Crude oil is a complex mixture of hydrocarbons with minor proportions of other chemicals such as compounds of sulphur, nitrogen and oxygen. To use the different parts of the mixture they must be separated from each other. This separation is called refining. Crude oil 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 - so viscous that they are difficult to pump from the ground.
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 . About hydrocarbons Hydrocarbons vary in structure depending on the number of carbon atoms and the way in which the hydrogen atoms combine with them. Hydrocarbons can be arranged as straight chains, branched chains, branched chains or closed rings. There are two main chemical families of hydrocarbons - the alkanes and the alkenes .