Oil and Gas Traps
Oil and Gas in Rocks
You may have heard that oil is found underground in "pools", or
"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. The stuff below is
extremely important!
Almost all oil and gas is found within the tiny spaces in sedimentary rocks, mainly
sandstone 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 either water,
oil or gas. 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,
and they stay there, for millions of years, until Petroleum Geologists come to find
it and get it out.
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.
Two new geologists might have a conversation that goes something like this:
David: Hey,
Steph!? What's the depth of that new sandstone we just drilled through?
J.D. wants to go upstairs and give them some good news.
Steph:
The top is at 11,622'. Here's a piece of the mud
log that just came in. Val already has it in the computer.
David: We sent
sidewall core samples of that sandstone off to the lab so they could accurately measure the contents of the pore
space. What did the lab say about that?
Steph: They said the
porosity was 10%, and the porosity was saturated with 75% oil and 25% gas. That's a pretty exact
measurement, you know. We don't get that many exact measurements.
David: OK, let me
think about this... 1 cubic foot of sandstone contains .10 cubic feet of
pore space (1 cubic foot X .10% porosity). If that pore space is 75%
oil-saturated and 25% gas-saturated, that means there are .075 cubic feet of
oil in place inside each cubic foot of sandstone, and also .025 cubic
feet of gas in place inside each cubic foot of sandstone. Steph, if we knew
the thickness of the sandstone and the width and length of the sand body, we
could estimate the total Oil and Gas In Place!
Steph: Well, I've
been working on that. I've completed several new maps.
That sand body is about 20 feet thick, 5000 feet
long, and 300 feet wide. We may find it's bigger if we drill more
wells. But let's stick to those numbers for now.
David:
Right. We'll know the exact thickness and a lot more when we get the electric
logs. But J.D. needs something to show the bigwigs right now
Always a crazy rush around here!. We don't have a lot of time to come up
with answers. Let's see, 20 feet thick X 5000 feet long X 300 feet wide is 30,000,000 cubic
feet of sandstone. Since we already figured out that each cubic foot of
sandstone contains only .075 cubic feet of oil, that means there is 30,000,0000
X .075...or 2,250,000 cubic feet...of oil in that formation. Since there
are 5.61 cubic feet to the US barrel, that's about 401,070 barrels of
Oil In Place (30,000,000 X .075) / 5.61). Heh, heh! That's a lot
of calculating for my old brain. Steph, you figure out the gas later!
Steph: OK.
Ken (the reservoir engineer) calculated there would be a 20 percent Recovery Factor for
oil. That means we will only be able to recover about 20% of the Oil In
Place.
David: That gives us
about 80,214 barrels of oil we can expect to recover (401,070 X .20%), plus the
gas we haven't figured up yet.
That's a pretty small oil field.
Steph
Yes, but every barrel counts! Our country needs energy! And every
barrel we can produce at home is a barrel we will not have to import! Who
wants to depend on outsiders for everything? They might cut off our
supply!
David: You're
right. Tell J.D we expect this well to produce oil and gas, and we'll propose
another well in this area over here.
With luck, the second well will be producing by October! That will provide more oil and
gas for people to heat their homes this winter!
Steph: OK.
I'll let J.D. know we have a winner.
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 deep, 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 gone away from 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
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.
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 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 trillions upon
trillions of
them. When these creatures die, they sink to the bottom and become part of
the shale sediments there.
The animals die 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 all shale rocks.
Sea-Floor Gunk
Of course, whales, sharks, and fish die too, and their bodies end up on the ocean bottom,
where they decay, and also become part of the shale. And, over the long periods of
geologic time, animals that are now extinct, like Trilobites and Ammonoids, lived and
died
in the oceans.
But, it is the trillions of tiny animals that have made up most of the living
gunk (the scientific name for this gunk is "ooze") deposited on the ocean
floor. You have probably heard of the Ozone Layer. You probably did not
know there was an "OOZONE LAYER", too! Well, it's not really called that,
but that's what it is! Just a mixture of sand, silt, mud, and the bodies of
ocean animals piled up on the sea floor. Sea-floor gunk!
Later, when thousands of feet of shale have piled up over millions of years, and the
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.
Movin' Out
At first, the oil and gas only exist between the shale particles as extremely tiny blobs.
Then, the intense pressure of the earth squeezes the oil and gas out of the shale,
and the oil and gas fluids move sideways 
many, many miles. On their way, they may
meet up with other traveling oil fluids.
Finally, the oil and gas may become "trapped" in a rock formation like sandstone
or limestone....a trap they can't escape! The oil and gas stay there, under
tremendous pressure, until the PG comes to get it. After they are formed, oil and gas must
be "trapped" in order to remain in place until it can be found. Without a trap, the PG has no place
to drill. All oil and gas deposits are held in some sort of trap.
There are two basic types of traps:
Structural 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.
Stratigraphic traps are depositional in nature. This means they are
formed in place, usually by a 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.
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 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 rocks.
The hole at the right has been drilled into a sandstone that was deposited in a
stream bed. This type of sandstone follows a winding path, and can be hard to hit
with a drill bit!
This type of sandstone is usually enclosed in shale, making this a
stratigraphic trap. This looks exactly like the kind of trap Steph and David were talking about at the top of this page.
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 !
Your well may find a producing reservoir very
near the surface. Or 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!
Even though oil and gas are not easy to find, they are found
in commercial quantities in many
areas of the United States. This map shows most of these areas. It's
really a crummy map, and not very accurate, and I need to replace it
sometime. But for now, this is the map.
Finally, structures in the earth can give the PG many challenges.
Look at the diagram to the
right. 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 well!
Too bad. Your second well is a DRY HOLE.
First 3 diagrams, A Primer of Oil and Gas Production , 4th diagram, Pennsylvanian
Sandstones of the Mid-Continent
