1. Petroleum migration involves the movement of oil and gas from source rock where it is generated into reservoir rock where it can accumulate. It occurs in three stages: primary, secondary, and tertiary migration. 2. Primary migration involves the movement of hydrocarbons out of the low permeability source rock due to overpressure buildup during burial. Secondary migration transports petroleum over longer distances through more permeable carrier beds. Tertiary migration is movement between traps or to the surface. 3. Key factors driving migration include buoyancy due to density differences between petroleum and water, capillary pressure, source rock compaction during burial, gas pressure reducing viscosity, and fractures and faults providing pathways. Evidence of migration

1. Petroleum System
( Part Two )
Migration of Petroleum
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Oil & Gas Engineering Dept. Petroleum Geology / 4 MSc. Hussein A.
29 /10 / 2019

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Oil and gas migration are the least understood processes
in the formation of hydrocarbon reservoirs. Much of the
current thinking of how it happens is hypothetical and
difficult to prove with either experiment or theory. When
potential source and reservoir rocks are buried, they contain
water in their pore space. The oil or gas, therefore, has to
replace this water in the migration process when it reaches
the reservoir rock.

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Types of Petroleum Migration
Transitional stage between place of oil origin and place oil collect. It
divided to three parts:
1- Primary migration: is the process by which petroleum moves from source
beds to reservoir rocks. Mature hydrocarbons first have to migrate out of the
source rock. This is in general a fine-grained rock that has a low
permeability, During burial, this rock gets compacted and its interstitial fluid
become over pressured with respect to surrounding rocks that have higher
permeabilities and from which fluids can migrate with greater ease upwards.
Therefore, a fluid pressure gradient develops between the source rock and
the surrounding, more permeable rocks. This causes the fluids the water and
the hydrocarbons to migrate along the pressure gradient, usually upwards.
This process is called primary migration, and it generally takes place across
the stratification.

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7. Physiochemical aspects of Primary migration
1 and 2) Pressure and Temperature during burial:
During burial, temperature and pressure increase, leading to Compaction
of the sedimentary rocks. The pore space, therefore, decreases and with it the
volume available for the fluids, principally formation water but also
hydrocarbons. They have to support an increasing load of the overburden stress
unless they escape. In shales, which are poorly permeable but which can be
richer in hydrocarbon source material, the fluids take a much longer time to
escape (and thus to equilibrate hydrodynamic pressures) than in porous and
permeable rocks. This can lead to overpressure in the shales, a so-called
compaction disequilibrium.
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2- Secondary migration: is migration of petroleum through permeable
rocks (carrier bed) after expulsion from the source rocks. Unlike
primary migration (expulsion), secondary migration generally involves
long distances from tens of meters to hundreds of kilometers. Distances
and directions of secondary petroleum migration vary depending on
the type and configuration of the sedimentary basin and the spatial
relationships between interbedded sandstones and shales, as well as
the abundance of fractures and faults. Short migration distances occur
in sedimentary sequences with intensive interbedding of source rock-
type shales and reservoir sandstones.

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3- Tertiary migration (Dismigration): occurs when petroleum moves from one
trap to another or to leakage, seepage, dissipation and alteration of petroleum as
it reaches the Earth’s surface.
Figure: Model to illustrate the primary and secondary migration and the accumulations

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Factors that lead to the petroleummigration
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1 Buoyancy which is due to the density contrast between petroleum
hydrocarbons and water. Oil densities can vary between 0.5 and 1.0 g/cm3,
natural gas densities are much less than 0.5 g/cm3 while pore waters have
densities varying between 1.0 and 1.20 g/cm3 depending on their salinity.
These density contrasts result in oil/water buoyancy gradients varying
between 0 and 6.8 kPa/m, while gas/water buoyancy gradients are much
higher, i.e. ranging from 4.5 to 11 kPa/m (Hunt, 1996). The greater the
volume of interconnected oil-saturated pores, the more the buoyancy forces
increase.

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2 Capillary pressure (Resistant force): is the pressure which oil or
gas has to overcome in order to displace the water from the pores of the
rock it is trying to penetrate. This displacement pressure depends very
much on the size of the pore throats. If a rock has very narrow pore
throats, capillary displacement pressures get so high that they cannot be
exceeded by the buoyancy of the oil stringer or gas bubble, and
entrapment occurs.
Types of Secondary Migration
 Lateral migration.
 Vertical migration.

14. 3- source rock compaction
Burial results in the downward motion of each
sediments package and is accompanied by a decrease of
porosity as it compacts
4- Specific gravity It is an Important factor especially
with the absence of water. When the oil moves down in
the porous rock under the Influence of weight
separated from the natural gas and continues until
become captured by impermeable seal rock
5. Natural gas pressure usually associated with oil
facilitates the petroleum migration process, because it
helps to reduce the viscosity of the oil and thus make It
easier to move from one place to another.
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15. 6. Cementation: When the rock fragments
consolidated and come together by pressure or by the
effect of the cementation materials (silica - calcium
carbonate Iron oxides) between the grains - the pores
become narrower and smaller in the rock and thus
escaping oil and migrates them.
7. Bacteria: Some believe that the bacteria can
sometimes help in the petroleum migration process by
melting limestone rocks (increase their vugs and
cavities) bearing crude oils. It may also generate
streams of carbon dioxide in the petroleum-bearing
limestone rocks and then push the oil.
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Theevidencethatsupportsthemigrationofpetroleum
1. Appearoilonearthsurfaceintheformofpetroleumnominated.
2. The presence of oil is always in the sandstone rocks and limestone (source rocks
usuallyismud).
3. Thepresenceofoilinsmallquantitiessometimesinigneousrocks.

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Migration Evidence: Oil Seepages

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Migration Evidence: Gas bubbles

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Figure: Migration pathways in the Cretaceous System of Southern Iraq

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Figure: Migration pathways in the oil fields Northern Iraq