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During postexpulsion migration, many processes can alter the chemical characteristics of the hydrocarbons expelled from the [[source rock]]. The geochemical similarity of reservoired hydrocarbons and hydrocarbons expelled from source rocks, however, indicates there is usually little compositional alteration along the postexpulsion migration routes. An exception to this is the selective trapping of gas- and liquid-rich phases due to the quality of the seal.

During postexpulsion migration, many processes can alter the chemical characteristics of the hydrocarbons expelled from the [[source rock]]. The geochemical similarity of reservoired hydrocarbons and hydrocarbons expelled from source rocks, however, indicates there is usually little compositional alteration along the postexpulsion migration routes. An exception to this is the selective trapping of gas- and liquid-rich phases due to the quality of the [[seal]].

==Alteration processes==

==Alteration processes==

Processes responsible for altering the composition of [[hydrocarbon]]s during migration include the following:

Processes responsible for altering the composition of [[hydrocarbon]]s during [[Hydrocarbon migration|migration]] include the following:

* Water-washing—selective removal of the more water-soluble components

* Water-washing—selective removal of the more water-soluble components

* Adsorption—selective removal and retardation of hydrocarbon migration rate by mineral and [http://www.thefreedictionary.com/kerogen kerogen] particles

* Adsorption—selective removal and retardation of hydrocarbon [[migration rate]] by mineral and [[kerogen]] particles

* Phase partitioning—concentration of different hydrocarbon species into gaseous and liquid phases with changes in pressure and temperature

* Phase partitioning—concentration of different hydrocarbon species into gaseous and liquid phases with changes in pressure and temperature

* Mixing—by (1) including hydrocarbons from  kerogen particles along the migration path; (2) mixing [[migration]] streams from two or more source rocks; or (3) precipitation of [[asphaltenes]] and other high-molecular-weight compounds by the addition of [http://scifun.chem.wisc.edu/chemweek/methane/methane.html methane]

* Mixing—by (1) including hydrocarbons from  kerogen particles along the migration path; (2) mixing [[migration]] streams from two or more source rocks; or (3) precipitation of [[asphaltenes]] and other high-molecular-weight compounds by the addition of [http://scifun.chem.wisc.edu/chemweek/methane/methane.html methane]

* Biodegradation—biologic alteration of the hydrocarbons

* [[Biodegradation]]—biologic alteration of the hydrocarbons

==Migration method and alteration==

==Migration method and alteration==

The migration method partly determines the extent of compositional changes that occur during secondary or [[Tertiary migration|tertiary]] migration, or remigraton. If the petroleum moves as a broad front—as would be expected for solution gas or light oil in water and perhaps for dispersed colloids or droplets—there would be a maximum probability of interactions. However, if the petroleum moved as a slug or filament, contact with elements that could alter its composition would be more limited.

The [[Hydrocarbon migration|migration]] method partly determines the extent of compositional changes that occur during [[secondary migration|secondary]] or [[Tertiary migration|tertiary]] migration, or remigraton. If the petroleum moves as a broad front—as would be expected for solution gas or light oil in water and perhaps for dispersed colloids or droplets—there would be a maximum probability of interactions. However, if the petroleum moved as a slug or filament, contact with elements that could alter its composition would be more limited.

==Seal leakage from traps with gas caps==

==Seal leakage from traps with gas caps==

[[file:migration-of-petroleum_fig7-8.png|300px|thumb|{{figure number|1}}Illustration of what happens when [[seal]]s leak from traps with gas caps. After Schowalter.<ref name=ch07r12>Schowalter, T. T., 1979, [http://archives.datapages.com/data/bulletns/1977-79/data/pg/0063/0005/0700/0723.htm Mechanics of secondary hydrocarbon migration and entrapment]: AAPG Bulletin, vol. 63, p. 723–760.</ref>]]

[[file:migration-of-petroleum_fig7-8.png|300px|thumb|{{figure number|1}}Illustration of what happens when [[seal]]s leak from traps with gas caps. After Schowalter.<ref name=ch07r12>Schowalter, T. T., 1979, [http://archives.datapages.com/data/bulletns/1977-79/data/pg/0063/0005/0700/0723.htm Mechanics of secondary hydrocarbon migration and entrapment]: AAPG Bulletin, vol. 63, p. 723–760.</ref>]]

In [[trap]]s with [http://www.glossary.oilfield.slb.com/en/Terms/g/gas_cap.aspx gas caps], the [[Buoyancy pressure|buoyancy]] of the gas and oil column can exceed the breakthrough pressure of the seal prior to the trap being filled to the spill point. If this happens, the trap will leak through the seal and preferentially lose the gas phase. This situation (deep oil, shallow gas) is observed but is opposite to the expected sequence of entrapment due to [[maturation]] (oil migrates first, then gas). [[:file:migration-of-petroleum_fig7-8.png|Figure 1]] illustrates what happens when seals leak from traps with gas caps.

In [[trap]]s with [http://www.glossary.oilfield.slb.com/en/Terms/g/gas_cap.aspx gas caps], the [[Buoyancy pressure|buoyancy]] of the gas and oil column can exceed the breakthrough pressure of the [[seal]] prior to the trap being filled to the spill point. If this happens, the trap will leak through the seal and preferentially lose the gas phase. This situation (deep oil, shallow gas) is observed but is opposite to the expected sequence of entrapment due to [[maturation]] (oil migrates first, then gas). [[:file:migration-of-petroleum_fig7-8.png|Figure 1]] illustrates what happens when seals leak from traps with gas caps.

==Differential entrapment==

==Differential entrapment==

[[Category:Critical elements of the petroleum system]]  

[[Category:Critical elements of the petroleum system]]  

[[Category:Migration of petroleum]]

[[Category:Migration of petroleum]]