What's the real origin of oil and gas?

I wrote the article below in early 1999.  Seems to have weathered 27 years fairly well.

For almost a century the scientific consensus has been that oil, gas and coal were produced from biological material.  Specifically, the theory has been that dead plants (or perhaps tiny marine organisms), buried under thousands of feet of sediment for millions of years at just the right temperature, were eventually transformed into one of these three fuels.

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. 

For 200 years scientists have known that when pressure on a gas drops, it cools instantly.  And going from a deep zone to any shallower zone--like the surface--pressure drops dramatically.  Since gases cool instantly when their pressure is reduced, an upward-exploding gas bubble would instantly cool the diamonds and prevent 'em from turning back into carbon.

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

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[For a full recounting of this theory see "Power from the Earth", Thomas Gold, 1987 (J.M. Dent & sons, London)]