ĐỊA CHẤT DẦU KHÍ ( PETROLEUM GEOLOGY ) - CHƯƠNG 5 pptx

CHAPTER 05
CHAPTER 05
GENERATION AND MIGRATION
GENERATION AND MIGRATION
OF HYDROCARBON
OF HYDROCARBON
1
1
-
-
GENERATION OF
GENERATION OF
HYDROCARBON
HYDROCARBON
1.1
1.1
-
-
Petroleum Source Material
Petroleum Source Material
1.1.1-Formation and Preservation of Organic
Matter
• In the nineteenth century, it was widely believed
that petroleum had a magmatic origin and that it
migrated from great depths along subcrustal faults.
• But the overwhelming evidence now suggests that
the original source material of petroleum is organic
matter formed at the earth's surface.
• The process begins with photosynthesis, in which
plants, in the presence of sunlight, convert water
and carbon dioxide into glucose, water and

oxygen:
6CO
2
+ 12H
2
O C
6
H
12
O
6
+ 6H
2
O + 6O
2
• Photosynthesis is part of the larger-scale carbon
cycle (Fig. 01). Ordinarily, most of the organic
matter produced by photosynthesis gets recycled
back to the atmosphere as carbon dioxide. This can
occur through plant and animal respiration, or
through oxidation and bacterial decay when
organisms die
Fig 01-CARBON CYCLE
1.1.2
1.1.2
-
-
Preservation and Organic Productivity
Preservation and Organic Productivity
• All organic matter in the ocean is originally

formed through photosynthesis. The main
producers are phytoplankton, which are
microscopic floating plants such as diatoms,
dinoflagellates and the blue-green algae.
Bottom-dwelling algae are also major
contributors in shallow water, shelf
environments.
1.1.3
1.1.3
-
-
Preservation and Organic Destruction
Preservation and Organic Destruction
• Areas of high productivity are not necessarily
those best suited for preservation. Destruction
of organic matter must also be prevented.
Complete biological recycling of organic
carbon is retarded by anything that limits the
supply of elemental oxygen.
• This occurs most favorably in either one of two
settings: rapid rate of deposition; and stratified,
oxygen-poor water bodies with anoxic bottoms
• First, rapid deposition may be necessary to keep the
organic material from being destroyed.
• Preservation is also favored by density stratification,
which produces oxygen-poor bottom waters.
• Water stratification and oxygen depletion are well
known in the modern Black Sea,
• The Eocene-age lakes of Utah, Colorado and Wyoming,
in which the Green River oil shale formation was

deposited, have been interpreted as seasonally stratified
water bodies which at a later stage become permanently
stratified (Fig 02)
Fig 02
• In the present-day world's oceans, there is a zone
of maximum oxygen depletion at a depth of about
200 meters, with oxygen more abundant in the
shallow surface waters and again at deeper levels
(Figure 03)
Fig 03
1.1.4
1.1.4
-
-
Diagenesis of Organic Matter
Diagenesis of Organic Matter
• There are three important stages in the burial and
evolution of organic matter into hydrocarbons:
– diagenesis;
– catagenesis;
– and metagenesis.
Diagenesis
Diagenesis
of Organic Matter
of Organic Matter
Diagenesis of organic matter begins as soon as sediment is
buried. However, the point at which diagenesis ends is
subject to how the term is used. Some geologists use the
term in a restricted sense to include only processes that
occur as sediment consolidates into sedimentary rock.

Others expand the realm of diagenesis to include all
processes extending up to, and blending imperceptibly
into, regional metamorphism.
In this discussion, diagenesis is defined on the basis of
organic matter, and it includes all changes that occur up to
the stage of petroleum generation.
• Freshly deposited muds are unconsolidated and
may contain more than 80% water in their
pores. These muds compact very quickly. Most
of the porosity is lost in the first 500 meters of
burial (Figure 04). After that, compaction to
form mudstones or shales continues much more
slowly.
Fig 04
1.1.5
1.1.5
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-
Kerogen Components
Kerogen Components
• Under the microscope, kerogen appears as disseminated
organic fragments. Some of this material is structured. It is
recognizable as plant tissue fragments, spores, algae, and
other pieces with a definite biological organization. These
plant-derived structured fragments can be grouped into
distinct biological units called macerals. Macerals in
kerogen are equivalent to minerals in rocks.
• Three major maceral groups are important: vitrinite,
exinite and inertinite.
Kerogen

Kerogen
Components
Components
• Vitrinite is the dominant maceral type in many kerogens and is the
major component of coal. It is derived almost entirely from woody
tissue of the higher land plants. Because it is derived from lignin and
is difficult to break down, vitrinite can appear in almost any
depositional environment, marine or nonmarine, and is generally the
most abundant type of structured particle.
• Exinite macerals are mainly derived from algae, spores, pollen, and
leaf-cuticle waxes. High percentages of exinite are not common, but if
present, they usually imply lacustrine or shallow marine
environments.
• Inertinite macerals come from various sources that have been
extensively oxidized before deposition. Charcoal, derived from woody
plants, is the major recognizable type. Inertinite is usually a minor
component of kerogen, and is abundant only where much of the
organic matter has been recycled.
• In addition to the structured macerals, some of
the components of kerogen are amorphous.
Amorphous particles have been so mechanically
broken up and/or chemically altered by bacteria
and fungi that their original maceral types and
cell structures have been obliterated -Amorphous
particles are not true macerals but alteration
products, although the maceral term
"amorphinite" has sometimes been applied to
these materials.
Kerogen
Kerogen

Components
Components
1.2
1.2
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-
Hydrocarbons and
Hydrocarbons and
Kerogen
Kerogen
Type
Type
• The macerals and amorphous particles in kerogen affect its
ability to generate hydrocarbons. Oil-prone kerogens
generally are made of more than 65% exinite and
amorphous particles (Figure 05).
• Kerogens with 65% to 35% of oil-prone components will
expel mostly condensate and wet gas. With less than 35%
oil-prone constituents, the kerogen will yield dry gas if
dominated by vitrinite and will be non-reactive and barren
if dominated by inertinite.
Figure
Figure
05
05
-
-
Types of petroleum generated from
Types of petroleum generated from
kerogen

kerogen
, based on
, based on
visual analysis with reflected light microscope
visual analysis with reflected light microscope
• The oil-prone kerogens can be divided into
two types.
• Type I, or algal kerogen (Table 1), is rich in
the algal components of exinite, and is
formed in either lacustrine or marine
environments. Type I kerogen is derived
mainly from lipids and tends to produce
crudes that are rich in saturated
hydrocarbons.
Mostly inertinite; some amorphous
decomposition products
Fossil charcoal and other oxidized
material of continental vegetation
IV Inert
Mostly vitrinite;some exinite ( not
algal ) and amorphous
decomposition products
Debris of continental vegetation
(wood, spores, leaf cuticle wax,
resin, plant tissue )
III Coaly
Amorphous particles derived mostly
from phytoplankton, zooplankton,
and higher organisms; also some
macerals from these groups

Decomposition in reducing
environments, mostly marine
II Mixed Marine
Mostly algal components: of exinite
(alginite); some amorphous material
derived from algae
Algae of marine,
lacustrine,boghead coal
environments
I Algal
Organic ConstituentsOriginKerogen Type
Table 1 Kerogen types, their origin, and organic particle
constituents
•Type II is a kerogen derived from mixed marine sources.
Its particles are mostly amorphous and result from the
decomposition of phytoplankton, zooplankton, and some
higher animals. Its chemical nature is intermediate between
Types I and III. Type II kerogens tend to produce
naphthenic and aromatic-rich oils, and they yield more gas
than Type I.
•Type III or coaly kerogen, is rich in vitrinite macerals,
and therefore has a very low capacity to form oil. It mainly
generates dry gas. Any oils generated from Type III
kerogens are mostly paraffinic waxy crudes derived from
its exinite and amorphous constituents.
• There is a fourth kerogen type which is extremely rare. It is rich in
inertinite macerals and produces very low hydrocarbon yields.
Inertinite is, as its name implies, inert and has practically no ability
to generate either oil or gas (Figure 05).
• Sedimentary rocks commonly contain mixtures of the kerogen

types. Many oil shales contain dominantly Type I, the algal
kerogens. Coals and some nearshore clastic source rocks, such as
those found in deltas, contain mainly Type III, coaly kerogen. In
some cases, coal deposits can be direct contributors to significant
natural gas accumulations, as for example the Carboniferous coals
of the North Sea. Many marine source rocks have either Type I algal
or Type II mixed marine kerogen, with Type II the more common.
For example, some of the excellent source rocks of Iran contain
mostly Type I, algal kerogen, while the Paleozoic source rocks of
the North African Sahara have Type II, mixed marine kerogen.
Chemical Changes with
Chemical Changes with
Kerogen
Kerogen
Maturation
Maturation
• In the stage of diagenesis, prior to the generation of oil
and gas, each of the kerogen types has a unique
chemistry (Figure 06).
• This is because kerogen composition is controlled by
the types of macerals and the original biopolymers
from which it was formed. This chemical variability of
immature kerogen types and the changes that occur as
petroleum is generated are usually presented as plots of
the atomic hydrogen to carbon ratio (H/C) against the
oxygen to carbon ratio (O/C) . This graph is often
called a Van Krevelen diagram ( Figure 07, and
Figure 08)