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'''Migration''' of hydrocarbons is a little-understood but critical process of the [[petroleum system]]. This chapter attempts to address the following questions:

'''Migration''' of [[hydrocarbon]]s is a little-understood but critical process of the [[petroleum system]]. This chapter attempts to address the following questions:

* How does oil escape from the [[source rock]]?

* How does oil escape from the [[source rock]]?

==Principles of migration==

==Principles of migration==

* Hydrocarbons migrate as a separate phase, primarily due to [[Buoyancy forces in reservoir fluids|buoyancy]]. This force causes them to move vertically at geologically rapid rates.

* [[Hydrocarbon]]s migrate as a separate phase, primarily due to [[Buoyancy forces in reservoir fluids|buoyancy]]. This force causes them to move vertically at geologically rapid rates.

* Lithologic layers slow or restrict the vertical movement of hydrocarbons. [[Seal]]s deflect the hydrocarbons laterally up [[dip]] through underlying beds to a [[trap]] or spill point. Lateral migration is also facilitated by meteoric groundwater flow. Flow rates for compaction-driven water generally are too slow to significantly affect hydrocarbon flow.

* Lithologic layers slow or restrict the vertical movement of hydrocarbons. [[Seal]]s deflect the hydrocarbons laterally up [[dip]] through underlying beds to a [[trap]] or spill point. Lateral migration is also facilitated by meteoric groundwater flow. Flow rates for compaction-driven water generally are too slow to significantly affect hydrocarbon flow.

* The properties of [[reservoir]]s and carrier beds (dip, [[relative permeability]], etc.) control the rate of migration and thus the specific direction of the bulk of hydrocarbons under seals.

* The properties of [[reservoir]]s and carrier beds (dip, [[relative permeability]], etc.) control the rate of migration and thus the specific direction of the bulk of hydrocarbons under seals.

==Observations of migration==

==Observations of migration==

Hydrocarbon migration has been observed only rarely and indirectly in the natural environment under atypical conditions. Observation is difficult because it occurs either too rapidly, too slowly, or elsewhere. As such, migration is generally inferred rather than demonstrated. Conclusions about migration are based on snapshots in reservoir and source-rock systems. Laboratory migration experiments are limited in their applications by the time frame and the ability to reproduce subsurface conditions.

[[Hydrocarbon]] migration has been observed only rarely and indirectly in the natural environment under atypical conditions. Observation is difficult because it occurs either too rapidly, too slowly, or elsewhere. As such, migration is generally inferred rather than demonstrated. Conclusions about migration are based on snapshots in reservoir and source-rock systems. Laboratory migration experiments are limited in their applications by the time frame and the ability to reproduce subsurface conditions.

==Migration studies==

==Migration studies==

The movement of hydrocarbons through an entire stratigraphic section is generally ignored. Geochemists usually focus on migration out of [[source rock]]s, and [[Subsurface team|reservoir engineers]] usually study migration within carrier beds ([[reservoir]]-quality rocks). Little is known, though much is inferred, about cross-facies flow required when source rocks and reservoir-quality rocks are not adjacent to one another.

The movement of [[hydrocarbon]]s through an entire stratigraphic section is generally ignored. Geochemists usually focus on migration out of [[source rock]]s, and [[Subsurface team|reservoir engineers]] usually study migration within carrier beds ([[reservoir]]-quality rocks). Little is known, though much is inferred, about cross-facies flow required when source rocks and reservoir-quality rocks are not adjacent to one another.

==Migration constraints==

==Migration constraints==

Physical conditions constraining migration through stratigraphic sections are [[Wikipedia:Pressure|pressure]], [[Wikipedia:Temperature|temperature]], [[permeability]], [[Capillary pressure|capillarity]], [http://hyperphysics.phy-astr.gsu.edu/hbase/surten.html surface tension], molecular size, and [http://physics.about.com/od/fluidmechanics/f/density.htm density]. The main chemical constraint is [[Wikipedia:Solubility|solubility]] of migrating hydrocarbons.

Physical conditions constraining migration through stratigraphic sections are [[Wikipedia:Pressure|pressure]], [[Wikipedia:Temperature|temperature]], [[permeability]], [[Capillary pressure|capillarity]], [http://hyperphysics.phy-astr.gsu.edu/hbase/surten.html surface tension], molecular size, and [http://physics.about.com/od/fluidmechanics/f/density.htm density]. The main chemical constraint is [[Wikipedia:Solubility|solubility]] of migrating [[hydrocarbon]]s.

==Chemistry of migrated hydrocarbons==

==Chemistry of migrated hydrocarbons==

Detailed chemical correlations made of reservoired hydrocarbons with source rocks strongly indicate that the migration process does not significantly affect the overall geochemistry of the migrated hydrocarbons. However, general differences exist between the chemical composition of oils and the source rocks to which they are chemically correlated. These differences must be explained.

Detailed chemical correlations made of [[reservoir]]ed [[hydrocarbon]]s with source rocks strongly indicate that the migration process does not significantly affect the overall geochemistry of the migrated hydrocarbons. However, general differences exist between the chemical composition of oils and the source rocks to which they are chemically correlated. These differences must be explained.

==How we observe migration==

==How we observe migration==

Materials trapped in diagenetic overgrowths offer snapshots of the migration process. Studies of these materials by microanalytical techniques such as fluid inclusion analysis, microfluorescence, and cathodoluminescence offer potential for great advances in our understanding of the migration process and our ability to recognize and perhaps predict migration pathways and timing.

Materials trapped in [http://bio-geo-terms.blogspot.com/2006/10/overgrowth.html diagenetic overgrowths] offer snapshots of the migration process. Studies of these materials by microanalytical techniques such as [http://www.geotrack.com.au/fluidinclusion.htm fluid inclusion] analysis, microfluorescence, and [[SEM,_XRD,_CL,_and_XF_methods#Cathodoluminescence_.28CL.29|cathodoluminescence]] offer potential for great advances in our understanding of the migration process and our ability to recognize and perhaps predict migration pathways and timing.

==Migration stages==

==Migration stages==

Hydrocarbon migration consists of four stages: primary, secondary, tertiary, and remigration. The list below contains their definitions.

Hydrocarbon migration consists of four stages: primary, secondary, tertiary, and remigration. The list below contains their definitions.

* '''Primary Migration'''—The process of loss of hydrocarbons from the source rock.

* '''Primary Migration'''—The process of loss of [[hydrocarbon]]s from the [[source rock]].

* '''Secondary Migration'''—Migration from source to reservoir along a simple or complex carrier system. Includes migration within the reservoir rock itself.

* '''Secondary Migration'''—Migration from source to [[reservoir]] along a simple or complex carrier system. Includes migration within the reservoir rock itself.

* '''Tertiary Migration'''—Migration to the surface, either from a reservoir or source rock. Also called dismigration.

* '''Tertiary Migration'''—Migration to the surface, either from a reservoir or source rock. Also called dismigration.

* '''Remigration'''—Migration from one reservoir position through an intervening section into another reservoir position in the same or a different reservoir.

* '''Remigration'''—Migration from one reservoir position through an intervening section into another reservoir position in the same or a different reservoir.