In
July, Mike Lewan had an unusual conversation with his new neighbor, who had
been reading lately about oil from deep sources that “can’t be explained.”
Lewan, though slightly amused, was not entirely surprised to hear this topic
in casual conversation.
In the 1980s, Thomas Gold, an astronomer
at Cornell University, received support from the Swedish government to drill
into the Siljan Ring, the site of an ancient meteorite crater in central Sweden
(shown here), in search of “inorganic” oil from Earth’s mantle.
Gold believed that natural gas migrates upward from the mantle where it transforms
into more complex gases and oil, and that the petroleum could be expelled through
large releases of energy such as a meteor impact. The experiment, which did
not find large quantities of such gas, still helped mobilize a small community
of scientists who reject the theory of organic origins of petroleum. Image copyright
Tom Johansson.
A petroleum geochemist at the U.S. Geological Survey, Lewan is an expert on
the origins of oil, and quite familiar with an idea that has been lingering
within some scientific circles for many years now: that petroleum — oil
and natural gas — comes from processes deep in Earth that do not involve
organic material. This idea runs contrary to the theory that has driven modern
oil exploration: that petroleum comes from the heating of organic material over
time in Earth’s shallower crust.
The so-called inorganic or abiogenic oil idea has been getting more attention
lately, at a time when it seems that energy is on everyone’s mind. With
oil more expensive than ever and many people citing future shortages, understanding
the origins of petroleum is increasingly relevant.
For the first time ever in North America, proponents of the inorganic origins
hypothesis, largely from Russia and the Ukraine, had a major forum for their
ideas at a meeting held in June in Calgary, Alberta — a city that has built
its wealth on the vast petroleum deposits found in the Canadian province. Held
in association with the annual meeting of the American Association of Petroleum
Geologists — a group of people whose livelihood depends on understanding
how and where oil and gas form — this was no ordinary forum.
Walking around the modest-sized ballroom in the Calgary Hyatt right before the
meeting started, words like “lunatic” and “crazy” were being
tossed between some participants. Other attendants had a giddy glow of excitement,
rarely seen at such gatherings. A co-convener of the meeting, Barry Katz, who
is a petroleum geologist at Chevron, hesitantly mentioned that before it even
began, the meeting was receiving attention from the Financial Times,
as well as publications as unlikely as Playboy — yes, Playboy.
One thing is for sure, Katz says: Scientists from the minority inorganic oil
camp, promoting myriad ideas and lacking a unified hypothesis, have a way to
go if they want to overturn the theory of organic petroleum genesis. Although
most scientists agree that inorganic petroleum exists, he says, they differ
widely in their thoughts on how it forms and how widespread it is versus organic
petroleum. The meeting was a vehicle to share these ideas and it highlighted
what geologists know — and don’t know — about the origins of
one of the most important resources on Earth.
Chemical fossils
Starting in the 1860s, scientists in Scotland found a way to make oil: They
took rocks known as oil shales, which contain solid organic material called
kerogen, and heated them to break down the kerogen and produce gas and oil.
Using this process, Scotland “stayed profitable and competitive with the
petroleum industry until 1960,” Lewan says.
Indeed, the idea that heated organic material results in petroleum, so-called
fossil fuels, has been around for hundreds if not thousands of years. In 1757,
Russian scientist M.V. Lomonosov wrote: “Rock oil originates as tiny bodies
of animals buried in the sediments which, under the influence of increased temperature
and pressure acting during an unimaginably long period of time, transform into
rock oil.”
By the mid-20th century, the organic model was widespread, given a big boost
by modern chemistry, paleontology and geology. Katz points to a 1936 paper by
Alfred Treibs, in which he showed that chemical compounds in oil called porphyrins
have the same chemical structure as chlorophyll, thus linking petroleum to plant
material.
Over
subsequent years, researchers would find evidence of other chemical signatures
present in petroleum that correspond to precursors of life. These “biomarkers”
are essentially molecular fossils, and they differ between hydrocarbons, Katz
says. “Oil tends to come from material that is largely from an algal source,
although not exclusively, and gas tends to come from material that is more woody
material.”
Hydrothermal fluids exude vigorously from
the “Lost City” system, located 15 kilometers away from the spine
of the Mid-Atlantic Ridge. Here, minerals precipitate from the fluids and create
white feathery spires and mounds of carbonate. Scientists believe that the vented
gases are from the mantle and are abiogenic (not made from life) in origin,
and some researchers are looking for terrestrial analogs to such systems to
find untapped gas and oil resources. Courtesy of University of Washington.
In explaining biomarker evidence at the Calgary meeting, Marcio R.
Mello of High Resolution Technology & Petroleum in Rio de Janeiro, Brazil,
compared different types of petroleum to different types of people, and biomarkers
to DNA. “Who is the mother?” he said jokingly.
To answer that question, geologists have linked biological molecules found in
petroleum of a certain age to precursors of specific life forms found in ancient
environments of the same age. For example, oleanane, a compound that led to
the evolution of flowering plants, is present in Tertiary and Upper Cretaceous
rocks and oil. Mello said that geologists have also linked biomarkers to the
specific environments in which they formed.
In addition to biomarker evidence, work done by the oil industry in the last
25 years has defined the petroleum systems concept, linking where petroleum
is found to where it originates. Wallace Dow, now a consultant based in The
Woodlands, Texas, recalled his days as a geologist at Amoco in 1970, when he
and others studied the Williston Basin along the eastern edge of the Rocky Mountains
to nail down the geographic and geologic trends in sedimentary basins, where
most petroleum is found. They tested and analyzed 22 oils and, based on chemical
composition, defined three oil types. To find out where those oils came from,
the researchers analyzed core samples in the basin and found three distinct
organic-rich intervals of rock, or source rocks, where the petroleum originated.
This work was some of the first to link oil accumulations to their source rocks.
Not long after that study, in 1978, Lewan and colleagues sought to replicate
oil generation and expulsion in the lab. The team took some crushed shale containing
organic matter, placed it in a 1-liter reactor and added enough water to submerge
the rock. After heating the material between 300 and 360 degrees Celsius for
several days, “there was beautiful oil floating on top of the water,”
Lewan says.
In nature, however, oil does not form at such high temperatures, and it forms
over thousands to millions of years. Thus in the experiment, the higher temperatures
compensated for the short timeframe. “The extrapolation works very well,
substituting temperature for time,” Lewan says, and the researchers have
successfully entered the back calculations into basin models to determine timing
and depths of petroleum generation.
Inorganic oil supporters, however, point to these experiments as a weakness
in the model, saying that no one has ever successfully produced organic oil
in the lab without a lab “time machine” accounting for the higher
temperature (which according to the organic model would take thousands to millions
of years). That’s in part why the inorganic camp follows a different model
for oil formation: Instead of breaking down kerogen at high temperatures into
lighter gases and oil, they think that hydrocarbons start off as simple compounds
and then “chain up” the ladder through a cooling process deeper in
the planet.
In the 1980s, Cornell University astronomer Thomas Gold made headlines with
a variation on this idea, saying that methane (a common natural gas) migrates
upward from the mantle where it transforms into more complex hydrocarbons and
then accumulates in igneous rocks. Biomarkers, he said, are from microorganisms
interacting with the petroleum along its migration route. The way to tap this
source, he said, would be through a cataclysmic release of energy that would
propagate through the mantle, releasing the petroleum.
Thus, Gold, with the support of the Swedish government in the 1980s, drilled
two deep wells into the Siljan Ring, the site of an ancient meteorite crater.
They did find some gas, but researchers have varied widely in their interpretation
of the results. While Gold heralded the find as proof of abiogenic petroleum,
most geologists dismissed the findings as too small to be economically viable.
Gold died last year, but a small and persistent group of scientists still passionately
promotes the idea that petroleum originates from a deep, inorganic source.
Signatures for Mars
At the June meeting in Calgary, the debate was fierce. Katz says, however, that
there was just as much, if not more, infighting among the inorganic folks than
between them and the organic petroleum proponents. That’s because the ideas
on how petroleum forms inorganically vary widely — from oil present as
inclusions in rocks at depth to gases forming in a setting analogous to hydrothermal
vents on the seafloor.
Alexander
Kitchka, a co-convener of the meeting and a geologist at the National Academy
of Sciences in the Ukraine, attributes the various ideas to the fact that it
is a “live, developing theory.” He says that outside North America
(where people prefer the “fast-food bio-organic paradigm”), scientists
are working on hybrid ideas that combine mantle-derived hydrogen with biologically
derived carbon to explain the origins of oil.
T.C. Onstott, a geomicrobiologist at Princeton
University, samples a borehole from a mine site in northern Canada for both
gases and groundwater, to determine whether the microbes in the groundwater
might be living off of inorganically derived gases (hydrogen or methane) as
their source for life. The work, funded through the NASA Astrobiology Institute,
could ultimately help researchers determine whether there is life on Mars and
other planetary bodies. Courtesy of Barbara Sherwood Lollar.
Alexei Milkov, however, a Russian petroleum geologist with BP in Houston, Texas,
who was educated in Russia, does not think of the debate as East versus West,
noting that the organic theory “is taught at universities, is practiced
at major research institutes that work on the fundamental problems of petroleum
geology, and is applied by companies exploring for and producing oil and gas.”
The success of Russian petroleum exploration, he says, did not come from the
inorganic model. He adds that the subject of petroleum’s origins is more
widely discussed in the former Soviet Union than elsewhere. “When I talk
with practicing Russian geologists, they often ask my views on the abiogenic
origin of petroleum,” Milkov says. “This never happens in the United
States, where organic is accepted fully and without scientific questioning.”
The actual question, says Barbara Sherwood Lollar, director of the Stable Isotope
Laboratory at the University of Toronto, is not whether abiogenic hydrocarbons
exist but whether they occur in the natural environment. “Abiogenic gases
are a clear fact,” she says. “I can make them on the lab bench today,”
with basic chemical reactions.
Indeed, last year, a team of researchers led by Henry Scott of Indiana University
at South Bend created methane in the lab at temperatures and pressures common
in the mantle. The results show that methane could be stable as deep as 100
kilometers below Earth’s surface.
Lewan has read these studies with interest, and says that while Scott’s
team’s work is elegant, it does not represent the mantle as scientists
currently know it, pointing to the fact that the experiment used calcite, a
mineral not found commonly in the mantle. “People can make abiogenic gas,
but whether the results can be applied to the Earth’s mantle or deep crust
remains to be seen,” he says.
Abiogenic gas in the crust, however, has been discovered. Working at the Kidd
Creek Mine in Ontario, Sherwood Lollar and colleagues analyzed gases found there
and determined they were inorganic. Using both carbon and hydrogen isotopes,
they identified a unique inorganic chemical signature that differs from the
signatures of gases in economic accumulations, as published in Nature
in 2002. “To date, nobody’s been able to show that same signature
in any of the economic deposits,” Sherwood Lollar says. “That would
argue that any of the economic deposits we’ve found to date are in fact
quite consistent with biological origins.” Still, she says, the work shows
that inorganic gas is “a real phenomenon.”
Her work, which is partially funded through NASA, has continued at other similar
sites in Canada and Africa, showing that such inorganic gas formation is common
in Precambrian shield systems that date back more than 3 billion years. NASA
is interested because the work could provide a test for whether methane and
other hydrocarbons found off Earth are either made by microorganisms or acting
as a “source for life,” as occurs at mid-ocean ridges, Sherwood Lollar
says. On Mars, for example, scientists have detected methane, but do not know
its origins.
Sherwood Lollar’s team has found three different isotopic signatures in
natural gas on Earth: organic, inorganic and a mixture of the two. The mixed
signal, she says, comes from microbes using up hydrogen in these Precambrian
systems and making their own methane, which then mingles with the inorganic
methane. Analyzing the methane on Mars using the team’s isotopic method
could indicate whether life exists on that planet.
This work, which is currently in press in Chemical Geology, could also
be important in the ongoing debate about the origins of petroleum on Earth.
“Here’s a signature we can use to actually look at some of these deposits
that have been claimed by Thomas Gold and others to be abiogenic in origin and
see whether it fits,” Sherwood Lollar says.
Back in the 1980s when Gold drilled the Siljan crater and in the 1970s when
the Russians began massive drilling into the Kola Peninsula in search of inorganic
petroleum, the technology was not available to look at hydrogen isotopes. The
limited carbon isotope data, Sherwood Lollar says, was not enough to prove or
disprove the origins of any gas that was discovered, which in both cases, she
adds, was present in miniscule amounts.
Now, Sherwood Lollar says, scientists are shifting away from the “very
expensive one-shot, one-location superholes to an attempt to take advantage
of any windows into the subsurface that are already available to us.” That
is leading researchers to two key areas: mining environments and oceanic vent
systems.
Time capsules
Stanley Keith has been reading with interest in recent years about odd ecosystems
that thrive at mid-ocean ridges. One that has especially caught his eye is a
hydrothermal system, 15 kilometers away from the Mid-Atlantic Ridge, called
the Lost City. There, magnificent white towers of minerals are home to bacteria
— and inorganic methane. Below this vent system, rocks from the mantle
react with seawater in a series of reactions called serpentinization, producing
the natural gas that is vented at the top for the microorganisms to consume.
Keith, a mineral geologist who owns a consulting company called MagmaChem in
Sonoita, Ariz., began thinking that perhaps he could find ancient systems below
the continents that were analogs to this oceanic system. If so, the hydrothermal
environment would also create inorganic methane, and perhaps other hydrocarbons,
terrestrially. “The geologic challenge is to figure out to what extent
serpentinization creates this kind of opportunity beneath basins in continental
settings,” he says.
According to his model, large slabs of oceanic crust that have pushed below
the continents contain “hydrothermal brine” that reacts with the mantle
rocks to form methane and hydrogen. As the gases move through the system, they
cool and transform to more complex hydrocarbons, ultimately landing in sedimentary
basins and other spots around the world. Keith sees his model as bridging the
gap between the observations of the organic model with known processes that
occur in the mantle. “People have recognized the trap configurations in
basin geology for a long time,” Keith says. “We’re not throwing
the biogenic model out; we’re simply building it into what we’re doing.”
Like Gold, Keith suggests that biomarkers are the result of microbes interacting
with the inorganic petroleum during its migration up into the crust. “The
biosphere has been coexisting with the petroleum system throughout geologic
history,” Keith says. But, Gold’s petroleum model “missed something
called ‘geology.’ That was always its weak point,” and what led
to such divisions in the inorganic/organic oil debate, he says.
Keith is “drawing on processes that no one would argue with,” Sherwood
Lollar says. Scientists have known about serpentinization for 30 years, she
says, “and it’s always been known that it can produce hydrogen and
methane as a byproduct.” She also says that the idea that microbes are
using the inorganic gases has been well-understood the past 10 years. But when
it comes to the broader implications of mantle-derived hydrocarbons, “the
jury’s still very much out,” Sherwood Lollar says, as there is no
consensus on the phase in which the gas would move through the mantle.
In her own work, looking at the “only sites in the continental crust where
abiogenic gases have been identified,” Sherwood Lollar says that there
is no mantle component; the methane instead comes from water and rock interactions,
including serpentinization in the crust. “The rock is in contact with water
in fracture systems, and in these old rocks, they sit in contact for millions
and millions of years,” she says, “until you get major alteration
of the rock and of the water because of that exchange.” Still, Sherwood
Lollar says that any mantle-derived gases would be abiogenic, and so it might
come down to testing Keith’s model using the isotopic signatures her team
has identified.
Keith recently found what he calls “uncontestable exposures in the field”
near Elko, Nev., that support the idea that hydrocarbon formations there are
the tops of giant seep systems above big hydrothermal systems. The outcrops,
he says, show physical evidence of a fossilized hydrothermal system that was
active in the Eocene about 40 million years ago.
“It’s just spectacular stuff,” Keith says. “It’s the
first evidence like this in a continental setting, and we can defend it.”
He and his colleagues plan to publish the work, which is being funded through
a client interested in economic development of the field.
Lewan says that he looks forward to seeing a write-up of the work in peer-reviewed
journals — he sees the work Keith presented at the June meeting as being
far from complete. “The proposed hydrocarbon-charged plume is never described
in terms of its phase or the driving force responsible for its ascent,”
he says.
“Once again, no one doubts that inorganic hydrocarbons may occur in association
with hydrothermal systems,” Lewan says. “The real issue is that they
do not appear to be a significant contributor to the economic accumulations
currently under production.”
Dollars and sense
“In my lifetime, there have been two main trends that I’ve observed:
the big debate with Gold in the early 1980s and then this one, and those sort
of correlate with times when petroleum is getting expensive,” Lewan says.
“Whether that’s actually a cause and an effect remains to be determined,
but it seems like this keeps coming up periodically.”
And at the end of the Calgary meeting, one thing was clear: The debate is far
from over. In the meantime, though, both groups say they can learn from one
another.
For example, although Katz says that he was not persuaded by any evidence on
the inorganic side, he points to studies that have been done to explore inorganic
petroleum that have contributed to his knowledge base. For example, scientists
have studied gas accumulations found with odd isotopic compositions, thought
to indicate an inorganic origin. But, he says, so far, they have mostly found
evidence for gas alteration, something quite useful in searching for oil. “If
the gas is being degraded in some fashion, that has implications on how much
gas is there or the type of gas that might be there.”
“Our story is not complete,” Katz says. “I know a lot more now
than I did when I started 26 years ago, and I hope that when I retire, I’ll
know more then than I know now.” Still, he says, “I think we have
the concepts down from an organic standpoint.”
The challenges Katz sees now to petroleum exploration are political — issues
of access — and economic. “Oil is not simple to find and it’s
very expensive to produce, and the places that we’re going now are more
difficult and more expensive than ever,” he says. “It is a finite
resource, so don’t waste it.” As for inorganic petroleum, “I
don’t see any evidence for that,” Katz says.
Keith, however, remains optimistic. If his model is correct, he says, “we’ve
barely tapped, from the exploration point of view, the hydrocarbon potential
that’s out there on this planet.” He looks forward to taking geologists
to the outcrops he’s discovered in Nevada and, over time, sees the two
sides coming together. “That process will take decades,” Keith says,
and will require a new way to explore for oil and natural gas.
The key word, though, that Katz and Lewan both emphasize is “economic.”
Both acknowledge the existence of abiogenic petroleum and say that it might
be an untapped source, but that it is likely present in small quantities only.
“The organic origin of petroleum is a theory based on field observations,
laboratory experiments and basin models; it explains currently known economic
occurrences of natural gas, crude oil and asphalt,” Lewan says. “The
inorganic origin remains a hypothesis; it has not been proven to be a significant
contributor to currently known economic petroleum accumulations.”
Indeed, Sherwood Lollar says, “a really exciting new idea” needs to
have a “very high standard of proof.” She is grateful, though, to
the Thomas Golds of the world who “throw out grand ideas” and get
people thinking in new directions. In the end, however, she says that the scientific
method is the final arbiter.
 |
Geotimes Home | AGI Home | Information Services | Geoscience Education | Public Policy | Programs | Publications | Careers  |