Explainer: Where fossil fuels come from | Science News for Students

Explainer: Where fossil fuels come from

Spoiler alert: It’s not from dinosaurs
Sep 20, 2018 — 6:35 am EST
a photo of pumping gas into a car

The liquid fuels that power most vehicles have been millions of years in the making.


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.

Power Words

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Apatosaurus     A dinosaur whose name means deceptive lizard. It has a long neck and thick, whip-like tail. Formerly known as a brontosaurus, it lived during the Jurassic period, about 150 million years ago. In adulthood, this plant-eater would have weighed some 36 metric tons (40 short tons) and had an average length of perhaps 23 meters (75 feet). That would have made it one of the largest animals to ever roam the Earth.

atom     The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

bitumen     A natural type of especially heavy, dense petroleum. This type of heavy crude oil must be diluted before it can flow through pipelines.

bog     A type of wetland that forms peat from the accumulation of dead plant material — often mosses.

bond     (in chemistry) A semi-permanent attachment between atoms — or groups of atoms — in a molecule. It’s formed by an attractive force between the participating atoms. Once bonded, the atoms will work as a unit. To separate the component atoms, energy must be supplied to the molecule as heat or some other type of radiation.

carbon dioxide (or CO2) A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter burns (including fossil fuels like oil or gas). Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.

chemical     A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.

chemical reaction     A process that involves the rearrangement of the molecules or structure of a substance, as opposed to a change in physical form (as from a solid to a gas).

climate     The weather conditions that typically exist in one area, in general, or over a long period.

climate change     Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.

crude oil      Petroleum in the form as it comes out of the ground.

crust     (in geology) Earth’s outermost surface, usually made from dense, solid rock.

dinosaur     A term that means terrible lizard. These ancient reptiles lived from about 250 million years ago to roughly 65 million years ago. All descended from egg-laying reptiles known as archosaurs. Their descendants eventually split into two lines. For many decades, they have been distinguished by their hips. But a new 2017 analysis now calls into question that characterization of relatedness based on hip shape.

engineer     A person who uses science to solve problems. As a verb, to engineer means to design a device, material or process that will solve some problem or unmet need.

environment     The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of components in some electronics system or product).

food web     (also known as a food chain) The network of relationships among organisms sharing an ecosystem. Member organisms depend on others within this network as a source of food.

fossil fuel     Any fuel — such as coal, petroleum (crude oil) or natural gas — that has developed within the Earth over millions of years from the decayed remains of bacteria, plants or animals.

global warming     The gradual increase in the overall temperature of Earth’s atmosphere due to the greenhouse effect. This effect is caused by increased levels of carbon dioxide, chlorofluorocarbons and other gases in the air, many of them released by human activity.

greenhouse gas     A gas that contributes to the greenhouse effect by absorbing heat. Carbon dioxide is one example of a greenhouse gas.

hydraulic fracturing, or fracking     The cracking open of underground rocks by introducing liquid at high pressure, especially to extract natural gas. Those cracks are then held open by sand that had been added to the fracking fluid.

hydrocarbon     Any of a range of large molecules containing chemically bound carbon and hydrogen atoms. Crude oil, for example, is a naturally occurring mix of many hydrocarbons.

hydrogen     The lightest element in the universe. As a gas, it is colorless, odorless and highly flammable. It’s an integral part of many fuels, fats and chemicals that make up living tissues. It’s made of a single proton (which serves as its nucleus) orbited by a single electron.

icon     (adj. iconic) Something that represents another thing, often as an ideal version of it.

kerogen     Fossilized organic material that is a starting ingredient in the formation of oil and natural gas. Found in oil shale and other sedimentary rock, it does not dissolve in common carbon-based solvents.

microbe     Short for microorganism. A living thing that is too small to see with the unaided eye, including bacteria, some fungi and many other organisms such as amoebas. Most consist of a single cell.

molecule     An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

natural gas     A mix of gases that developed underground over millions of years (often in association with crude oil). Most natural gas starts out as 50 to 90 percent methane, along with small amounts of heavier hydrocarbons, such as propane and butane.

organism     Any living thing, from elephants and plants to bacteria and other types of single-celled life.

oxygen     A gas that makes up about 21 percent of Earth's atmosphere. All animals and many microorganisms need oxygen to fuel their growth (and metabolism).

peat     Largely decomposed plant material that develops in the absence of oxygen within a water-saturated site, such as a bog. When dried out, peat can be burned as a low-grade fuel.

petroleum     A thick flammable liquid mixture of hydrocarbons. Petroleum is a fossil fuel mainly found beneath the Earth’s surface. It is the source of the chemicals used to make gasoline, lubricating oils, plastics and many other products.

photosynthesis     (verb: photosynthesize) The process by which green plants and some other organisms use sunlight to produce foods from carbon dioxide and water.

plankton     A small organism that drifts or floats in the sea. Depending on the species, plankton range from microscopic sizes to organisms about the size of a flea. Some are tiny animals. Others are plantlike organisms. Although individual plankton are very small, they form massive colonies, numbering in the billions. The largest animal in the world, the blue whale, lives on plankton.

plastic     Any of a series of materials that are easily deformable; or synthetic materials that have been made from polymers (long strings of some building-block molecule) that tend to be lightweight, inexpensive and resistant to degradation.

pressure     Force applied uniformly over a surface, measured as force per unit of area.

sea     An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.

sediment     Material (such as stones and sand) deposited by water, wind or glaciers.

tar     A thick, viscous black flammable goo derived from coal or wood. It consists of a range of hydrocarbons, resins, alcohols and more.

technology     The application of scientific knowledge for practical purposes, especially in industry — or the devices, processes and systems that result from those efforts.