Metagenesis (geology)

The metagenesis is the final stage of the formation of hydrocarbons of finely divided organic material in sediments . This phase lies between catagenesis and metamorphosis . The term originating from the Greek (μετα = after and γινεσις = origin) was introduced in 1969 by the Russian oil geologist Nikolai Vassoevich .

Processes in Metagenesis

When sediments are deposited, organic particles such as algae, remains of higher plants and bacteria are embedded, which are retained in the pore water of the sediments if the oxygen content is too low . This organic material is known as kerogen and, if it contains more than 0.5%, marks a petroleum parent rock . First, in the event of tectonic subsidence and temperatures above 50 ° C, catagenesis splits off crude oil from the kerogen. If the temperature continues to rise above 150 ° C and a lithostatic pressure of more than 1500 bar pressure, methane and carbon dioxide , the main components of natural gas , escape from the kerogen during metagenesis . At the same time, changes take place in the minerals , including the splitting off of crystal water in clay minerals . Therefore the metagnesis of the organic material corresponds to the beginning metamorphosis ( anchimetamorphosis ) in rocks with silicate minerals. At even higher temperatures during metamorphosis, the kerogen is converted into crystalline graphite .

Demarcation

The exact delimitation between catagenesis, metagenesis and metamorphosis 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 . Metagenesis is defined as the range of vitrinite reflection between 2.0 and 4.0%.

literature

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