How is oil formed? From ancient plankton to petrol

Oil is formed from the remains of tiny marine organisms, mostly plankton and algae, that died in ancient seas, sank into mud on the seabed and were buried. Over millions of years, as the mud sank kilometers deep, heat and pressure turned that organic matter first into a waxy solid and then into liquid oil and gas. That is how oil is formed: slowly, in narrow conditions, over geological time, and not from dinosaurs.

In short

• Most oil comes from plankton and algae buried in oxygen-poor mud, not from dinosaurs or land animals.
• It forms only in a narrow band of temperature, roughly 60 to 120°C, usually a few kilometers underground; hotter rocks make natural gas instead.
• Oil that forms then has to move into porous rock and be trapped under a seal, or it leaks away, which is why usable oil fields are rare.

What is oil made from?

Crude oil is a mix of hydrocarbons, molecules made of hydrogen and carbon. The U.S. Energy Information Administration (EIA) describes it as formed from the remains of plants and animals, such as diatoms, a kind of single-celled algae, that lived millions of years ago in marine environments. The carbon in those organisms came from carbon dioxide they absorbed from the air and sea while alive, using sunlight. In a sense, oil is stored ancient sunlight.

It isn't made from dinosaurs. "Oil comes from trillions of tiny algae and plankton," the University of Oslo geologist Reidar Müller told Science Norway. The myth probably comes from the phrase "fossil fuel" and from advertising, such as the green dinosaur in Sinclair Oil's logo. Some of the world's oil formed while dinosaurs lived, from plankton in the seas of the Jurassic and Cretaceous periods, and much of the North Sea's oil comes from algae and plankton that lived about 150 million years ago. Some source rocks are older than the first dinosaurs. For what did happen to the dinosaurs, see what killed the dinosaurs.

How does dead plankton become oil?

The process runs in stages, each with conditions that have to be just right:

1. Bloom and burial. Plankton and algae thrive in sunlit surface water. When they die, they sink and collect on the seabed with fine clay and silt washed in from rivers.
2. No oxygen. Usually, bacteria and scavengers eat dead organic matter, using oxygen. Where the bottom water has little or no oxygen, as in some deep basins and still seas, the organic matter survives and is buried in layers of black mud.
3. Kerogen. As more sediment piles on top, the mud is squeezed and slowly hardens into rock, usually shale. In the first several hundred meters of burial, mild heat and pressure turn the organic remains into kerogen, a waxy solid spread through the rock, according to an American Chemical Society explainer hosted by Penn State.
4. The oil window. Deeper still, the rock warms. Between about 60 and 120°C, kerogen breaks down into liquid oil. This range is called the oil window; its exact edges vary with the type of kerogen and how long it is heated. Colder, and the kerogen stays solid. Hotter, and the long oil molecules crack into the short molecules of natural gas, which is why the deepest, hottest source rocks hold gas rather than oil.
5. Migration. Oil and gas are lighter than water, so they seep upward through cracks and pores out of the dense source rock.
6. The trap. If they meet a layer they can't pass, such as dense shale or salt, they collect in porous rock beneath it. That is an oil field. If nothing stops them, they reach the surface and leak away, as in natural tar pits and seeps.

The type of buried material matters too. Energy Education, from the University of Calgary, notes that kerogen made mostly from algae is the type most likely to produce oil, while kerogen made from woody land plants tends to produce natural gas. That is why coal, which comes from land plants, and oil, which comes mostly from sea life, are different fuels.

How deep and how long? A worked example

The numbers make the timescale concrete. Take typical, round values:

• Rock gets warmer with depth, on average by about 25°C per kilometer according to Energy Education, and faster in many sedimentary basins. Take 30°C.
• Start with a seabed at about 15°C.
• Suppose mud piles up at 0.1 millimeters a year, which is 100 meters every million years. That is a round figure chosen for the example; real basins fill far slower or far faster.

Depth	Temperature (at 30°C per km)	Time to be buried that deep	What happens
0.5 km	about 30°C	5 million years	Mud hardens; kerogen forms
1.5 km	about 60°C	15 million years	Top of the oil window
3.5 km	about 120°C	35 million years	Bottom of the oil window
5 km and deeper	165°C or more	50 million years	Mostly gas

On these assumptions, a layer of plankton takes 15 million years just to reach the oil window, and spends 20 million more years passing through it. Real basins vary a great deal, sinking faster or slower and heating unevenly, but the order of magnitude holds: tens of millions of years.

Where does oil collect?

An oil field needs four things to line up, which petroleum geologists call a petroleum system: a source rock rich in kerogen, a porous reservoir rock such as sandstone or limestone with tiny spaces between its grains, an impermeable seal or cap rock above it, and a trap shape that stops oil escaping sideways. Common traps are arches of folded rock, called anticlines, faults that seal one side, and domes of salt that push up through the layers.

Not all oil leaves its source rock. Some stays locked in tight shale, and that is what U.S. drillers reach with horizontal wells and hydraulic fracturing, cracking the rock open with high-pressure fluid. This "shale oil" made the United States the world's largest oil producer in the late 2010s, a shift that changed the power of exporters' groups such as OPEC.

Why will oil run out?

Oil is still forming today, in places like the Black Sea where the deep water lacks oxygen. But at the pace described above it takes tens of millions of years, while the world used more than 100 million barrels a day in the mid-2020s. On any human timescale, the oil in the ground is a fixed stock, not a flow. Burning it returns to the air, in decades, carbon that took millions of years to bury, which is why oil is central to climate change.

How much remains is a moving target. Estimates of reserves, the oil that can be produced profitably with today's technology, rise when prices or methods improve and fall when they don't, so any figure needs a date and a source. The oil is also unevenly spread, which is why much of the world's supply depends on routes like the Strait of Hormuz.

How is crude oil turned into petrol?

Crude oil as it comes out of the ground is a mixture of thousands of hydrocarbons, from gases to tar. A refinery separates and reshapes them. First it heats the crude and feeds it into a tall distillation column, where lighter molecules rise higher and condense at lower temperatures: gas at the top, then petrol, jet fuel, diesel, and heavy oils and asphalt at the bottom. Then it cracks some of the heavy molecules into lighter, more valuable ones, and treats the products to remove sulfur.

According to the EIA, a 42-gallon barrel of crude yields, in U.S. refineries, about 19 to 20 gallons of motor gasoline (petrol) and 11 to 13 gallons of distillate fuel, mostly diesel. Other EIA figures add 3 to 5 gallons of jet fuel, with the rest going to products such as heating oil, asphalt, lubricants and the chemical feedstocks used to make plastics.

Check yourself

• Without looking back: why does buried plankton need to avoid oxygen to become oil?
• What happens to kerogen that is buried too shallow, and to kerogen buried too deep?
• Why can't oil just be left to form again once a field is used up?

Sources

• Oil and petroleum products explained — U.S. Energy Information Administration
• How many gallons of gasoline and diesel fuel are made from one barrel of oil? — U.S. Energy Information Administration
• Refining crude oil — U.S. Energy Information Administration
• No, oil does not come from dinosaurs — Science Norway, 2022
• Kerogen — Energy Education, University of Calgary
• The formation of petroleum — American Chemical Society Division of Fuel Chemistry, via Penn State