Migration of petroleum involves transporting hydrocarbons from the source rock to accumulation points within reservoirs, divided into three stages: primary, secondary, and tertiary migration. Primary migration allows hydrocarbons to move within source rocks to permeable carrier beds, influenced by expulsion models like saturation, pressure-driven, and chemical diffusion. Secondary migration involves hydrocarbons moving within carrier beds towards accumulations, whereas tertiary migration refers to the remobilization from existing accumulations, potentially altering volumes and fluid characteristics.

1. Migration

3. Migration
The process of transportation of petroleum from its place of origin, the source rock, to its place
of accumulation into the reservoir upto the trap is termed as Migration.

4. Migration Stages:
The process of migration can be divided into three stages
1. Primary Migration.
2. Secondary Migration
3. Tertiary Migration

5. Primary Migration
Primary migration or expulsion is the movement of oil and gas within the source rocks and its
expulsion into permeable carrier beds or reservoirs.
Distances travelled = metres to 10’s of meters.

6. Expulsion Models
1. Saturation Model
2. Pressure Driven Model
3. Diffusion Model.

7. 1.Saturation model
Hydrocarbon is expelled from source rock once a
threshold saturation limit is reached, According
to Mckenzie & Quigley (1988) at least 40%
saturation is required.
Expulsion efficiency: Expulsion efficiency is a
measure of the percentage of a particular
hydrocarbon that can escape from the source
bed during primary migration.

8. 2. Pressure driven
model
In this model two types of pressures are
applied
one internal pressure in which micro fracture
networks are developed to facilitate
migration
and the second is external overburden
pressure which squeezes the source rock.
In this migration model migration
depends on fracture strength, pore
pressure, expulsion potential of fluid
phases.

9. 3. Chemical diffusion
 Expulsion is controlled by thermally activated diffusion of bitumen molecules through the
holes in the solid organic matter polymer networks.
 Diffusion is the spreading of HC as a result of a concentration gradient.
 This process leads to dispersal rather than accumulation.
 Diffusion rates in porous media are very low.
Driving Forces: Shale source rocks act like sieves during primary migration. Capillary pressure is
the excess pressure required for oil or gas to displace water from pores. Small HC molecules can
migrate through all but the smallest micropores. Large complex molecules are retained by small
pores.

11. Secondary Migration:
Secondary migration is the movementof hydrocarbons within carrier beds and reservoir-type
rocks leading to a petroleum accumulation.
Distances travelled- can be up to hundreds of kms.

12. Secondary Migration Models:
1. Steady-state or “updip” / Raypathmigration:
This model is only valid for normal non-overpressured conditions and is very fast.

13. 2. Lateral or Vertical Migration:
Most accumulations result from a combination of lateral and vertical migration
Lateral Migration:
Hydrocarbons normally moves latterly, at permeabilities >10-15 m2 (1 mD) lateral migration
dominates.
Vertical Migration:
Hydrocarbons don’t moves vertically until adid by fractures. At permeabilities <10-15 m2 (1
mD) only fracture controlled vertical migration can occur (shales, silts)
Driving Forces:
Once out of the source rock secondary migration begins. Secondary migration is driven by buoyancy
and modified by water movement.

14. Tertiary Migration:
Many petroleum systems displayevidence for tertiary migration or the remobilization of
hydrocarbons from a pre-existing accumulation.
Tertiary migration may result in the reduction of hydrocarbon volumes as well as changes in
fluid character.