Hydrocarbon migration

In fact, 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 most productive source rocks and they are often filled with remnants of oil.

Evidence for Migration    There are 3 primary observational evidence that suggests that the hydrocarbons migrated into reservoir rocks at considerable depth below the surface and at some time after burial:

1. Oil and gas occur in soluble pores and fractures in host rock (reservoir rock) that formed these pores and fractures after its transformation (lithification) into solid rock
2. Oil & gas are trapped at the highest point in a permeable rock unit which necessitates lateral and upward migration through a reservoir rock
3. 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

• 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.

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

Primary migration mechanisms

When we talk about primary migration there are two main questions we need to answer;

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

1-How could 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 fig(1)

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1. The most water expulsion by the compaction in the upper 2 KM of burial
2. Pore expelled by the compaction is minimal below this depth.
3. 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 first phase of water's emission occurs. When the montmorillonite is buried the first phase occurred and after that the second type of water-bonded water-is expelled when the compaction cause montmorillonite change to illite.

• Burst (1969)

Burst made some illustration to the montmorillonite-----illite transformation. He found that the digenesis process occurs at temp 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 are bonded the clay minerals but also HC and it will detached from the lattice surface when dewatering occur.

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 make micro fractures that help the fluid to flow inside & out the source rock.
3. There are some causes for over pressure zone for example; Clay dehydration itself, inhibition of normal compaction resulting from rapid sedimentation.

Conclusion

The dewatering in its two stages and overpressure in the source rock are the main parameters which make the path of the fluid in the impermeable source. Only one of them can't be dominant.

Migration's theorem

Expulsion as protopetroleum

Figure 3 : graph show the relation between number of carbon atoms and the solubility of component in the water]]

1. The solubility of oil in water at surface conditions are negligible but if the temp increased and the carbon content decreased it will produce high solubility of oil in water.

Figure 2graph show the relation between number of carbon atoms and the solubility of component in the water According to fig1 and Fig2, The solubility is neglected below about 150 ○c which exceeds the window of oil formation. Figure 2 Shows that component which contains lower number of carbon is more soluble than any component. The question here is what about heavy components which their solubility in high temp is low. So this theory can explain a part of the mystery; the part of gas migration which have high solubility because of its low carbon content.

Within micelles

This theory was suggested by Baker (1963) and Cordell (1973) which states that ‘to enhance the solubility of oil in the water they assumed the presence of colloidal acid soaps whose molecules have hydrophilic & hydrophobic ends”. Many scientists have reviewed this theory and listed a lot of objection, for example the diameter of micelles molecule is greater than the diameter of pore throats and many objection like this.  

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 participates in oil migration in direct way and another indirect one such that, directly, the existence of carbon dioxide in solution makes the oil lighter because it makes the heavier component precipitate, increases GOR and also decreases the viscosity. Indirectly, by acting with calcium ions to produce calcite cement which precipitate in the pores then the pressure increase and this will help HCs to get out of the source rock. The main objection on this theory is "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 that ways.

Globules of oil in water &Continuous phase migration

These theories claimed that oil emigrates from the source rock to the carrier rock as a discrete oil phase, there are two types

• The expulsion of discrete droplet associated with pore water
• The expulsion Three dimensional continuous phase of oil, (oil wet source rock)

References

1. Rechard ,C.selley : Elements of Petroleum Geology (1998 ) .
2. Clark, N.J.: Elements of Petroleum Reservoirs, E. J. Storm Printing Co., Dallas, TX. (1960).

Migration 

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