Migration of hydrocarbons is a little-understood but critical process of the petroleum system. The short definition is: Movement of petroleum from source rock toward a reservoir or seep. Primary migration is expulsion of petroleum from fine-grained source rock, while secondary migration moves petroleum through a coarse-grained carrier bed or fault to a reservoir or seep. Tertiary migration occurs when petroleum moves from one trap to another or to a seep.
The following questions are vital for understanding the complications of the process:
- How does oil escape from the source rock?
- Does oil migrate out of the trap?
- Why are there marked differences in oil gravity, wax content, and sulfur content in lateral and stratigraphically successive sands?
- Why are there differences in water salinity for multiple sands in one structural trap?
- What is the role of faults in transporting and trapping hydrocarbons?
- Why are there barren sands within sequences of productive sands?
- How is cross-formational flow of hydrocarbons accomplished?
- Does the form change during migration and, if so, which form is dominant under what conditions?
- How can we estimate the timing, volumes, and compositions of transported hydrocarbons?
Principles of migration
- 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.
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.
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.
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.
- Factors that cause migration
- Factors that oppose hydrocarbon migration
- Migration pathways
- Calculating migration rate
- Calculating charge volume
- Estimating expulsion efficiency
- Migration by solution in water
- Migration by separate phase
- Peters, Kenneth E., David J. Curry, and Marek Kacewicz, 2012, An overview of basin and petroleum system modeling: Definitions and concepts, in Peters, Kenneth E., David J. Curry, and Marek Kacewicz, eds., Basin modeling: New horizons in research and applications: AAPG Hedberg Series no. 4, p. 1-16.