Flexi Says: A combustion reaction occurs when fuel reacts quickly with oxygen (O2). The fuel that burns in a combustion reaction contains compounds called hydrocarbons. Petroleum contains a mixture of hydrocarbons. Petroleum is especially flammable because it also has high vapor pressure and low flash point. This means that a vapor (gas) layer easily forms over liquid petroleum. Fuel in the gas phase can react rapidly (and even explosively) with oxygen in the air to form flame.