Millions of years ago, the microbes in river bottoms disappeared into the earth, buried by successive layers of sediment. Over hundreds of millions of years, these microbes—syntrophic bacteria and methanogenic Archaea—evolved to thrive in this underworld, slowly consuming the rich hydrocarbons that surrounded them in the form of oil. As a result, a large amount of the planet's petroleum stash has been ruined, becoming instead a "hot spot" for deep microbial life that consumes it and taints the rest with sulfur and other byproducts. Over time, however, these microbes finish their feast, leaving behind oil transmuted into methane—another, cleaner fossil fuel. Now researchers think they may have figured out how this process works and how to accelerate it to create a vast new energy resource.
"You're looking at an increase equivalent to the same amount of energy as conventional oil reserves in the world today," says petroleum geologist Steve Larter of the University of Calgary in Canada, a member of the team investigating the microbial process. "It's potentially a game changer if it can be demonstrated."
Larter and his colleagues used test tubes to demonstrate exactly how the microbes, over time, convert oil into methane. The tubes, laced with nondegraded oil from the North Sea and microbes from present-day river sediment that are accustomed to living without oxygen, produced the same levels of methane and other byproducts typical of degraded or "heavy" oil in reservoirs worldwide, the researchers report in Nature.
By fermenting unrecoverable heavy oil into methane, the microbes could boost energy supplies; methane can be burned in power plants to produce electricity. "At a conventional petroleum reservoir, you get 35 percent of the oil out of the ground and 65 percent remains in the ground," says microbial ecologist Ian Head of Newcastle University in England, another team member. "The equivalent figure for gas is 70 percent and 30 percent. If we can convert oil to methane, then the recovery of energy goes up."
A trillion cubic feet of methane can be generated from a billion barrels of heavy oil, Larter estimates, and the world contains at least six trillion barrels of such oil. "It will take less energy to recover," team member and Newcastle organic geochemist Martin Jones says. Plus, just knowing how the microbes operate allows the pinpointing of the better oils within a given field. "Biodegradation models are actually key for targeting the best quality oils, or sweet spots, in biodegraded oil fields," notes team member Jennifer Adams, a Calgary geologist.
Further, burning methane, itself a potent greenhouse gas, to generate electricity produces less carbon dioxide than burning oil or coal. "It's not environmentally neutral but it's certainly an improvement," Head notes.
The researchers plan to determine if this microbial conversion can be boosted in existing reservoirs by 2009 by adding nutrient-rich wastewater to them. "The microorganisms feed on the oil hydrocarbons. We just added some fertilizer, including nitrate and phosphate to enable them to grow faster," Head says. "This is a bit like giving the microbes a balanced diet."
If giving the microbes a balanced diet proves effective, then a vast new store of climate-friendlier fossil fuel might become available. But that gas will have to be recovered quickly after the microbes do their work. "Methanogenesis is still active today or was until recently," Adams notes. "Because there is very little gas found in these reservoirs, that also means that most of the generated gas leaked out of these reservoirs."
Farther down the road, direct production of hydrogen—a potential energy carrier with few known environmental drawbacks—might be possible. "Zapping the methanogens and accelerating the syntrophs, plus some clever engineering, may enable hydrogen production—but that's much more difficult, " Larter says. "If you could recover molecular hydrogen from the reservoir, you've got a zero-carbon energy source from fossil fuel."
In the meantime, the process shows the power of microbes to literally change the world. "Most of the world's oil has been generated by microorganisms over geologic time scales," Head says. "Very small microorganisms have global effects."