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Marine or lacustrine sediments may be considered as subaquatic soils, in which organic matter is destroyed, transformed, or preserved to a certain extent, according to the conditions of sedimentation. Small amounts of hydrocarbons can be found, inherited from living organisms, either directly (like certain n-alkanes of high molecular weight) or through an early transformation (like steroid and triterpenoid types). When humic material is present in muds, it may represent a large amount of total organic matter and constitute the support of various functional types of compounds. The whole is able to evolve by loss of functional groups towards several types of kerogen, according to depositional environment.
Elevation of temperature and pressure during burial of sediments results in a physicochemical transformation of the various kerogen types, along different evolutionary paths. The products formed include oil, gas, and other compounds like water and carbon dioxide. Nature and abundance of these products depend on particular evolutionary path and grade of diagenesis. The bulk of oil is formed at that stage of diagenesis in petroleum source rocks: the greatest rate of oil generation can be identified on the evolutionary paths, and is followed by gas generation as burial increases. Other types of organic matter produce little petroleum, but produce methane at great depth.
Identification of the nature of kerogens resulting from depositional environment and of the grade of diagenesis resulting from the burial history allows the evaluation of the petroleum potential of a given formation.
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