SOURCE ROCK AND KEROGEN
In the process of oil and gas exploration, we must know elements of the petroleum system because it’s very important to help exploration process. The petroleum system consists of source rocks, reservoir rocks, cap rocks, trap, and migration. Each system has characteristic which can help exploration activities. From the five systems, we already know that source rock as the trigger how petroleum systems can work. Why does the source rocks become an important part of the systems?
Source rocks is the origin of petroleum. Its composition contains an organic material which has elements like C, H, O, N, S, and P. The organic material usually called as dispersed organic matter (DOM). Occurence of source rocks from accumulation several organic source into reductive environment of deposition and low energy so it has limited O2 circulation. Therefore, the grain size of the source rocks have a small size as shale. In the dynamics of rift basin formation, the formation process of source rocks are in the stage of synrift with lacustrine environment. The organic material can be humic and sapropelic. After the supply of organic mineral, there is burial process with increasing pressures and temperatures. This stage is the part that affects maturity on petroleum. The maturation process needs several stages, there are consists of :
- 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.
The result of source rocks maturation will generate kerogen. Kerogen is organic material in sedimentary rocks which are capable of producing oil and gas after the heating process. Kerogen is composed of four types, such as type I, II, III, and IV. Maturity of kerogen can be seen from the percentage of H / C and O / C on the van Krevelen diagram. Van Krevelen split three kerogen types, namely:
- Type I: a primary aliphatic nature and generally derived from algae. This type has the potential of forming gas and oil are very large.
- Type II kerogen is dominated by Alicyclic (Naphthenic) mainly formed by nature and marine organic material (plankton) in the reductive environment. Oil-forming potential is high but not as high as type I.
- Type III kerogen primary aromatic nature, formed on land rich in plants. Kerogen is similar to humic coal. Formation of low oil. Primarily formed hydrocarbon gas cleaning.
There are also kerogen type IV (inertinit) which can’t react anymore and composed of reworked organic matter.