Natural-gas processing begins at the well head. The composition of the raw natural gas extracted from producing wells depends on the type, depth, and location of the underground deposit and the geology of the area. Oil and natural gas are often found together in the same reservoir. The natural gas produced from oil wells is generally classified as associated-dissolved, meaning that the natural gas is associated with or dissolved in crude oil. Natural gas production absent any association with crude oil is classified as “non-associated.” In 2004, 75 percent of U.S. wellhead production of natural gas was non-associated.^{[citation needed]}

Most natural-gas production contains, to varying degrees, small (two to eight carbons) hydrocarbon (any class of compound containing only hydrogen and carbon; examples include methane gas (CH₄), benzene (C₆H₆), and butane (C₄H₁₀). Although they exist in a liquid state at underground pressures, these molecules will become gaseous at normal atmospheric pressure. Collectively, they are called condensates or natural gas liquids (NGLs). The natural gas extracted from coal reservoirs and mines (coalbed methane) is the primary exception, being essentially a mix of mostly methane and carbon dioxide (about 10 percent).

Natural-gas processing plants purify raw natural gas produced from underground gas fields or extracted at the surface from fluids produced by oil wells. A fully operational plant delivers pipeline-quality natural gas that can be used as fuel by residential, commercial and industrial consumers. In the plant, contaminants are removed and heavier hydrocarbons are captured for other commercial uses. For economic reasons, however, some plants may be designed to yield an intermediate product typically containing over 90% pure methane and smaller amounts of nitrogen, carbon dioxide, and sometimes ethane. This can be further processed in downstream plants or used as feedstock for chemicals manufacturing.