UA Hydrologist Studies Huge Natural Gas Reserves Created By Microbes

While oil is a finite resource — at least in the short term of thousands, rather than hundreds of millions of years — some of the largest natural gas reserves in the world are biogenic, which means they’re being created by microbes today and could potentially be a renewable resource.

Jennifer McIntosh, a groundwater geochemist in The University of Arizona's Hydrology and Water Resources Department, is studying the factors that influence how microbes create these gas fields and how fluids migrate in the subsurface.

She also is exploring how conditions miles underground could be modified to create more gas resources and how they might be used to sequester carbon dioxide from the atmosphere.

Her work is of vital interest to the oil and gas industry in the areas of exploration and exploitation of microbially generated gas fields.

The biogenic gas deposits are found in sedimentary basins worldwide, including in the mid-continent United States and Canada — specifically the Michigan, Illinois, and Appalachian basins. These biogenic gas deposits are also found in basins in the West, such as the San Juan Basin in northern New Mexico and the Powder River Basin in Wyoming and Montana.

SUMMER FIELD WORK

McIntosh has ongoing, funded projects in all three mid-continent basins, and she and her students will spend this summer sampling oil, gas and groundwater wells in the Michigan, Illinois and Appalachian basins. Her team (which includes Ph.D. student Stephen Osborn and undergraduate Justin Clark) also will be sampling in Southwestern Ontario, studying the sustainability and recharge history of the area's major aquifer system.

McIntosh's research focuses on understanding how groundwater flow affects microbes that generate methane within organic-rich shales and coal beds that are found up to 4 kilometers underground.

The sedimentary basins she is studying were inland seas during the Paleozoic Age (540 to 250 million years ago). These seas eventually filled with sedimentary rocks — sandstones, shales and carbonates — and sank beneath the Earth's surface.

These basins also contain highly saline brine, with a salinity ranging from about 100,000 mg/liter to 400,000 mg/liter, making it about 10 times saltier than seawater, which registers about 35,000 mg/liter.

Devonian Age (415 to 360 million years ago) black shales within these basins contain high concentrations of organic carbon, which is a food source for microbes called methanogens, McIntosh explained. Methanogens consume the shale, producing methane as a byproduct along the less-salty, shallow margins of the basins where they live. Fresh water has diluted the brine along these margins, creating a methanogen-friendly environment, whereas undiluted brine is toxic to the microbes.

ICE SHEET DROVE WATER DEEP UNDERGROUND

Fresh water was driven into these basin margins during Pleistocene glaciation when pressure from the Laurentide ice sheet drove dilute waters deep underground. This occurred multiple times over a period of approximately two million years, and as recently as 18,000 years ago, when the ice sheet was melting and large amounts of fresh water suddenly became available.

In addition to providing a friendly environment for methanogens, these Pleistocene-age meltwaters are an important groundwater resource for large metropolitan areas, such as Chicago because of the large volumes of high-quality water available in these basins.

Today, the basins are isolated from shallower groundwater aquifers.
Without the weight of glaciers to drive fresh water deep underground, the gas deposits and associated fresh water supplies are out of communication with the modern groundwater system, McIntosh said. So they represent a finite resource that is being mined to provide fresh water for many metro areas.
“We sample the gas and water that comes out of the wells and look at its chemistry and its isotopic composition, and then we're able to determine how the gas was created,” she said. “We can determine whether it was created by methanogens or if it was created by thermogenic (heat and

pressure) processes. The gases created by these different processes have different isotopic signatures.”

It’s important to determine how the gas was created because this helps geologists determine where they should explore for gas in a specific region, McIntosh said.

The research also is important for determining the source and timing of freshwater recharge and how that recharge affects the water quality and sustainability of the underground water resources being pumped by Midwestern cities.

WHERE DID THE METHANOGENS COME FROM?

One of the questions McIntosh is studying is whether methanogens were deposited with the sediment during the Devonian and were sitting dormant in a saline environment before the ice sheet melted or if they were surface microbes that migrated with the water from the ice sheet and evolved to exploit their new environment.

“If the microbes came with the groundwater, you would want to explore for new deposits in areas that have active groundwater recharge,”

McIntosh said. “If they were dormant within shale, you could possibly dilute the brine to encourage growth by adding fresh water to wells. So this work has a lot of consequences for gas exploration and production.”

Regardless of their origin, methanogens have been producing methane in the mid-continent United States and Canada for about the past 18,000 years, unlike other thermogenic gas reserves that were created over geologic time scales measuring millions of years.

IT'S A HUGE RESOURCE

Biogenically produced natural gas is a huge resource, McIntosh noted.
There are more than 10,000 shallow gas wells in northern Michigan alone, representing the largest natural gas production in the state.

“There are large, unexplored areas, such as the western Canada sedimentary basin and the Hudson Bay,” she said. “So it's important in those areas to determine the source of water recharge — if it's modern recharge or if the water was recharged beneath this continental ice sheet,” she said. “Again, this has to do with where you explore for natural gas, the rates of microbial activity in the subsurface, and groundwater resources.”

McIntosh's research also is important to radioactive waste storage and carbon dioxide absorption.

“Understanding how groundwater flows in these saline aquifers and sedimentary basins affects the security of the carbon dioxide in these aquifers,” she said. “Once you get it into these saline aquifers, you want to know how much fluid flow you're going to have over a geologic time scale. When you're interested in things like radioactive waste or carbon dioxide in deep aquifers, you need to take things like the next ice age into account.”

CARBON DIOXIDE STORAGE

McIntosh noted that carbon dioxide is much more strongly absorbed in organic matter than methane. So flushing carbon dioxide through a coal bed or a black shale, will cause the carbon dioxide to be absorbed onto the coal or shale, while displacing the methane.

“So you actually sweep the methane out of the reservoir,” she said.
“This is the only proposed mechanism for CO2 storage that would produce an economically valuable byproduct.”

“I'm interested not only in how CO2 will displace methane in these coals and shales, but if it could potentially generate more methane,” she said. Methane is produced in black shales and coal beds through CO2 reduction. So it's possible that bacteria could consume the carbon dioxide and produce more methane.

“These are anaerobic bacteria,” she said. “They can exist in the subsurface and reproduce forever as long as they have sources of energy and the environmental conditions — pore space, temperature, salinity, sulfate concentration — are favorable.”

McIntosh conducts her research in collaboration with microbiologists Klaus Nüsslein and Steven Petsch, of the University of Massachusetts Amherst; hydrologist Mark Person, at Indiana University; geologist Peter Warwick, at the USGS; hydrogeochemist Stephen Grasby, at the Geological Survey of Canada; hydrogeochemist Anna Martini, at Amherst College; and noble gas geochemist Chris Ballentine, of the School of Earth and Atmospheric and Environmental Sciences at the University of Manchester.

Her research is funded through the Geological Survey of Canada, the USGS, NSF, the American Chemical Society and the New York State Energy Research Development Authority.

CONTACT INFORMATION:
Jennifer McIntosh
Assistant Professor
Hydrology and Water Resources
(520) 626-2282
mcintosh@hwr.arizona.edu