In the first blog
of this two-part series, I made the following points: (1) There is a lot of carbon in the earth, probably left
over from the formation of our planet; (2) theoretical calculations indicate
that hydrocarbons can be stably formed from methane and C02 at
geologically-realistic temperatures and pressures; (3) laboratory experiments at
realistic temperatures and pressures show that a range of hydrocarbons can be formed
from marble—within an HOUR.
Before I go
on, though, there is one additional piece of laboratory evidence I just have to
share. There is a very famous
chemical reaction called the Fischer–Tropsch process. At high pressure and high temperatures
(300F to 500F), it produces methane and higher hydrocarbons from carbon
monoxide (CO) and/or carbon dioxide (CO2). Varying the temperature and the pressure results in a
different mix of end products.
Additionally, a number of different metals, including iron, nickel,
cobalt, and ruthenium, can be used as catalysts. (A catalyst is something that speeds up a chemical reaction.)
Now, the
Fischer-Tropsch reaction was discovered and patented in the 1920’s by German
scientists. The Nazis used it in World
War II to convert coal into fuel, since they had more coal than petroleum. For this process to work, they first
had to convert the coal into CO2 and/or CO through a process known
as gasification. The
Fischer-Tropsch reaction is still used today at several refineries around the
world for the purpose of making petroleum products.
So, what does
the Fischer-Tropsch reaction have to do with abiogenic oil? Maybe plenty. That is because it shows that you can take CO2 and/or
CO and make methane and higher hydrocarbons as long as you have hydrogen and
suitable catalysts. But to fit Fischer-Tropsch
into our abiogenic oil theory, all the necessary ingredients have to be present
deep in the earth’s mantle. So are
they?
First, let’s just
assume that quantities of CO2 and/or CO are available to fuel the
reaction, as I haven’t seen the presence of these gases disputed anywhere. Next, how do we get hydrogen? It is well known that hydrogen is produced when the mineral
fayalite (iron silicon oxide) is exposed to water. Fayalite is common in the kind of rocks found deep in the
earth (igneous rocks), and we can assume the presence of water. And last, the necessary catalysts are
minerals such as olivine (one of the most common minerals by volume) and
magnetite (found in almost all igneous and metamorphic rocks). Put these ingredients in the pressure cooker that is the
earth’s mantle and perhaps we now have all the conditions necessary to make hydrocarbons
from simple starting materials.
Results of laboratory experiments indicate that the
abiogenic production of hydrocarbons is possible, and we discussed a couple of
them in Part 1 of this blog series.
However, questions do remain concerning the ability of naturally-occurring
catalysts to produce these hydrocarbons and the stability of the resulting compounds.
So is there “real life” evidence outside the laboratory that oil and
gas are derived from non-biological (that is, non-fossil fuel) sources? First, the two processes—abiogenic and
biogenic—are not mutually exclusive. Both could be going on at the same time, with each
contaminating the other. Microbes
living in a petroleum reservoir whose oil was originally abiogenically produced
could introduce contaminates that give the oil a biological signature. And oil can move around—certainly
fractures must exist throughout even the deepest regions of the earth’s
interior—so whether or not oil is found near visible fractures would not seem
to be determinative. What we
really need is some way to tell whether oil was made biogenically or abiogenically just by
analyzing it.
Fortunately,
there are “signatures” within the oil itself that just might do the trick. One of them results from the ratio
between two specific types (isotopes) of carbon that are present in petroleum : 12C and 13C. To understand how these two
carbon isotopes fit into the big petroleum picture, we need to know how they
got in there in the first place.
It starts with plants, which extract carbon from the atmosphere in the
form of CO2 (carbon dioxide) and use it to build plant tissues. It turns out that even though plants
can, and do, use either 12C or 13C for this purpose, most
of them prefer 12C (with the exception of C4 plants, which don’t discriminate
between the two—but were probably NOT associated with oil production anyway*). Therefore, you would predict that
petroleum originating from plants would have a particular carbon signature,
reflecting the fact that when the plants were still alive, they used more 12C
as a building material than 13C. That is, you would expect petroleum with plant origins
to have a 12C /13C ratio similar to that of plants.. And in general, it does. So what is all the fuss about
then? In order to identify
plant-based oil, all we have to do is determine whether its carbon signature is
consistent with the oil having started out as a 12C–loving
plant. And, in fact, all oil
reserves found so far DO have a 12C /13C ratio consistent
with plant origins. Case closed .
. . right?
Wait just a
minute—what about limestone, marble, and other rocks high in calcium carbonate
that were formed primarily from the skeletons of marine animals? Since animals can’t extract carbon from
CO2 in the air, they get the carbon used in their own bodies from
plants, either by eating the plants themselves or by consuming other animals
that in turn got their carbon from plants. As a result, the carbon signature of all life, on
earth at least, tends to have a 12C /13C ratio reflecting
the preference for 12C exhibited by most plants. And therefore, so does limestone and marble and any other
rock whose carbon originally came from something that was once alive. For this reason, any oil made from
these rocks will also tend to have a plant-based carbon signature—even if it
was produced abiogenically from a chunk of marble in a laboratory. So the bottom line is that we can’t use
the 12C /13C
ratio in oil to differentiate biogenic petroleum from abiogenic petroleum if
the oil originated from limestone or similar rock. Or at least so it seems to me.
Further, some Fischer–Tropsch reactions
show that if you start with a material that is depleted in 13C (such
as rock made from plants or animals), you can unexpectedly end up with higher
hydrocarbons that have INCREASED amounts of 13C—or, as you would predict,
you could end up with higher hydrocarbons that are DEPLETED in 13C. The amount of 13C found in hydrocarbons can depend on the
reaction conditions and the type of catalysts that are used rather than the
amount of 13C in the starting material. This is important because researchers have used the presence
of depleted 13C as evidence that the hydrocarbons are “fossil
fuels,” or, said more properly, that they came about by “thermogenesis”—the
scientific term for the process that converts plants etc. into oil by the
“normal” biogenic route. So I’m
not so sure that 13C depletion is the “smoking gun” that proponents
of the fossil fuel theory would like to claim that it is.
But what about
methane? Unlike oil, methane is
known to have a very wide range of 13C, meaning that some sources of
methane are rich in 13C and others are not. Since almost all terrestrial life
is lower in 13C than 12C, one would think that it’s safe
to assume that any methane without a lot of 13C has biological
origins, especially considering the many bacteria that produce methane from
once-living things. But what about
methane that is high in 13C?
A meteorite
fell to earth on September 28, 1969 in the vicinity of Murchison, Australia, that
is both interesting and gratifying.
Interesting because it contains at least 10,000 different organic
compounds, and gratifying because the methane produced from those compounds is
richer in 13C than 12C. This is exactly what we would expect to find in methane from
meteorites, since the extraterrestrial carbon that produced that methane almost
certainly didn’t have biological origins. So the Murchison meteorite is a nice check on the 13C
theory. If scientists had instead found
higher amounts of 12C
than 13C in the Murchison methane , they’d have some explaining to
do—and maybe we’d have to throw out 13C as evidence.
Knowing that methane
without biological origins is high in 13C, one would posit that
carbon deep in the earth (such as in graphite, and any of its degradation
products such as methane) would be also be richer in 13C.
So, has anyone
found sources of methane produced on earth that are rich in 13C? They have. It comes bubbling up from a rip in the sea floor on the side
of the Atlantis Massif, 2000 feet below the surface. These rips are called “hydrothermal vents,” and they emit
high concentrations of methane and hydrogen. This particular vent is called the
“Lost City” vent (lost city = Atlantis, get it?), and in 2008 it was reported
that its methane was rich in 13C. Scientists have
concluded it is NOT of biological origin, but that it possibly originated from
source rocks that date back to the beginnings of the earth.
An earlier
2002 study of methane from the Kidd Creek formation in Canada also shows an
abiogenic signature, as does the Potato Hills gas field in southeastern
Oklahoma. Similar reports come
from China. More recently, a 2010
report concluded that gases emitted from a Socorro Island volcano (in the South
Pacific about 400 miles west of central Mexico) were abiogenic.
Another
element that might possibly help differentiate biogenic from abiogenic oil is the
3He isotope of Helium. 3He
occurs in space at concentrations 200 times higher than it does here on earth,
and scientists believe it was trapped deep underground when the earth was first
formed. It turns out that some oil
fields have high concentrations of 3He, suggesting an abiogenic
origin. A 2009 report on the
Songliao Basin in China looked at 3He and 13C methane,
and concluded that the source of the oil was from deep crustal “kitchens.”
And so the
debate goes. Given that the
non-fossil theory of oil formation is supported by theoretical results, experimental
evidence, and field observations, the real question is how common is it? If the majority of our “fossil”
fuels in fact have a non-fossil origin, there might be a whole lot more oil and
gas available than we now believe.
And if abiogenic oil is being produced deep in the earth at this very
moment, it could in fact be a “sustainable” resource. That would certainly change our view of the world—and it
would have far-reaching political and practical ramifications.
But right now,
if you were to advocate an abiogenic theory for oil and gas, you’d be in the
minority. And in general, those
who take a minority position have the burden of proof—that is, it is up to the
advocates of the abiogenic theory to prove that there really are hydrocarbons with
abiogenic origins. But scientists who
support the “normal” fossil fuel theory don’t seem to be too interested in
engaging in a debate on this question.
I guess that is since theirs is the dominant theory and they are in the
majority, the game is already over as far as they are concerned.
But, speaking
as a scientist with no specific expertise in this particular field, I can say
that after looking long and hard, I haven’t run across any definitive test that
can conclusively tell us whether a petroleum sample has biogenic or abiogenic
origins. The 13C work
seems to be the most promising because finding an abundance of 13C
hydrocarbons appears to constitute evidence for non-biological origins. The problem is that the reverse is not
true. In other words, finding a
paucity of 13C does not seem to constitute evidence for biological
origins. Even the existence of
biosignatures in oil consisting of compounds made by plants does not prove that
the oil actually came from plants since bacteria may have
the chemical pathways to make many plant compounds and bacterial contamination
of oil is ubiquitous.
So what is the
critical evidence that supports the fossil fuel theory? I don’t know, but
almost everyone seems to believe in it.
Just ask any 6th
grader.
* C4 metabolism is found only in terrestrial
plants, but the majority of those who support the fossil fuel theory believe that
most of our oil came from marine algae.
Further, C4 metabolism evolved 25 to 30 million years ago, and the
majority of oil is found in rock formations that are 65 to 500 million years old.
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