GIAO TRINH CHAPTER 5

CHAPTER 05
GENERATION AND MIGRATION
OF HYDROCARBON

UA-2011


1- GENERATION OF
HYDROCARBON


1.1-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:
6CO2 + 12H2O

C6H12O6 + 6H2O + 6O2

• 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-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 bluegreen algae. Bottom-dwelling algae are also major
contributors in shallow water, shelf environments.


1.1.3-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


Oxygen-minimum
layer (OML)
This is an oxygendeficient layer, often
within a large body of

water. The OML, in
varying
oxygen
deficiencies,
is
common to oceans; it
generally lies below the
photic zone and it can
be
underlain
by
deeper,
oxygenated
water
(Figure
…,
Oxygen
minimum
layer.


1.1.4-Diagenesis of Organic Matter
• There are three important stages in the burial and
evolution of organic matter into hydrocarbons:

– diagenesis;
– catagenesis;
– and metagenesis.

Modern Organic Processes at
the Earth’s Surface
• Surface
– 82% C locked into CO3 in carbonates
– 18% occurs as organic C in coal, oil & gas
– When death occurs, a plant or animals remains are
normally oxidized and CO2/ H2O released

• Subsurface
– When death occurs, a plant or animals remains are
normally oxidized and CO2/ H2O released
– Under exceptional conditions: organic matter is buried
and preserved in sediments
– The composition of the organic matter strongly
influences whether the organic matter can produce coal,
oil or gas.


Basic components of organic
matter in sediments
• PROTEINS
– More abundant in animals: O, C, N, H

• CARBOHYDRATES

– Occur in both. Cn(H2O)n
– sugars, cellulose, starch

• LIPIDS (Fats)

– Occur in both: C, H, O
– Fats, oils, waxes (e.g. leaf cuticles)

• LIGNIN

– Occurs in plants: complex aromatic ring structures, large
molecules

All of these + Time + Temperature + Pressure = KEROGEN


Diagenesis 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-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 plantderived 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 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.


Kerogen Components

• 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.


1.2-Hydrocarbons and Kerogen
•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 05- Types of petroleum generated from
kerogen, based on visual analysis with reflected
light microscope


The oil-prone kerogens can be divided into four
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.


Kerogen Type

Origin

Organic Constituents

I Algal

Algae of marine, lacustrine,
boghead coal environments

Mostly algal components: of
exinite (alginite); some
amorphous material derived
from algae

II Mixed Marine

Decomposition in reducing
environments, mostly marine

Amorphous particles derived
mostly from phytoplankton,
zooplankton, and higher
organisms; also some macerals
from these groups

III Coaly

Debris of continental vegetation Mostly vitrinite; some exinite
(wood, spores, leaf cuticle wax, (not algal) and amorphous
resin, plant tissue )
decomposition products

IV Inert

Fossil charcoal and other

oxidized material of continental
vegetation

Mostly inertinite; some
amorphous decomposition
products

Table 1 Kerogen types, their origin, and
organic particle constituents