This "biogenic theory" got started when British naturalists
noted that peat--a combustible substance recognizably composed of compressed
plant material--was often found above coal deposits. And coal often
contained plant fossils. It wasn't much of a leap to conclude that
coal was probably just peat that had been compressed at a higher pressure
for a much longer time.
Oil, in turn, was often found under or near coal deposits.
Oil certainly looked like liquified coal, and burned even more readily
than coal or peat. Because of these similarities and the frequent
proximity of oil to coal, it was reasonable to believe there might be some
connection between all three fuels. If so, then since peat was recognizably composed of biological material, and coal seemed to be like peat, then oil
also probably came from biological sources.
This "biogenic theory" got a big boost about a century ago
when it was discovered that decaying plant and animal matter gave off methane.
Since natural gas is mostly methane, the biogenic theory was looking better
and better.
About 80 years ago the biogenic theory seemed to be further
confirmed when researchers found that most crude oil contained a variety
of molecules that were also found in the cell walls of bacteria.
Researchers also found that most petroleum can rotate polarized light--a
well-known characteristic of biological molecules. This discovery
pretty much ended most debate on the origin of oil, gas and coal.
But from the outset a handful of scientists questioned
this conclusion. In 1877, Mendeleev (of periodic-table fame) published
a paper explaining why most hydrocarbons probably didn't come from biological
sources, but rather from a source below all sediment layers. Since
then other scientists have found bits of evidence that seemed to contradict
the idea of biologic origin. For the most part, though, anyone who
openly questioned the biogenic theory was loudly ridiculed.
But gradually over the past century, evidence has been
found suggesting that the conventional wisdom about the origin of oil and
gas is wrong--that these substances don't come from biological sources
after all.
If this is true, it could radically change world events
for the better.
The first challenge to the theory that coal came from
compressed plants was the discovery in coal beds of an occasional fossil
that was three-dimensional and only slightly compressed. If coal
was produced by squeezing plant debris at high pressure for millions of
years, everything found in coal beds should be squashed completely flat.
How had these rare fossils managed to remain three-dimensional?
In the 1870s the crushed-plant theory of coal was further
confounded by the discovery of a large vertical crack filled
with coal. If the compressed-plant theory of coal formation was correct,
coal should not have been able to form in a vertical crack.
The biogenic-source theory also has a hard time explaining
why some geographic regions contain so much more oil and gas than others.
For example, in the Middle East a mere 2% of the Earth's surface contains
60% of all the oil discovered so far. If the biogenic theory were
correct, such a huge amount of oil would require unusually large volumes
(or unusually high qualities) of both source and reservoir rocks.
But the subsurface formations actually found in the Middle East so far
are no different--either in quality or amount--from those in other parts
of the world.
Another problem with the biogenic theory is that although
oil from shallow reservoirs contains molecules identical to those found
in the bacteria, oil from deeper formations doesn't. If all
oil came from biologic sources, why are these telltale biological molecules
missing from deep oil?
Some of the other arguments for the non-biologic origin
of most oil and gas are:
1. Oil and gas fields are often found in long lines or
arcs stretching hundreds of miles. Such long lines are on the scale
of large crustal structures rather than the much smaller scale of
sedimentary deposits.
2. Most hydrocarbon-rich areas have vertically stacked
deposits at many different levels all the way down to the "basement" rock,
roughly a billion years old. These different levels correspond to
widely separated time periods. For the biogenic theory to be correct,
exactly the same small area would have to have repeatedly played host to
large quantities of biological material at several different periods separated
by millions of years, while areas just a few miles away *never* had similar
luck. This seems most unlikely. [Of course one could argue
that a single biologic source of oil or gas at great depth would diffuse
upward and charge several different reservoirs above it.]
3. Methane is found in many locations where a biogenic
origin is unlikely: in fissures in igneous and metamorphic rocks; in active
volcanic regions even where there is little sediment; in fractures in basement
rock, etc. If methane, like oil, comes from biological sources, how
did it get into these locations?
4. Oil from a given region of the globe often has a unique
chemical "signature", regardless of the particular field or the age of
the formation that produced the oil. For example, oil from the Middle
East is chemically distinguishable from oil from California. This
suggests that all the oil in the entire region came from a common source.
This is consistent with a deep source but inconsistent with shallow biologic
origins.
5. Both oil and natural gas are often associated with
unusually high levels of helium--an inert gas not associated with any compound.
Nothing in the biogenic theory accounts for this. However, helium
is produced by a common radioactive decay process deep in the earth.
Finding the true origin of oil and gas isn't just a matter
of academic interest, because if most oil and gas DOESN'T come from biologic
sources it would mean that huge areas of the Earth that are known to have
little sediment--and have therefore been ignored in the search for oil
or gas--might instead contain commercial quantities. If true, this
could radically alter the geopolitical situation.
So if oil, gas and coal don't come from crushed plants,
where DID they come from? Mendeleev and dozens of other scientists
believed the source is carbon and methane deep within the earth, far below
the crust and all sedimentary deposits. This carbon and methane got
there when the Earth was formed from the accretion of cosmic debris.
The best evidence that carbon exists below the crust is
found in certain volcanoes. Those associated with lava often expel
huge volumes of CO2, while cooler "mud volcanoes" often vent methane.
Based on the current rate of methane flow from these mud volcanoes and
the calculated age of some mud volcano areas, Gold calculated that the
total amount of methane expelled in some of these fields is many times
larger than the largest gas field ever discovered. Yet the volume
of sediment in those areas is far too small to have produced or contained
even a fraction of the calculated volume of gas.
Another clue that large quantities of carbon are constantly
coming to the surface from deep in the Earth is found by examining carbon
dioxide (CO2) in the oceans and atmosphere. At the ocean's surface,
carbon dioxide from the air is continuously dissolving into the water.
Eventually this CO2 precipitates out of the seawater as various carbonates,
which fall to the sea floor and ultimately form limestone and other rocks.
The rate at which dissolved CO2 precipitates out of seawater
is known from experiment. At this rate, all the CO2 now dissolved
in the oceans would have precipitated out as carbonates in around 10,000
years. Since the level of dissolved CO2 in ocean isn't falling, apparently
the ocean is constantly being recharged with CO2 from the air.
But all available evidence suggests that, like CO2 levels
in the oceans, the amount of CO2 in the atmosphere has also stayed fairly
constant over the last few thousand years. (We'll ignore the rise
over the last century, which some scientists believe is the result of humans
burning fossil fuels.) If atmospheric CO2 is constantly being lost
to the oceans, what's been keeping atmospheric CO2 levels relatively constant?
Gold thinks the answer is CO2 from deep inside the Earth.
(To get an idea of the scale for the processes being considered
here, Gold estimates that the total amount of carbon in carbonate rocks
is equivalent to about 17 kg for every square centimeter of Earth's surface.
By comparison, the carbon in atmospheric CO2 is barely 0.0001 kg per sq.cm.
Thus an infinitesimal annual rate of expulsion of "deep" CO2--made by heating
carbonate rocks--would be enough to replenish all the CO2 in the atmosphere.)
But the most unusual argument for the presence both of
unoxidized carbon and various carbon-containing gases at depths far below
the basement is the existence of diamonds. These can only be formed
at the pressures found at depths of 100 to 300 kilometers. Although
the temperature at such depths is well above that needed to make diamonds
change back to the more stable graphite form of carbon if they were at
normal atmospheric pressure, the high pressures 100-plus km. below the
surface keep the diamonds intact.
If the hot diamonds were slowly raised to the surface
they'd change to the graphite form as the pressure decreased.
Since diamonds clearly exist, this can't be happening. Tom Gold believed the only way diamonds can avoid this is if they're raised from
great depth almost instantly, in gas bubbles.
This theory would explain a mystery: Diamonds are
often found in formations called Kimberlite pipes--funnel-like structures
that look much like volcanic throats, but no lava is associated with them
and their walls show no evidence of having been exposed to the heat of
lava. Gold believes these funnel-shaped structures were created when
large quantities of high-pressure gas--sometimes containing diamonds--
blasted to the surface. Although the gas would initially be very
hot, it would cool instantly as it expanded, which explains why the walls
of the pipe near the surface show no evidence of heat.
Interestingly, diamonds also provide direct evidence
that gases containing carbon exist at depths greater than 100 km, because
microscopic pores in diamonds have been found to contain both CO2 and methane.
Gold believed packets of high-pressure gas at depth work
their way toward the surface in a series of short jumps. Successive
packets (which Gold calls "domains") are sporadically recharged from the
next lower packet until they accumulate enough pressure to rupture the
local rock and thus start the next jump. As a packet of gas rises,
it expands and its pressure drops. Finally when the pressure of the
gas falls too low to displace the surrounding rock, the jump stops.
Gold believed these upward jumps of deep gas may be the
source of the frequent deep, small-amplitude earthquakes detected near
many oil and gas fields. If true, the great frequency of occurrence
of these deep, non-damaging quakes shows that the upwelling phenomenon
is still occurring in many parts of the world today.
This cascade effect would explain another enduring mystery:
the existence of unexpected--and dangerous--"overpressured zones" sometimes
encountered by drillers. Drillers know from experience about how
much pressure to expect at any given depth. They then try to control
this "formation pressure" by keeping the drill hole filled with dense drilling
mud as they drill. But if the mud is too dense, the hole will be
damaged--sometimes so badly that it won't produce oil if any is found.
So the driller must control the density of the mud so as to just barely
exceed the pressure at the bottom of the hole.
This works as long as the pressure at the bottom of the
hole behaves in the predictable way. But once in every few thousand
wells the drill bit penetrates a region containing gas at a far higher
pressure than should exist at that depth. When this happens, the
gas begins flowing into the hole and up toward the surface--expanding as
it goes.
If the drill crew recognizes what's happening in time
they can trigger a device that slams a thick steel blade across the top
of the hole, cutting through the drill pipe but sealing the hole.
But if they're a fraction of a second late, the gas bubble bursts out the
top of the hole with enough force to throw 20 tons of drillpipe hundreds
of yards. It's called a blowout, and the unchecked gas roars like
a tornado. Usually the gas finds a source of ignition and turns the
rig into a charred, twisted wreck. In a few cases the roaring flow
of burning gas has continued for over a year, shooting a torch a hundred
or more feet in the air.
If upward migration of mantle gas is an accurate model,
large faults in basement rock should provide a path of lower resistance
which would presumably be used by greater-than-average quantities of gas.
And indeed, geologists have found that once a discovery has been made,
they're most likely to find additional fields by drilling along any deep
fault that can be identified. Supporters of the biogenic theory have
never come up with a convincing explanation for this fact.
Upward migration of deep oil or gas would also explain
the well-known fact that areas having oil or gas production at one level
often have productive zones at many levels down to the earliest sedimentary
rocks. (In fact, it appears that in ALL areas with shallow production,
the underlying formations at least show evidence of hydrocarbon passage,
even if no commercial quantities remain.)
In the Hugoton field in southwestern Kansas one can see
this vertical stacking not only down TO the basement rock (i.e. below the
earliest sediment) but actually into it, with several wells
producing commercial quantities from fractured basement rock. When
supporters of the biogenic theory are asked to explain this, they speculate
about basement faults opening up and sucking gas down from shallower zones.
Since this would involve movement from areas of lower pressure to areas
of generally higher pressure, this explanation seems a bit strained.
Continuing upward migration of deeper gas would also explain
the observation that some coal fields produce much more methane than could
be expected from gasification of the coal itself. In fact, some coal
mines in Japan have had to be abandoned because even with maximum forced
ventilation, explosive quantities of methane kept flowing in. Since
no natural gas has been found adjacent to the mines, this would seem to
suggest a deeper gas source.
As noted earlier, if the true source of most oil and gas
is deep within the Earth instead of in sediments, a lot more areas are
likely to be productive of commercial quantities of oil and gas.
But if the theory outlined above is true, the story has two other, wider
lessons for us: 1) the "experts" aren't always right; and 2) a great
deal that we *think* we know may not actually be settled after all.
In the author's humble opinion, the century-long misconception
of the true source of oil and gas ranks with a handful of long-lived errors
by the scientific establishment, like the official belief that the Earth
was flat or that ulcers were caused by eating spicy foods. As such,
it's a story everyone should hear.
--sf, February 16, 1999
[For a full recounting of this theory see "Power from
the Earth", Thomas Gold, 1987 (J.M. Dent & sons, London)]
