<P> In petroleum production gas is often burnt as flare gas . The World Bank estimates that over 150 cubic kilometers of natural gas are flared or vented annually . Before natural gas can be used as a fuel, most, but not all, must be processed to remove impurities, including water, to meet the specifications of marketable natural gas . The by - products of this processing include: ethane, propane, butanes, pentanes, and higher molecular weight hydrocarbons, hydrogen sulfide (which may be converted into pure sulfur), carbon dioxide, water vapor, and sometimes helium and nitrogen . </P> <P> Natural gas is often informally referred to simply as "gas", especially when compared to other energy sources such as oil or coal . However, it is not to be confused with gasoline, especially in North America, where the term gasoline is often shortened in colloquial usage to gas . </P> <P> Natural gas was discovered accidentally in ancient China, as it resulted from the drilling for brines . The natural gas was used to boil brine to make salt . Natural gas was first used by the Chinese in about 500 BC (possibly even 1000 BC). They discovered a way to transport gas seeping from the ground in crude pipelines of bamboo to where it was used to boil salt water to extract the salt, in the Ziliujing District of Sichuan . The world's first industrial extraction of natural gas started at Fredonia, New York, United States, in 1825 . By 2009, 66 000 km3 (or 8%) had been used out of the total 850 000 km3 of estimated remaining recoverable reserves of natural gas . Based on an estimated 2015 world consumption rate of about 3400 km3 of gas per year, the total estimated remaining economically recoverable reserves of natural gas would last 250 years at current consumption rates . An annual increase in usage of 2--3% could result in currently recoverable reserves lasting significantly less, perhaps as few as 80 to 100 years . </P> <P> In the 19th century, natural gas was usually obtained as a by - product of producing oil, since the small, light gas carbon chains came out of solution as the extracted fluids underwent pressure reduction from the reservoir to the surface, similar to uncapping a soft drink bottle where the carbon dioxide effervesces . Unwanted natural gas was a disposal problem in the active oil fields . If there was not a market for natural gas near the wellhead it was prohibitively expensive to pipe to the end user . </P>

When was natural gas first used for energy