Kerogen
Kerogen is a mixture of polymeric organic material from which hydrocarbons are formed with increasing geological subsidence and heating . It occurs in sedimentary rocks in the form of finely divided organic macerales and is by far the most common form of organically bound carbon in the earth's crust . It is insoluble in organic solvents, non-oxidizing acids ( HCl and HF ) and alkalis . Kerogen is divided into four types of maceral:
Type 1 Kerogen (Liptinite Type)
Liptinite is relatively rare and has a high petroleum formation potential , at low ripeness a high H / C ratio (> 1.5) and low O / C ratio (<0.1). In its composition it shows high contributions of lipid substances from algae material and bacterial residues , but few aromatics and NSO components . It is formed as oil shale in fine-grained marine and lacustrine muds .
Examples: Green River Formation , oil shale from the Messel pit , Swabian Posidonia slate .
Type 2 Kerogen (Exinit Type)
Exinit is the most widely used kerogen with a medium H / C ratio (0.8–1.5) and medium O / C ratios . It contains a high proportion of aromatics and carboxylic acids , as well as up to 14% sulfides .
Exinit is mainly formed from marine organic matter, including autochthonous phytoplankton , zooplankton and bacterial material that were deposited under reducing conditions . In some cases it can also contain allochthonous spores , pollen and resins from higher plants on the mainland.
Kerogens of this type form oil and, due to their widespread distribution, are often sources of large oil and gas fields, such as the Paris Basin .
Type 3 Kerogen (Vitrinit Type)
The vitrinite type has a low H / C ratio and a high O / C ratio at low ripeness . It contains many polyaromatics , carboxylic acids and ketones . Ester compounds are completely absent. Aliphatic groups are only present - if at all - in methyl and other short-chain groups. Long chains come from the grow and cutin of the higher plants.
Sources are basically higher plants, it often contains woody plant residues. In terms of its behavior with increasing sinking, it is very similar to coal . It is a bad oil producer and a sufficient producer of gas (especially methane ).
Examples: Douala basin .
Type 4 Kerogen (inertite type)
This kerogen consists primarily of black, opaque particles of almost pure carbon , which are in the form of inertite . Since it cannot form hydrocarbons , it is often not considered a real kerogen. It arises in the terrestrial environment from heavily oxidized material from higher plants .
literature
- Bernard P. Tissot, Dietrich H. Welte : Petroleum Formation and Occurrence . 2. Auf., Springer-Verlag, Berlin, Heidelberg, New-York, Tokyo 1984. pp. 131-159.