This article will explain the overall of how to transform organic matter into petroleum. Chemical processes will take place in order to transform organic matter preserved in source rocks into petroleum. This can be called a cooking process. Three phases regarding maturation of the organic matter to form petroleum are diagenesis, categenesis and metagenesis.
This occurs at shallow surfaces and begins during initial deposition. It normally takes place from a shallow depth to 1,000 m and the temperature range is less than 60 C (140 F). Non-biogenic reaction and biogenic decay aided by bacteria turns organic matter to Methane (CH4), Carbon Dioxide (CO2), water (H2O) and Kerogen. Kerogen is a precursor to the creation of petroleum. Types of kerogen depend on the original type of organic matter.
Chemical components of kerogen consist of hydrogen, carbon and oxygen with a trace of nitrogen and sulphur. Three types of kerogen which are categorized by depositional environment are as follows;
Type 1 – Fine algae in fresh water lake: Type1 has the highest H:C ratio and it tends to be oil with a low amount of gas.
Type 2 – Single cell plankton, algae and bacteria in marine environment: Type 2 has a moderate H:C ration about 1.4 and this is the main type of organic matter. This type tends to have a mixture between oil and gas.
Type 3 – Organic material from swamp area: The organic material comes from land plants, spores and pollen. It has H:C ratio of less than 1.0 and this will be mostly dry gas.
With an increasing depth of deposition, pressure and temperature increase and bacteria cannot live in this environment. The critical temperature is around 60 C (140 F) that will start to crack molecules of kerogen and then oil begins to form. The deeper the depth of burial, the higher the temperature that will crack molecules of oil and heavy oil will become lighter oil or gas.
This phase occurs at very high temperature and pressure. Gas molecules will be broken down and become only carbon in the form of graphite. Figure 1 illustrates petroleum maturation process.
Figure 1 – Summary of oil formation process
(Ref Image: http://www.ems.psu.edu/~pisupati/ACSOutreach/Petroleum_2_files/image007.gif)
Figure 2 has shown that significant oil generation occurs between 60 C (140F) and 160C (320 F), whilst significant gas generation occurs between 160C (320 F) and 225 C (437 F). Above 225 C (437 F) only graphite can exist.
Figure 2 – Organic Maturity
(Ref Image http://2.bp.blogspot.com/-uLt9tdgeTFY/VX0BzNKbSlI/AAAAAAAAAp8/_M6wwVJeUjs/s1600/process_chart.gif)