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 Despite
the popular beliefs that oil and gas came from dinosaurs, Tyrannosaurs Rex did not have a
lot to do with forming fossil fuels. Fossil fuels are crude oil, natural gas, and coal.
There are two theories about how crude oil and natural gas were formed. Some scientists
believe that petroleum is the result of the decomposition of plants and animals, while
others suggest that decaying plants and animals had nothing to do with it.
Organic
Theory
The organic theory says that
oil and gas were formed from the remains of plants and animals. Scientists, who support
this theory, think that oil and gas were formed from the remains of small or microscopic
plants and animals that lived in prehistoric rivers and seas. When these plants and
animals died they combined with mud, silt, and sand to form layers of the mixture called
sediments. After thousands of years, a thick layer of sediment formed on the bottom of the
sea. As more layers were added the weight of the new layers applied pressure to the lower
layers and turned them into sedimentary rock. Scientists believe that high heat and
pressure, bacteria, chemical reactions, and other forces transformed these sediments into
oil and gas.
Inorganic
Theory
Other scientists believe in the
inorganic
theory that says that oil and gas were produced during the formation of the
solar system and the earth. The inorganic theory is often used to explain why oil and gas
are found in unexpected places and differences in chemical composition.
The
Science of the Earth
 Geology
is the science that involves the study of
the earth and the earth's origin,
composition, structure and history. Geology is the key
to finding new sources of useful earth materials and to understanding earth processes that
affect our lives. Scientists who study the origin, history, composition, and structure of
the earth and its life as recorded in rocks and other solid matter are called
geologists.
Because they study earth, geologists are
important in the search for mineral resources and petroleum. Geologists who are employed
by oil companies to determine whether a region may produce oil or gas are called
petroleum geologists.
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Unfortunately oil and gas do not exist is liquid pools
underneath the earth's surface. In fact, you could hold a rock containing oil in your hand
and would not be able to see the oil.
Important
Properties of Rocks
 For
rocks to contain oil and gas they must have two very important properties. The rock must
have a place for the oil and gas to be stored. These storage areas are called
pores. These pores are not visible to the naked eye
and must be viewed using a microscope. The number of pores that a rock contains indicates
how much oil or gas can be stored. The more pores the more oil or gas that can be
contained in the rock.
Scientists measure the amount of pore space by determining the
rock's porosity. Porosity is expressed
as the volume percent of the rock that contains open space and can range
from 5 to 30%.
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The
greater the porosity, the more oil and gas can be stored in the rock. Porosity does not
mean that there is oil or gas in the the rocks. In fact the pores could be filled with
water.
Not only must the rocks have pores, the pores must be connected so that
the oil and gas can move through the rock. Scientists measure the ability of oil and gas
to move through rock by determining its
permeablity.
Oil and gas in low permeability rock have difficulty moving out of the rock and into a
well. Permeability is measured in thousandths of a
darcy
or
millidarcys.
Sandstone
and carbonate rocks are generally the most
porous and permeable rocks and are therefore where scientists usually find oil and gas.
How Does Oil
Collect Under the Earth's Surface
 The
oil and gas formed millions of years ago did not stay in the same place.
The permeability of the rocks allowed oil and gas to move through the
rocks until they reached a layer of rock that had low permeability. This
is like having a sponge filled with water. Water can be added to the
sponge and will drip out of it.
However, if we put the sponge in a plastic
container, the water will stop dripping and will be trapped. Oil and gas will move toward
the surface until they reached a barrier.
Scientists call these barriers that hold oil and gas traps.
A trap prevents oil and gas from rising to the earth's surface. A trap consists of
a layer of rock that oil and gas cannot penetrate (impermeable) above a porous, permeable
layer that holds the oil and gas. These traps can take many forms but a common type would
be in the shape of an inverted bowl formed from impermeable rock. Traps are formed by
changes in the earth's crust such as folding or faulting. Scientists categorize traps by
how they were formed or how they look. The major types of traps are anticlinal,
fault,
plug,
stratigraphic, and
lenticular.
 In an anticlinal trap, the layers of rock are altered by geological
forces to create a concave shape or dome. Hydrocarbons move into the porous layers and are
trapped by an impermeable rock layer or
caprock.
A fault trap is formed by a shift or movement of rock layers that caused an impermeable
layer to create a barrier next to a layer of porous rocks containing hydrocarbons. Plug
traps are traps where a core of impermeable rock has moved through layers of rock to form
a trap.
These plugs are often composed of salt and salt domes
are an important formation that scientists look for when trying to find oil and gas. A
stratigraphic trap is created by either an impermeable rock layer over the top of a porous
rock layer or by a change of porosity and permeability within the reservoir itself.
Lenticular traps are formed by a change of permeability within the formation. These
changes in the amount of connected pore space can create hydrocarbon pockets within a
formation. The different trapping mechanisms can also occur in combination.
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It would be nice if scientists had some magical x-ray equipment that would
let them look under the earth's surface to locate oil and gas reservoirs. Unfortunately
this technology is not yet available and so scientists use other techniques to try to
develop a picture of the underground rock layers and crude oil and natural gas deposits.
Seismology
 Scientists use seismology to learn
about the formations below the earth's surface. They perform seismic
surveys to develop a map of the underground rock layers. These scientists,
primarily geologists and geophysicists, can identify the layers because each layer has a
different thickness and density.
Energy is directed into the surface and passes through
some layers and is reflected off of layers at different depths. This energy returns to the
surface at different angles and with different energy levels.
The path the energy takes
will be different depending on the shape of the surface it hits and the density of the
surface. The energy is in the form of sound waves.
The concept is similar to dropping a ball on a surface. If the surface is
flat the ball will bounce straight up. If the surface is curved the ball will return at an
angle. If the ball is bounced on a flat concrete surface it will return to nearly the same
height from which it was dropped. If the ball is dropped on the grass it may not return at
all. In seismic surveys the energy is returned to surface with a specific set of
properties.
The first step in performing a seismic survey is to create energy directed
into the surface. This energy is produced by small explosive charges, mechanical methods,
or sound waves. In the past scientists placed small explosive charges at various locations
on the surface to create sound waves. Today mechanical equipment is used to produce these
sound waves. In one device a steel slab is dropped onto the surface to create the energy
while another method generates low-frequency sound waves.
The next step is to measure the energy that is returned from the earth's
layers. Sensors on the earth's surface called
geophones
receive the reflected sound waves, translate them into electronic impulses, and send them
to the seismograph. The seismograph amplifies
and records the electrical signal and produces a picture or
seismogram.
The seismogram is a two-dimensional picture of the subsurface. It is generate a seismic
section, which is a two-dimensional slice from the surface of the earth downward. This
section is known as 2D seismic because it shows
the width and depth. The information from a seismic survey indicates the types of rock,
their relative depth, and whether a trap is present. The seismogram indicates the types of
rock, relative depth, and the presence of structural traps.
Scientists are now applying seismic technology to produce
three-dimensional images of the earth's subsurface. Three-dimensional seismic or 3D seismic creates an image that includes length,
width, and depth. Seismic crews obtain data on a 2 or 3 mile square of the earth's
surface. Large, high-speed computers analyze the data and create a picture of the surface.
Scientists can then take slices of this in a variety of directions to examine the
formation.
Learn how 3D
seismic surveys are conducted and how
the data helps to pinpoint underground oil and gas deposits.
This technique is also be used at different time periods to study the
movements of hydrocarbons. This produces a 4D seismic
history of the formation with time as the fourth dimension.
Although explorationists usually use geophones on the surface of the earth
to gather seismic data, they may also use them in an existing well. In this method,
scientists lower geophones into the well and attach them to the wall at intervals of 20 to
100 feet. This method is useful to obtain information about the geological structures
close to the well.
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 To
learn more about exploration, please check out the following
PetroStrategies
classes:
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For more information about how
scientists search for oil and gas, check out the following references:
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