is a place where oil andgas are barred from further movement. The word trap was first applied
to a hydrocarbon accumulation by Orton ( 1889 ). A trap may contain oil,
gas or both depending on the chemistry and level of maturation of the
source rock and on the pressure and temperature of the reservoir itself.
It's one of five essential requirements for a commercial accumulation
for oil or gas which are :
- Organic-rich source : to generate oil and/or gas.
- Heat : source rock must have been heated sufficiently under high pressure with bacterial reaction to yield its petroleum.
- Reservoir rock : to contain hydrocarbons, this reservoir must have.
- Cap rock : which is impermeable rock that prevent the upward escape of petroleum to earth's surface.
- Arrangement : source, reservoir and seal must be arranged in such a way to trap petroleum.
Types of Petroleum Traps
Most classifications of traps are based on geometry of the trap but traps are generally classified into:
- Structural traps.
- Stratigraphic traps.
- Combination traps.
- Diapiric traps.
- Hydrodynamic traps.
Those traps whose geometry was formed by tectonic processes
after the deposition of beds involved. Basically structural traps are
caused by folding or faulting. Sometimes it's possible to find a trap
where a single process took place, but frequently the two processes can
be involved with equal importance in the creation of trap. Structural
traps are divided into :
- Anticline traps.
- Fault traps.
Where salt or mud have moved upward and domed the overlying
strata. They can be considered as structural traps, but since they are
caused by local lithostatic movement not regional tectonic forces they
should be differentiated. They are classified into :
- Salt diapirs.
- Mud diapirs.
Occurs when downward movement of water prevent the upward
movement of oil, thus trapping the oil without normal structural or
stratigraphic closure. Such traps are very rare.
Traps have a geometry that is formed by changes in lithology.
The lithological variations may be depositional (channels, reefs, and
bars) or post depositional (truncations and digenetic changes). levorsen
(1934) defined a stratigraphic trap as " one in which chief trapmaking
reservoir rock, such as a facies change, variable local porosity and
permeability, or an up-structure termination of the reservoir rock".
Stratigraphic traps are divided into :
- Pitchout traps : oil and gas are trapped where a layer of reservoir rock ends in a wedge surrounded by impermeable rocks.
- Unconformity traps : have been formed due to break in the depositional sequence of sediments.
- Isolated, lenticular bodies
: commonly of sandstone, form closed traps. Some of them are produced
because they are filled with oil without free water. Most of them are
small ex: channels and bars.
- Massive traps : consists of reef traps and massive erosion traps.
Many oil and gas fields around the world are not due to
structure or stratigraphy or hydrodynamic flow, but a combination of two
or more of these forces. Such fields may be termed as combination
traps. Most of these traps are caused by a combination of structural and
stratigraphic processes. Structural-hydrodynamic and
stratigraphic-hydrodynamic are very rare.