Coal vs Oil Formation: What's the Difference?

[iamconfused]

So I'm learning in a class that coal is made when land plants (in swamps) die and then get buried in sediments, and with the Earth's pressure/temperature the plants become coal.

Then I learned that when ocean plants (like algae and plankton) die, they also get buried in sediments in the sea floor, and with temperature/pressure they become oil.

Why don't the ocean plants become coal? What is the difference?

 

[Mark44]

So I'm learning in a class that coal is made when land plants (in swamps) die and then get buried in sediments, and with the Earth's pressure/temperature the plants become coal.

Then I learned that when ocean plants (like algae and plankton) die, they also get buried in sediments in the sea floor, and with temperature/pressure they become oil.

Why don't the ocean plants become coal? What is the difference?

The difference, as I understand things, is the amount of heat, which corresponds roughly with how deeply the vegetative material is buried under the overlying sediment layers. More heat, the buried material becomes coal; less heat, it remains liquid. This was the thesis put forward by a petroleum geologist who was interviewed extensively in one of John McPhee's geology books, possibly "In Suspect Terrain." I don't recall which one, but the others in this series include "Basin and Range, " "Rising from the Plains," and "Annals of the Former World."

 

[iamconfused]

I also read that oil, if at a higher temperature, becomes natural gas. I'm confused then where coal fits into this. Intuitively, I could imagine a scenario where more heat would make liquid from solid, and even more heat makes gas...but it seems you are saying then that you go from liquid, coal, and then gas... unless maybe it's more about the pressure? Maybe more pressure turns the liquid to coal? But more heat turns liquid to gas?

And then I guess I should just assume that the temperature/pressure conditions of sediment buried from the land is different than the sediment buried from the ocean?

 

[256bits]

You can expand your search with the term kerogen, which is the decomposed organic material that becomes oil or coil.
https://en.wikipedia.org/wiki/KerogenNote the different H and O content of each type of kerogen.

 

[Mark44]

I could imagine a scenario where more heat would make liquid from solid, and even more heat makes gas...but it seems you are saying then that you go from liquid, coal, and then gas

I didn't say anything about a progression from liquid to solid, then to gas - what I said was a progression from vegetative matter to liquid, and then to a solid.

You seem to be thinking in terms of state changes via heat: solid state --> liquid --> gas, as for example, when ice melts to a liquid, and then becomes a gas in the form of steam. What I described is a situation of the liquid portion boiling off so that a liquid becomes a solid. If you mix sugar and water (a liquid) and boil it, what you'll eventually be left with is a solid.

 

[russ_watters]

What I described is a situation of the liquid portion boiling off so that a liquid becomes a solid. If you mix sugar and water (a liquid) and boil it, you'll eventually be left with is a solid.

The first semi-scientific experiment I remember performing was in Junior High and called The Distillation of Wood:
https://www.google.com/url?sa=t&sou...Vaw3hgqF22Z2HAE7v_p6vUW8w&cshid=1557346224922

Heating wood drives off water and various hydrocarbons, leaving charcoal. Similar idea.

 

[symbolipoint]

Were microorganisms involved in the organic matter turning to coal or petroleum? Or just heat and pressure?

 

[256bits]

Were microorganisms involved in the organic matter turning to coal or petroleum? Or just heat and pressure?

Coal, as an example, starts out with generations of higher order plants dying one on top of another. You may have heard of peat bogs - the organic matter accumulating over 100's of years in a wet environment where microorganisms can do some decomposition. The decomposition process though is interrupted by the burial of the bio-mass with sediment resulting in an increase in pressure and temperature, offering a protection from oxidation and a reduction and complete stoppage of bio-degradation. The peat will turn into lignite, bituminous coal, and then anthracite over millions of years, with gassing off of CO2, CH4, and loss of water.

 

[Asymptotic]

The peat will turn into lignite, bituminous coal, and then anthracite over millions of years

One point - it isn't simply a matter of time - anthracite requires considerably more pressure to form than bituminous. For instance, Pennsylvania coal fields were deposited at the same time, although in the flatter, western part of the state they are primarily bituminous, but it took being folded into ridge and valley systems by the Appalachian mountains to turn it into anthracite. It isn't unusual for the seams over here to have 30 to 45° dip angles.

 

[trainman2001]

I'm finding it hard to understand how the oil and gas going from deep in the crust is the result of biological decamp only. With an entire moon of Saturn being composed of hydrocarbons, is it not out of possibility that some of the copious amounts of hydrocarbons in the Earth's crust aren't from non-biological sources. Could not the presence of methane, heat and natural catalysts reform them into higher molecular weight hydrocarbons? If Titan can have non-biological hydrocarbons why can't we?

 

[Felipe good guy]

The first semi-scientific experiment I remember performing was in Junior High and called The Distillation of Wood:
https://www.google.com/url?sa=t&sou...Vaw3hgqF22Z2HAE7v_p6vUW8w&cshid=1557346224922

Heating wood drives off water and various hydrocarbons, leaving charcoal. Similar idea.

Hi russ_waters !

Thanks for your note on wood. I think it is super relevant to the question on this thread.

I guess I will just add this: A portion of matter can be decomposed into different compounds (chemical/mineralogical subsets or subproducts) via ''navigating the P-T space''. In the case of wood on a bonfire, or distillation of wood; the wood looses water/volatiles by, at constant pressure, rising its temperature; and you are finally left with charcoal (and the volatiles in the surrounding air). But, in a different way, moving in the P-T space increasing both the temperature and the pressure, one could get similar results..
In subduction zones, oceanic lithosphere minerals/rocks (basalt/gabro) are subjected to higher P and T, and they undergo phase changes were volatiles are expelled (dehydration of the original mineral) to the surrounding mantle, and the solid residue is another mineral/rock (such as eclogite).

In the case of organics/plants being buried under sediments and subjected to tectonism, the ''plants'' are subjected to higher P and T, with some specific path for each case. The specific path and final conditions are what (I think) determine the generation of gas, oil or coal.

I am no expert on petrology, whatsoever. But, I would like to ask the following:

is the generation of these phases (gas, oil, coal) only state(P-T) dependent?, or is it more complex, being state- and path-dependent?

cheers,

Felipe

 

[Mark44]

I'm finding it hard to understand how the oil and gas going from deep in the crust is the result of biological decamp only. With an entire moon of Saturn being composed of hydrocarbons, is it not out of possibility that some of the copious amounts of hydrocarbons in the Earth's crust aren't from non-biological sources. Could not the presence of methane, heat and natural catalysts reform them into higher molecular weight hydrocarbons? If Titan can have non-biological hydrocarbons why can't we?

The idea that hydrocarbons can be formed by means other than the decomposition of vegetable matter was put forth by T. Gold and S. Soter in a paper back in 1980 - https://www.osti.gov/biblio/6070499. I've heard this described as "Gold's Hypothesis".

 

[KevinMcHugh]

I'm finding it hard to understand how the oil and gas going from deep in the crust is the result of biological decamp only. With an entire moon of Saturn being composed of hydrocarbons, is it not out of possibility that some of the copious amounts of hydrocarbons in the Earth's crust aren't from non-biological sources. Could not the presence of methane, heat and natural catalysts reform them into higher molecular weight hydrocarbons? If Titan can have non-biological hydrocarbons why can't we?

There is abiotic oil, but it is not what we pump out of the ground. I worked with the organic geochemistry group analyzing core samples from DSDP and ODP from over the globe, and we never a sample that did not contain biomarkers.

 

[Felipe good guy]

There is abiotic oil, but it is not what we pump out of the ground. I worked with the organic geochemistry group analyzing core samples from DSDP and ODP from over the globe, and we never a sample that did not contain biomarkers.

Hello Kevin McHugh,

Thanks for your response, it is very relevant.

Do you know how similar or different Abiotic Oil vs. Biotic Oil are?

It would be cool to know how much similarity there is at the molecular level..

cheers many,
Felipe

 

[KevinMcHugh]

Hello Kevin McHugh,

Thanks for your response, it is very relevant.

Do you know how similar or different Abiotic Oil vs. Biotic Oil are?

It would be cool to know how much similarity there is at the molecular level..

cheers many,
Felipe

I have never analyzed abiotic oil, so I cannot attest to the differences. Wiki has info on abiotic oil.

 

[trainman2001]

Well, I guess that answers it. Hard to imagine how many dead animals and plants existed to create the billions and billions of barrels of oil that's been found and yet to be found.

 

[Tish62]

Humans have a hard time in conceptualizing what millions or hundreds of millions of years mean; You can build up organic deposits several kilometers deep just with a modest deposition of organic plant matter of a centimeter or so per year.

Think of the outflow of river deltas like the Amazon, Mississippi, Yangtze, Nile or the Mekong. Sediments are deposited so rapidly that a river can change its course over the period of just one rainy season. Millions of tons of organic sediments are entrapped by just this one activity. The same goes on with forests and bogs; All over the world, for hundreds of millions of years, organic materials are trapped in sediments and get buried deeper and deeper.

 

[KevinMcHugh]

Albert Treibs developed a scheme describing depositional environments. Pressures and temps increase with time and depositional rate. He monitored the fate of chlorophyll in the deposits, and looked at the products formed at increasing depth. Good source rocks contain nickel and vanadyl erythroetioporphyrins derived from chlorophyll. These porphyrins produced are a function of T and P, which are a function of deposition rate.

 

[KevinMcHugh]

Well, I guess that answers it. Hard to imagine how many dead animals and plants existed to create the billions and billions of barrels of oil that's been found and yet to be found.

Coal is produced from terrestrial deposition of fauna. Oil is produced from flora and fauna (phytoplankton and zooplankton) in marine anoxic environments. No multicellular animals were used in the making of these products.

 

[KevinMcHugh]

So I'm learning in a class that coal is made when land plants (in swamps) die and then get buried in sediments, and with the Earth's pressure/temperature the plants become coal.

Then I learned that when ocean plants (like algae and plankton) die, they also get buried in sediments in the sea floor, and with temperature/pressure they become oil.

Why don't the ocean plants become coal? What is the difference?

IIRC, marine environments are anoxic, and terrestrial deposits are oxic.