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Discussion > Help required on the Earth's Crust

Sedimentary rock is where I would expect to find fossils and fossil fuels but it only accounts for 8% of the crust. The bulk of the crust is igneous rock (volcanic rock).
Volcanic magma can be created within a few kilometres of the surface or it can have come from the mantle.
The Tectonic plate mechanism means that what is now the surface will one day become part of the mantle and vice versa.
A great deal of 'fossil' fuel is found in igneous rock which seems to suggest that a large part of the fossil deposits made during the 4.5 billion year history of the planet are no longer in the crust but actually in the mantle.
Can some knowledgeable poster explain this for me?

Oct 29, 2014 at 4:54 PM | Registered CommenterDung

Could the same knowledgeable poster or another one tell me to what extent it is firmly established that 'fossil fuels' did in fact originate from organic life, rather than some process not involving living things. I assume that natural diamonds have an origin that did not involve living organisms.

Oct 29, 2014 at 5:41 PM | Registered CommenterMartin A

I confess to a degree of ignorance as well (a large one!) but I was given to understand that coal is fossil material while gas and oil are hydrocarbons which is not the same thing, or at least not necessarily the same thing.
However, wikipedia classifies coal as a hydrocarbon also so I'm just going round in circles!
There is certainly a theory (Russian?) that oil deposits are formed quicker than coal.
Like you, Dung, I'd be keen to be enlightened by someone who actually has studied the subject and knows what he's talking about.

Oct 29, 2014 at 7:14 PM | Registered CommenterMike Jackson

I think it depends upon which fossil fuel you're talking about. Coal is plant based, oil is plankton based and gas might be from a number of processes. Methane, ethane, etc aren't restricted to the Earth so there is a purely chemical reaction that can create them but it also it can be made from organic decomposition.

Oil doesn't necessarily end up where it was made and seeps upwards.

This article states "It is very unlikely that coal has been moved from the crust down to a depth well below the base of a continental plate. The carbon source for these mantle diamonds is most likely carbon trapped in Earth's interior at the time of the planet's formation."

Oct 29, 2014 at 8:16 PM | Unregistered CommenterTinyCO2


The current paradigm is that all oil ( and much, but not all  natural gas – mainly CH4) is ultimately of biogenic origin ( ie derived by the "maturation" of buried biological material in sediments)   and there is a “consensus” that the science about the origin of oil is “settled”. There is however a growing body of evidence that oil can be, and is formed by non biogenic processes  and the occurrence of heavy hydrocarbons in extraterrestrial environments  is now well documented. It is also well documented that Methane –CH4 – is evolved in many volcanic environments , frequently  in ones where the temperatures  and pressures fall outside the stability envelope of possible  biological pre-cursers .

Like  the “climate change” paradigm  there is a great reluctance   amongst the great majority of  adherents  of the biogenic paradigm to seriously consider  any hypothesis which  which tends to undermine it  and   those who do so  are  often deemed as unqualified to participate in the scientific debate.( I have not yet heard the term “biogenic oil denier”!)

As a professional geologist ( but not a petroleum geologist ) I would conclude that the evidence that methane is produced by BOTH biogenic and abiogenic processes is overwhelming and it is clear that complex hydrocarbons are also produced and preserved in extra terrestrial environments. We know  that CH4 can be hydrogenated  to yield more complex hydrocarbons and the chemistry of these processes does not require biogenic processes.

I would thus incline to the  view that some oil ( perhaps even a significant proportion of all oil) may well result from non biogenic  sources. I would not go so far as to conclude that all oil is non-biogenic in origin however.

I do not think there is any dispute about the origin of coal - which is always biogenic being the compressed remains of plant material buried in sedimentary environments where it has not been broken down by biological processes. In the case of coal laid down in the Carboniferous period ( a huge proportion of the total resource) it is theorised that the vegetal matter resulting fern forests was partly preserved from biogenic breakdown because the fungi which today breakdown woody tissues had not at that time evolved.

As a result of burial under pressure and higher temperatures at depth a proportion of the volatile hydrocarbons resulting from maturation of coal ( including oil precursors) are driven off and a variety of hydrocarbon rich gasses and liquids migrate upward through the sedimentary pile until they accumulate in some geological "trap" thus forming a reservoir such as are found in the younger rocks overlying the Carboniferous rocks in the North Sea. The remaining coal thus contains less volatile hydrocarbons. The more volatiles lost in this process the higher is the " rank" of the coal. The lowest rank being lignite an the highest anthracite. remaining

I hope this is helpful.

Oct 29, 2014 at 9:03 PM | Unregistered CommenterGlebekinvara

Thanks all for the responses.

Glebekinvara how do you account for gas deposits being found mainly in igneous rock?

I know that there are theories that state that the production of hydrocarbons in the mantle is 'plausible' and that oil 'basins' are refilled by from 'beneath'

It just seems that this area is far from settled science and that it does tend to undermine the idea that fossil fuel use is unsustainable.

TinyCO2 your wiki quote backs up the idea that the mantle contains huge deposits of hydrocarbons. When you consider what proportion of the Earth is constituted by the Mantle versus what proportion is the crust; I would wager that fossil fuels will last a hell of a lot longer than the current estimates.

Oct 29, 2014 at 10:00 PM | Registered CommenterDung

from my own notes a few years ago :
Thomas Gold's theory - Complex, but believable argument that the theory hydrocarbons mostly come from old lifeforms is wrong as actually oil doesn't have fossils in. Instead theorises hydrocarbons are part of the Earths crust just like iron. Methane is common on other planets and there was no life there. Oil companies like the other idea that oil is a irreplaceable fossil fuel cos it keeps oil prices high.
- The article was in Nine crazy ideas in science Robert Ehrlich - He analysed them and found 3 were not crazy.. e.g. Also faster than light particles exist.

Oct 29, 2014 at 10:14 PM | Registered Commenterstewgreen


Reports on the presence of hydrocarbons in igneous rocks have been on the increase and generating greater interest in the scientific community over the last 20 years. Most of the occurrences are due to the incorporation of organic material into the magmatic systems. However, reports on the presence of hydrocarbons formed by abiogenic processes have also increased in recent years, suggesting that these hydrocarbons may not be as rare as previously thought and may have implications for natural gas resources in the future.

The origin of these hydrocarbons remains controversial, whether they are (1) derived directly from the mantle, (2) formed during late crystallization stages by respeciation of a C-O-H fluid below 500°C, or (3) formed during postmagmatic alteration processes involving Fisher-Tropsch type reactions catalysed in the presence of Fe-oxides and silicates. The reports suggest that a direct mantle origin for the hydrocarbon fluid is unlikely.

Oct 29, 2014 at 10:17 PM | Unregistered CommenterGlebekinvara

With shale gas and methane clathrates I don't think we're short of gas, it's just a matter of working out how to extract it. It doesn't really matter what generated the gas unless we thought we could duplicate it. Given that any process probably involves geological type pressure and temperatures and a few million years, it doesn't seem entirely useful.

Oct 29, 2014 at 11:08 PM | Unregistered CommenterTinyCO2

Methane is quite common throughout the universe without an organic source. Not as common as I thought. One of the tests for life on Mars was testing for methane, they haven't found any so far. On the other hand both Jupiter and Saturn have Methane in their atmosphere. Titan and other moons of the gas giants have Methane and Ethane in the form of ice created by, with current knowledge, a non-organic process.

I'm not sure what this adds to the discussion, but I find it of interest.

BTW Ammonia seems quite common on these bodies too.

Oct 30, 2014 at 7:40 AM | Unregistered CommenterSandyS

Maybe it's simply my lack of knowledge of chemistry, but it always struck me as odd that petroleum, said to have originated from microorganisms, contains benzene.

Oct 30, 2014 at 9:00 AM | Registered CommenterMartin A

By coincidence not exactly off topic.

Nasa has spotted a mysterious methane cloud high in the atmosphere of Saturn's moon Titan.

They say the phenomenon is similar to exotic clouds found far above Earth's poles.

This lofty cloud, imaged by Nasa's Cassini spacecraft, was part of the winter cap of condensation over Titan's north pole.

Oct 30, 2014 at 10:09 AM | Unregistered CommenterSandyS

Martin A

Photosynthesis and chemosynthesis tend to trap the Carbon12 isotope in preference to Carbon13. Thus hydrocarbons in wood, peat, coal, and sedimentary rockall of organic origin tend to have a lower proportion of carbon13 than the inorganic carbon pool.

Test diamond and you find an isotope ratio typical of inorganic carbon.

Test oil or natural gas and you see an isotope ratio typical of organic carbon, hence it is probably of biological origin.

Hydrocarbons in igneous rocks tend to show a biotic isotope ratio, implying that the rocks act as reservoirs for hydrocarbons produced elaewhere.

Oct 30, 2014 at 11:25 AM | Unregistered CommenterEntropic man

EM - thank you for that.

Is there possibly a circular argument involved there?

I assume that the preference of biological processes to 'trap' (as you express it) 12C is due to the higher chemical reaction rate of molecules with lower mass. Don't you think that the same sort of reaction rate differences could apply in non-organic chemical processes that could result in the generation of hydrocarbons?

Oct 30, 2014 at 12:30 PM | Registered CommenterMartin A

" does tend to undermine the idea that fossil fuel use is unsustainable."

Even if a lot of the fuel is of abiogenic origin, I'm not aware of any significant resevoirs that refill at the speed the fuel is extracted from them.

Oct 30, 2014 at 1:42 PM | Unregistered CommenterRob Burton

Revisionism in that Wikipedia page .. I see after many years it now says "Abiogenic petroleum origin is an obsolete body of thought that proposed an alternative mechanism for the origin of petroleum. "
that would be fine to say obsolete if it had been disproven, but it hasn't . When I check the Wikipedia TALK button I see there is anger by other editors at the current version.

Oct 30, 2014 at 2:17 PM | Registered Commenterstewgreen

The discrimination between isotopes takes place as Co2 is absorbed. An extra carbon atom is added to a 5-carbon molocecule called ribulose biphosphate. The details are hideously complicated, though the enzyme most directly involved is familiarly known as rubisco.

Rubisco enzymes from different species shows small variations in structure and they show different degrees of discrimination. I don't know if anyone has pinpointed the mechanism, but this suggests that the discrimination is due the different way in which C13O2 and C12O2 interact with the active site of the enzyme. If it were purely an inorganic effect all rubiscos would be expected to behave in the same way. Another candidate enzyme is pyruvate dehydrogenase which reacts slightly more easily with pyruvate molecules with C12 at the no. 2 position in the chain.

Interestingly photosynthesis discriminates in oxygen isotope production too. Perhaps rubisco discriminates between CO2 containing O16 and O18?

There is quite a lot of research on the subject.

Inorganic reactions do show rate variations based on atomic mass, but enzymes tend to be much more rate sensitive to such small differences.

Oct 30, 2014 at 2:30 PM | Unregistered CommenterEntropic man

Martin A

I don't think the argument is circular.

You can demonstrate the ∆C13 effect in the laboratory.The proportion of C13 in the carbohydrate produced by photosynthesis is consistently lower than in the CO2 supplied.

When you see a similar ratio in the photosynthetic products in coal the default option is to presume a similar cause. When you see it in oil as well a biological cause is again the most likely explanation.

Oct 30, 2014 at 2:42 PM | Unregistered CommenterEntropic man

When it comes to abiogenic petroleum... dontcha think we got enough contoversy on our plates without looking for another? Trust me, if oil companies think it's got legs as an idea, they'll be trying to exploit it. I suspect we'd be more likely to see a liquid fuel created from other sources before this is relevant. Or who knows, maybe an electrc revolution. Won't hold my breath.

Oct 30, 2014 at 2:50 PM | Unregistered CommenterTinyCO2

@TinyCO2 I think you are confused
abiogenic petroleum is not a new or different type of petroleum rather it's just an alternative explanation for the origin of fossil fuels that they may not come from biology, but rather be natural product coming from the mantle.

Oct 30, 2014 at 3:40 PM | Registered Commenterstewgreen

EM - I'm asking, I don't know.

If you try some non-organic non-photosynthesis reaction in the lab - absorption of gaseous CO2 from dry air by CaO for example to produce calcium carbonate - won't the CaCO3 produced also be richer in 12C than the CO2 supplied?

[Essentially because of the higher speed of the lower mass molecules resulting in a higher rate of suitable collisions thus a higher reaction rate. ]

If not, how can the difference be explained? Vital Force?

Oct 30, 2014 at 3:56 PM | Registered CommenterMartin A

"Don't you think that the same sort of reaction rate differences could apply in non-organic chemical processes that could result in the generation of hydrocarbons?"

Oct 30, 2014 at 12:30 PM | Registered CommenterMartin A

Yes, Martin, they do. Isotopic differences can be observed in the laboratory, at least in the sense that the fractionation of isotopically enriched compounds can be observed in analytical instruments when not expected. These occur over a process of minutes or less in HPLC. It would be an unwise chemist that said they cannot occur to a significant extent over geological timescales as fluids slowly move and diffuse through the pores in rocks*.

Then there is the implicit assumption that biological (as opposed to physico-chemical) isotope-fractionation has always been constant. Given the relatively small differences involved in isotope-fractionation ratios as compared to the relatively large absolute numbers, then I think the question remains open. That is also why I attach little credence to the 12C/13C isotope ratios often cited as proving something about the carbon cycle. The uncertainties leave too much scope for everybody to be wrong.

[*Analytical chromatography is founded on the fact such similar such types of processes do occur!]

Oct 30, 2014 at 4:30 PM | Unregistered Commentermichael hart

stewgreen, nope I understand. I also understand it's not proven and hasn't found much support outside Russia. That doesn't mean it isn't true but let's leave it to the oil geologists to work out the fact from fiction. Eh?

Oct 30, 2014 at 6:17 PM | Unregistered CommenterTinyCO2

@TinyCO2 "only in Russia" that is not the impression I had ..I refer you to my first answer above
Next time you pass your local library look for the book

Oct 30, 2014 at 7:22 PM | Registered Commenterstewgreen

Inorganic reactions do show rate variations based on atomic mass, but enzymes tend to be much more rate sensitive to such small differences.
Oct 30, 2014 at 2:30 PM Entropic man

Could you give some sort of explanation of how that could be? Why should the reaction rate of reactions involving enzymes depend more on the ratio of CO2 molecular masses than other chemical reactions? I can't see it (unless there is some physical principle that I'm missing).

Oct 30, 2014 at 7:33 PM | Registered CommenterMartin A