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 bacteriaís 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 Earthís surface
transformed organic material into oil and natural gas.
According to Martini, biogenic natural gas deposits like
those in Michiganís 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, youíll 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.