Catagenesis

The Catagenesis is the main phase of the petroleum formation of finely divided organic material in sediments . This phase lies between diagenesis and metagenesis . The Greek term (κατα = completely and γένεσις = origin) was introduced in 1969 by the Russian petroleum geologist Nikolai Vassoevich .

Processes in catagenesis

When sediments are deposited, organic particles such as algae, remains of higher plants and bacteria are also embedded. If the oxygen content in the pore water is insufficient to oxidize these residues , they are retained and are known as kerogen . In diagenesis , rock is submerged by the overlaying of new sediments or tectonic processes and is subject to rising temperature and growing lithostatic pressure . Only at temperatures between 50 and 150 ° C and 300 to 1500  bar pressure, corresponding to a sinking of around 1500 to 4000 m, do liquid hydrocarbons and methane escape from the kerogen during catagenesis . This process is known as pyrolysis . As the temperature increases, the chain-like hydrocarbons are split more strongly, so that in the end only short-chain hydrocarbons, the basic components of natural gas , are released. The pyrolysis is facilitated by the catalytic effect of clay minerals such as montmorillonite . The process is much more dependent on temperature than on pressure. The change in the organic material at even higher temperatures is known as metagenesis .

Demarcation

The exact delimitation between diagenesis, catagenesis and metagenesis is carried out microscopically with the help of the reflectivity of certain organic particles ( vitrinite ). The property of organic particles is used to reflect more light with increasing temperature . Catagenesis is defined as the range of vitrinite reflection between 0.5 and 2%.

literature

• BP Tissot, DH Welte: Petroleum Formation and Occurrence. Springer, 1984, ISBN 3-540-13281-3 , pp. 71-72.
• NB Vassoevich et al .: Principal phase of oil formation. In: International Geological Reviews. 12, 1970, pp. 1276-1296.