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In some areas, the decomposing materials were covered by ancient seas, then the seas dried up and receded. During the millions of years that passed, the dead plants and animals slowly decomposed into organic materials and formed fossil fuels. Different types of fossil fuels were formed depending on what combination of animal and plant debris was present, how long the material was buried, and what conditions of temperature and pressure existed when they were decomposing. For example, oil and natural gas were created from organisms that lived in the water and were buried under ocean or river sediments. Long after the great prehistoric seas and rivers vanished, heat, pressure and bacteria combined to compress and 'cook' the organic material under layers of silt.
In
most areas, a thick liquid called oil formed first, but in deeper, hot
regions underground, the cooking process continued until natural gas
was formed. Over time, some of this oil and natural gas began working
its way upward through the earth's crust until they ran into rock
formations called 'caprocks' that are dense enough to prevent them from
seeping to the surface. It is from under these caprocks that most oil
and natural gas is produced today. The same types of forces also
created coal, but there are a few differences. Coal formed from the
dead remains of trees, ferns and other plants that lived 300 to 400
million years ago. In some areas, such as portions of what-is-now the
eastern United States, coal was formed from swamps covered by sea
water. The sea water contained a large amount of sulfur, and as the
seas dried up, the sulfur was left behind in the coal.