Petroleum is a fossil fuel. It is called a fossil fuel because it was formed from the remains of tiny SEA plants and animals that died millions of years ago. When the plants and animals died, they sank to the bottom of the OCEANS. They were buried by thousands of feet of sand and silt.

Over time, this organic MIXTURE was subjected to enormous pressure and heat as the LAYERS increased. The mixture changed chemically, breaking down into compounds made of hydrogen and carbon atoms—hydrocarbons. Finally, an oil-saturated rock—much like a wet household sponge—was formed.

All organic material does not turn into oil. Certain geological conditions must exist within the oil-rich rocks. First, there must be a trap of non-porous rock that prevents the oil from seeping out, and a seal (such as salt or clay) that keeps the oil from rising to the SURFACE. Even under these conditions, only about two percent of the organic material is transformed into oil.

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