Migration Definition / Meaning
In petroleum geology, migration refers to the movement of hydrocarbons (oil and natural gas) from their origin in a source rock to a reservoir rock where they can accumulate. This process is essential for forming commercial oil and gas fields and is divided into three distinct phases: primary, secondary, and tertiary migration. Understanding migration helps exploration geologists predict where hydrocarbons might be trapped and reduce drilling risk.
Types of Migration
- Primary Migration (expulsion): The initial movement of hydrocarbons out of the fine-grained source rock into a more permeable carrier bed or fracture system. This happens because of increasing pressure and temperature as the source rock is buried deeper. Kerogen (organic matter) transforms into oil and gas, creating small oil droplets or gas bubbles that are squeezed out of the source rock.
- Secondary Migration: The movement of hydrocarbons through permeable carrier beds, faults, or fractures from the source rock to the reservoir rock or trap. This phase relies on buoyancy (oil and gas are lighter than water), capillary pressure, and the geometry of the rock layers. Hydrocarbons migrate upward until they encounter a seal or a trap.
- Tertiary Migration (remigration): The redistribution of hydrocarbons after they have already accumulated in a trap, often due to changes in the basin geometry, faulting, or tilting. This can result in the oil or gas moving from one trap to another or leaking to the surface.
Driving Forces
Migration is driven by a combination of physical and chemical forces. The table below summarizes the main factors:
| Factor | Description | Role in Migration |
|---|---|---|
| Buoyancy | Oil and gas have lower density than the surrounding formation water. | Primary driver for upward movement of hydrocarbons during secondary migration. |
| Capillary Pressure | Pressure difference across the interface between two immiscible fluids (e.g., oil and water) in small pores. | Controls entry of hydrocarbons into pore spaces; can impede or assist migration depending on pore throat size. |
| Overpressure | Abnormally high pore pressure in rocks, often due to rapid burial or hydrocarbon generation. | Helps expel hydrocarbons from source rocks (primary migration) and can drive migration through fractures. |
| Compaction | Reduction in rock volume due to overburden pressure. | Expels pore fluids, including hydrocarbons, out of compacting sediments. |
Migration Pathways and Timing
Hydrocarbons migrate along paths of least resistance: porous and permeable rock layers (e.g., sandstones, carbonates), fault planes, and fractures. The timing of migration relative to trap formation is critical. If migration occurs before a trap exists, hydrocarbons will continue moving and may be lost. If migration happens after the trap is sealed, accumulation can occur. Geologists use basin modeling software to simulate burial history, temperature changes, and fluid flow to predict when and where migration likely occurred.
Practical Importance
In exploration, understanding migration helps identify prospective areas. For example, if a source rock is known to be mature but no traps are present in the immediate vicinity, geologists look for migration pathways that could carry hydrocarbons to distant structures. Conversely, if a trap is identified but migration is deemed unlikely, the prospect may be abandoned. Migration also affects oil quality: long-distance migration through fine-grained rocks can remove heavier components, leaving lighter, more valuable oil.
Usage Example
“During the basin analysis, the geoscientists modeled the primary migration of oil from the Kimmeridge Clay source rock and then traced secondary migration along the tilted fault blocks to the Jurassic sandstone reservoir. The resulting map showed a high probability of accumulation in the eastern closure.”