This presentation discusses petroleum traps, which are subsurface reservoirs that prevent petroleum from migrating. There are three main types of traps - structural, stratigraphic, and combination. Structural traps are created by folding or faulting of reservoir rocks. Stratigraphic traps result from variations in rock layers. Combination traps involve both structural and stratigraphic elements, like salt domes. For petroleum to accumulate, a trap must form before or during migration from the source rock. The timing of trap formation is important for a reservoir to contain producible quantities of oil or gas.

1. Presentation on
PETROLEUM TRAPS

2.  Introduction
 Migration of petroleum
 Petroleum traps
 Structural traps
 Stratigraphic traps
 Combination traps
 Timing of trap formation
 Conclusion
 References

3.  “Petroleum "is the general term used for all the natural hydrocarbons found in
rocks.
 Petroleum "refers only to the liquid oil. Gaseous varieties are called “natural gas
"and highly viscous to solid varieties are called “bitumen”.
 The fine grained muddy sediments in which petroleum originates are called
“source rocks".
 The source rocks of petroleum are generally shales,silts and limestones.
petroleum migrates from the source rock into adjacent porous and permeable
rocks and accumulates there to form a pool. Such permeable rocks are called
“reservoir rocks".

4.  The common reservoir rocks are sandstones,conglomerates,porous
limestones,fractured shales,and jointed igneous and metamorphic
rocks. Most of the known hydrocarbons are trapped in source rocks less
than 200 million years.

5.  There are two types of migration when discussing the movement of
petroleum, primary and secondary.
 Primary migration refers to the movement of hydrocarbons from within
the source rock and into reservoir rock.
 Secondary migration refers to the subsequent movement of
hydrocarbons within reservoir rock; the oil and gas vacated the source
rock and has entered the reservoir rock

6. Petroleum trap, is subsurface reservoir of petroleum. The oil is
always accompanied by water and often by natural gas; all are confined
in porous rock, usually such sedimentary rocks as sandstones, arkoses
and fissured limestones. The natural gas being lightest, occupies the top
of the trap and is underlain by the oil and then the water. A layer of
impervious rock, called the roof rock, prevents the upward or lateral
escape of the petroleum.

7.  The conditions necessary for the formation of an oil trap are as follows,
1. The porous reservoir rocks must have a favorable structure such as
an anticline fold or dome, to hold oil.
2. There must be an impervious cap rock or roof rock to check the
upward migration of oil.
3. The structural deformation of rocks must not be very severe.
Intensely fractured rocks may render traps ineffective by causing
leakage.

8. Contd.
 A hydrocarbon reservoir has a distinctive shape or configuration,
that prevents the escape of hydrocarbons that migrate into it.
Geologists classify reservoir shapes, or traps into the following
types.
 Structural traps
 Stratighraphic traps
 Combination traps

9.  Structural traps are created when the seal or barrier is concave
upward[looking from below].The geometry is formed by tectonic
processes after deposition of the reservoir beds involved. This concave
nature may be due to local deformation as a result of folding, faulting or
both of the reservoir rock. Some of the important structural traps are as
follows.

10. a) Anticlines and domes
An anticline is an example of rocks which are
previously flat, but have been bent into an arch. Oil that finds its
way into a reservoir rock that has been bent into an arch will flow
to the crest of the arch, and get stuck. Folds result in the
physical bending[deformation]of the rock units without breaking.
The rock units undergo bending very slowly over a long periods
of geologic time. These types of traps are often found adjacent
to mountain ranges.

11. b)Fault trap
Fault traps are formed by the movement of rock
along fault line. In some cases, the reservoir rock has moved opposite
a layer of impermeable rock. The impermeable rock thus prevents the
oil from escaping. In other cases, the fault itself can be a very effective
trap, when a fault affects inclined strata, a reservoir rock may be
blocked off by an impervious shale there by creating an oil
trap.Commonly,faults form traps in combination with other structural
features such as folding.

12. c)Salt domes
This kind of trap originates when salt is deposited by
shallow seas.Later,a sinking seafloor deposits organic rich shale over the
salt, which is in turn covered with layers of sandstone and shale. Deeply
buried salt tends to rise unevenly in swells or salt domes, and any oil
generated with in the sediments is trapped where the sandstones are
pushed up over or adjacent to the saltdome.Where,saltdomes intruded into
the sedimentary rocks, good oil traps are formed. Here the oil accumulates
near the upturned edges of the reservoir rock which are sealed by the salt.

13. The main trap- making element in a stratigraphic trap is
some variation in the lithology or stratigraphy,or both of the reservoir rock.
The variation may be facies change, variable porosity and permeability or an
up-structure termination of the reservoir rock.

14. Contd.
 The following may be the setup to give rise to such traps.
i. A reservoir rock may be truncated by an unconformity or
overlapped.
ii. There may be changes along the bedding.
iii. The boundary between two kinds of rocks may favour the formation
of a reservoir. The boundary may be sharp or gradational.
 The two classes of stratigraphic traps are,
a) Primary stratigraphic traps and
b) secondary stratigraphic traps

15. a)Primary stratigraphic traps
Such traps are also known as ‘digenetic’
and ‘depositional’traps.Thus primary stratigraphic traps formed during
deposition or digenesis of the sediments.
b)Secondary stratigraphic traps
These are the result of some stratigraphic
variation that developed after the deposition and digenesis of the reservoir
rock. They are almost always associated with unconfirmities,they may be
called unconformity traps.
eg:for stratigraphic traps is the Monroe gas field in Morehouse
parish, Louisiana[U.S.A]

16.  The combined(or mixed)traps are combinations of structure
and lithology. In such traps, a stratigraphic element may be the cause
for the permeability of a reservoir rock. A structural element caused by
deformation may combine with the stratigraphic element to give rise to a
trap.Additionally,the down-dip flow of formation water may increase the
trapping effects. A great variety of traps, which are combinations of
structural &stratigraphic traps, is associated with intrusion of deep-
seated rocks into overlying sediments.

17.  A common trap that would be an example of a combination trap is a
salt dome. The great majority of such traps are in sediments associated
with rock salt intrutions,traced in the Gulf coast region,U.S.A,Northern
germany,The north sea and in Russia.
 Oil pools associated with them are known as salt- dome or salt-plug
pools.
 When salt plugs rise through soft sedimentary rocks, they affect the
stratigraphy and structure of adjacent rocks and traps for the
accumulation of petroleum are created. This has been well observed in
the Gulf coast,Texas,Louisiana and Missisipi,where the plugs are still
moving.

18. Timing of trap formation
 The timing of trap formation is related to petroleum migration and the formation
of the trap. This is an important aspect to be considered in the accumulation of
petroleum.
 If the trap formation is before migration, then they will be the productive.
 on the other hand, where trap formation post-dates its petroleum migration, they
could be barren. In post migration, structural changes by faults, they permit petroleum
to undergo further migration.

19. Oil is found in reservoirs in sedimentary rock. It is composed of
compressed hydrocarbons, and was formed millions of years ago in
a process that began when aquatic plant and animal remains where
covered by layers of sediments(particles of rock and mineral).As
bacteria and chemicals broke down the organic plants and animal
material,inccreasing layers of sediments settled on top. Heat and
pressure transformed the layers of sediment into sandstone,
limestone and other types of sedimentary rocks, and transformed
the organic matter into petroleum. Tiny pores in the rock allowed the
petroleum to seep in. These ‘reservoir rocks' hold the oil like a
sponge, confined by other non-porous layers that form a trap.
In India, reservoirs of petroleum and natural gas are found
in the belts of tertiary rocks of Assam,Gujarath,Offshore regions of
Bombay high, and in the Cauvery and Godavari deltaic areas.

20.  Principles of engineering geology, k.M. BANGAR,Standard
publishers,2009,(338-341)
 Geology of petroleum, second edition,A.I.LEVORSEN,C B S
Publishers,1986,(14-64).
 Principles of physical geology, ARTHUR HOLMES, Ronald
Publishers,1945,(343-351).
WEBSITES
 www.wikipedia.com.
 http://www.topogeo.com
 http://oilandgas.com
 http://sc-geoscience.com
 http://geology.com