How Oil and Gas Deposits Are Formed

Deep beneath the earth's surface, oil and methane gas are formed from organic matter that come from dead animals and plants. They take tens of millions of years to form and require a specific set of physical, chemical, and geographical conditions.

The formation of a deposit
© Keblow

Where Are Deposits Found?

Oil and gas are made of molecules that contain hydrogen and carbon atoms - known as hydrocarbons.











When exposed to air, these molecules come into contact with oxygen and are consumed by aerobic bacteria (microorganisms that live in environments where air is present). This contact triggers various chemical reactions.  First, the hydrocarbons are oxidized by oxygen and eaten by bacteria (biodegradation), which converts them into water and carbon dioxide.

Oil and gas deposits are located hundreds of meters underground.

So, the reason why there are no oil deposits on the earth's surface is because it is more or less in direct contact with the atmosphere. To find liquid or gas hydrocarbon deposits, we need to drill hundreds of meters below ground.

Oil and gas deposits can be found at any depth underground.

False. There are no hydrocarbons below a depth of 10 km because they are destroyed by the high underground temperature - every 100 meters or so, it increases by an average of 3°C.

Where Do Oil and Gas Come From?

Hydrocarbons are the result of the transformation of organic matter from dead plants or animals tens of millions of years ago.

When a living organism dies above ground, it is generally recycled in two different ways:

   • it is either eaten by predators, scavengers or bacteria.

or

   • the remaining matter is oxidized when exposed to ambient air.  Hydrogen and carbon atoms contained in the matter combine with oxygen atoms present in the air. Thus, the organic matter is transformed into water (H₂O) and carbon dioxide (CO₂) that feeds the growth of new plants.

However, a tiny proportion of this organic mass - about 0.1% - escapes from this cycle. Transported by currents, this matter sometimes sinks to the bottom of the sea. It is partly preserved in this poorly oxygenated environment, well away from tidal currents. It mixes with mineral matter (clay particles and very fine sand) and dead marine plankton (microscopic animals and plants). This mixture is transformed into dark, foul-smelling mud by anaerobic bacteria - microorganisms that don't need air to survive.

Over time, this mud accumulates and hardens. Mud that contains at least 1-2% of organic matter can be transformed into source rock which eventually produces oil and gas deposits.
While this percentage may seem low, the site must have one or more exceptional features:

   • A hot climate that favors the development of large quantities of plankton

   • A location near the mouth of a major river carrying a lot of plant debris

   • No nearby mountains, as these could limit the volumes of mineral sediments within the rock.

Nevertheless, as long as the rock remains on the sea bed, it cannot produce oil.

Sedimentation, a Process that Takes Millions of Years

The weight of accumulating sediment pushes the source rock further beneath the earth's crust -from a few meters to a few hundred meters- every million years or so. This gradual sinking, which can reach up to 8000 meters, is called subsidence and leads to the formation of a sedimentary basin. A sedimentary basin is formed from superimposed rocky layers made up of sediments that have accumulated over millions of years.

By sinking into the ground, the source rock is subject to ever increasing temperatures. At the same time, the organic matter that makes up the rock is crushed by the weight of the accumulating sediments and pressure increases by 25 bars every 100 meters. Thus, 1 km underground, the temperature is already 50°C with a pressure of 250 bars.

In these physical conditions, organic matter gradually changes:

   • The carbon and hydrogen atoms are reorganized and combine with the oxygen remaining in the rock.

   • The nitrogen, sulfur, and phosphorus atoms, which are other elements essential for life, dissipate gradually.

In this way, organic matter is converted into kerogen. This is an intermediate material made up of water, CO2, carbon, and hydrogen, which is then converted into oil or gas. At depths of 2200 meters, when the underground temperature reaches 100°C, kerogen starts to generate hydrocarbons.

   • From 2200- 3800 meters, it turns into oil. This depth interval is known as the oil window.

   • When the source rock sinks further, to depths of 3800 -5000 meters, liquid hydrocarbon production peaks. The liquids produced get lighter and lighter and gradually turn into gas, producing methane gas, the lightest hydrocarbon. This depth interval is known as the gas window.

Eventually, the source rock exhausts its potential and produces no more hydrocarbons.

The proportion of liquids and gas generated in this way depends on the type of source rock:

   • If the organic debris composing it is mostly of animal origin, it will produce more oil than gas.

   • If it is composed mainly of plant debris, the source rock will produce mostly gas.

So, with estimated average sedimentation of 50 meters every million years, 60 million years are required to change dead animals into liquid hydrocarbons, now located 3000 meters underground. It is hardly surprising, therefore, that oil is classified as a non-renewable energy source.