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A fault is a break in the layers of rock. A fault trap occurs
when the formations on either side of the fault move. The formations
then come to rest in such a way that, when petroleum migrates into
one of the formations, it becomes trapped there. Often, an
impermeable formation on one side of the fault moves opposite a
porous and permeable formation on the other side. The petroleum
migrates into the porous and permeable formation. Once there, it
cannot get out because the impervious layer at the fault line traps it.

Anticlinal Traps

An anticline is an upward fold in the layers of rock, much like a
domed arch in a building. The oil and gas migrate into the folded
porous and permeable layer and rise to the top. They cannot escape
because of an overlying bed of impermeable rock.
Stratigraphic Traps

Stratigraphic traps form when other beds seal a reservoir bed or
when the permeability changes within the reservoir itself. In one
stratigraphic trap, a horizontal, impermeable rock layer cuts off, or
truncates, an inclined layer of petroleum-hearing rock. Sometimes a
petroleum-bearing formation pinches out – that is, an impervious layer
cuts it off. Other stratigraphic traps are lens-shaped. Impervious layers
surround the hydrocarbon-bearing rock. Still another occurs when the
porosity und permeability change within the reservoir itself. The upper
reaches of the reservoir are nonporous and impermeable; the lower
part is porous and permeable and contains hydrocarbons.

Other Traps
Many other traps occur. In a combination trap, for example,
more than one kind of trap forms a reservoir. A faulted anticline is an
example. Several faults cut across the anticline. In some places, the
faults trap oil and gas. Another trap is a piercement dome. In this case, a
molten substance – salt is a common one – pierced surrounding rock

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