The first fraction to separate out of petroleum is natural gas. Natural gas is a mixture of hydrocarbons that exist in the gaseous phase at atmospheric conditions. It is composed primarily of methane (CH4), with smaller amounts of ethane (C2H6), propane (C3H8), and butane (C4H10). Natural gas is separated from petroleum through a process called fractional distillation, which is based on the different boiling points of the various hydrocarbons present in petroleum.