GEOLOGICAL FEATURES

There are three geological features that need to be present before oil may be present
underground: source rock, reservoir rock, and geological traps. The source rock is where
the oil was formed (if you accept the organic theory), but because it is relatively nonporous, it cannot hold oil in appreciable amounts. Instead, the oil migrates to more porous
rock like sandstone or limestone. These are examples of reservoir rock. It is possible for
the oil to move through the reservoir rock all the way to the surface of the earth.
However, this rarely happens because its progress is blocked by some impermeable rock
barrier. This causes the oil to accumulate to form a reservoir. The barrier and the resulting
reservoir form what is known as a trap.

 Reservoir Rock
The oil that migrates through the reservoir rock is not pure oil. Rather it is a mixture of
oil, water, and natural gas. When the reservoir forms, the three components will separate,
with the gas at the top, the oil in the middle, and the water at the bottom. Depending on
the pressure in the reservoir, the gas may stay in solution. If the gas does form a separate
layer at the top, it is referred to as the gas cap. It is important to note that the
oil/water/gas mixture does not form a large pool of liquid as some people often envision;
it is actually dispersed throughout the reservoir rock. 

Traps
There are two basic kinds of traps: structural and stratigraphic. Structural traps are the
result of deformations of the rock layer. Examples of structural traps are anticlines and
fault traps. The fault trap is associated with the shifting of fault layers along a fault line,
something that we are familiar with as the cause of earthquakes if the shifting motion is
strong enough. Stratigraphic traps form when reservoir rock is cut off by a horizontal
layer of impermeable rock. The figure above shows oil pooling in the two different types
of structural traps. The dome-like structure on the right is an anticline, while the structure
on the left is a trap formed along a fault.
There are three basic forms of a structural trap in petroleum geology:
• Anticline trap
• Fault Trap
• Salt Dome Trap
The common link between these three is simple: some part of the earth has moved in the
past, creating an impedence to oil flow.
Anticline Trap
An anticline is an example of rocks which were 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 (provided, of course, that there is an
impermeable trap rock above the arch to seal the oil in place).

Fault trap

Fault traps are formed by movement of rock along a 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. Clays within the fault zone are smeared as the layers of rock slip past one another.
This is known as fault gouge.

A cross section of rock showing a fault trap - in this case, an example of gouge.This is because the reservoir
rock on both sides of the fault would be connected, if not for the fault seperating the two. In this example, it
is the fault itself that is trapping the oil.
Thrust Fault
Thrust faulting occurs when one section of the Earth is pushed up and over another
section, and they most often occur in areas where two continental plates are running into
one another. However, the photos below show sediments that were deposited by glaciers
only 10,000 years ago, and these sediments were then run over by a glacial readvance.
When the glacier moved back over the sediments, faulting occured. The faults below can
be clearly seen.

Below you can see the faults and rock horizons drawn in If the conditions were right, oil
might become trapped in this rock.

Also drawn in is the possibility of oil being trapped by the shale above it, as well as by
the fault and the shale to the left of it. Of course, this outcrop is only a couple of meters
wide, there really is no oil here, and the layers that we've assigned to the rock are mostly
imaginary in this case. But the point is, this is exactly how many structural traps are set
up below the Earth's surface.
Salt Dome Trap
Salt is a peculiar substance. If you put enough heat and pressure on it, the salt will slowly
flow, much like a glacier that slowly but continually moves downhill. Unlike glaciers, salt
which is buried kilometers below the surface of the Earth can move upward until it
breaks through to the Earth's surface, where it is then dissolved by ground- and rainwater. To get all the way to the Earth's surface, salt has to push aside and break through
many layers of rock in its path. This is what ultimately will create the oil trap. 

Here we see salt that has moved up through the Earth, punching through and bending rock along the way.
Oil can come to rest right up against the salt, which makes salt an effective trap. However, many times, the
salt chemically changes the rocks next to it in such a way that oil will no longer seep into them. In a sense,
it destroys the porosity of a reservoir rock.
Stratigraphic Trap
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 good
reservoirs because of the many pore spaces contained within. On the other hand, shales,
made up of clay particles, do not make good reservoirs, because they do not contain large
pore spaces. Therefore, if oil migrates into a sandstone, it will flow along this rock layer
until it hits the low-porosity shale, thus forming a stratigraphic trap.

The above series of diagrams is an attempt to illustrate a type of stratigraphic trap. In the
diagram at the upper left, we see a river that is meandering. As it does so, it deposits sand
along its bank. Further away from the river is the floodplain, where broad layers of mud
are deposited during a flood. Though they seem fairly constant, rivers actually change
course frequently, eventually moving to new locations. Sometimes these new locations
are miles away from their former path.
In the diagram at the upper right, we show what happens when a river changes its course.
The sand bars that were deposited earlier are now covered by the mud of the new
floodplain. These lenses of sand, when looked at from the side many years later (the
bottom diagram), become cut off from each other, and are surrounded by the mud of the
river's floodplain - which will eventually turn to shale. This makes for a perfect
stratigraphic trap.