Eszterházy Károly College Faculty of Natural Sciences
The starting material for the formation of petroleum and natural gas is the organic matter of dead organisms. In the process the elements which are made up the organisms such as protein, fat and carbohydrate molecules are decomposed into elements(C,H,N,O), to build up hydrocarbon molecules beside the increased temperature and pressure conditions. As seen above the starting materials: algae with high-protein content or materials made of dead animals, are suitable for hydrocarbon formation. The accumulation of organic matter can be occur in oxygen-poor environment similar to the formation of coal. Such conditions may occur in inland seas or in isolated lagoons. The reductive environment of sediment formation is good for the persistence of organic materials, on the other hand because of the lack of oxygen, there are no benthos moulds which are consumed it. The burial of organic matter create putrid sludge (sapropel) which will develop to the main composite of dark gray bituminous rock, petroleum, natural gas.
With the increased burial the transformation of organic matter occurs in the following sections.
Digenesis: Geopolimers are formed by biopolymers and the organic material converted into kerogen. Kerogen is a transition state between the organic matter and hydrocarbons. The organic origin wreckage is recognizable under the microscope., but it is different from the organic material because it is not soluble in organic solvents. At the beginning the decomposition is carried out by bacterial, so biogenic methane is formed , but it escapes into the atmosphere. The digenesis runs until 60 ° C (1-2 km depth).
Catagenesis: The separation of petroleum and natural gas stars from small carogen droplets. This stage lasts from 60 to 175 ° C, which is around 4 km depth corresponding to the maximum. The phase is also called the oil-window, referring to the separation of petroleum.
Metagenesis: The direct separation from carogen is ceased. Only methane is formed with the thermal transformation of hydrocarbons. Temperature has a crucial role in the transformation, the role of time and pressure is subordinated. The intensity of hydrocarbon formation has exponential relation with the temperature and linear relation with the time.
Due to the layer charging pressure the separated(from the parent rock) petroleum and natural gas is starting to migrate. The migration has two stages: primary and secondary migration. The primary migration, is the migration from the source rock, which lasts until they reach to the storage rock. Layer charging, ie, the effects of compaction. The secondary migration is the wandering from the storage rocks, that is the accumulation which takes until the trapping. The buoyancy (the hydrocarbons lighter specific gravity than water),occurs due to the capillary pressure (the pores size is microscopic as well as the small channels between them) and the hydrodynamic effect (formation water or groundwater flow).