How oil is formed

How oil is formed?

We know that, we usually extract oil from oceans and many time question rises in mind, how oil is formed? Let understand oil formation in detail.

How oil is formed - a short answer?

In sedimentary ocean basins, the oil and gas normally forms. Organic plant and animal material which is called as dead plankton gets deposited along with mud, sand and other natural sediments. In this initial stage the plankton/sediment layer is named as sapropel. With the rise in temperature the sapropel is converted into the kerogen. With the increase in temperature and more sediment pressure the kerogen converts to petroleum. This formed oil further trapped in sandstone reservoir.
Oil rig for oil extraction
Oil rig for oil extraction

Oil formation in detail

  • The beginning of formation of fossil fuel started at approximately 400 million years ago.

  • Dead plankton i.e. organic plant and animal materials sinks to the floor of ocean and gets deposited.

  • A fine mud also mixed with sinked plankton at the ocean floor. The resulting sediment because of plankton, mud and anaerobic bacteria is further called as sapropel.

  • Many layers of new sediments buried the sapropel, which increases the pressure of sapropel and rise the temperature of the sapropel layer.

  • With the help of anaerobic bacteria and chemical processes, at approx. 50°C the sapropel is converted into the kerogen.

  • Because of tectonic movements in the basin, the sediment layers get folded. Now more sediments fall on top of kerogen, this increases the pressure and temperature on the kerogen.

  • At 80°C to 120°C the kerogen converts into liquid petroleum.

  • The oil came out from of the kerogen into sandstone reservoir which is also called as storage beds.

  • Natural gas forms in between temperature range 120°C to 150°C and starts to flow from the kerogen to the sandstone reservoir.

  • Salt water also migrate into the sandstone reservoir and lie under the oil, in some sedimentary basins or storage beds.

  • When the liquid or gas moves through the pores and spaces of permeable rock results in the formation of oil and gas. Which is collected in the pore spaces under an retentive trap.


It took many-many years to form oil from dead plants and animals with the help of proper temperature, pressure and floating condition. Now we understand, how oil is formed? This natural formation process is very long, that’s why if once oil extinct from earth surface we can’t do any thing to get it back.


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