<P> Natural gas production in the US reached a peak in 1973, and went over a second lower peak in 2001, but recently has peaked again and is continuing to rise . </P> <P> Because of its low density, it is not easy to store natural gas or to transport it by vehicle . Natural gas pipelines are impractical across oceans, since the gas needs to be cooled down and compressed, as the friction in the pipeline causes the gas to heat up . Many existing pipelines in America are close to reaching their capacity, prompting some politicians representing northern states to speak of potential shortages . The large trade cost implies that natural gas markets are globally much less integrated, causing significant price differences across countries . In Western Europe, the gas pipeline network is already dense . New pipelines are planned or under construction in Eastern Europe and between gas fields in Russia, Near East and Northern Africa and Western Europe . See also List of natural gas pipelines . </P> <P> Whenever gas is bought or sold at custody transfer points, rules and agreements are made regarding the gas quality . These may include the maximum allowable concentration of CO 2, H 2S and H 2O . Usually sales quality gas that has been treated to remove contamination is traded on a "dry gas" basis and is required to be commercially free from objectionable odours, materials, and dust or other solid or liquid matter, waxes, gums and gum forming constituents, which might damage or adversely affect operation of equipment downstream of the custody transfer point . </P> <P> LNG carriers transport liquefied natural gas (LNG) across oceans, while tank trucks can carry liquefied or compressed natural gas (CNG) over shorter distances . Sea transport using CNG carrier ships that are now under development may be competitive with LNG transport in specific conditions . </P>

Is north sea gas a pure substance compound or mixture