The generation of hydrocarbons (oil and natural gas) can be explained in a simple way by a slow process that can be divided into 3 main stages: 1) Formation of the Source Rock; 2) Burial of the Source Rock; and 3) Formation of gas and oil.
It should be noted that the “Source Rock” is a sedimentary rock derived from erosion processes that is naturally rich in organic matter and in which hydrocarbons are formed. Not to be confused with Reservoir Rock which is a rock in which hydrocarbons accumulate.(From : Dictionnaire de géologie 4eédition, Foucault et Raoult).
1- Formation of the Source Rock
After the death of a living organism (plant or animal), the material of which it is composed is recycled either by being eaten by scavengers or bacteria, or by being directly transformed into carbon dioxide (CO2), nitrate, sulphate, etc. to be released in the atmosphere or in the groundwater. However, 0.1% of this material escapes this fate and ends up being deposited in sediments at the bottom of the seas or lakes which are low-agitated and low-oxygenated environments, thus favorable to preserve this organic matter. Due to anaerobic bacteria, this mixture is transformed into dark and smelly mud that accumulates and hardens over time to become a source rock with at least a 1-2% organic matter content.
The environments most conducive to significant accumulations of organic matter have the following characteristics: i) They are located in warm climate conducive to the formation of plankton II) are near the mouth of rivers or deltas where a greater amount of organic matter is carried; and III) are far from a mountain range which limit the presence of heavier mineral sediments.
2- Burial of the Source Rock
Under the weight of the accumulating sediments, the source rock sinks into the earth’s crust at a rate of a few meters to a few hundred meters per million years. This progressive phenomenon is called subsidence and creates what is called a sedimentary basin.
Burial gradually submits the source rock to increasingly strong pressures and temperatures. The organic matter it contains is crushed by the weight of the sediments (at a depth of 1000 m, temperature is 50 °c and the pressure reaches 250 bars).
Under these physical conditions, the nitrogen, sulfur and phosphorus atoms end up disappearing and the organic matter is gradually transforming into kerogen, a mixture of water, CO2, carbon and hydrogen that will allow for the genesis of oil and gas.
3- Formation of gas and oil
At depth of burial of about 2000m and at a temperature of 100 ° C, the kerogen begins to transform into hydrocarbons.
At depths between 2000m and 3800m, we are in what we call the oil window(liquid oil) in which the kerogen generates on average more oil than gas.
And at depths between 3800m and 5000m, the peak of theoil windowis reached and we enter into the gas windowin which the kerogen gives more and more light parts and the generation of gas (methane) takes over the formation of oil.
There you go! It is by being subjected to these different phases that some organisms end up being transformed into hydrocarbons!
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