Diagenesis is a process of compaction under mild conditions of temperature and pressure. When organic aquatic sediments (proteins, lipids, carbohydrates) are deposited, they are very saturated with water and rich in minerals. Through chemical reaction, compaction, and microbial action during burial, water is forced out and proteins and carbohydrates break down to form new structures that comprise a waxy material known as “kerogen” and a black tar like substance called “bitumen”. All of this occurs within the first several hundred meters of burial.
The bitumen comprises the heaviest components of petroleum, but the kerogen will undergo further change to make hydrocarbons and, yes, more bitumen…
Step 2: Catagenesis (or “cracking”) turns kerogen into petroleum and natural gas
As temperatures and pressures increase (deeper burial) the process of catagenesis begins, which is the thermal degradation of kerogen to form hydrocarbon chains. Importantly, the process of catagenesis is catalyzed by the minerals that are deposited and persist through marine diagenesis. The conditions of catagenesis determine the product, such that higher temperature and pressure lead to more complete “cracking” of the kerogen and progressively lighter and smaller hydrocarbons. Petroleum formation, then, requires a specific window of conditions; too hot and the product will favor natural gas (small hydrocarbons), but too cold and the plankton will remain trapped as kerogen.
This behavior is contrary to what is associated with coal formation. In the case of terrestrial burial, the organic sediment is dominated by cellulose and lignin and the fraction of minerals is much smaller. Here, decomposition of the organic matter is restricted in a different way. The organic matter is condensed to form peat and, if enough temperature (geothermal energy) and pressure is supplied, it will condense and undergo catagenesis to form coal. Higher temperatures and pressures, in general, lead to higher ranks of coal.
Source : http://www.ems.psu.edu/~pisupati/ACSOutreach/Petroleum_2.html
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