Oil is a fossil fuel.
It was formed from chemicals from ancient living things.
To make the chemicals in oil, the temperatures and pressures needed to be just right.
The oil (and gas) will only stay in the reservoir if there are suitable structures in the rocks:
The carbon compounds from the plants and animals have been trapped for hundreds of millions of years.
They have been burned to make carbon dioxide in a very short time.
The amount of carbon dioxide in the air has changed very quickly.
Diagram of how oil is made
Source: Library and Archives Canada
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