Hydrocarbon migration
The principles of hydrocarbon migration, discussed in this section, can be summarized as follows: • Hydrocarbons migrate as a separate phase, primarily due to buoyancy. This force causes them to move vertically at geologically rapid rates. • Lithologic layers slow or restrict the vertical movement of hydrocarbons. Seals 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 reservoirs and carrier beds (dip, relative permeability, etc.) control the rate of migration and thus the specific direction of the bulk of hydrocarbons under seals.
Less is known about migration than any other process involved in the accumulation of hydrocarbons in the subsurface. It is generally described as that unknown process or group of processes that enable petroleum to move from a source to a reservoir.
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.
Migration studies
The movement of hydrocarbons through an entire stratigraphic section is generally ignored. Geochemists usually focus on migration out of source rocks, and 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
Physical conditions constraining migration through stratigraphic sections are pressure, temperature, permeability, capillarity, surface tension, molecular size, and density. The main chemical constraint is solubility of migrating 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.
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.
Migration stages
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.
- 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.
- Remigration—Migration from one reservoir position through an intervening section into another reservoir position in the same or a different reservoir.
See also
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