When organic material is buried it is compressed and “cooked” because the deeper you go beneath the surface of the earth the hotter it gets. This causes the breakdown of the organic matter and the production natural gas and oil. The stages of decomposition are:
Diagenesis:
- Decomposition of biological material produces methane gas. At slightly higher temperatures and pressures the organic matter is converted to kerogen – an unorganized (amorphous) material of carbon, hydrogen, and oxygen.
Catagenesis:
- At higher temperatures and pressures kerogen is altered and the majority of crude oil is formed. During this phase and the next, the larger molecules break down into simpler molecules such as octane and propane (a process called cracking).
Metagenesis:
- In the final stage of alteration (at higher temperatures and pressures) of kerogen and crude oil, natural gas (mostly methane) is produced and residual carbon is left in the source rock.