gas. With its relatively low emissions, natural gas is a logical bridge between traditional fossil fuels like oil and coal and newly emerging renewable resources such as ethanol and wind. But natural gas may be “green” for other reasons. There is a clear potential that a certain type of gas can be “raised” like an agricultural product. An ultra-shallow glacial aquifer in eastern South Dakota provides insight into the distinctive hydrocarbon system that has the capacity to deliver this new “agricultural product.”

BACKGROUND

Most economic accumulations of shallow natural gas are biogenic gas, of which there are two types. Early-generation biogenic gas was formed in the distant geologic past by microbial action near the depositional interface between water and sediments. Early-generation biogenic gas is old gas that has been trapped in the host rock since the time of deposition.

The second type of biogenic gas is late-generation microbial methane, in which methanogenesis occurs in the relatively recent geologic past-during the Pleistocene ice ages and even into the present time. Modern methanogenic microbes can use food sources stored in organic-rich bedrock to generate gas. Thus, in contrast with static, trapped old gas, this new microbial methane is in a dynamic contemporary system that continuously generates gas in real time.

Microbial methane is an unconventional energy resource that is largely untested and underutilized. There are economic accumulations that provide some information about the system, but useful science and technology are only now emerging. Microbial methane systems are more similar to swamps, landfills and waste digesters than to the systems that generate the trapped old biogenic gas or deep thermogenic gas traditionally targeted for exploration and production. Modern methanogenesis that uses food sources in bedrock is an ongoing, real-time process, and the resulting methane needs to be “harvested.”