natural gas . (a) Hydrocarbons that exist as a gas or vapor at ordinary pressures and temperatures. Methane is the most important, but ethane, propane, and others may be present. Common impurities include nitrogen, carbon dioxide, and hydrogen sulfide. Natural gas may occur alone or associated with oil. Syn: gas. (b) Gaseous hydrocarbons trapped in the zone of groundwater saturation, under pressure from, and partially dissolved in, underlying water or petroleum (Meinzer, 1923, p.21). Cf: subsurface air; included gas.