AINA ZA TRAP ZA MAFUTA NA GESI (OIL AND GAS TRAPS)

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”

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HIVI NDIVYO VITU 5 AMBAVYO PETROLEUM GEOLOGIST HUVIFANYA

Prospecting is the work the petroleum geologist does to locate a place to drill a test hole, which hopefully can be turned into a producing oil or gas well!
Most petroleum geologists work in an office, where they have access to a lot of data. This includes electric logs, core records, drilling records, scout tickets, and production data. They use the data to construct maps, cross-sections, and databases.  These tools help them locate the best places to drill their test holes.
The geologist studies his maps and cross-sections and runs computer simulations that help him select the next best location to drill. He is always thinking about the next drilling location…or prospect!
He will want to know what type of trap he is dealing with, and the composition of the sedimentary rocks he will be drilling through. He needs to estimate the porosity of his prospective “pay zone.” He wants to know if dangerous high pressures can be expected in the new hole. If seismic data is involved in the prospect, he will consult with the geophysicist and get his opinion of the prospect.
The geologist is always interested in anything that happens in her area, particularly news of new discoveries by other companies!  If she sees a promising new area, she will recommend to the land department that an attempt be made to lease the land; the leased acreage will then be available for drilling later.
When the geologist has finally found the correct spot, she spends much time cross-checking to ensure she has not missed anything. She wants to make sure she is not “surprised” by any of the following:

• Discovering the selected location was already drilled by another company 40 years ago (and was dry)
• Discovering that her company has no legal right to drill on the location (lease problems)
• Unexpected faults or other geologic problems that crop up during drilling and ruin the prospect
• Discovering that the hole is being drilled in the wrong place after drilling begins (a very bad thing that has actually happened!)

Step 2 – The Petroleum Geologist Packages The Deal

Once the new location is defined, and the geologist is satisfied the prospect is a good one, the work is just beginning. He has a large amount of rough data available in the form of work maps, that he used to satisfy himself of the feasibility.  Now, he must condense this large data mass into a set of presentation materials that can be shown to non-geologists.
To package the deal, he will prepare sets of simplified maps and cross-sections, highly-colored and attractive to the eye. He may use Powerpoint, or other presentation software. Creativity, design sense, and art skills are important during this phase. He needs to anticipate all possible questions, and be prepared to answer each one of them. He must be very sure of himself and his facts before he moves to the next step.

Step 3 – The Petroleum Geologist Sells the Deal

Now the geologist must step into a role that is often uncomfortable for him … selling his prospect. It may be uncomfortable because geologists are scientists, with scientific backgrounds and schooling. They are used to talking to other scientists. But now the geologist must become a salesman in order to convince people who are not geology experts of the value of the prospect.  These people may include managers, bankers, engineers, and oil and gas investors.
Of course, the geologist wants to see her prospect drilled.  But she will take great pains to ensure all her data is presented accurately and concisely.  She is bound by her own integrity, the integrity of any professional group she is a member of, and the integrity of her profession.

She is looking to convince her clients that

• the prospect is worth drilling,
• investors will get a fair return for their money
• the provided financing will be money spent wisely

Even an inexpensive test hole can cost a couple of million dollars, and some exploration tests may run into many tens of millions! So the geologist wants to be very sure of her facts.  All her clients must believe the proposed well has a reasonable chance of being successful.
The geologist will meet with the landman (females in the business are also called “landmen”), who will ensure the company has the legal right to drill in the chosen spot. He will consult with the engineer, who will determine the exact cost of drilling and completing the hole.  Marketing personnel will ensure the company has a market (buyer) for the oil, or a pipeline for the gas. Managers, responsible for ensuring the company’s drilling budget is spent wisely, will also approve the test. If outside financing will be used, the geologist will explain the prospect to representatives of the bank or other individuals or partnerships that put up the money.
When he’s done, the geologist will have “sold” his prospect to anywhere from a few to several dozen people.

Step 4 – The Petroleum Geologist Monitors the Drilling of the Test

Next comes the part that every geologist enjoys the most! Drilling the hole! It has now been several months since the geologist started working on his prospect. Now the surface owners have been paid, permits acquired, and money raised.  Roads and the drilling location have been built, pipe and supplies have been ordered.  Also, the energy company has engaged a drilling contractor who owns and operates the drilling rig.
The drilling contractor will drill the hole in the manner specified by the company. The contractor will have leeway to select the type of drill bits to be used, hire a drilling crew, and make many other decisions concerning the actual drilling.  Virtually all holes are drilled by contractors.
Drilling a hole is a very complex procedure involving many people and many critical steps. Nearly everything must go right. Dangerous machinery, bad weather, and continuous mechanical failures are faced daily. The work goes on for weeks to months, 24 hours a day, nonstop. A slip-up at any point can ruin the very expensive hole, cost a fortune, or get people killed.
The geologist will closely monitor all aspects of the drilling as it takes place. She will select an electric-logging company, and the proper wireline logging tools to evaluate the hole. She will usually hire a mud logging contractor to “sit” the well day and night.  The mud logger will study the well cuttings, report shows of oil and gas, and keep track of other things on the location. The geologist will monitor the formation tops as they are encountered, and discuss the progress of the drilling with the investors. The geologist will decide where and when to take cores or drill-stem tests. Finally, after the hole is logged with electric logs, she will examine the logs and recommend the hole be completed or plugged.

Step 5 – The Petroleum Geologist Works With The Engineer to Complete the Well

At last the hole is drilled!  At this time, a decision must quickly be made to attempt a completion and make the hole a well, or plug the dry hole.  Completion costs are extremely high, so it must be believed the expensive completion will be worth the money. No one wants to throw good money after bad. Justifying a completion can be a grueling process. It almost always takes place in the middle of the night!
The job of completing the hole is mainly in the hands of the petroleum engineer. The engineer will decide the type of casing to use, and the method of cementing, He will design the completion procedure (which may involve perforating, breakdowns, acid jobs, fracks, and pumps). However, he will depend on the geologist to advise him on various topics. To start, the geologist will give the engineer a list of formation tops, and tell the engineer exactly which zones should be tested.
The geologist is often the person most familiar with the technical practices of other oil companies in the area. No two companies are exactly alike; some may come up with better ideas for certain processes.  The geologist may be familiar with the most successful fracturing or breakdown procedures of other companies. He might suggest a certain style or method of perforation, or offer advice on cementing techniques. He will relay this information to the petroleum engineer, who will usually be thankful for the help! Working as a team, the geologist and petroleum engineer will get the new well completed, and put it to work providing energy for all of us!

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GEOLOGIST START WITH SEDIMENTARY ROCK

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!

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