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.

http://en.wikipedia.org/wiki/Abiogenic_petroleum_origin

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

http://geology.com/articles/diamonds-from-coal/

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."

Dung

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.

Dung

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.

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.

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.

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.

http://www.dailymail.co.uk/sciencetech/article-2807217/The-mysterious-methane-ice-cloud-high-Titan-s-atmosphere-Researchers-baffled-Saturn-s-moon.html

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.

"...it 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.

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.

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.

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.

"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!]

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?