Algae, Bacillariophyta
Taken by a scanning electron microscope
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www.shell.com/us/alaska
CHAPTER ONE:
The Origins of
Oil and Gas
Sandstone
Close-up of textured pattern on a sandstone
Organic Origins
The story of oil and gas begins hundreds of millions of years ago when the Earth was
covered in swamplands filled with huge trees and the seas were teaming with microscopic
plants and animals. The oil and gas deposits started forming about 350 to 290 million
years ago during the Carboniferous Period, which gets its name from the basic element
in oil and gas: carbon. A popular belief is that oil comes from dead dinosaurs. It doesn’t.
The giant reptiles lived mostly from 250 to 65 million years ago, and most scientists
believe oil actually comes from the tiniest plants and animals that preceded them.
Source and Reservoir Rocks
As they died and sank to the ocean floor, the
decomposing organisms, along with mud and silt,
created hundreds of feet of sediment. Sand, clay
and minerals settled over this organic-rich mud
and solidified into rocks. The weight of the rocks
above pressed the mud into a fraction of its original
thickness. Heat from within the Earth cooked the
mud’s organic remains into a soup of hydrocarbons,
the main element of petroleum and natural gas.
Those liquids and vapors were emitted from these
source rocks, moved upward through the sediment
pores, and accumulated between the grains of the
sediment, or “reservoir rock.” The reservoir rocks often
contained water, which then pushed the lighter oil
and gas upward until they hit an impermeable rock
layer, such as mudstone or salt rock, which becomes a
“seal” or “cap rock.” The oil and gas are thus trapped
in “reservoir rocks,” usually sandstone or limestone.
So as you can see, petroleum* reservoirs aren’t
underground pools as is commonly believed. They
are actually rocks soaked in oil and gas, just as water
is held in a sponge. (The word petroleum comes
from the Latin for “rock oil.”) And they’re not
alone in that sponge-like home. Other substances
such as water, salt, carbon dioxide and hydrogen
sulfide can get trapped in the rocks too. Oil and gas,
however, contain mostly two elements: hydrogen
and carbon. How those elements are arranged
determines the form of the hydrocarbon. For example,
natural gas contains the simplest hydrocarbon,
methane, while crude oils can be made up of
more complex liquid and solid hydrocarbons.
Oil and Gas Varieties
The fluids and gases caught in these “traps” separate
into three layers: water at the bottom, oil in the middle
and natural gas on top. The oil, sometimes referred
to as “crude,” is usually black or dark brown, but it
can also be yellowish or even greenish. The different
* The terms “petroleum,” “hydrocarbons” and “oil and gas” are often used interchangeably throughout this text. For definitions and distinctions, refer to the glossary at the end of this chapter.
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THE ORIGINS OF OIL AND GAS
varieties of crude include “sweet,” (lower sulfur) and
“sour” (higher sulfur). The terms “light” or “heavy”
refer to the density of the crude and how easily it
flows. The heavier the crude, the harder and more
expensive it is to refine for use in products. Oil, rather
than natural gas, is what produces the gasoline used
in cars. Diesel, jet fuels, heating oil, lubricants and
asphalt also come from oil. Natural gas is a vapor,
but it can be pressurized and cooled to become a
liquid (LNG stands for liquid natural gas) for easier
transportation. The vapor is colorless and odorless, but
gas companies add a chemical to make it smell bad so
leaks can be more easily detected. Natural gas is a fuel
used to generate electricity; it is also used in plastics,
fertilizers, fabrics and other products.
The Migration of Hydrocarbons
As the tectonic plates of the Earth’s crust shifted over
millions of years, some of them would rise and others
sink, collisions between plates were common as the
land became sea and sea became land. Tremendous
THE ORIGINS OF OIL AND GAS
pressure from thousands of feet beneath the Earth’s
surface continued to push the oil and gas into areas
of less resistance. Sometimes the caps are not perfect
seals and petroleum escapes to the Earth’s surface
as natural seepage, which can be spotted by oily
residue on the surface soil and
rocks. Underwater seeps
can bubble up to the
surface and leave an
oily sheen. Early
consumers such as
Native Americans
used blankets to
skim oil off the
surface of streams and
lakes to use in medicines
and to water-proof canoes. While
seeps can signal that there is oil below ground, they
also tell us that much of the oil may have already
escaped and there may not be much left to find.
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Geological History of Alaska’s Oil
Beneath the Earth’s Surface
Most oil and gas fields are buried anywhere from one to
four miles under the Earth’s surface, with the record well
at more than seven miles. Reservoirs under the ocean
can be covered by as much as two miles of water on
top of that. Oil and gas are generally trapped together,
but sometimes they separate and form fields containing
predominately one or the other. If a reservoir lies deeper
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than three miles, the liquid petroleum can become
“overcooked,” leaving only the natural gas.
Types of Traps
The Earth’s tremendous forces can change the original
form of the reservoir rocks, moving them up, down
or sideways, and folding the layers into arches or
troughs. These shifts can determine the type of trap in
THE ORIGINS OF OIL AND GAS
which the petroleum resides. A “fold” trap is formed
when rock layers are pushed upward into an arch or
“anticline.” The hydrocarbons move into the uppermost
porous layer of the arch until they hit the impervious
rock seal. In a “fault” trap, the reservoir rock is sealed
off along a fault or fracture when earth movements
shift the impermeable rock layer over the reservoir
rock. Another kind of trap occurs when salt or some
other non-porous material is pushed up by pressure
THE ORIGINS OF OIL AND GAS
from within the Earth to create a “dome.” The salt
dome breaks through the layers and when it meets the
reservoir rock it blocks the path of the hydrocarbons,
which then accumulate around the pillar. Traps are
often a combination of these varieties because the forces
creating them can deform the rocks in so many ways.
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Glossary
Reserves Around the World
While most of the known oil and gas reserves
produced in Russia, Canada, China, Brazil,
are held in the Middle East, they can be
Norway, Mexico, Venezuela, Great Britain,
found in many places around the world,
Nigeria and the United States — chiefly
such as Australia, Italy, Malaysia and New
Texas, California, Louisiana, Oklahoma,
Zealand. The leading petroleum producers
Kansas and Alaska. Offshore reservoirs have
include Saudi Arabia, Iran, Iraq, Kuwait
been discovered in the North Sea, Africa,
and the United Arab Emirates. Oil is also
South America and the Gulf of Mexico.
Anticline – an arch of stratified rock in which the layers bend downward
in opposite directions from the crest. Anticlines form many excellent
hydrocarbon traps.
Cap – a layer of impermeable (non-porous) rock that seals hydrocarbons that
have migrated up into reservoirs.
Fault trap – a trap for oil or gas in which the closure forming the trap results
from the presence of one or more faults.
Hydrocarbons – organic chemical compounds of hydrogen and carbon
atoms forming the basis of all petroleum products. They may exist as gases,
liquids or solids. An example of each is methane, hexane and asphalt. For
this document the terms “hydrocarbons,” “petroleum” and “oil and gas” are
interchangeable.
Methane – a colorless, odorless gas that is stable under widely varying
pressure and temperature conditions in the absence of other compounds.
It is the lightest and most abundant hydrocarbon gas and the principal
component of natural gas.
Reserves – the unproduced but economically recoverable oil or gas in a
formation that has been proved by production.
Reservoir (Rock) – a subsurface body of rock having sufficient porosity and
permeability to store and transmit fluids.
Seal – a layer of impermeable rock that forms a barrier to hydrocarbons that
have migrated upward into a reservoir rock.
Seep – the leakage of hydrocarbons that have not been sealed into
reservoirs by impermeable cap rocks.
Source (Rock) – a rock rich in organic matter that if heated sufficiently will
generate oil or gas.
Tectonic – pertaining to the structure of the Earth’s crust.
Trap – a rock formation that contains petroleum that has migrated upward
from a source rock and became trapped by an impermeable layer above
and sealed into a reservoir.
Petroleum – a substance occurring naturally in the earth in solid, liquid, or
gaseous state and composed mainly of mixtures of chemical compounds of
carbon and hydrogen, with or without other nonmetallic elements such as
sulfur, oxygen and nitrogen. In some cases, especially in the measurement
of oil and gas, petroleum refers only to oil — a liquid hydrocarbon — and
does not include natural gas or gas liquids such as propane and butane. For
this document the terms “hydrocarbons,” “petroleum” and “oil and gas” are
interchangeable.
Sources
•
American Petroleum Institute
•
Schlumberger Oilfield Services
•
Encyclopedia Britannica
•
Society of Petroleum Engineers
•
Oil and Gas UK
•
The Learning Space (http://openlearn.open.ac.uk)
•
Natural Gas Supply Association
•
U.S. Minerals Management Service, Department of the Interior
•
New World Encyclopedia
Photo credits for Geological History of Alaska’s Oil and Gas timeline: Space, courtesy of NASA; Rock formation, courtesy of seakayakphoto.com; Zebra mussels, by Amy Benson/ U.S. Geological
Survey; Sea Lamprey, courtesy of the Environmental Protection Agency; Stoneflies, courtesy of the Environmental Protection Agency; Spider, by James Solomon, USDA Forest Service; Japanese
honeysuckle, by John J. Mosesso, National Biological Information Infrastructure (NBII); Amphibian, by John J. Mosesso, (NBII); Crocodile, by Thomas A. Hermann, (NBII); Lizard, by Randolph Femmer,
(NBII); Forest, by Randolph Femmer, (NBII); Bird, by John J. Mosesso, (NBII); Flock of birds, by John J. Mosesso, (NBII); Fox, by John J. Mosesso, (NBII); Flowers, by John J. Mosesso, (NBII); Fossilized
tracks, by Thomas A. Hermann, (NBII); Monkey, by John J. Mosesso, (NBII); Lion, by Thomas A. Hermann, (NBII); Glacier, by Russels Gletscher, © Free Software Foundation, Inc.; Pangeo map, by Kieff,
© Free Software Foundation, Inc.; Globes, by Antonio Snider-Pellegrini; Earth, courtesy of U.S. Department of Defense; Dead tree, by Magnus Manske; Allosaurus, by Grzegorz Wysocki; Skate, courtesy
of U.S. National Oceanic and Atmospheric Administration.
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THE ORIGINS OF OIL AND GAS
THE ORIGINS OF OIL AND GAS
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Shell in Alaska
To find and develop commercial
hydrocarbon resources in the Chukchi
and Beaufort Outer Continental Shelf.
To support communities where we
operate in benefiting from any potential
offshore activities economically
and socially. To respect the way of
life of the residents of Alaska.