Chapter I: General Introduction
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The nature of oil and gas
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Petroleum
 The word petroleum comes from the Greeks: petro means
rock, and oleum means oil.
 In strict sense petroleum includes only crude oil; by usage it
includes both crude oil and natural gas
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What is Oil?
Oil is a natural substance, found in sedimentary rocks. The oil has
been generated, over a very long time, by the decomposition of little
organisms which settled on the sea bottom and were then covered by
waterproof sediments.
Oil is a renewable resource on a geological time scale (millions of
years), but we are using it up much faster than it is being created.
Oil has been well known since the ancient times when it was used as
medicine, lubricant, for waterproofing and for lighting. It has been
used as fuel only since the beginning of the 1900’s.
Petroleum occurs
as a liquid (oil)
as a gas (gas)
as a solid (bitumen)
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Crude oil
 Both crude oil and natural gas are mixtures of molecules
formed by carbon and hydrogen atoms
 Different types:
 Heavy crude oils are very thick and viscous and are difficult
to produce
 Light crude oils are very fluid and relatively easy to produce
 Sour crude oils that contain sulphur and sour natural
gasses that contain hydrogen sulphide are less valuable
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What Makes up Oil?
Chemistry
Hydrocarbon
Oil and gas are made of a mixture
of different hydrocarbons
Crude Oil
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As the name suggests these are
large molecules made up of
hydrogen atoms attached to a
backbone of carbon
What Makes up Oil?
Oil is predominantly made up by hydrocarbons, which are
chemical compounds of carbon and hydrogen.
methane
carbon
hydrogen
ethane
proton
electron
METHAN
E
ETHAN
E
PROPAN
E
BUTANE
PENTA
NE
ESANE
OTTANE
C1…C4negli Idrocarburi
gas
Il Carbonio
C5…C15Gas oil
C1....C4
C5….C15
C16…
Liquid
solid
C16 ….
Solid
More carbon atoms, higher hydrocarbon density
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Hydrocarbon molecules
 Four types of hydrocarbon molecules, called the hydrocarbon
series, occur in each crude oil
 The relative percentage of each hydrocarbon series molecule
varies from oil to oil, controlling the chemical and physical
properties of that oil
 The hydrocarbon series include:

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Paraffins
Naphthenes
Aromatics
Asphaltics
 Hydrocarbons that have only single bonds between carbon atoms
are saturated; if they contain one or more double bonds, they are
unsaturated
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Chemical composition of typical crude
oil and natural gas
Crude oil
Natural gas
Carbon
84 – 87%
65 – 80%
Hydrogen
11 – 14%
1 – 25%
Sulphur
0.06 – 2%
0 – 0.2%
Nitrogen
0.1 – 2%
1 – 15%
Oxygen
0.1 – 2%
0%
Modified from Lovorsen, 1967
The two most important elements in both crude oil and natural gas are
carbon and hydrogen. Because of this crude oil and natural gas are
called hydrocarbons
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 °API
 Most common density scale used to describe and compare crude
oils
 Stands for American Petroleum Institute
 Formula for computing °API is:
 °API = [(141.5 + specific gravity at 60°F) – 131.5]
 Fresh water has °API of 10
 Crude oils has °API of 5 – 55
 Average weight (25 – 35), Light oils (35 -45), heavy oils (<25)
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 Sulphur
 Undesirable impurity in crude oil and natural gas
 When sulphur is burned, it forms sulphur dioxide, a gas that
pollutes the air and forms acid rains
 During refinery process sulphur must be removed
 Crude oils are classified as sweet or sour on the basis of their
sulphur content.
 Sweet crudes have less than 1% sulphur by weight
 Sour crudes have more than 1% sulphur
 In general, heavy oils tend to be sour whereas light oils tend to be
sweet
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Benchmark crude oils
A benchmark crude oil is a standard against which other crude
oils are compared and prices are set
 In the United States:
 West Texas Intermediate (WTI) is 38 – 40 °API and 0.3% S
 West Texas Sour is 33 °API and 1.6% S
 In North Sea:
 Brent is 38 °API and 0.3% S
 Middle East:
 Dubai is 31 °API and 2% S
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Pour Point
 The amount of wax in crude oil is indicated by the pour point of
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


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the oil
A crude oil that contains a significant amount of wax is called a
waxy crude oil
Waxes are solid at surface temperature; in the subsurface
reservoir where it is very hot, waxy crude oil occurs as liquid. As
it is being brought up the well, it cools and the waxes can solidify
The lowest temperature at which the oil will still pour before it
solidifies is called the pour point
Crude oil pour points vary from +125° to -75°F (+52° to 60°C); Higher pour points reflect higher oil wax content
Pour Point (cont.)
 Very waxy crude oils are yellow in colour; slightly waxy crude
oils have a greenish colour; low or no wax oils are black
 Examples in the North Sea
 Ekofisk oil has a pour point of +10°F
 Brent oil has a pour point of +27°F
 Statfjord field oil has a pour point of +40°F and has a higher wax
content
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Properties
 The colour of crude oils ranges from colourless through
greenish-yellow, reddish, and brown to black
 In general the darker the crude oil the lower the °API
 The smell varies from gasoline (sweet crude) to foul (sour
crude) to fruity (aromatic crude)
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Crude Streams
 A crude stream is
oil that can be
purchased from
an oil exporting
country. It can be
from a single
field or a blend of
oils from several
fields
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Crude
Stream
Country
°API
S%
Pour
point
Arabian
light
Saudi
Arabia
33.4
1.80
-30°F
Bachequero
Venezuela
16.8
2.40
-10°F
Bonny
light
Nigeria
37.6
0.13
+36°F
Brass
River
Nigeria
43.0
0.08
-5°F
Dubai
Dubai
32.5
1.68
-5°F
Ekofisk
Norway
31.2
0.18
+15°F
Kuwait
Kuwait
31.2
2.50
0°F
??
Ghana
??
??
??
Measurement
 The English unit of crude oil measurement is a barrel (bbl)
that holds 34.97 Imperial gallons or 42 U.S. gallons
 Oil well production is measured in barrels of oil per day (bopd)
 The metric units of oil measurement are metric tons and
cubic meters
 A metric ton of average crude oil (30°API) equals 7.19 barrels
 A metric ton of heavy oil (20°API) equals 6.75 barrels
 A metric ton of light oil (40°API) equals 7.64 barrels
 A cubic meter (m3) of oil equals 6.29 barrels of oil
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Sedimentary basins
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Petroleum Geology
 For a commercial oil accumulation to occur, five conditions must
be fulfilled:
 There must be an organic rich source rock to generate the oil
and/or gas
 The source rock must have been heated sufficiently to yield its
petroleum
 There must be a reservoir to contain the expelled hydrocarbons.
This reservoir must have porosity, to contain the oil and/or gas, and
permeability, to permit fluid flow
 The reservoir must be sealed by an impermeable cap rock to
prevent the upward escape of the petroleum to the Earth’s surface
 Source, reservoir and seal must be arranged in such a way as to trap
the petroleum
Distribution of Petroleum
 Economic accumulations of petroleum only occur when all of
these conditions are met
 These conditions and the sequence of occurrence do not
occur everywhere
 Conditions are most likely where there are thick
accumulations of sedimentary rock – in sedimentary
basins
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What is a basin?
 A sedimentary basin is an
area on the earth’s surface
where sediments have
accumulated to a greater
thickness than they have
been in adjacent areas.
 Most sedimentary basins
cover tens of thousands of
square km and may contain
more than 5 km of
sedimentary fill.
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Types of Basins
 A basin can be formed by subsidence of the basement rock
 The depression may be originally filled with ocean water and
eventually, with sediments
 The Michigan basin in USA is an example of this type of basin
 Some basins are formed by grabens
 An example is southern California oil basins
 The Los Angeles basin is the most prolific petroleum basin on
earth
 Half-graben basins are formed by subsidence along one
side of a normal fault
 These basins are common and are productive in the North Sea,
offshore western Africa, and offshore Brazil
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 Intermontane basins form when mountain ranges are
created
 The basin is located between the mountain peaks and is often
occupied by a lake
 Algae growing in the lake contribute organic matter to the
bottom sediments for source rocks
 Streams eroding the surrounding mountains deposit numerous
channel sandstone reservoir rocks in the basin
 When the Rocky mountains were uplifted during the
cretaceous time, several intermontane basins were formed
 Many of these basins are good petroleum producers today
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 Basins also form along the edges of mountains
 As the mountains are eroded by streams, sediments fill in the
areas adjacent to the mountain
 Coastal plains are formed by thick sediments deposited
adjacent to an ocean
 They originate when mountains are uplifted adjacent to a coast
 As erosion lowers the mountain, streams deposit sands along
the beaches
 The sandy beaches are deposited into the ocean forming the
coastal plain
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Mechanism of Basin Formation -Local
 On a small scale, hundreds to thousands of meters laterally,
fault movements can create relief of hundreds to thousands of
meters, resulting in small but often deep basins (some of
these are called intermontane basins;
 You might guess that it takes dip-slip fault movements to
create new relief, but that’s not true: steps (in the proper
sense) along strike-slip faults can produce small pull-apart
basins.
 Relief of this kind is on such a small scale that it tends not to
be isostatically compensated
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Mechanism of basin formation Regional
 Basin relief can be created mechanically on a regional scale in
two very important ways: thermally or flexurally, or by a
combination of those two effects.
 Each of these is discussed briefly below. Keep in mind that
basins can also be made just by making mountain ranges, on
land or in the ocean, by volcanism
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Thermal
 If the lithosphere is heated from below, it expands slightly
and thus becomes less dense. This less dense lithosphere
adjusts isostatically to float higher in the asthenosphere,
producing what we see at the Earth’s surface as crustal uplift.
If the lithosphere cools back to its original temperature, there’s
isostatic subsidence back to the original level.
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 But suppose that some erosion took place while the crust was
elevated.The crust is thinned where the erosion took place (and
thickened somewhere else, where there was deposition; that might be
far away, at the mouth of some long river system), so when the crust
cools again it subsides to a position lower than where it started, thus
creating a basin available for filling by sediments.
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 But the magnitude of crustal lowering by this mechanism is
less than is often observed in basins thought to be created
thermally. It has therefore been proposed, and widely
accepted, that in many cases extensional thinning of the
lithosphere accompanies the heating. Then, upon recooling, the
elevation of the top of the lithosphere is less than before the heating
and extension.This kind of subsidence has been invoked to explain
many sedimentary basins.
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Flexural
 Another important way to make basins is to park a large load
on some area of the lithosphere.
 The new load causes that lithosphere to subside by isostatic
adjustment. But because the lithosphere has considerable
flexural rigidity, adjacent lithosphere is bowed down also
 The region between the high-standing load and the
lithosphere in the far field is thus depressed to form a basin.
 This model has been very successful in accounting for the
features of foreland basins which are formed ahead of
large thrust sheets that move out from orogenic areas onto
previously undeformed cratonal lithosphere
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Some of the world’s most productive
sedimentary basins
 Saudi Arabia
 Kuwait
 Alaska – north slope
 Texas – Louisiana Gulf Coast
 Iraq and Iran
 Mexico
 Venezuela
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Sedimentary basins of the world
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Today’s Major Oil and Gas Producing Areas
USGS
Global oil and gas occurrences are now well understood
(Main producing areas shown in green). Only the
Antarctica and the Arctic remain unexplored.
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