Oil and other natural hydrocarbons, such as natural gas, are formed by the decomposition of organic matter under high pressure and high temperature in a geological process that lasts hundreds of thousands to several million years.
This is the accepted and widespread idea among scientists and the general population, and this is what happens in nature, as shown by the study of biomarkers found in oil.
However, in the laboratory Petroleum-like hydrocarbons have been obtained in a matter of dayseven minutes, although it is not an economically very profitable procedure nor does it avoid the pollution problems derived from its combustion.
How is oil formed?
Oil is formed mainly from sedimentary deposits of the ocean floor. When plankton, algae and other marine organisms die, they fall to the seabed and form accumulations of decomposing organic matter along with mineral deposits.
In the subduction zones of the tectonic plates, these sedimentary clusters sink to deep layers of the earth's crust, being subjected to a progressive increase in pressure and temperature.
Proteins and carbohydrates, including lignin and cellulose, are rapidly broken down to amino acids and sugars. Another fraction of the organic matter, mainly the lipidsare preserved in sediments and contain precursors to substances found in crude oil.
As more and more layers of sediment accumulate, the lower layers are compacted by the pressure rise. And when the pressure increases, it also goes increasing temperature.
Heat catalyzes the transformation of these organic molecules into much more complex and heavier molecules called kerogens. The kerogens, together with the lipids preserved in the sediments, constitute the raw material that will become oil.
When kerogens are buried to a depth of 1.5 – 5 kmthe temperature reaches 30 – 150ºC. Over the course of several million years, the carbon bonds in kerogens break down, a process known as crackingand the simple hydrocarbons that we know as oil are formed.
Methane, propane, asphaltenes, or propylene are all examples of hydrocarbons that can be found in crude oil. Some of these hydrocarbons are liquid and others are gaseous. The gases form what is known as natural gas and the liquids oil.
When kerogens are transformed into gas or liquid, an expansion occurs due to the greater volume they occupy in gaseous and liquid states. The expansion generates an increase in pressure until eventually surrounding rocks fracture and natural gas or oil can escape and form pockets in areas where the pressure is lower.
Usually, oil rises through oil-permeable rocksfor instance sandstone type rocksuntil they find an impermeable rock layer that traps it, mainly clay rocks.
The entire process can take hundreds of thousands to several million yearseven tens of millions of years, depending on the amount and speed of sedimentation of organic matter and the speed of burial and tectonic movements.
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