Hydrocarbon Traps & Seals

1. Hydrocarbon Traps and
Seals

2. Revision
What are geological elements of petroleum
system?
What are geological processes of petroleum
system?
How organic matters is converted into
petroleum?

3. • Source Rock - A rock with abundant hydrocarbon-prone organic
matter
• Reservoir Rock - A rock in which oil and gas accumulates:
- Porosity - space between rock grains in which oil accumulates
- Permeability - passage-ways between pores through
which oil and gas moves
• Seal Rock - A rock through which oil and gas cannot move
effectively (such as mudstone and claystone)
• Migration Route - Avenues in rock through which oil and gas moves from
source rock to trap
• Trap - The structural and stratigraphic configuration that focuses
oil and gas into an accumulation

4. The processes
Top of oil window
Top of gas window
Generation Expulsion Accumulation
GOC
OWC
Migration Preservation

5. (modified from Tissot and Welte, 1984)
Organic Debris
Kerogen
Carbon
Initial Bitumen
Oil and Gas
Methane
Oil Reservoir
Migration
Thermal Degradation
Cracking
Diagenesis
Catagenesis
Metagenesis
Progressive Burial and Heating
Schematic Representation of the Mechanism
of Petroleum Generation and Destruction

6. i. Diagenesis is chemical, physical, or biological
change undergone by a sediment after its initial
deposition and during and after its lithification,
exclusive of surface alteration (weathering) and
metamorphism. These changes happen at relatively
low temperatures and pressures and result in
changes to the rock's original mineralogy and
texture. There is no sharp boundary between
diagenesis and metamorphism, but the latter occurs
at higher temperature and pressure than the former.
ii. Catagenesis
Catagenesis is the cracking process which results in
the conversion of organic kerogens into hydrocarbons

7. iii. Metagenesis is the last stage of maturation and
conversion of organic matter to hydrocarbons.
Metagenesis occurs at temperatures of 150° to
200°C. At the end of metagenesis, methane, or
dry gas, is evolved along with nonhydrocarbon
gases such as CO2, N2, and H2S, as oil molecules
are cracked into smaller gas molecules.

8. What is Trap
A trap is the place where oil and gas are barred
from further movement….(Levorsen, 1967)
.

9. Seals or Cap Rocks
For a trap to have integrity, it must be overlain
by an effective seal.
Any rock that is impermeable can act as seal or
cap rock but commonly mudstone

10. Introduction: Oil Traps
• Some rocks are permeable
and allow oil and gas to freely
pass through them
• Other rocks are impermeable
and block the upward passage
of oil and gas
• Where oil and gas rises up
and capped by impermeable
rocks it can’t escape. This is
one type of an Oil Trap.
Impermeable
Permeable

12. • The permeable rocks than contain oil and gas within
the oil trap are known as the Reservoir Rock.
• Reservoir rocks have lots of interconnected holes
called pores. These allow them to absorb the oil and
gas like a sponge.

13. Earth Science World Image Bank Image #h5innl
This is a highly magnified picture of
a sandy reservoir rock (water-filled
pores are shown in blue)
As oil migrates it fills up the pores
(oil-filled pores shown in black)

14. Carbonate as Reservoir

19. How to find oil: Source rock, reservoir rock, traps

20. Hydrocarbon Traps
i. Structural traps
Structural traps are caused by structural features.
They are usually formed as a result of tectonics.
ii. Stratigraphic traps
Stratigraphic traps are usually caused by changes in
rock quality.
iii. Combination traps
Combination traps that combine more than one type of
trap are common in petroleum reservoirs.
Other types of traps (such as hydrodynamic traps)
are usually less common.

21. Structural Hydrocarbon Traps
i. Fold
ii. Fault
iii. Salt Dome

22. Petroleum trapped in top of dome
iii. Salt Dome
Rocks above salt
dome are bowed up
Oil and gas
trapped on
sides of
impermeable
dome
Salt flows up as a weak mass

23. Fault Trap
Oil / Gas
In this normal fault trap, oil-bearing
sandstone is juxtaposed against
impervious shale.

24. Stratigraphic Hydrocarbon Traps
Stratigraphic hydrocarbon traps occur where
reservoir facies pinch into impervious rock such
as shale, or where they have been truncated by
erosion and capped by impervious layers above
an unconformity.
(modified from Bjorlykke, 1989)

26. Other Traps In hydrodynamic
Asphalt Trap
Water
Hydrodynamic Trap
Meteoric
Water
Biodegraded
Oil/Asphalt
Partly
Biodegraded Oil
Shale
Oil
Water
traps, the
hydrocarbon is
trapped by the action
of water movements.
Tilted contacts are
common in this case.
The water usually
comes from a source
such as rain falls or
rivers.
Hydrostatic
Head
(modified from Bjorlykke, 1989)

28. Summary

30. Questions?