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
Most oil and gas deposits are found in sandstones and coarse-grained
limestones. A piece of sandstone or limestone is very much like a hard
sponge, full of holes, but not compressible. These holes, or pores, can
contain water or oil or gas, and the rock will be saturated with one of
the three. The holes are much tinier than sponge holes, but they are
still holes, and they are called porosity.
The oil and gas become trapped in these holes, stays there, for
millions of years, until petroleum geologists come to find it and
When you hold a piece of sandstone containing oil in your hand, the
rock may look and smell oily, but the oil usually won’t run out, and you
can’t squeeze sandstone like a sponge! The oil is trapped inside the
Oil Formation and Oil Movement
The very fine-grained shale we talked about previously is one of the
most common sedimentary rocks on earth. In many places, thousands upon
thousands of feet of shale are stacked up like the pages in a book, deep
underground. It is not unusual to have layers in the earth’s crust
made up mostly of shale that are 4 miles thick. These shales were
deposited in quiet ocean waters over millions of years time.
During much of the earth’s history, the land areas we now know as
continents were covered with water. This situation allowed tremendous
piles of sediment to cover huge areas. The oceans may have left the
land we now live on, but the great deposits of shale and sandstone
remain deep underground….right under our feet!
The Tiny Gigantic Kingdom
In the deep ocean, far from land, about the only sediment deposited is the fine-grained clastic rock known as shale.But
what about the oil and gas? For the answer, we need to move to the
ancient oceans that once covered almost all of the earth.
A lot of other material is deposited along with the clay or mud-sized
sediments. We often think of sharks and whales as being the kings of
the deep oceans. Actually, there are other animals that have
established giant kingdoms in the sea…the largest and most impressive
kingdoms of all! These animals are the various kinds of microscopic
creatures….both plant and animal. Most of them would fit on the head of
a pin. They are tiny, but there are uncountable trillions of them.
When these creatures die, they sink to the bottom and become part of
the sediments there that will eventually turn into shale.
The animals die by the trillions and rain down on the ocean floor all
the time. And since the beginning of life on earth, they have been
living their exciting lives in the ocean, dying, sinking to the bottom,
and becoming part of the once-living matter that is part of most shale
It is the trillions of tiny animals that make up most of the gunk
(the scientific name for this gunk is “ooze”) deposited on the ocean
floor. It’s a very fine-grained goop containing a lot of organic
material mixed with the clay-sized particles that form shale. It is
called organic-rich shale.
Later, when thousands of feet of organic-rich shales have piled up
over millions of years, and the dead animal bodies are buried very deep
(more than two miles down), an amazing thing happens. The heat from
deep inside the earth “cooks” the animals, turning their bodies into
what we call hydrocarbons……oil and natural gas.
At first, the oil and gas only exist between the shale particles as
extremely tiny blobs, left over from the decay of the tiny animals.
Then, the intense
pressure of the earth squeezes the oil and gas out of the shale, and
the oil and gas fluids gather together in a porous layer and move
sideways many miles. On their way, they may meet up with other
traveling oil or gas fluids.
Finally, the oil and gas may become “trapped” in a rock formation
like sandstone or limestone….a hydrocarbon trap. The oil and gas stay
there, under tremendous pressure, until the petroleum geologist comes
looking for it. Without a trap, the geologist has no place to drill. All oil and gas deposits are held in some sort of trap.
The Two Types of 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 at the right. 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
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 are depositional in nature. This means they are
formed in place, often by a body of porous sandstone or limestone
becoming enclosed in shale. The shale keeps the oil and gas from escaping
the trap, as it is generally very difficult for fluids (either oil or
gas) to migrate through shales. In essence, this kind of stratigraphic
trap is surrounded by “cap rock.”
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 below)
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.
Some diagrams from “A Primer of Oil and Gas Production” and “Pennsylvanian Sandstones of the Mid-Continent”