Eli5: Why is oil referred to as fossil fuel?
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Crude oil exists as a byproduct of decomposing organic materials from extremely long ago, just like fossils are remnants of those same organisms.
Oil comes from the ground because large deposits of organic matter were buried by various events (earthquakes, glaciers, etc.) and trapped underground to decompose. Because there wasn’t the typical exposure to bacteria and oxygen and what not, the buried organic matter decomposed in a wildly different way than regular decomposition on the surface of the ground.
The original meaning of the term “fossil” meant anything acquired by digging from the ground and then became extended to referring to any long dead organism.
So they are called fossil fuels because they come from long dead organisms (primarily plankton).
The energy in oil came from the Sun long ago, when the plants that became oil were alive. The layers where oil is found are related to the layers where fossils were found, because the plants that became oil were living when the animals that became fossils were walking around.
if you take organic matter and put it into a pressure cooker, then slow roast it for millions of years, you get crude oil. it is literally the fossils of plants and animals. the key to forming it is the pressure cooker part that seals it away from other decomp processes. there needs to be an event that rather quickly sealed off a large amount of organic matter, like earthquake, volcano, etc.
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