By heat and pressure, both coal and oil can produce gas. But what is the most important substance for gas? Is there a relation to the amounts of marine organism (sea life) which died and the amount of plants and trees which was covered by sand or is oil (sea life) or coal (plants) just more capable of producing gas?

Else what is the alternative contributing source?

  • $\begingroup$ Coal and oil can be converted to nat gas but it is very rarely done. Nat gasses are present in reservoirs by themselves or dissolved in oil and coal . That is why most wells have a separator at the well head or nearby, to give the gas time to come out of solution at the (low) atmospheric pressure. And gas wells are drilled into coal deposits to collect gas. $\endgroup$ Commented May 26, 2019 at 22:49
  • $\begingroup$ I guess the vast deposits of methane hydrates on the deep ocean floors are not part of the discussion ? $\endgroup$ Commented May 26, 2019 at 22:51
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    $\begingroup$ How do you define "important"? It's not clear what you're trying to find out. $\endgroup$
    – jpmc26
    Commented May 27, 2019 at 12:27
  • $\begingroup$ The most convenient source is, indisputably, beans. $\endgroup$
    – Vikki
    Commented May 28, 2019 at 1:07
  • $\begingroup$ "They mixed together tiny quantities of three things that we know exist in the mantle of the earth, and observed them at the pressure and temperature appropriate to the mantle about two hundred kilometers down. The three things were calcium carbonate which is sedimentary rock, iron oxide which is a component of igneous rock, and water. These three things are certainly present when a slab of subducted ocean floor descends from a deep ocean trench into the mantle. The experiment showed that they react quickly to produce lots of methane, which is natural gas." — Freeman Dyson $\endgroup$ Commented Jun 23, 2021 at 9:44

3 Answers 3


The most important source of natural gas is natural gas. This is why it is called natural. It is not "made" from oil or coal.

Natural gas forms by decomposition of organic material. Whether the organic material decomposes to coal, oil, or gas depends on the composition of the original material, time, pressure, and temperature. In theory, oil (=long chains of hydrocarbons) can decompose to gas (=mostly methane and small amounts of very short chains of hydrocarbons). I am not certain as to how much of the natural gas in existence went through this oil-like stage.

Coal cannot "become" natural hydrocarbon gas, because it contains no hydrogen. It can be burnt to form carbon dioxide, which is a gas, but this is not what you're talking about.

  • $\begingroup$ ok, can gas also be 'made' by plants and trees organisms as the only substance? $\endgroup$
    – Marijn
    Commented May 26, 2019 at 11:05
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    $\begingroup$ @Marijn: As Gimelist states natural gas is methane, CH4. Methane is made from organic matter via fermentation. The resulting gas is called biogas. Carbon likes to react with oxygen, but when there is no oxygen it will react with hydrogen to form methane. Natural gas (methane) can occur in coal seams & is called coal seam gas. In some parts of the world it forms a resource for natural gas. Most coal mine explosions are caused by methane igniting . $\endgroup$
    – Fred
    Commented May 26, 2019 at 14:44
  • $\begingroup$ @Marijn: Natural gas is naturally produced when crude oil reservoirs are naturally heated to between 90C & 160C. $\endgroup$
    – Fred
    Commented May 26, 2019 at 14:51
  • $\begingroup$ @Fred were does this hydrogen come from? $\endgroup$
    – Marijn
    Commented May 26, 2019 at 16:12
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    $\begingroup$ @Marijn: all living organisms contain carbon, hydrogen & oxygen $\endgroup$
    – Fred
    Commented May 26, 2019 at 17:38

A significant part of the natural gas used around the world today comes from oil fields.

On top of the oil is a layer of natural gas, and in the crude oil is dissolved natural gas. This will be released when the pressure drops during the extraction of the crude oil so the natural gas will be separated from the oil during the extraction process.

Earlier this gas was flared (burned off) during extraction, but this is no longer alowed for many oil fields to do so the natural gas is now collected and processed before the gas is sold to users around the globe.

Link to pictures of flaring around the world https://geology.com/articles/oil-fields-from-space/ A lot of work remains to limit the flaring from oil and gas fields,Some flaring is to avoid over pressure for safety reasons and is unavoidable.

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    $\begingroup$ Are you sure it's illegal to burn off the gas? The fact that i's an extremely valuable product would be more than enough reason to not set fire to it. (But, rather, sell it to people so they can set fire to it.) $\endgroup$ Commented May 26, 2019 at 20:21
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    $\begingroup$ @DavidRicherby gas is harder to store and transport than oil, and much more dangerous. Flaring it is an easy and relatively safe option to prevent it accumulating anywhere. Sure it's waste of valuable product, also to the oil company, but if it requires lots of extra infrastructure expense and risks literally exploding the entire operation, you can see why they might just go for the easy, dirty option and make their profit with oil only... $\endgroup$ Commented May 26, 2019 at 22:28
  • $\begingroup$ There are many gas flaring regulation in most countries that involve the amount, location, type of well , time periods, pipeline construction and many other considerations of whether or not, and how much gas may be flared. $\endgroup$ Commented May 26, 2019 at 22:56
  • $\begingroup$ in norway the oil and gass companies are not allowed to flare the gass,and many countries are putting regulations in place to limit the flaring of gass. $\endgroup$ Commented May 27, 2019 at 4:10
  • $\begingroup$ that didn't sound correct. It is semantics. It is against the law to flare in Norway UNLESS the government ( Norway Petroleum Directorate ) gives you permission . So what I commented is true. Most recent data I found :Norway flared 400,000,000 cubic meters in 2002. $\endgroup$ Commented May 27, 2019 at 17:07

The most important factor for petroleum becoming gas isn't a substance, it's temperature and pressure. Most reservoirs have varying degrees of both; the T&P dictates how much of each will form. Knowing the type of field tells you what you should expect, be it a natural reservoir (black gold), or peat bogs (oil sands).

Geologists often refer to the temperature range in which oil forms as an "oil window". Below the minimum temperature oil remains trapped in the form of kerogen. Above the maximum temperature the oil is converted to natural gas through the process of thermal cracking. Sometimes, oil formed at extreme depths may migrate and become trapped at a much shallower level. The Athabasca Oil Sands are one example of this. – Petroleum

The oil in Texas, where 40% of America's crude comes from, is under a caprock, which traps a "three-layer cake with a layer of water below the oil layer and a layer of gas above it" (Wiki). This type of reservoir will contain much natural gas.

The oil sands of Canada, which has the largest mining operation by area in the entire world, extracts bitumen; aka: peat. To extract natural gas from oil sand it must be cracked, which costs money, as opposed to the freely available stuff under a capstone.

Peat forms in wetland conditions [terrestrial life, not aquatic], where flooding or stagnant water obstructs the flow of oxygen from the atmosphere, slowing the rate of decomposition.

A fossil fuel, petroleum is formed when large quantities of dead organisms, mostly zooplankton and algae [aquatic life], are buried underneath sedimentary rock and subjected to both intense heat and pressure.

TL;DR: the good stuff comes from poorly decomposed aquatic life. The harder stuff comes from poorly decomposed plant life.... Apparently the dinosaurs did not all die and turn into oil.

Apologies but, the most important source IMO would need to be determined by computing the amount of barrels produced, while knowing what type of field they came from. E.g., "Oil sands were the source of 62% of Alberta's total oil production and 47% of all oil produced in Canada." (Wiki) Now you just have to figure out how many barrels Canada produces, contrasted against the same information for every country or oil company throughout the entire world.

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    $\begingroup$ This answer could be improved by adding references for the quoted sections. $\endgroup$
    – njuffa
    Commented May 26, 2019 at 22:40
  • $\begingroup$ "Sometimes, oil formed at extreme depths may migrate and become trapped at a much shallower level.The Athabasca Oil Sands are one example of this." - I was under the impression that the Canadian oil sands were 'Peat' having formed in "wetland conditions". Perhaps it formed the normal way, but with nothing acting as a capstone it lost, and continually losses, all its gas. $\endgroup$
    – Mazura
    Commented May 28, 2019 at 23:03

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