Explainer: Where fossil fuels come from

One of the most widespread beliefs about fossil fuels — oil, natural gas and coal — is that these substances started out as dinosaurs. There’s even an oil company, Sinclair, that uses an Apatosaurus as its icon. That dino-source story is, however, a myth. What is true: These fuels got their start long, long ago — at a time when those “terrible lizards” still walked the Earth.

Fossil fuels store energy in the bonds between the atoms that make up their molecules. Burning the fuels breaks apart those bonds. This releases the energy that originally came from the sun. Green plants had locked up that solar energy within their leaves using photosynthesis, millions of years ago. Animals ate some of those plants, moving that energy up the food web. Others plants just died and decayed.

Any of these organisms, when they die, can be turned into fossil fuels, notes Azra Tutuncu. She’s a geoscientist and petroleum engineer at the Colorado School of Mines in Golden. But it takes the right conditions, including an oxygen-free (anoxic) environment. And time. A whole lot of time.

The coal we burn today got its start some 300 million years ago. Back then, dinosaurs roamed the Earth. But they didn’t get incorporated into coal. Instead, plants in bogs and swamps died. As this greenery sunk to the bottom of those wet areas, it partially decayed and turned into peat. Those wetlands dried out. Other materials then settled down and covered the peat. With heat, pressure and time, that peat transformed into coal. To extract coal, people now have to dig deeply into the earth.

Petroleum — oil and natural gas — comes from a process that started in ancient seas. Small organisms called plankton lived, died and sank to the bottom of those oceans. As debris settled down through the water, it covered the dead plankton. Microbes dined on some of the dead. Chemical reactions further transformed these buried materials. Eventually, two substances formed: waxy kerogen and a black tar called bitumen (one of the ingredients of petroleum).

The kerogen can undergo further changes. As debris buries it deeper and deeper, the chemical becomes ever hotter and subjected to more pressure. If conditions become just right, the kerogen transforms into the hydrocarbons (molecules formed from hydrogen and carbon) that we know as crude oil. If temperatures become hotter still, kerogen becomes the even smaller hydrocarbons that we know as natural gas.

The hydrocarbons in oil and gas are less dense than the rock and water in Earth’s crust. That prompts them to migrate upward, at least until they get trapped by some ground layer that they can’t move past. When that happens, they gradually build up. This forms a reservoir of them. And they will stay in it until people drill down to release them.

How much is there?

There is no way to know how much coal, oil and natural gas lie buried within the Earth. Even putting a number on that amount would not be very useful. Some of these fossil fuels simply will be in places from which people cannot safely or affordably extract them.

And even that can change over time, Tutuncu notes.

Some 20 years ago, she says, scientists knew where they could find what they call “unconventional resources.” These were accumulations of oil and gas that couldn’t be obtained through traditional drilling techniques. But then companies figured out new and less costly ways to bring up these resources.

One of these methods is hydraulic fracturing. Better known as fracking, it’s when drillers inject a mix of water, sand and chemicals deep into the ground to force out the oil and gas. In the foreseeable future, Tutuncu says, “I don’t think we will run out [of fossil fuels]. It’s just a matter of improvements in the technology [to extract them affordably].”

The burning of fossil fuels creates carbon dioxide and other greenhouse gases. These can contribute to climate change and global warming. For that reason, many scientists have warned that people should stop using fossil fuels. Alternatives, such as wind and solar power, don’t produce greenhouse gases.

Giving up fossil fuels entirely, though, won’t be easy, at least in the near future, Tutuncu says. These substances are used for more than just producing energy. Plastics and many other products include fossil fuels in their recipes. Scientists and engineers will have to come up with environmentally friendly replacements for all those products if society chooses to wean itself off of its current reliance on fossil fuels.