Formation of fossil fuels. Computer artwork showing how fossil fuels, such as coil, oil and gas, are formed. Fossil fuels are rich in hydrocarbons and form over millions of years from organic remains. It is thought that the process starts when large amounts of plankton debris forms a mud on the seabed rich in organic material. Over millions of years, layers of sediment build up on top of this organic layer, compressing it and heating it and eventually turning it into fossil fuels.