What is petroleum in geology ?

Petroleum : definition

Bitumen dripping from a fracture - Mines des Rois, Dallet, France
Petroleum is a rock in the form of a mineral oil, composed of 3 natural hydrocarbons : paraffinic carbides, naphthenic hydrocarbons, and aromatic hydrocarbons, mixed with various oxygenated, nitrogenous or sulphurous compounds.

The raw material for oil is essentially zooplankton, or animal plankton, sometimes associated with a little phytoplankton. An oil basin is a sedimentation basin where sludge consisting of fine clay particles and at least 2% organic matter is deposited. The conservation of organic matter requires oxygen-poor waters (reducing) and rapid burial, and therefore fairly strong subsidence. The temperature increases with the burial and leads to transformations of the organic matter : we first observe the formation of kerozene, large insoluble organic molecules dispersed in the clay, which then breaks up into solid hydrocarbons, liquids (oils) and gaseous.

The formation of oil is optimal in a temperature range between 60°C and 120°C and called the oil window. At the rate of a burial of 1 cm per century, which is a rapid rate, it takes 15 million years between the deposition of zooplankton and the appearance of liquid hydrocarbons.
Former oil drilling well from the 1930's rediscovered in the Trois-Fées ZAC in 2012 (Cébazat, Puy-de-Dôme, France) during earthworks.
© Fred Marquet - La Montagne

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