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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 (or even most of it).

Almost all oil and gas is found within the tiny spaces in sedimentary rocks, mainly sandstone and coarse-grained limestones.  Imagine that a sponge is a hunk of sandstone or limestone.  The sponge is full of holes, or "pores", that can contain water or oil or gas.  Limestone and sandstone, even though hard, also contain lots of holes.  The holes are much tinier than sponge holes, but they are still holes, 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.

How do oil and natural gas get into the rocks in the first place?   There are several ideas about how this happens, but one idea is very popular , and it is called...

The Big Idea of Oil Formation and Oil Movement

This sounds very important, and it is, but it's not hard to understand.   If you know this, you will know more than most everyone else about where oil comes from and how it gets there.

The very fine-grained shale we talked about before 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 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.  When these creatures die, they sink to the bottom and become part of the shale sediments there. 

The animals are tiny, but there are trillions and trillions of them, and they 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 rot, 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 dies 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, 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 oil.  Natural gas is also produced during this time.
 
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 that 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 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, 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 sandstone ending up enclosed in shale.  The shale keeps the oil and gas from escaping the trap.

Two kinds of strat traps are shown here.

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.

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.  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?

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