Generation and accumulation happen in different rocks, often far apart. Migration is the link between them — the element of the petroleum system that carries hydrocarbons from the source rock to the reservoir and ultimately into a trap.
Because oil and gas are less dense than the water that saturates the subsurface, buoyancy pushes them upward until an impermeable seal stops them — concentrating hydrocarbons into traps.
Primary vs secondary migration
Geologists divide migration into two stages:
The expulsion of hydrocarbons out of the fine-grained source rock, where they were generated, into more permeable adjacent carrier beds.
The subsequent movement of hydrocarbons through permeable carrier beds and reservoirs — driven by buoyancy — until a trap and seal halt them.
Primary migration is the harder, slower step: squeezing oil and gas out of a tight source rock. Once the hydrocarbons reach a permeable carrier bed, secondary migration takes over and they can travel long distances laterally and vertically.
What drives migration
The main engine of secondary migration is buoyancy. The pore space of the subsurface is saturated with water, and both oil and gas are less dense than that water. Just as a bubble rises through a liquid, hydrocarbons rise through the water-filled pore network, seeking the highest point they can reach.
This upward drift continues until the hydrocarbons meet an impermeable barrier — a seal or cap rock — within a trapping geometry. There the buoyant rise is halted and the oil and gas accumulate, with gas (lightest) on top, then oil, then water below. Without a trap and seal in the migration path, hydrocarbons keep rising and may seep all the way to the surface and be lost.
Because migration must arrive after the trap and seal are in place, timing is critical — a theme covered in the petroleum system overview. In unconventional plays, migration barely happens at all: the hydrocarbons stay locked in the tight source rock and must be produced where they formed.
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Frequently asked
Primary migration is the expulsion of hydrocarbons out of the source rock into adjacent permeable carrier beds. Secondary migration is the subsequent buoyancy-driven movement through those carrier beds and reservoirs until a trap and seal halt the hydrocarbons.
Buoyancy is the main driver. Oil and gas are less dense than the water saturating the subsurface, so they rise through the water-filled pore network until an impermeable seal within a trap stops them.
Migrating oil and gas rise until they meet an impermeable seal or cap rock within a trapping geometry. The seal blocks further upward movement, so the hydrocarbons accumulate there instead of escaping to the surface.