Natural gas is one of the world's most important fossil fuels. The denotation of "fossil fuel" comes from the belief that most scientists share, that natural gas and other "fossil fuels" were formed from the remains of tiny sea animals and plants that perished 200-400 million years ago. Scientists believe that when these tiny organisms died, they sank to the bottom of the ocean, where over many years, the pressure created by the ever-thickening layers of ocean floor combined with the earth's heat to change the molecular structure of the organic remains into deposits of petroleum and natural gas.

More: continued here