ANN ARBOR, Mich.---Freshwater from melting ice sheets set the stage several thousand years ago for production of natural gas along the margins of sedimentary basins.
Now researchers at the University of Michigan and Amherst College are reading chemical signatures of water in those areas to pinpoint places where gas is most likely to be found. Their most recent work is described in a paper published in the May/June issue of the Geological Society of America Bulletin.
Natural gas forms when organic materials trapped in sediments decompose. This can happen when the materials are exposed to high temperatures, producing thermogenic gas, or when bacteria break down the organic matter and, through the process of methanogenesis, produce microbial gas.
Finding and exploiting microbial gas deposits, which account for as much as 20 percent of the world's natural gas resources, is "becoming more and more important," said U-M doctoral student Jennifer McIntosh, lead author of the paper. "And if you're exploring for microbial gas, you need to know what areas have been affected by methanogenesis, because that's how the microbial gas is produced."
McIntosh and coauthors Lynn Walter, U-M professor of geological sciences, and Anna Martini, assistant professor of geology at Amherst College, studied Antrim Shale deposits in the Michigan Basin, a deep depression filled with sediments that date back to the Paleozoic Era. While thermogenic gas forms far below the surface in the centers of such depressions, microbial gas is produced along the shallow edges. In previous work, the researchers showed that freshwater seeping into basin edges from melting ice sheets made it possible for methanogenesis to occur. "The fluids in the Michigan basin are some of the most saline fluids in the world," McIntosh said. "When freshwater penetrated into these basin margins, it suppressed the salinity and created an environment that was conducive to methanogenesis within organic-rich black shales."
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.
Source: Eurekalert & othersLast reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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