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Materials trapped in diagenetic overgrowths offer snapshots of the migration process. However, general differences exist between the chemical composition of oils and the source rocks to which they are chemically correlated. The main chemical constraint is solubility of migrating hydrocarbons.ĭetailed 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.

Physical conditions constraining migration through stratigraphic sections are pressure, temperature, permeability, capillarity, surface tension, molecular size, and density. 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. Geochemists usually focus on migration out of source rocks, and reservoir engineers usually study migration within carrier beds ( reservoir-quality rocks). The movement of hydrocarbons through an entire stratigraphic section is generally ignored. Laboratory migration experiments are limited in their applications by the time frame and the ability to reproduce subsurface conditions. Conclusions about migration are based on snapshots in reservoir and source-rock systems. As such, migration is generally inferred rather than demonstrated. Observation is difficult because it occurs either too rapidly, too slowly, or elsewhere. Hydrocarbon migration has been observed only rarely and indirectly in the natural environment under atypical conditions. It is generally described as that unknown process or group of processes that enable petroleum to move from a source to a reservoir. Less is known about migration than any other process involved in the accumulation of hydrocarbons in the subsurface.

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.

Flow rates for compaction-driven water generally are too slow to significantly affect hydrocarbon flow. Lateral migration is also facilitated by meteoric groundwater flow. Seals deflect the hydrocarbons laterally up dip through underlying beds to a trap or spill point.

Lithologic layers slow or restrict the vertical movement of hydrocarbons.

This force causes them to move vertically at geologically rapid rates.

Hydrocarbons migrate as a separate phase, primarily due to buoyancy.

How can we estimate the timing, volumes, and compositions of transported hydrocarbons?.

Does the form change during migration and, if so, which form is dominant under what conditions?.

How is cross-formational flow of hydrocarbons accomplished?.

Why are there barren sands within sequences of productive sands?.

What is the role of faults in transporting and trapping hydrocarbons?.

Why are there differences in water salinity for multiple sands in one structural trap?.Why are there marked differences in oil gravity, wax content, and sulfur content in lateral and stratigraphically successive sands?.How does oil escape from the source rock?.The following questions are vital for understanding the complications of the process:

Tertiary migration occurs when petroleum moves from one trap to another or to a 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. The short definition is: Movement of petroleum from source rock toward a reservoir or seep. Migration of hydrocarbons is a little-understood but critical process of the petroleum system.