Most coal originated as peat in ancient swamps created many millions of years ago. Crude oil is a complex mixture of hydrocarbons with small amounts of other chemicals such as sulphur. The crude oil is useless as a mixture and must be sent to an oil refinery to be separated. Crude oils from different parts of the world, or even from different depths in the same oilfield, contain different mixtures of hydrocarbons and other compounds. This is why they vary from light coloured volatile liquids to thick, dark oils.
Natural gas is a mixture of hydrocarbons with small molecules.
These molecules are made of atoms of carbon and hydrogen. For example, natural gas used in the home is mainly methane , CH 4 . Hydrocarbons only contain hydro gen and carbon atoms. There are two main chemical families of hydrocarbons - the alkanes and the alkenes . Thousands of synthetic products can be manufactured from hydrocarbons with many different properties.