Generation of Petroleum

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GENERATION OF PETROLEUM
LECTURE OUTLINE
 Origin of petroleum: Organic versus inorganic
 Processes leading to petroleum generation in source rocks
a) Organic matter formation in water column and preservation
in sediments
b) Early sediment diagenesis and degradation of organic
matter: formation and types of kerogen
c) Catagenesis: maturation of kerogen and formation of
petroleum
d) Metagenesis
 Methods of source rock evaluation 1
Oil and gas are generated in sedimentary basins
1) The first main stage is the generation of organic matter in the upper
water column by photosynthesis.
Sediment burial diagenesis (T/P) sedimentary rock
Org. Matter diagenesis- catagenesis (T/P) oil and gas
in sediments
2) Second stage is burial and maturation of organic matter within
source rocks to generate petroleum at depths of ~3-6 km.
3) Third stage: Migration of petroleum from source rocks to reservoir
rocks and its accumulation in traps.
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ORIGIN OF PETROLEUM: ORGANIC OR INORGANIC
Any theory of petroleum genesis must explain the facts
about the geological occurrence and chemical properties of
the petroleum
Early theories of petroleum generation until early 20th
century postulated an inorganic origin (magmatic or cosmic
source).
The geological and chemical evidence strongly supports an
organic origin of petroleum.
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Evidence for the organic origin of petroleum
Geological evidence
1. Most petroleum occurs in sedimentry rocks
2. Petroleum is enclosed by impermeable sedimentary rocks
Chemical facts and evidence
1. Preference for even-numbered carbon chains in crude oil,
whereas modern HCs commonly occur in odd-numbered
chains
2. Crude oils contain >50% light HCs, which are absent in
modern sediments
3. Young oils show optical activity which is an evidence of
biosynthesis
4. Crude oils contain complex molecules such as porphyrins
and steroids that are also found in modern organic matter.
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In very rare cases oil may occur in or close to igneous rocks
Serpentinite (igneous rock)
Sedimentary rock
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ORGANIC MATTER FORMATION AND PRESERVATION IN
SEDIMENTARY BASINS
Carbon is present in the earth’s crust as:
 Organic carbon: 18% of total carbon occurs as coal, oil and gas
 Inorganic (carbonate carbon): 82% of total carbon occurs in
limestone (CaCO3) and dolomite (CaMg (CO3)2).
Organic Matter
Organic matter in sediments and sedimentary rocks consists of
organic molecules formed from the soft tissues of plants and
animals. CaCO3 shells making up the carbonate rocks are not
regarded as organic carbon.
Organic matter can be of two different origins:
 authoctonous: generated within the basin of deposition, and
 allochtonous: transported to the basin by rivers and wind from
continental areas. 6

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The first stage in oil generation:
Production of organic matter in the sedimentary basins by photosynthesis
This process requires sunlight and nutrients (N, P, and Si)
Sedimentary basin:
Sea, Lake
CO2 + H2O + NO3
- + HPO4
2-  CH2O (N, P) + O2
Org. matter
light
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The first stage in oil generation:
Production of organic matter in the sedimentary basins
Photosynthesis: production of algae
in surface waters
Nutrient poor
waters
Nutrient-rich
waters
106 CO2 + 16 NO3
- + HPO4
2- 122 H2O + 18 H+  C106H263O110N16P + 138 O2
Nutrients Sun light Phytoplankton
algae
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Organic Matter Production: Photosynthesis
In the photic zone of the water column in the sea and lakes,
organic matter is synthesized by plants by the process of
photosynthesis:
light
CO2 + H2O + NO3
- + HPO4
2-  CH2O (N, P) + O2
674 kcal Phytoplankton
In water, organic matter is produced by various algae
(e.g.,blue-green algae, coccolithophore, diatoms,
dinoflagellate).
Photosynthesis produces the first step, and is followed by
zooplankton and higher forms of animals (e.g., fish) in the
food chain:
Phytoplankton → Zooplankton → Fish and other higher organisms 10

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Factors affecting primary productivity (amount of org-C):
a) Presence of nutrients (nitrate, phosphate and silica)
b) Light
Some seas and ocean areas have higher organic
productivity than others. Such areas are known as
eutrophic areas:
1) Upwelling areas on the eastern margins of oceans (e.g.,
off Peru ~250 gC m-2 y-1) and divergence zones (e.g.,
equatorial belt of Pacific; ≥100 gC m-2 y-1).
2) Marginal basins and estuaries with large river input (e.g.,
Black Sea and Japan Sea; 150-300 gCm-2 y-1).
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The ocean areas between 10º and 40º latitude, central
oceanic gyres, and in areas of well developed thermocline has
low organic carbon production (<50 gC m-2 y-1). Such areas
are termed oligotrophic seas (e.g., Mediterranean Sea).
Table 1. Global net primary organic matter production
in different marine areas (Romankevich, 1984)
Ecosystem
Area (106
km2)
109 t Corg y-1 g Corg m-2 y-1
Estuaries 1.4 1.0 714.3
Upwelling
areas
0.4 0.1 250.0
Shelf areas 26.6 4.3 161.6
Open ocean 332 18.7 56.3 13
Factors Controlling Organic Matter Accumulation in
Sedimentary Rocks
The following factors are important in the rate of
accumulation of organic matter:
1) Organic matter production in the surface waters
2) Preservation of organic matter in the water column and in
the sediment (> 95% of organic matter decays by
microbiologic activity which uses mainly oxygen).
The preservation of organic carbon depends on:
 Oxygen content of the water-column
 Depth of water column
 Sedimentation rate
 Terrestrial (allochtonous) organic matter input in the basin
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Oxygen consumption by organic matter decay: Oxygen minimum zone
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Organic matter preservation is more efficient in anoxic basins like the Black Sea
This is because oxygen and aerobic bacteria are absent.
But anaerobic bacteria is present
Anoxic basins are ideal for accumulation and preservation
of organic causes
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Oxidative degradation versus preservation of organic material
in the water column and sediment is controlled by:
1) Oxygen content of the bottom waters
2) Sedimentation rate (SR)
Terrestrial Organic Matter Input
The allochtonous organic matter transported by rivers are
deposited mainly in the estuaries and shelf areas. Deep open
ocean sediments contain small amount of organic carbon
(<0.1%).
Significant part of this organic matter in deep marine
sediments is of terrestrial origin and transported by wind. 17
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Importance of sedimentary organic matter
Organic matter in sediments are important because:
a) Constitutes source of fossil fuels such as oil, gas and coal.
b) Controls the diagenetic processes,
c) Causes formation of economic metal deposits (e.g., U, Cu,
Pb, Zn) in sedimentary rocks
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COMPOSITION OF SEDIMENTARY ORGANIC MATTER AND
KEROGEN
There are four major group of compounds in organic matter:
a) proteins,
b) carbohydrates,
c) lipids and
d) lignins.
Proteins are found largely in animals and to a lesser extent in
plants. They contain H, C, O, N, with some S and P.
The simplest form of protein is aminoacids.
Carbohydrates are present in both plants and animals and
consists of C, H and O with H-O ratio being 2 as in the water
molecule. They include sugars, glucose, cellulose, starch and
chitin,
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Lignin
 Found in higher plants (e.g., tree)
 A polyphenol of high molecular weight
 Consists of various types of aromatic carbon rings.
Lipids
 Found in both animals and plants
 Include fats, oils and waxes.
 Contain C, H and O.
 The basic molecule of lipids have five C atoms (C5H8).
 From this molecule steroids are built.
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Mechanism of generating oil
and gas
 It is the thermal transformation of organic matter that
causes a source rock to generate petroleum
 Following deposition of organic-rich sediments, microbial
processes convert some of the organic matter into biogenic
methane gas
 Greater depths of burial are accompanied by increases in
heat
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Mechanism of generating oil
and gas
 This heat causes the organic matter to gradually transform
into an insoluble organic matter known as kerogen
 Further heating converts the kerogen, yielding bitumen
and petroleum
 Increasing maturity causes the petroleum compounds to
undergo structural changes – typically starting with oil,
then wet gas and ending at dry gas
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Stages in the generation of
petroleum from sedimentary
organic matter
With burial in sedimentary basins
• Diagenesis: formation of kerogen
• Catagenesis: formation of oil and wet gas
• Metagenesis: formation of dry gas
Burial
depth
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EARLY SEDIMENT DIAGENESIS, DEGRADATION OF
ORGANIC MATTER AND FORMATION OF KEROGEN
The early diagenesis of sediments takes place in the top 1km
of the sediment column and involves a series of microbial
degradation reactions that take place in three zones:
1) Oxic-suboxic zone
2) Sulphate reduction zone
3) Fermentation zone (methanognesis) zone
In each of these zones biogeochemical reactions are mediated
by a special bacterial group. The thicknesses of the zones and
the rates of the reactions in the specific zones are controlled by:
1) the type and amount of sedimentary organic matter and
2) the rates of sedimentation and bioturbation. 30

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Sea floor
Bacteria present
Bacteria absent
Methane is produced
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Oxic -Suboxic Zone
In this zone, aerobic bacteria degrade the organic matter using
the dissolved oxygen in water:
CH2O + O2 → CO2 + H2O → HCO3
- + H+
Org. matter
Under normal marine conditions the thickness of this zone: a
few cm to about 15 cm.
Under exceptional conditions the thickness may extend to 1-2
m if there is high rate of bioturbation by benthic organisms.
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As the oxygen is consumed, then in the suboxic zone
the bacteria starts using nitrate and Mn(IV) and Fe(III)
Oxyhydroxide as oxidants to break down the sedimentary
organic matter (suboxic diagenesis).
This happens in the sequence:
Oxygen → Nitrate → Mn(IV) → Fe(III)
In anoxic basins such as the Black Sea, the oxic and suboxic
zones are located in the water column below 100-150 m water
depth.
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Sulphate Reduction Zone
Sulphate reducing bacteria (Desulphovibrio) use sulphate
as an oxidant to decompose the organic matter:
CH2O + SO4
2-  H2S + 2HCO3
- (4)
Org. matter
In this zone hydrogen sulphide combines with dissolved
Fe(II) ions to form Fe-sulphides and pyrite.
Increased alkalinity (bicarbonate ions) results in
deposition of carbonate cements.
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Fermentation (Metanogenesis) Zone
At the depth of sulphate consumption a new group of
bacteria (Archea) causes the biogenic production of methane:
CH2O  CH4 + CO2 (5)
Org. matter
This zone can extend to depths of about 1 km and can
contain Fe- and Mg-rich carbonate minerals as ankerite,
dolomite and siderite as cement minerals or concretions.
At the boundary between the sulphate reduction and
methanogenesis zones another important diagenetic reaction
is the anaerobic methane oxidation:
CH4 + SO4
2-  HCO3
- + HS- + H2O (6)
In this zone pyrite and carbonate minerals with different S
and C isotopic compositions form as cement minerals. 35
Stages in the generation of
petroleum
With further burial in sedimentary basins
• Diagenesis: Formation of kerogen
• Catagenesis: Formation of oil and wet gas
• Metagenesis: Dry gas
Burial
depth
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DIAGENESIS
Kerogen
The early diagenesis of organic matter modifies the organic matter and
converts to the biopolymers to geopolymers collectively known as
kerogen: sedimentary organic matter insoluble in organic solvents.
Bitumen is the soluble fraction of the organic matter.
Elementally kerogen consists of C, H and O with minor amounts of N
and S.
The following conversions take place near the sediment/water interface:
The proteins => aminoacids and peptides.
The lipids => glycerol and fatty acids.
The lignin => phenols, aromatic acids and quinones.
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Diagenesis of organic matter: formation of
kerogen by polycondensation and polymerization
The residue not used by bacteria recombines by
polycondensation and polymerization to form brown
compounds similar to fulvic and humic acids.
Insolubilization of the previous constituents occur over the first
tens or hundreds of meters of burial, and finally by
polycondensation and loss of functional groups lead to
formation of kerogen from fulvic and humic acids.
Organic matter Humic and Fulvic acids Kerogen
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There are four types of kerogen:
Type I is essentially algal origin. It has a high H/C ratio (1.65 or 1.6-1.8) and low O/C
ratio (0.06). Lipids (aliphatics) are the dominant compounds in this kerogen. Many oil
shales has type I kerogen. Abundant in lacustrine source rocks. Oil prone; yields high
(up to 80%)
Type II is liptinic and rich in aliphatic compounds. It consists of a mixture of marine
algal material (phytoplankton, zooplankton) and terrestrial material (spores, pollens).
Its H/C ratio is greater than one. Intermediate H/C (about 1.3) and intermediate O/C
(0.1). Sulphur, where present, influences the timing and rate of maturation of Type II
kerogen. Oil and gas prone; yields 40-60%.
Type III is humic with a much lower H/C ratio (0.8 or lower from 1.0) and high O/C
ration (0.15). It is produced from the lignin of the higher woody plants. It is enriched
in aromatic compounds and low in aliphatics. Low yield for oil but gas prone.
Type IV is high in carbon and very low in hydrogen. Often termed ‘dead-carbon’. No
effective potential to generate petroleum.
Types of kerogen
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CATAGENESIS: MATURATION OF KEROGEN AND
FORMATION OF PETROLEUM
In the sedimentary basins during the burial of sediments, the
increase in temperature and pressure results in progressive
changes in the kerogen.
This change is marked by an important decrease in the H
content and H/C ratio (from 1.25 to 0.5 in Type II kerogen).
This stage corresponds to the principal stages of oil and wet
gas formation. By catagenetic processes hydrocarbon chains
and cycles are broken down (cracking), and first crude oil and
then gas are successively formed. Medium and low molecular
hydrocarbons (n- and iso-alkanes) become dominant.
Catagenesis takes place at different depths (3-6 km) in
different basins. The oil window is at about 60-120ºC. 45
METAGENESIS
With increasing burial after catagenesis, further increase in
temperature (150-225ºC) results in:
 Rearrangement of the aromatic sheets: previously randomly
distributed stacks of aromatic sheets in kerogen, now gather to
form larger clusters.
 In this stage only dry gas (methane) is generated.
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Source rock evaluation methods
Evaluation of the quality of source rocks involves determination of:
 Total organic carbon (TOC) content
 Origin of organic matter and type of kerogen
 Maturity of the kerogen
SOURCE ROCK EVALUATION
Evaluation of source rocks is important in basin evaluation for petroleum
exploration purposes.
Source rock is as any fine-grained, organic-rich rock that is capable of generating
petroleum
Source rock is the most important part in the petroleum system as it refers to rocks
from which hydrocarbons have been generated.
Source rock examples as shale, clay and fine limestone while shale is one of the
dominant source rocks for hydrocarbon.
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Organic carbon content
Potential of a source rock to generate petroleum is related to
the organic carbon (Corg or TOC) content of the source rock:
 Corg < 0.5 wt %: low potential
 Corg = 0.5-1 wt % : good potential
 Corg > 1 wt %: high potential
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Type of organic matter (kerogen):
Methods:
1) Microscopic method
2) Elemental C / N ratio (marine vs. terrestrial)
3) C-isotopes (marine vs. terrestrial)
4) Type of kerogen based on H/C and O/C ratios
or Rock Eval pyrolysis
Microscopic methods
1) Liptinite (algae, resins, spores): high transmission,
intense fluorescence, low reflectance.
2) Vitrinite, huminite: angular to subangular particles.
Cell structure. Intermediate transmittance and
reflectance, usually absence of fluorescence.
3) Inertinite: angular, high reflectance, cellular outline,
no fluorescence, opaque in transmitted light.
Transmittance
Fluorescence
Reflectance
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Type of organic matter: Atomic C / N ratio
C/N <8 Marine
C/N: 8-15 Mixed
C/N: > 20 Terrestrial
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Type of organic matter: C-isotopes vs. C / N ratio
C-4
C-3
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MATURITY OF KEROGEN IN SOURCE ROCKS
It is important to determine if a possible source rock has been
buried to the depth of oil window during diagenesis (i.e., state
of maturity)
During the catagenesis kerogen matures and produces oil
and gas.
 When kerogen is immature no petroleum has been
generated.
 With increasing maturity first oil and then gas are generated
and expelled;
 When the kerogen is overmature neither oil nor gas remains.
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Methods to determine the the maturity of kerogen:
Chemical methods:
1. Elemental (C/H and C/O) ratios
2. Pyrolysis: Rock-Eval analysis
3.Electron spin resonance (ESR)
4. Gas chromatography
Mineralogical methods:
1. Crystallinity index of illite
2. Percentage of illite (i) in illite-smectite (i/s) mixed-layer clay
mineral
Biological methods:
1. Pollen and spore coloration index (visual kerogen
analysis)
2. Vitrinite reflectance 55
Van Krevelan diagrams based on elemental (H/C and O/C) ratios
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Van Krevelan diagrams based on elemental (H/C and O/C) ratios
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 S1 peak: free and adsorbed HCs already present in the rock
 S2 peak: is the thermal decomposition of kerogen. HCs generated as a result of
pyrolysis (heating)
 S3 peak: is the quantity of O2 in kerogen. CO2 and water generated from kerogen,
and then carbonates at a high T
Important parameters:
a. Hydrogen index: as indicator of oil vs. gas, (S2 / TOC)*100
b. Oxygen index: oxygen richness of samples used as indicator of the kerogen
type/degree of weathering, (S3 / TOC)*100
c. S1+S2: genetic potential (HCs per ton of rock); >6 kg/ton
good source rock
d. Tmax: This the max temperature of S2 peak, peak temperature of HC generation
e. S2/S3: is an indicator of hydrogen richness in the kerogen.
Pyrolysis: Rock-Eval analysis
Sample is gradually heated to
550ºC at a given rate, and the
amount HC products given off
are measured by flame
ionization detector and thermal
conductivity detector for CO2.
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Mineralogical methods: % illite in İllite/smectite clay
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Vitrinite reflectence
and spor coloration
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