How oil is formed.
The origin of oil goes back several million years. Oil is a fossil fuel, like natural gas and coal, which means that it originated from tiny animals and plants that died millions of years ago. All fossil fuels are finite resources, which means that there is a limited supply of each type of fuel in the world today, and that it is not being replaced. All living material is based on chains of carbon and hydrogen atoms. Crude oil, the form in which oil is found in the ground, is also based on chains of carbon and hydrogen atoms. The chains of carbon and hydrogen atoms that make up crude oil are called hydrocarbons. The hydrocarbons in crude oil were formed by the decaying of tiny animals and plants, known as phytoplankton and zooplankton. a long, long time ago. When the tiny animals and plants (phytoplankton and zooplankton) died their remains would sink to the bottom of the sea.
As time went by they were covered by more dead animals, and eventually by layers of clay and sand. As the layers of clay and sand built up the remains of the tiny animals and plants were buried deeper and deeper, with the pressure, and the temperature, increasing. The increase in pressure and temperature turned the remains of the tiny animals and plants (organic matter) into the hydrocarbons that make up oil.
The oil that was formed by the compression of the organic matter travelled up inside the earth. Some oil managed to make its way up to the earth surface, and this flowed away, but some of the oil became trapped under rock that would not allow the oil to pass through it. When the oil is trapped below the surface it is known as a reservoir, and is in fact stored within porous rock. Porous rock is rock that has holes in it, rather like cheese with holes in it. The oil is trapped in the holes in the rock, unable to pass up toward the earth surface, unless a path through the rock is made to the earth surface.