Changes

The migration method partly determines the extent of compositional changes that occur during secondary, tertiary, 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 migration method partly determines the extent of compositional changes that occur during secondary, tertiary, 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.

[[file:migration-of-petroleum_fig7-8.png|thumb|{{figure number|1}}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|thumb|{{figure number|1}}Illustration of what happens when seals 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 traps with gas caps, the 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 traps with gas caps, the 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.

[[file:migration-of-petroleum_fig7-9.png|thumb|{{figure number|2}}.]]

 

[[file:migration-of-petroleum_fig7-9.png|thumb|{{figure number|2}}Illustration of what happens when traps preferentially spill oil and retain gas.]]

The differential entrapment of gas in downdip traps<ref name=ch07r1>Gussow, W., C., 1954, [http://archives.datapages.com/data/bulletns/1953-56/data/pg/0038/0005/0800/0816.htm Differential entrapment of gas and oil: a fundamental principle]: AAPG Bulletin, vol. 38, p. 816–853.</ref> is achieved by successively filling a sequence of traps in the same formation with oil and gas. As each trap fills to its spill point, the phase that is spilled first is the liquid leg. Thus, the gas is retained in the structurally lower traps and the oil is trapped farther up the migration path. This situation is the expected sequence of entrapment (shallow oil, deep gas) from the maturation sequence. [[:file:migration-of-petroleum_fig7-9.png|Figure 2]] illustrates what happens when traps preferentially spill oil and retain gas.

The differential entrapment of gas in downdip traps<ref name=ch07r1>Gussow, W., C., 1954, [http://archives.datapages.com/data/bulletns/1953-56/data/pg/0038/0005/0800/0816.htm Differential entrapment of gas and oil: a fundamental principle]: AAPG Bulletin, vol. 38, p. 816–853.</ref> is achieved by successively filling a sequence of traps in the same formation with oil and gas. As each trap fills to its spill point, the phase that is spilled first is the liquid leg. Thus, the gas is retained in the structurally lower traps and the oil is trapped farther up the migration path. This situation is the expected sequence of entrapment (shallow oil, deep gas) from the maturation sequence. [[:file:migration-of-petroleum_fig7-9.png|Figure 2]] illustrates what happens when traps preferentially spill oil and retain gas.