'''Natural gas''' is a [[fossil fuel]] that can be a byproduct of [[petroleum]] extraction and production (known as associated natural gas), extracted from separate natural gas fields (non-associated natural gas), or extracted from other areas such as coal-beds or shale (continuous or unconventional gas)<ref>U.S. Geological Survey. <i>Natural Gas Production in the United States</i>. 2002. http://pubs.usgs.gov/fs/fs-0113-01/fs-0113-01.pdf</ref>. It consists of about 75% [[methane]], and the rest is made up of [[ethane]] (15%) and other [[hydrocarbon|hydrocarbons]],<ref>http://scifun.chem.wisc.edu/chemweek/methane/methane.html</ref> but this varies by region and degree of refinement. It is naturally oderless, so odor is added to make a leak detectible.