Topic 5 Migration of Petroleum from Source Rock to Reservoir
It is now generally accepted by geochemists that in sand-shale sequences some of the petroleum generated in organic rich “source rocks” can move to a porous and permeable reservoir rock where it may accumulate:
Thus migration links the source rock to the reservoir. An understanding of the migration mechanism is important in many aspects of exploration for the following reasons.
Correlation of reservoired crude oils to their source rocks is based on the assumption that the composition of the source rock extract should closely resemble that of the reservoired oil. This will be true if the migration process causes only minor chemical fractionation. If migration does produce considerable changes in the chemical composition then many source rock-crude oil pairs may be missed by current techniques.
Many traps, particularly structural ones, develop later than the onset of generation. Some knowledge of the time of migration is important in establishing whether the trap was available to accumulate the migrating petroleum.
The mechanism of migration is one of the factors controlling the distance of migration. This in turn sets limits on the maximum possible distance between the source rock and the trap.
Source rocks contain the material left after migration has occurred. A residue can only be interpreted if the composition of the migrated material is known and this depends on the migration mechanism.
The mineralogy of the source rock could have an important influence if internally generated water is necessary for migration. The presence of montmorillonite (which releases water