Petroleum Generation and Migration

Petroleum Generation (Maturation Process)
Petroleum is generated from productive source beds which have high organic carbon content. This high organic carbon content resulted from the burial of plants and animals. Maturation process occurs through millions of years and the higher the rock quality is, the more mature it is.

Diagenesis, the chemical reaction process that occurred first few thousand years after burial at temperatures less than 50°C. During this process oxygen, nitrogen and sulfur are removed from the organic matter leading to an increase in hydrogen content of the sedimentary organic matter. About 10 to 20% of petroleum is formed during this process.

Catagenesis, the reactions that occur between 60 and 200°C. During catagenesis, the organic compounds are exposed to diverse thermal degradation reactions that include double bonds reduction by adding sulfur or hydrogen atoms, cracking reactions and condensation reactions.

Metagenesis, takes place at temperatures over 200°C and is considered to be a type of very low-grade metamorphism. Most petroleum is formed during the catagenesis and metagenesis of the residual biogenic organic matter. Converting biomolecules into petroleum is called maturation.
After petroleum generation it migrates until it is trapped. So, a brief overview about the migration process in indicated in the next section.
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Picture
This figure shows each type of maturation processes along depth (km).
Petroleum Migration
After fracturing of source rock due to pressure increase that happens due to the generation of oil and gas. Since gases and liquids are less dense than solids, they take up more volume causing pressure to increase. So it is important for a productive source rock to be easily fractured to let the fluids move through it.

Driving Mechanisms of Migration
Hydrocarbon migration is primarily driven by physical forces acting within the subsurface:
-Buoyancy Force : The most important mechanism, resulting from hydrocarbons being less dense than water. This density contrast drives hydrocarbons upward through porous and permeable rocks. The buoyancy force can be written as 
                                                                              F=Δρ⋅g⋅h
where Δρ is the density difference between water and hydrocarbons.
-Capillary Pressure : Hydrocarbons must overcome the surface tension that holds fluids in pore throats. If the pore throat is too small, migration cannot happen. The effect is strongly influenced by the wettability of the rock​.
-Hydrodynamic Flow : Movement of groundwater in the basin can carry hydrocarbons sideways and sometimes even push against buoyancy flow. This is less common, but in some basins it can strongly affect the migration path.
-Overpressure and Compaction : When too much pressure builds up in source rocks during hydrocarbon generation, the rock can fracture. This helps hydrocarbons escape and start primary migration.​

Migration occurs in two stages :
Primary Migration
During primary migration, gas and oil travel together as a single liquid phase because the high pressures in the source rock which usually become higher than the bubble point pressure at which gases start to be liberated from liquid. Expulsion from the source rock may occur in pulses and after some time from the first expulsion. With time, petroleum creates enough pressure to reopen fractures and pores causing a second expulsion, and the pulses continue to happen as long as the petroleum can rebuild enough pressure to reopen its path out of the source rock. After migration the pressure decreases and the fractures and pores close. Finally, petroleum migrates out of the source rock, pressures decline, especially if the petroleum migrates vertically.

Secondary Migration
Secondary migration is the movement of petroleum outside of the source rock through carrier beds toward reservoir or trap. During secondary migration, the gas and oil separate with the gas traveling ahead of oil. In most cases, differences in permeability between adjacent stratigraphic layers inhibit migration. This causes petroleum to flow within geologic units. For petroleum to accumulate in a trap, it must encounter a cap rock or seal to become an oil or gas reservoir.

Tertiary Migration
Tertiary migration refers to the movement of petroleum from a reservoir or trap to the surface or into another trap. This typically occurs when the seal is breached or when hydrocarbons escape along faults, fractures, or wellbores. Some petroleum geology texts describe tertiary migration as a third stage of petroleum migration. However, most researchers emphasize that it is more accurately a leakage or remigration process rather than a fundamental stage of petroleum migration.
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Picture
This figure shows the paths of primary and secondary migration.
If you want to learn more,

                                 Reference                                           Picture
                                               petrowiki.org                                                              offshoreengineering.com
                                               Hydrocarbon migration - AAPG Wiki
Author
Napat Tangjaichuychat ​
Editor
Poomisant Srimaharaja
Kittitanet Kongkanchanathip