(e) liquefied hydrocarbons are heavier than natural gas (methanol) and may, if available in sufficient quantities, be exploited economically before sale. If they remain in the gas stream, the calorific value of the gas current improves, which leads to an increase in the price for the producer. Some examples are butane, ethane, pentan, propane and natural gas. (i) Associated gas: the associated gas is also called dissolved gas. It is produced at the same time as crude oil. It must be disposed of at the same time as the oil by injection again, by exploitation of the oil field or by fire. Fire is considered ecologically bad because it wastes natural resources and is illegal in many countries. The associated gas is normally rich in C3-C5 hydrocarbons which can be recovered as liquefied natural gas before they are marketed.