There are two main types of traps that can contain oil and gas deposits underground: structural traps and stratigraphic traps. Structural traps form when rock layers are bent or broken through geologic processes like folding or faulting. Stratigraphic traps form when permeable rock layers, like sandstone, become surrounded by impermeable layers, like shale, that trap the oil and gas inside. It is important for identifying potential oil and gas reservoirs that petroleum geologists understand the complex variations in rock layers and how different rock types can trap hydrocarbon deposits in different ways either structurally or stratigraphically.

1. Oil and Gas Traps
All oil and gas deposits are found in structural or stratigraphic
traps. You may have heard that oil is found underground in
“pools,” “lakes,” or “rivers.” Maybe someone told you there
was a “sea” or “ocean” of oil underground. This is all
completely wrong, so don’t believe everything you hear.
Beneath the earth's surface, oil will ooze through rocks if there
is enough space between them, but this oil will not accumulate
into large quantities unless something traps it in a particular
place. There are a variety of geologic traps, which themselves
can be broken into categories:
 Structural trap types: anticline, fault, salt
 Stratigraphic trap types: unconformity, lens, pinch-out
It is important to remember that oil is not all by itself in some
sort of underground cave, but is, instead, contained within solid
rock - which has enough room within it to actually soak up (or
become soaked in, however you look at it) oil.
Structural Traps
These traps hold oil and gas because the earth has been bent and
deformed in some way. The trap may be a simple dome (or big
bump), just a “crease” in the rocks, or it may be a more complex
fault trap like the one shown below. All pore spaces in the rocks
are filled with fluid, either water, gas, or oil. Gas, being the
lightest, moves to the top. Oil locates right beneath the gas, and
water stays lower.
Once the oil and gas reach an impenetrable layer, a layer that is
very dense or non-permeable, the movement stops. The
impenetrable layer is called a “cap rock.”

2. Stratigraphic Traps
Stratigraphic traps are depositional in nature. This means they
are formed in place, often by a body of porous sandstone or
limestone becoming enclosed in shale.
A stratigraphic trap accumulates oil due to changes of rock
character rather than faulting or folding of the rock. The term
"stratigraphy" basically means "the study of the rocks and their
variations". One thing stratigraphy has shown us is that many
layers of rock change, sometimes over short distances, even
within the same rock layer. As an example, it is possible that a
layer of rock which is a sandstone at one location is a siltstone
or a shale at another location. In between, the rock grades
between the two rock types. From the section on reservoir rocks,
we learned that sandstones make a good reservoir because of the
many pore spaces contained within. On the other hand, shale,
made up of clay particles, does NOT make a good reservoir,
because it does not contain large pore spaces. Therefore, if oil
migrates into the sandstone, it will flow along this rock layer
until it hits the low-porosity shale. Voilà, a stratigraphic trap is
born!

3. Here are four traps. The anticline is a structural type of trap, as
is the fault trap and the salt dome trap.

4. The stratigraphic trap shown at the lower left is a cool one. It
was formed when rock layers at the bottom were tilted, then
eroded flat. Then more layers were formed horizontally on top
of the tilted ones. The oil moved up through the tilted porous
rock and was trapped underneath the horizontal, nonporous
(cap) rocks.
Another Stratigraphic Trap
This hole has been drilled into a sandstone that was deposited in
a stream bed. This type of sandstone follows a winding path,
and can be very hard to hit with a drill bit! The plus is that old
stream beds make excellent traps and reservoir rock, and some
of these fields are tens of miles long!
This type of sandstone is usually enclosed in shale, making this
a stratigraphic trap.
Just because you drill for oil or gas does not mean that you will
find it! Oil and gas reservoirs all have edges. If you drill past
the edge, you will miss it ! This might explain why your
neighbor has a well on his land, and you do not!

5. Stratigraphic Problems When Drilling
When you drill, you may find a producing reservoir very near
the surface. But many other things can happen:
You might drill into a reservoir that has been depleted (all the
oil and gas removed) by another well. There may be a new infill
reservoir between two wells that could be developed with a third
well. Or one that was incompletely drained. Maybe if you drill
a little deeper you might hit a deeper pool reservoir! You might
be able to back up and produce a bypassed compartment. The
petroleum geologist has to think of all these things when
planning a new well!

6. Structural Problems When Drilling
Finally, structures in the earth can give the PG many challenges.
Look at this diagram. Imagine you first drilled the hole on the
left into the green layer which represents a nice oil and gas-bearing
rock. YES! You have a great well, producing lots of oil
and gas!
Then you drilled your second hole to the east (right) of the first
one. What happened to that hole?
Answer: The oil reservoir has been split in two by the fault,
which is nothing but a place in the earth where rock layers break
in two. The arrows on the diagram show that the rocks moved
DOWN on the LEFT side of the fault and UP on the RIGHT
side of the fault. This created a GAP in the oil field……right
where you drilled your second hole! Incredibly bad luck! Or,
bad seismic! Your second hole is a DRY HOLE.