How was petroleum formed?

Just how petroleum was formed is still something of a mystery. Petroleum – both liquid oil and natural gas – does not contain any obvious clues that tell us about its origins. In contrast, we can follow the development of coal because we can find traces of prehistoric plant life in coal fragments at all stages of their formation.

All petroleum products are “complete” products and each one is fully developed in the earth and different from all the others. Some petroleum is thick and waxy and it could be used as axle grease without any changes. But some petroleum is so light that it could be used to fuel a diesel engine.

Some scientists, known as petroleum geologists, have developed a theory about the formation of this important energy source. The theory states that petroleum was formed from decayed organic material, both animal and vegetable, that lived close by – or within - the ocean. The dead material was built up on the ocean floor. Then certain chemical al­teration processes turned the dead material (debris) into fatty and waxy substances.

Over time, the buried substances - dead marine animals and plants – became petroleum. It is estimated that this process (the conversion of decayed organic material into petroleum) took up to 10 million years. The creation of petroleum occurred sometime between 200 and 400 million years ago.

Above - Living matter under the ocean surface.


Above - As plants and animals died and the dead material built up in the muddy sediment in the stagnant parts of the world's oceans, it was quickly covered by sediment, preventing oxidation.


Above - Chemical alteration and microbiotic processes changed the dead matter into fatty and waxy substances.


Above - Substances became more deeply buried by layers of sediment, and as the layers increased, the pressure and temperature increased, causing petroleum to form. This process took millions of years.
Source: Western Power’s World of Energy.

Another, but less-accepted theory, is that volatile fluids have “cooked out” from the earth’s rock mantle when the planet was forming. The fluids work their way to the upper layers of the earth's crust. Petroleum is formed in rocks that are fine-grained and soft. There are great forces of water and pressure acting on these porous rocks. This pressure eventually forced the petroleum from the pores and cracks of the rocks.

The petroleum was gradually “squeezed” into the water’s depths and it continued to rise until it reached a barrier known as a “seal”. The petroleum then moved along the barrier until it reached the highest point, known as a “trap”. Then it could go no further. It began to accumulate as more and more was squeezed out of the porous rock. This movement or “migration”, so long and so slow, often means that petroleum is discovered far away from the place where it was formed, and in a great variety of rocks. Often, it is found in large amounts in places where rocks have been forced by earth movements into the form of a dome. Petroleum rises into the dome and it is trapped there. The rocks that capture the petroleum are called reservoir rocks.

A typical petroleum deposit is made up of three layers:

• There is a zone where the pores of the rocks are filled with natural gas.
• Underneath the gas is a zone where the rocks have their inter­stices (or pores) full of oil. This oil also contains dissolved gas, as the gas is under great pressure (Similar to bottled gas, propane and butane gases which are liquids through pressure.)
• Beneath the oil is water.

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