Components of natural gases may come from a variety of sources, but the composition and extent of diagenesis of sedimentary organic matter probably control regional patterns of gas composition. Chemical analyses of suites of samples from different depths within single shale units indicate that the kerogen in ordinary shales may be the source of the order of 1 Tcf of methane gas and smaller amounts of carbon dioxide and nitrogen per cubic mile of shale. The process of generation is similar to coalification, as are the products.

Relative proportions of gases depend on differences in original constitution of starting materials and on the stage (i.e., early or late) of maturation. The progression is from relatively more nitrogen in early stages, to more carbon dioxide, and finally to very large quantities of methane. Other sources must be called on to account for local anomalous gases, such as those very rich in H₂S and CO₂.

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