Hydrocarbon migration

There are numerous factors controlling the hydrocarbon migration procsess like kerogen expansion, increase in pressure and hydrocarbon expulsion out of source rock.  The expulsion of the oil out of the source rock is a dynamic process driven by the oil generation itself. Good source rocks have TOC (total organic content) ranging from 3 to 10%. At low TOC, the kerogen may occupy a position within the matrix porosity of the rock. At high TOC, the kerogen can form connected bands within the rock. Then, the kerogen is bearing part of the lithostatic load. As the organic matter transforms into oil, this load-bearing kerogen turns into liquid. The fluid pressure of the oil within the black shales can become high enough to produce microfractures in the rock. Once the micro fractures form, the oil is squeezed out and the source rock collapses. So, primary migration can be viewed as a second episode of compaction. Microfractures of this type can be seen in the most productive source rocks and they are often filled with remnants of oil.

Evidences for migration

There are 3 primary observational evidences which suggest that the hydrocarbons migrated into reservoir rocks at considerable depth below the surface and at some time after burial. These three evidences are as follows:

• Oil and gas occur in soluble pores and fractures in a host rock (reservoir rock) that formed these pores and fractures after its transformation (lithification) into solid rock
• Oil and gas are trapped at the highest point in a permeable rock unit which necessitates lateral and upward migration through a reservoir rock
• Oil, gas, and water occur together in a stratified relationship in porous and permeable reservoir rock. Stratification requires freedom to migrate laterally and vertically within a porous and permeable reservoir rock.

Migration Types

There are three types of hydrocarbon migration :

Primary Migration

Primary migration is the process by which hydrocarbons are expelled from the source rock into an adjacent permeable carrier bed.

Secondary Migration

Secondary migration is the movement of hydrocarbons along a "carrier bed" from the source area to the trap. Migration mostly takes place as one or more separate hydrocarbons phases (gas or liquid depending on pressure and temperature conditions). There is also minor dissolution in water of methane and short chain hydrocarbons.

Tertiary migration

It is a migration that occurs when petroleum moves from one trap to another or to a seep.

Hydrocarbon migration mechanisms and driving forces

When we talk about the causes and forces leading to the hydrocarbons migration, we should differ between the mechanisms of primary migration and those which led to secondary movement of oil and gas .

Primary migration mechanisms

1. How could the fluid find its path out of the impermeable source rock?
2. What is the nature of HC during its migration?

Secondary migration driving forces

The movement of oil and gas through the permeable rocks seems referable to five main causes. These are:

1. Gravitation
2. Capillary attraction
3. Displacement
4. Gas pressure
5. Difference in specific gravities

How can the fluid find its way out of the impermeable source rock?

To answer this question, it would be easy to say that HC squeezed from the source rock early before the compaction had occurred permeability was destroyed. However, it's simple answer; it's a wrong one because oil maturation window wouldn't be reached unless the compaction occurred.

According to the fig 1:

• The most water expulsion by the compaction in the upper 2 KM of burial.
• Pore expelled by the compaction is minimal below this depth.
• The geothermal gradient is 25◦c per KM, the oil generation window (pressure &temp) will begin below the depth at which whole compacted pore water has been expelled.

These evidence increase wrong's probability for the first answer.

To go further in the answer we will take two other factors in the consideration

1. Supernormal conditions of pressure and temp. 
2. The variety in water clays

Clay's water types

• Powers (1967)
  He assumed that there are two types of water: pore water and bonded water. When illitic clay is buried, only the first phase of water's emission occurs. When the montmorillonite is buried, the first phase occurred. Then, the second type of water (bonded water) is expelled when the compaction causes montmorillonite to change into illite.
• Burst (1969)
  Burst made some illustration to describe the montmorillonite-illite transformation. He found that the digenesis process occurs at temperatures ranging from 100◦c to 110◦c at the middle of oil window.
• Barker (1975)
  Barker has pursued the idea of Burst and showed that not only the water bonds clay minerals together but also HCs do the same, and these minerals will detach from the lattice surface when dewatering occurs.

Over pressure as a migration factor

1. It maintains the porosity and permeability of the rock besides inhibiting the formation of rigid framework for the rock.
2. Some geologists say that the overpressure induces micro fractures that help the fluid flow inside and outside the source rock.
3. There are some causes for over pressure zone including: clay dehydration itself, and inhibition of normal compaction resulting from rapid sedimentation.

Migration theorem

Migration is controlled by the following four factors or parts of the general theory:

Expulsion as protopetroleum

The evolutionary sequence from kerogen to crude oil and gas is very complex. Determination of whether the transformation was completed during, before or after migration is also very difficult. In addition, oil is not soluble in water. So, all these reasons pushed Hunt (1968) to assume that the migration process was completed before kerogen was transformed into crude oil. While the migration was occurring, kerogen had changed into esters, acids, and ketones. All these components dissolve in the water so the crude oil migrates. Cordell (1975) said that this mechanism contains several problems:

1. The concentrations of esters, ketones, and acids are low.
2. These components are likely to absorbed as petroleum is expelled.

Dissolved oil in water

The solubility of oil in water at surface conditions are negligible, but if the temperature increased and the carbon content decreased, it will produce high solubility of oil in water. According to figures 2 and 3, The solubility is neglected below about 150 C which exceeds the window of oil formation. Figure 2 Shows that components with lower number of carbon are more soluble than any others. The question here is about the heavy components whose solubility in high temp is low. So, this theory can explain a part of the mystery; the part of gas migration which has high solubility because of its low carbon content.

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  Fig 2. Solubilities of different crude oils against different temperatures

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  Fig 3. The relation between number of carbon atoms and the solubility of component in it

Within micelles

This theory was suggested by Baker (1963) and Cordell (1973) which states that "Enhancing the solubility of oil in the water is explained by assuming the presence of colloidal acid soaps whose molecules have hydrophilic and hydrophobic ends." Many scientists have reviewed this theory and listed a lot of objections including the diameter of micelles molecule which is greater than the diameter of pore throats.  

Solution of oil in gas

This theory included the gas phase as a catalyst. Momper (1978) has discussed the role of carbon dioxide which is produced from kerogen maturation. Carbon dioxide causes heavier hydrocarbons to participate during oil migration in a direct way and another indirect one. Therefore, the existence of carbon dioxide in solution makes the oil lighter because it makes the heavier component precipitate, and hence increases GOR and also decreases the viscosity. Indirectly, it reacts with calcium ions to produce calcite cement which precipitate in the pores. Then, the pressure increases and this will help HCs to get out of the source rock. The main objection facing this theory is that "the main phase of decarboxylation of kerogen is known to occur before hydrocarbon generation and the concentration of carbon dioxide may be too low to assist migration in these ways."

Globules of oil in water and continuous phase migration

These theories claimed that oil emigrates from the source rock to the carrier rock as a discrete oil phase. However, there are two mechanisms for the expulsion of oil. These mechanisms are:

1. Expulsion of discrete droplets associated with pore water
2. Expulsion of three dimensional continuous phase of oil (oil wet source rock)