ANN ARBOR—Large deposits of methane and other natural gases, generated by billions of munching microbes and conveniently stored in shallow, easy-to-access reservoirs, could be bacteria’s gift to an energy-hungry world, according to an article by University of Michigan geologists published in the Sept. 12 issue of Nature.

“Our research on the Antrim Shale deposits in northern Michigan shows that, under the correct conditions, microbial activity can generate significant volumes of natural gas in organic-rich shales at shallow depths between 300 and 1,800 feet,” said Anna M. Martini, U-M graduate student in geological sciences. “By identifying the physical and chemical conditions the bugs like best, we can help locate areas where similar deposits are likely to be found.”

While pockets of biogenic methane are common in many parts of the world, commercial production firms have largely ignored them in the past, because they were thought to be too small to be economically viable, Martini explained. Instead, commercial drillers focused on huge deposits created millions of years ago when intense heat and pressure miles beneath the Earth’s surface transformed organic material into oil and natural gas.

According to Martini, biogenic natural gas deposits like those in Michigan’s Antrim Shale could be an environmentally sound local energy resource for developing countries, because methane burns more cleanly than coal or oil and the shallow reservoirs are easy and inexpensive to access.

Martini and associates Joyce M. Budai, U-M assistant research scientist; Lynn M. Walter, U-M professor of geological sciences; and Martin Schoell, a Chevron Petroleum Technology Company geologist started the research project by comparing the ratio of two isotopes of carbon dissolved in water pumped from natural gas wells.

“We found high concentrations of carbon-13 relative to carbon-12 in wells throughout the production area,” Martini said. “This ratio occurs as a result of bacterial action, and gave us our first clue as to the origin of the gas.”

By analyzing the chemical isotopic fingerprint of carbon and hydrogen in methane and water produced from Antrim Shale wells, Martini and her colleagues also determined the chemical “digestion” process microbes use to transform organic compounds and ground water in the shale into carbon dioxide and methane.

Carbon-14 dating of dissolved inorganic carbon in water from Antrim Shale reservoir wells indicated the methane was created quite recently?no more than 22,000 years ago. “It appears the gas production is quite modern, in geological terms, and the reservoir is continually replenishing itself,” Martini said. Results from additional carbon-14 dating of methane samples taken from the wells, due in mid-October, should answer the question of how quickly the deposits are being renewed.

U-M researchers are currently looking for the same microbial signature in the gas chemistry of the New Albany Shale deposit located beneath southern Illinois, Indiana and Kentucky.

“Whenever you find permeable, shallow, organic-rich sediments with a regular supply of reasonably fresh water percolating through, you’ll find colonies of microbes capable of generating significant volumes of methane,” Martini said.

“This economically important and unexpected discovery is a great example of a successful academic/industrial partnership,” said Prof. Walter, co-author of the study. The Antrim Shale research project was funded by the Gas Research Institute, the American Chemical Society, Shell Oil Company, Chevron Petroleum and Technology Company and Amoco Production Company.

ontact: Sally Pobojewski U-M News and Information Services University of Michigan

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