Diagenesis-catagenesis-metagenesis
The maturation process needs several stages, they are:
Diagenesis, this stage is
the decomposition process occurs and there is a reduction in the oxygen content of organic material with abiotic reactions that produce methane and carbon dioxide kerogen. At this stage the organic material is still immature.
Catagenesis, burial process continues and the fluid content of hydrocarbons starts out with an initial form of fluid and then the temperature rise resulting gas. At this stage, the percentage of H / C decreases but the O / C is not too reduced.
Metagenesis, the process continues as a large burial pressure and temperature almost reached metamorphic phase. The end result can be either graphite.