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.”

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!

Here are four traps. The anticline is a structural type of trap, as

is the fault trap and the salt dome trap.

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!

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!

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.