Biogenic theory

Most geologists view crude oil, like coal and natural gas, as the product of compression and heating of ancient organic materials over geological time scales. According to this theory, it is formed from the decayed remains of prehistoric small marine animals and algae.

One of the most prolific periods of oil production was during the Cretaceous. During this time, marine algae died and built up in great quantities on the seafloor. When this organic material is buried, it is heated and put under pressure. If this continues to the right temperature and pressure, then oil is produced. The Persian Gulf region is rich in oil because it contains large quantities of rock from this time period.

Over millennia this organic matter, mixed with mud, is buried under thick sedimentary layers of material. The resulting high levels of heat and pressure cause the remains to metamorphose, first into a waxy material known as kerogen, and then into liquid and gaseous hydrocarbons in a process known as catagenesis. Because hydrocarbons are less dense than the surrounding rock, these migrate upward through adjacent rock layers until they become trapped beneath impermeable rocks, within porous rocks called reservoirs. Concentration of hydrocarbons in a trap forms an oil field, from which the liquid can be extracted by drilling and pumping.

Geologists also refer to the "oil window". This is the temperature range that oil forms in-below the minimum temperature oil does not form, and above the maximum temperature natural gas forms instead. Though this corresponds to different depths for different locations around the world, a “typical” depth for the oil window might be 4 - 6 km. Note that oil may be trapped at much shallower depths, even if it is not formed there. Three conditions must be present for oil reservoirs to form: a rich source rock, a migration conduit, and a trap (seal) that concentrates the hydrocarbons.

The reactions that produce oil and natural gas are often modeled as first order breakdown reactions, where kerogen breaks down to oil and natural gas by a large set of parallel reactions, and oil eventually breaks down to natural gas by another set of reactions.

Дата добавления: 2015-07-21