How fossil fuels formed

When I was quite young, someone told me that all the fuel that exists in the form of oil and coal came from long dead dinosaurs. What amazes me is that I never questioned this preposterous piece of information for a long time. I think the fact that they were referred to as ‘fossil fuels’ confirmed in my mind that dead animals were the source and dinosaurs, being the largest dead animals, seemed to fit the bill.

It was only much later that the thought occurred to me that given the amount of oil and coal that exists and that we have already extracted, there must have been a hell of a lot of dinosaurs to produce it and that was just not possible, and that while dinosaurs were big, they could not be the sole source. This informative page from the US Department of Energy suggests that I was not alone in my goofy misapprehension, so I thought that I would spread the word.

Contrary to what many people believe, fossil fuels are not the remains of dead dinosaurs. In fact, most of the fossil fuels we find today were formed millions of years before the first dinosaurs.

Fossil fuels, however, were once alive!

They were formed from prehistoric plants and animals that lived hundreds of millions of years ago.

Think about what the Earth must have looked like 300 million years or so ago. The land masses we live on today were just forming. There were swamps and bogs everywhere. The climate was warmer. Ancient trees and plants grew everywhere. Strange looking animals walked on the land, and just as weird looking fish swam in the rivers and seas. Tiny one-celled organisms called protoplankton floated in the ocean.

When these ancient living things died, they decomposed and became buried under layers and layers of mud, rock, and sand. Eventually, hundreds and sometimes thousands of feet of earth covered them. In some areas, the decomposing materials were covered by ancient seas, then the seas dried up and receded.

During the millions of years that passed, the dead plants and animals slowly decomposed into organic materials and formed fossil fuels. Different types of fossil fuels were formed depending on what combination of animal and plant debris was present, how long the material was buried, and what conditions of temperature and pressure existed when they were decomposing.

For example, oil and natural gas were created from organisms that lived in the water and were buried under ocean or river sediments. Long after the great prehistoric seas and rivers vanished, heat, pressure and bacteria combined to compress and “cook” the organic material under layers of silt. In most areas, a thick liquid called oil formed first, but in deeper, hot regions underground, the cooking process continued until natural gas was formed. Over time, some of this oil and natural gas began working its way upward through the earth’s crust until they ran into rock formations called “caprocks” that are dense enough to prevent them from seeping to the surface. It is from under these caprocks that most oil and natural gas is produced today.

The same types of forces also created coal, but there are a few differences. Coal formed from the dead remains of trees, ferns and other plants that lived 300 to 400 million years ago.

This is what I like so much about science, that being wrong is the precursor to finding out interesting new things. But it requires being willing to look a little below the surface of folklore and not accept the glib answers that are often proffered for questions about our world.


  1. moarscienceplz says

    I think I remember a TV commercial from back in the ’60s for some gasoline company that had a cartoon brontosaurus (I know the proper name is apatosaurus, but I still like brontosaurus better, and hey, it’s a cartoon) sticking its head out of a car’s gas fill pipe and roaring. Sinclair gas had a brontosaurus as its logo, but I think the commercial was for another brand.

  2. moarscienceplz says

    I should remember that one can find anything on Youtube. The commercial was from 1977 for Chevron and it may be partially to blame for giving you that false impression, Mano.

  3. Mano Singham says


    Thanks for that commercial. Even if I did not see it (I don’t recall) the fact that it was used suggests that this belief was widespread at the time.

  4. dysomniak "They are unanimous in their hate for me, and I welcome their hatred!" says

    I know the proper name is apatosaurus

    Nothing to do with “proper,” but rather “only.” The creature you call brontosaurus was nothing more than an apatasaur skeleton that some fool stuck the wrong skull onto.

  5. colnago80 says

    Re #5

    Yeah, but Stephen Jay Gould said it was OK to keep referring the the animal as Brontosaurus.

  6. colnago80 says

    Relative to this, the late physicist Thomas Gold hypothesized that the fossil fuels found on the earth were primordial and were present at the time of the earth’s formation. Gold was known for being a curmudgeon but, relative to natural gas, he might not have been entirely wrong.

  7. moarscienceplz says

    The creature you call brontosaurus was nothing more than an apatasaur skeleton that some fool stuck the wrong skull onto.

    Yes, but why do we have to toss a really cool (and apt) name in the garbage can just because Othniel Charles Marsh made a mistake?

  8. DennisinBaltimore says


    I am a geophysicist. Almost all the oil, coal, and gas available on this planet is from before land animals existed. Maybe before animals existed, including fish. It is exclusively composed of plant remains, plants invaded land before fish (I had to say that :). Plants changed the atmosphere (created an oxygen denominated atmosphere by just a few percent above 0 in the CO2 dominated atmosphere of the time). It was the first ecological disaster. Allowed the evolution of higher metabolism animals that ate plants (think, Plants created their predators). We need to be smarter than plants to survive the future, are we capable of that! Do we let Republicans or their proxies determine that. Vote dem.

  9. Mano Singham says


    Thanks for that information. Isn’t it interesting how plants managed to survive such a dramatic change in the atmosphere from a CO2 dominated one to the O2 dominated one that they themselves created? Do we know if those early plants were significantly different from the current ones in terms of how they functioned?

  10. corwyn says

    Isn’t it interesting how plants managed to survive such a dramatic change in the atmosphere from a CO2 dominated one to the O2 dominated one that they themselves created?

    IANAB, but I was under the impression that the ‘plants’ that converted the atmosphere were single celled organisms. Wikipedia agrees:

  11. permanentwiltingpoint says

    I’m a geoscientist in training (aspiring MSc) and have some quibbles with the US Department of Energy and DennisinBaltimore.
    The DoE text is mostly right, but the part on oil generation is somewhat muddled. I never heard of bacteria playing a significant role in that process, and since the onset happens at some 2000 metres depth / 60 °C and peaks at 4000 metres / 120 °C, I don’t see how they could. Of course, near the surface bacteria will break down the original organic matter, but when we come to oil formation as such, it’s a simple chemical process.
    If oil or gas forms, is a function of burial depth and so the amount of sediment stacked on top. Beyond some 6000 metres / 150 °C, oil breaks down and only gas will be produced. All that can be drawn from the following graphic:

    Now you’ll understand if “In most areas, a thick liquid called oil formed first, but in deeper, hot regions underground, the cooking process continued until natural gas was formed.” doesn’t make my list of good science journalism.

    @ DennisinBaltimore: I’m sorry, but it’s flat wrong that “Almost all the oil, coal, and gas available on this planet is from before land animals existed. Maybe before animals existed, including fish.” Most coal is from the carboniferous and permian era, when animals in general and land animals in special were long present, and almost none formed before the devonian. As for oil, I’m not quite as sure about the time range, but a lot of it is very young: Gulf of Mexico oil, for instance, as young as 12.000 years and none older than the Neogen era (~ 2.6 – 23 million years).
    And while it’s right that oil is made mostly from plants, i.e. algae, it’s not exclusivly so: Zooplankton also contributes to it.

    This has gotten long. But given the normal rate at which our host corrects his misapprehensions in this field, I don’t want let him go away with these 😉

  12. Francisco Bacopa says

    DennnisinBaltimore left out the best part: The last and most important coal making period was the Carboniferous. Land plants were first able to make the lignin protein at the beginning of this period, but no microbes at the time could break it down very well. When plants died, they did not rot. Thus plant life acted as a HUGE carbon sink and there were wild fluctuations in climate as basically all the carbon in the atmosphere ended up underground and oxygen would hit the the 35% level that would lead to wildfires of unimaginable scale.

    Eventually wood eating fungi came along and the climate became much more stable. There’s also been no new coal since this time.

  13. permanentwiltingpoint says

    @ Francisco Bacopa

    I too like the lignin theory (Floudas et al. 2012, for anyone interested). It very neatly explains the sharp drop in carbon burial at the end of the permian, and the old argument which talked of changing biogeographic conditions seems very weak to me. But it’s not something as established as, for instance, the K/T – Impact.

    As for the climate becoming more stable after the carboniferous, though, I disagree:

  14. Mano Singham says


    I am afraid that it is hopeless. I acquire new misapprehensions faster than I get rid of old ones.

  15. lpetrich says

    Anoxic event – Wikipedia Oceanic anoxic events are due to bouts of global warming, with average temperatures going as high as 25 C (it’s currently 13 C). This makes the oceans dissolve less oxygen, making it more difficult for decay organisms to live off of organic matter.

    Another, economically significant consequence of oceanic anoxic events is the fact that the prevailing conditions in so many Mesozoic oceans has helped produce most of the world’s petroleum and natural gas reserves. During an oceanic anoxic event, the accumulation and preservation of organic matter was much greater than normal, allowing the generation of potential petroleum source rocks in many environments across the globe. Consequently some 70 percent of oil source rocks are Mesozoic in age, and another 15 percent date from the warm Paleogene: only rarely in colder periods were conditions favorable for the production of source rocks on anything other than a local scale.

    What causes them is uncertain. A possible cause is the atmosphere getting a lot more CO2, like from volcanoes. Some Large Igneous Provinces are about the age of oceanic anoxic events, and that suggests a link.

    But when I think about this, I think “Don’t tell the Koch brothers. They may want global warming to get worse so that their company will have plenty of oil to extract.”

  16. says

    Biodiesel made today from plants is an obvious example. Pour vegetable oil into a diesel engine and it will work (in the right conditions).

    And scientists warn of methane in melting permafrost entering the atmosphere. The gas in the ground wouldn’t have come from dinosaurs.

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