Natural gas is more ubiquitous than oil. It is derived from both land plants and aquatic organic matter and is generated above, throughout, and below the oil window. Thus, all source rocks have the potential for gas generation. Many of the source rocks for significant gas deposits appear to be associated with the worldwide occurrence of coal dated to Carboniferous and Early Permian times (roughly 358.9 million to 273 million years ago).
During the immature, or biological, stage of petroleum formation, biogenic methane (often called marsh gas) is produced as a result of the decomposition of organic material by the action of anaerobic microbes. These microorganisms cannot tolerate even traces of oxygen and are also inhibited by high concentrations of dissolved sulfate. Consequently, biogenic gas generation is confined to certain environments that include poorly drained swamps and bays, some lake bottoms, and marine environments beneath the zone of active sulfate reduction. Gas of predominantly biogenic origin is thought to constitute more than 20 percent of the world’s gas reserves.
The mature stage of petroleum generation, which occurs at depths of about 750 to 5,000 metres (2,500 to 16,000 feet), includes the full range of hydrocarbons that are produced within the oil window. Often significant amounts of thermal methane gas are generated along with the oil. Below 2,900 metres (9,500 feet), primarily wet gas (gas containing liquid hydrocarbons) is formed.
In the postmature stage, below about 5,000 metres (16,000 feet), oil is no longer stable, and the main hydrocarbon product is thermal methane gas. The thermal gas is the product of the cracking of the existing liquid hydrocarbons. Those hydrocarbons with a larger chemical structure than that of methane are destroyed much more rapidly than they are formed. Thus, in the sedimentary basins of the world, comparatively little oil is found below 5,000 metres. The deep basins with thick sequences of sedimentary rocks, however, have the potential for deep gas production.