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Where does crude oil originate from?

Although there are different debates around the origin of crude oil and natural gas, according to the most accepted theory these hydrocarbons appear through the decomposition of large quantities of organic residues in an oxygen free environment, which largely occurs during sediment formation under the sea. The organic material piles up with inorganic fine materials(clay, marl).

The growing layer of residue becomes hydrocarbon under the weight of high pressure and high temperature while due to high pressure the materials move up in the porous structure of the rocks. This migration lasts until the substance reaches a non-impermeable layer, where it halts, and gets separated: in the bottom there is water, the oil appears above it and if there is enough gas than natural gas arises on the top.

Since the oil is not located in the place of origin - because of the migration - but in the accumulative rock, subsequently it is difficult to determinate where it was formed. The major oil sites from the current economic utilization point of view originate from the tertiary period in the history of Earth (tertiary, about 66-2 million years ago).

As far as it concerns the tropical rain forests, they played a significant role in the appearance of coal. The coal, originates from the decomposition of organic matter of vegetal origin in anaerobic (oxygen free) conditions. In the history of Earth the organic materials for this purpose arised in large quantities primarily in the marsh forests of tropical and subtropical areas.

Of course, the Earth's climate was not uniform in the earlier periods of history either. Due to the migration of continents the former coal generating tropical areas are not necessarily in the same place where they used to be. The processes modifying the surface and the wildlife always reflected and still reflect the climate adequate to the location of the continents, which changed continuously due to the migration of the land.

Source: National Geographic Online (2006), the original article here
Published in the consent of National Geographic Online.

Translated by Mónika Galántai

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