(1) (n.) Geology. A naturally occurring mixture of hydrocarbon gases that is highly compressible and expansible. Methane [CH₄] is the chief constituent of most natural gas (constituting as much as 85% of some natural gases), with lesser amounts of ethane [C₂H₆], propane [C₃H₈], butane [C₄H₁₀] and pentane [C₅H₁₂]. Impurities can also be present in large proportions, including carbon dioxide, helium, nitrogen and hydrogen sulfide. See: Carbon Dioxide, Coal, Condensate, Dry Gas, Hydrate, Hydrocarbon, Hydrogen Sulfide, Maturity, Methane, Petroleum, Retrograde Condensation, Source Rock, Sweet, Wet Gas. (2) (n.) Shale Gas. Natural gas produced from shale reservoirs is known as shale gas. The composition of the gas stream is a function of thethermal maturity of the rock. Thermally immature rocks will contain heavier hydrocarbon components, possibly even liquid components. Overmature reservoirs typically contain appreciable quantities of carbon dioxide [CO₂]