<P> In the past, the natural gas which was recovered in the course of recovering petroleum could not be profitably sold, and was simply burned at the oil field in a process known as flaring . Flaring is now illegal in many countries . Additionally, higher demand in the last 20--30 years has made production of gas associated with oil economically viable . As a further option, the gas is now sometimes re-injected into the formation for enhanced oil recovery by pressure maintenance as well as miscible or immiscible flooding . Conservation, re-injection, or flaring of natural gas associated with oil is primarily dependent on proximity to markets (pipelines), and regulatory restrictions . </P> <P> Natural gas can be indirectly exported through the absorption in other physical output . A recent study suggests that the expansion of shale gas production in the US has caused prices to drop relative to other countries . This has caused a boom in energy intensive manufacturing sector exports, whereby the average dollar unit of US manufacturing exports has almost tripled its energy content between 1996 and 2012 . </P> <P> A "master gas system" was invented in Saudi Arabia in the late 1970s, ending any necessity for flaring . Satellite observation, however, shows that flaring and venting are still practiced in some gas - extracting countries . </P> <P> Natural gas is used to generate electricity and heat for desalination . Similarly, some landfills that also discharge methane gases have been set up to capture the methane and generate electricity . </P>

When was the smell added to natural gas