How does natural gas form from land plants and aquatic organic matter?

Natural gas is derived from both land plants and aquatic organic matter, which serve as the source material for its formation through two primary processes: biological and thermal.1

The Biological Stage

In the immature stage, anaerobic microbes decompose organic material in environments lacking oxygen and low in sulfate, such as swamps, bays, and lake bottoms. This process produces biogenic methane, often referred to as marsh gas. This biogenic origin accounts for approximately 20 percent of the world’s known gas reserves.2

The Thermal Stage

As organic matter is buried deeper within sedimentary basins, it undergoes thermal transformation.34
* The Oil Window: At depths between approximately 750 and 5,000 metres, organic matter (specifically kerogen) is subjected to temperatures that produce a range of hydrocarbons, including significant amounts of thermal methane.24
* Postmature Stage: Below about 5,000 metres, where oil is no longer stable, the primary product is thermal methane gas, created through the thermal cracking of existing liquid hydrocarbons.34

Because natural gas can form above, throughout, and below the oil window, it is more ubiquitous than oil.15