GIAO TRINH CHAPTER 5

1.2-Hydrocarbons and Kerogen Type • The macerals and amorphous particles in kerogen affect its ability to generate hydrocarbons.. The oil-prone kerogens can be divided into four types:

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:

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-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 green algae Bottom-dwelling algae are also major contributors in shallow water, shelf environments

blue-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 deficient layer, often within a large body of water The OML, in

common to oceans; it generally lies below the photic zone and it can

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

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

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 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 Components

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

(wood, spores, leaf cuticle wax, resin, plant tissue )

Mostly vitrinite; some exinite (not algal) and amorphous 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

• 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 rich oils, and they yield more gas than Type I

aromatic-• 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

Types of Kerogen

• Type I : algal kerogen

– “best” oil source

– Lipid-rich

• Type II: herbaceous kerogen

– Good oil source

– Includes zooplankton (sapropelic)

• Type III: woody kerogen (coaly)

– Good gas source

– Rich in humic components

• Type IV: amorphous kerogen

What happens when we

subject kerogen to subsurface

• Overall decrease in O

• Overall increase in H and C

Deeper subsurface Increased pressure and temperature

Released: oil & gas

• Overall decrease in H and C

Metamorphism High temperature and pressure Only C remains: becomes graphite

Chemical Changes with Kerogen

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)

Figure Kerogen types

06-Figure 07

• As any of these kerogens are heated, they may

reach the second stage in the evolution of organic matter, the stage of catagenesis (Figure 08)

Catagenesis is defined as the stage at which oil

and natural gas is generated from kerogen

• Since oil and gas molecules have very high H/C

ratios, generation of petroleum will cause the H/C

of the residual kerogen to decrease Ultimately, all kerogen types will converge along a common path during the final stage in the evolution of organic

matter, the stage of metagenesis

Figure 08

• During metagenesis, oil and gas generation

directly from kerogen ceases, but considerable

methane gas can still be generated from the

thermal alteration of previously generated crude The kerogen residue of this stage approaches the pure carbon state, that is, graphite.

• Since it starts out with a lower H/C ratio

(Figure 07 & Figure 08), Type II kerogen can

generate less hydrocarbons than Type I, even

though both are oil-prone Similarly, Type III is less significant in the total quantity of

hydrocarbons it can generate, and Type IV is

almost barren.

1.3-Depth, Temperature and

Time in Petroleum Formation

• The generation of hydrocarbons can be related to burial depths of source rocks, since temperature

increases with increased depth The actual

generation depths for particular source rocks will depend on the local geothermal gradient, as well as kerogen type and burial history The depths given

in Figure 09 are average, maximum and minimum generation depths.

• During diagenesis and at very shallow depths, only biogenic methane, or marsh gas, is generated by the action of anaerobic bacteria.

•

1.3-Depth, Temperature and Time

in Petroleum Formation (Cont.)

stage begins The early stage of catagenesis, down to a depth of about 3 kilometers, corresponds to the principle zone of oil formation Source rocks buried within this

depth range are said to be within "the oil window"

kilometers to 3.5 kilometers This is the principle zone of gas formation, and both wet gas and methane are

produced But below depths of about 4 kilometers, the source rocks become overmature At this point,

metagenesis begins and only methane is produced

GENERATION OF PET RELATION TO AVERAGE, MAX.,

MIN OF BURIAL DEPTH

Figure 09

•The correlation of petroleum generation to depth is primarily a function of the increased Temp., and the graph in Fig 09 can also be constructed with Temp as the ordinate axis (Figure 10)

•Major oil generation does not occur until source rocks are heated above approximately 60°C These low Temp oils which form at shallower depths tend to

be heavy and rich in NSO-compounds With increasing temperature and greater depth, the oils become lighter Maximum oil generation occurs at temperatures of about 100°C Above this temperature oil generation gradually declines and condensates form.

•The oil window closes, and the principal zone of gas generation begins, at temperatures of about 175°C Generation directly from kerogen stops at about 225°C, but methane is still generated from the cracking of previously formed oil at temperatures up

to 315°C, the point at which source rocks begin to undergo regional metamorphism At those elevated temperatures, however, porosity may be so reduced that gas generated at this stage might not be economically recoverable.

GENERATION OF PET RELATION TO TEMPERATURE

Figure 10

An example of the maturation

progression is found in the

western Canada basin

•Immature source rocks are present in the east within shallow Upper Cretaceous sediments (Fig 11) These yield dry gas with a high N 2 content Deeper burial has resulted in Cretaceous and Devonian rocks rich in oil and wet gas Evans and Staplin (1971) have estimated that the wet gas and liquid hydrocarbons in the

western Canada Basin were formed in the

temperature range of 60 to 170°C Near the basin's

western margin, Paleozoic rocks are deeply buried and the dominant gases produced are methane and

hydrogen sulfide.

The maturation progression in the western Canada basin Figure 11