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.

Oil Moving Through Pore Space In Sandstone

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
extract it.

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
rock’s porosity.

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.

Tiny Microfossils Make Up the Sea-Floor Ooze

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
rocks.

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

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 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
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.  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.

Four Types Of Structural and Stratigraphic Traps

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”

And, the main type of rocks they study are called sedimentary rocks. 
Most sedimentary rocks are formed in lakes, rivers, or oceans.
Rains
fall in higher elevations, forming streams, then rivers.  Rivers and
streams carve out tiny bits of solid rock from the landscape and carry
them downstream.  If the rock bits are fairly coarse (about the size of
salt grains, or larger), they are called sand.  If they are a little finer, they are called silt.  If the rock bits are really fine (like flour) they may be called mud, or clay.  Remember, sand, silt, and mud/clay refer to the size of the grains, not what they are made of.
At some point in their travel, the rivers slow down.  This may be
because the surrounding land is very flat, or the river may enter a
lake, or the ocean.  When the water slows down,  the grains of sand,
silt, or mud being carried by the river drop to the bottom and form layers of sediment.  Usually a layer will be mostly sand, mostly silt, or mostly mud, but they are often mixed up.
If you have been to a beach, you stood on a pile of sand that was eroded by the forces of rain and wind
from rocks many hundreds of miles away, was transported by a stream or
river for a long distance, was broken into tinier and tinier bits as it
traveled,  and was then spread out in a long, continuous bar by the work
of waves, tides, and wind.
A beach is just one type of many sand deposits that may become deeply buried and later become a huge oil or gas field!

Satellite Photo – Mississippi River Delta

Here’s a slightly more complicated example. Take a look at the satellite photo of the Mississippi River’s delta.  Delta is a fancy word for a big pile of sand that forms in an ocean or lake at the end of a stream or river.
In this case, the Mississippi River is bringing down a huge amount of
sediment that has been scoured from all over eastern North America, and
is forming new land (many miles long) right before our eyes, south of
the city of New Orleans, Louisiana.
Note the main channel of the Mississippi River snaking down through
the delta.  The darker areas on the picture show where land sticks up
(just barely, no more than a foot or two) above the surface of the ocean
and allows plant life to grow.  The lighter, whitish areas show
sediments (sand, silt, and shale) that are just under the surface of the
water.  The darker blue to the left shows deeper water.
The portion of the delta visible in the photograph is about 46 miles
long and 21 miles wide (74 X 33 km)!  Imagine if that gigantic pile of
sand and silt was buried thousands of feet deep.  We would have the
potential for an absolutely tremendous oil or gas field!

An Ancient Delta – Now a Gas Field!

This is where the study of geology starts to get very cool.  It
happens when we take modern-day examples like the Mississippi Delta
(above), and find ancient systems that are very much the same.

Ancient Red Fork Delta in Roger Mills County, Oklahoma – Now a Gas Field!

During Red Fork time (about 300 million years ago), most of Roger Mills County, Oklahoma was under water. 
Sediments from a river to the east poured into the ocean near the green
arrow.  The sand from the river spread out on the sea floor and formed a
huge stratigraphic trap called the Red Fork delta.  The Red Fork Delta
is represented by the greenish area.    
The brown triangles indicate oil and gas (mainly gas) wells that produce from the Red Fork formation.
Each of the blue squares is one
section of land, or one square mile.  So, the Red Fork Delta is about 24
miles long in the north-south direction and about 22 miles wide in the
east-west direction.  Not only is it big, the sediments of the Red Fork
Delta are now buried very deep…over two miles deep!

Look on the Scout Ticket page to see the scout ticket from the Carrel #1-11, a well completed in the Red Fork formation in this very field!