Natural gas is a mixture of hydrocarbons with various quantities of non-hydrocarbons, which exists either in the gas phase or in solution with petroleum in natural underground reservoirs. The principal hydrocarbon component is methane. In most parts of the world, by the time the gas has been treated and distributed to consumers, it consists almost entirely of methane.

Gas produced during catagenesis usually migrates through porous rocks in the Earth's crust until it encounters a formation of non-porous rock. This non-porous rock prevents further migration of the gas, effectively trapping it in the porous rock below. The porous rock becomes a reservoir for the gas. The gas can be classified according to how it is found. Associated gas is found in conjunction with accumulated oil, either dissolved in the oil, called dissolved gas, or as a separate gaseous phase above the oil, gas-cap gas. Non-associated gas is found without accompanying oil. A reservoir of non-associated gas could arise from gas migrating to a different location than that to which oil migrated, or from formation of gas in the gas window, i.e. without oil. About 60% of the world's natural gas is non-associated.

Other sources of methane-rich gases occur in nature. Biogenic gas, produced during diagenesis, comes from the action of anaerobic bacteria on accumulated organic matter. Landfill gas is produced in the same way, but differs in that the feedstock is the organic residues of civilization, accumulated as solid waste in landfills. Excrement from humans or other animals also reacts in the same way, providing a useful source of fuel for farms or even domestic use [A]. Methane forms and accumulates in coal seams; its deliberate removal prior to mining provides another source, coalbed methane.