Fossil natural gas can be "associated" (found in oil fields) or "non-associated" (isolated in natural gas fields), and is also found in coal beds (as coal bed methane). It sometimes contains significant quantities of ethane, propane, butane, and pentane—heavier hydrocarbons removed prior to use as a consumer fuel—as well as carbon dioxide, nitrogen, helium and hydrogen sulfide. Natural gas is commercially produced from oil fields and natural gas fields. Gas produced from oil wells is called casing head gas or associated gas. The natural gas industry is producing gas from increasingly more challenging resource types: sour gas, tight gas, shale gas and coal bed methane.
The world's largest proven gas reserves are located in Russia. Russia is also the world's largest natural gas producer, through the Gazprom company. Major proven resources (with year of estimate) (in billion cubic meters) are world 175,400 (2006), Russia 47,570 (2006), Iran 26,370 (2006), Qatar 25,790 (2007), Saudi Arabia 6,568 (2006) and United Arab Emirates 5,823 (2006).
The world's largest gas field is Qatar's offshore North Field, estimated to have 25 trillion cubic meters of gas in place—enough to last more than 200 years at optimum production levels. The second largest natural gas field is the South Pars Gas Field in Iranian waters in the Persian Gulf. Connected to Qatar's North Field, it has estimated reserves of 8 to 14 trillion cubic meters of gas.
Because natural gas is not a pure product, when non-associated gas is extracted from a field under supercritical (pressure/temperature) conditions, it may partially condense upon isothermic depressurizing--an effect called retrograde condensation. The liquids thus formed may get trapped by depositing in the pores of the gas reservoir. One method to deal with this problem is to reinject dried gas free of condensate to maintain the underground pressure and to allow reevaporation and extraction of condensates.
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