In the current work, the research team compared the chemistry of water from wells drilled in the deeper center of the basin with that of water from wells at the edges. Their analysis not only provided further evidence that melting ice sheets made it possible for methane-producing bacteria to inhabit the shallow deposits, but also showed that methanogenesis has significantly changed water chemistry in those areas.
"We see large decreases in the calcium-to-magnesium and calcium-to-strontium ratios in high bicarbonate waters associated with microbial gas deposits, indicating methanogenesis caused calcite to precipitate within the Antrim Shale," McIntosh said. "So you can use the elemental chemistry of these shale wells to be able to tell if there was methanogenesis, and that guides gas companies in terms of where to explore for microbial gas. It's a relatively inexpensive analytical tool, compared to other methods that have been used, such as stable isotope chemistry."
The method has potential not just in Michigan, but also in the Illinois basin and in other parts of the world that have similar black shale deposits, said McIntosh. "There are organic-rich deposits in many basins throughout the world, and a lot of these have been covered by continental ice sheets, so these may represent areas where freshwater has penetrated into basins and microbial gas has been generated." To explore that idea, McIntosh compiled water chemistry data from basins in Africa, Asia and North America. "I was able to see similar trends in the water chemistry in other areas with microbial gas deposits, showing how important microbial processes may be in changing the fluid chemistry within the earth's crust," she said.