Oil and gas is a fossil fuel because it was formed in the same geological time as fossils. It was formed around 100 million years ago when tiny marine plants and animals called phytoplankton and zooplankton floated on the seas surface. When these died they sank to the seabed along with other larger plants and animals that had died.

Their remains were covered with mud and silt that washed into the sea from the shore. Many layers, called sediments, were built up this way over millions of years and buried deeper and deeper. As the layers of sediment got closer to the earth’s core they were heated and put under extreme pressure. The heat and pressure gradually turned the layers of sediment into different types of rocks, some really hard and others with holes, called porous rocks.

The layers of plants and animals slowly broke down into simple substances called hydrocarbons, compounds of hydrogen and carbon. The result was oil and gas. This then seeped through the holes in the porous rocks until it reached a rock that was impermeable and could go no further where it became trapped and pooled. These pools are what oil and gas prospectors look for.

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