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The Common Oil and Gas Traps
The 5 W’s Writing Assignment (Oil and Gas
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7/21/2009
Jocelyn Manlongat
In my research almost all oil and gas is found within the tiny spaces in sedimentary rocks, mainly
sandstone and coarse-grained limestone. 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. 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. In my
essay I will write about all the exploration and discovery of oil and gas traps. In the body of my essay I
will cover the topic that include “Who, What, When, Where, and Why” the common oil and gas traps
are very important in the industry. The last part is the summary of my essay.
Geologists, geochemists, and palynologists, scientists who study pollen and small pollen and
small fossils, examine the cuttings to learn more about the age, chemistry, porosity, permeability and
other properties of the sub surface rock formations.
People who explore for oil and gas traps are called Exploration geologists and Geophysicists.
Their main job is to find subsurface traps that could contain hydrocarbons.
Geologists use a different technique for discovering oil and gas traps. Typically, a geologist will
take known points of control from well logs of existing wells. With this data, the geologist will plot the
wells on a map, noting the presence, thickness, quality and subsurface depths of the various strata.
Then, through a combination of experience, knowledge and (to varying degrees) instinct, the geologist
extrapolates the data to form a three dimensional picture of what lies between the known control
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points. These days, most geologist and geophysicists overlap this process using seismic data to confirm
their geologically rendered maps or vice versa.
In seismic exploration, geophysicists are looking for traps buried deep beneath the surface. So,
the sound has to travel not through air but through rock. Also, the sound has to have the right
characteristics to go through thousands of feet of rock and back up to the surface. Further, the seismic
work of a geophysicist usually covers many miles of surface area in order to increase the chances of
finding a potential reservoir.
Regardless of how the sounds are made, they enter the layers of rock
below the surface, and each rock layer reflects some of the sound back, just as a
cliff reflects the sound of a voice back. These reflections of sound, or echoes,
must be heard or detected. Several sensitive detectors called geophones in land
exploration and hydrophones in offshore exploration are used to pick up the echoes. On land, the
geophones are usually strung out in a line or array behind truck and are connected together with
electric cable. The cable goes to recording equipment that is usually housed in a truck. Offshore, the
hydrophones are trailed in a line behind a boat, and the boat carries the recording equipment.
In either case, when the reflected sound comes bouncing back from the rock layers, it reaches
the geophones (or hydrophones, as the case may be) on the surface. The geophones convert the sound
vibrations to electric current, which is sent along the cable to a tape recorder. The tape holds a record of
the echoes. In a laboratory, the tapes are played back into computer, and record sections (also called
seismic sections) are made. Expert interpretation of record sections - a sort cross section of the earth
can reveal what may be a trap for petroleum.
It is important to realize that exploration techniques only indicate where an oil and gas trap may
be. In most cases, record sections do not give any direct indication of the presence of hydrocarbons in a
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trap. However, a seismic phenomenon known as bright spot shoes up on a seismic section as a sound
reflection that is much stronger or weaker than usual. A bright spot sometimes indicates natural gas in a
trap. It is safe to say, however, that the only sure way to find out whether hydrocarbons are in a trap is
to drill a hole down to it.
The animals die and rain down on the ocean floor all the time, and lives in the ocean, dying,
sinking to the bottom, and becoming part of the once-living matter that is part of all shale rocks. 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 become oil and natural gas.
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 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 Professional Geologist 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 Professional Geologist has no place to drill. All
oil and gas deposits are held in some sort of trap.
In a typical trap, gas accumulates on top of the reservoir as a gas cap over the oil leg which in
turn overlies the water-saturated zone in the reservoir. This occurs because natural gas is lighter than oil
which is lighter than oil which is lighter than water. However, all three fluids are oft en intermingled in
parts of the reservoir. Porosity is the ability of a rock to hold oil and gas like water in a sponge.
Permeability indicates how easily fluids can flow through the rock.
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A trap requires three elements:

A porous reservoir rock to accumulate the oil and gas—typically sandstone, limestones and
dolomites

An overlying impermeable rock to prevent the oil and gas from escaping

A source for the oil and gas, typically black waxy shales
This topic is very interesting to research and easy to understand. I have more knowledge of how
oil and gas traps stayed and develop to become hydrocarbons. This topic is very unique of how oil and
gas traps have different styles of form to develop reservoir rock formations that halt the natural upward
migration of hydrocarbons from the source rock to the surface of the earth. This topic is so amazing to
research because it involved the people who discover and studies the reservoir rock. Today, the
technologies have developed better tools to search for oil and gas traps.
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References
http://flash1r.apa.org/apastyle/basics/index.htm
http://search.datapages.com/data/doi/10.1306/2F9185AE-16CE-11D7-8645000102C1865D
http://www.fossiloilandgas.com/exploration-guide.htm
http://www.geomore.com/Oil/%20and%20Gas%20Traps.htm
http://www.abheritage.ca/abresources/inventory/resources_hydro_exploration_traps.html
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