InfoBank
What made oil?
Living plankton under a microscope (about 150 x bigger than real life).
Crude oil was made over millions of years from tiny plants and animals, called plankton. The plankton on the left would form oil in about 150 million years time if the sea bed is not disturbed.

The plankton that lived in the Jurassic period made our crude oil.

This was the time of the dinosaurs. It was about 180,000,000 years ago.

When plankton die, they fall to the bottom of the sea.

The plankton are trapped under many layers of sand and mud. Over millions of years, the dead animals and plants got buried deeper and deeper. The heat and pressure gradually turned the mud into rock and the dead animals and plants into oil and gas.

A photograph down a microscope of sandstone. There are tiny spaces, called pores. These pores would have held the oil like a sponge. The pores have been stained blue.
Oil is formed over millions of years. It is held in tiny spaces in underground rock, like water in a sponge.

Some rocks have tiny spaces called pores. The rocks with these pores can hold the oil like a sponge.

The cap rock stops the oil from escaping.

Over millions of years, the rocks fold. Sometimes they form a dome shape that can catch the oil.

Oil can float up through the tiny spaces in the rock. We call these tiny spaces pores and we say that the rock is porous. This takes millions of years.

The oil can't get through the cap rock and is trapped in the dome of rock underneath.

The cap rock stops the oil from escaping.

The rock layer underneath has tiny spaces, or pores, in it. Although it is solid, it can hold the oil like a sponge.

The oil companies can drill into this rock to recover the oil.