1. Suez University
Faculty of Petroleum & Mining Engineering
The Conversion of Organic Matter to Petroleum
Student
Belal Farouk El-saied Ibrahim
Class / III
Section / Engineering Geology and Geophysics
The Reference / Pet. Geology
(F.K.North)
Presented to
Prof. Dr. / Shouhdi E. Shalaby

2. Organic Matter
When an organism (plant or animal)
dies, it is normally oxidized 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.

3. Basic components of organic
matter in sediments
• PROTEINS
• CARBOHYDRATES
• LIPIDS (Fats)
• LIGNIN
All of these + Time +
Temperature + Pressure =
KEROGEN

4. Biomolecules in Living Organisms
Lipids, mostly fats, oils and waxes, have the greatest
potential to be hydrocarbon sources. They are combinations
of the fatty acids of the general formula CnH2nO2 with
glycerol, C3H5(OH)3. An important example is glyceride
C17H35COOCH3 formed from the stearic acid.
Proteins are giant molecules that make up the solid
constituents of animal tissues and plant cells. They are rich
in carbon but contain substantial amounts of N, S and O.
Carbohydrates are based on sugars Cn(H2O)n and their
polymers (cellulose, starch, chitin). They are common in
plant tissue.
Lignin is a polymer consisting of numerous aromatic rings. It
is a major constituent in land plants and converts to coal
through desoxygenation.

5. Environment of the Transformation
We have examined the type of raw material needed and
how it must accumulate in the natural environment. The
next link in the process is to examine what happens to this
organic matter (OM) when buried and subjected to
increased temperature and pressure. One thing to
remember is that not all of the organic carbon (OC) in
sedimentary rocks is converted into petroleum
hydrocarbons. A portion of the Total Organic Carbon (TOC)
consists of Kerogen. We will look at the transformation of
OM first to kerogen, then to petroleum hydrocarbons.

6. The only elements essential to the transformation of organic matter (OM) into petroleum
are hydrogen and carbon. Thus the nitrogen and oxygen contained in the OM must
somehow be removed while at the same time preserving the hydrogen-rich organic
residue. The formation of petroleum at this point must occur in an oxygen-deficient
environment, not be subjected to prolonged exposure to the atmosphere or to aerated
surface or subsurface waters containing acids or bases, come into contact with elemental
sulfur, vulcanicity, or other igneous activity, and have a short transportation time from the
time of death to that of burial. All of these conditions must be met in order to avoid
decomposition of the OM. All of this implies that as dead organic matter falls to the sea
floor (organic rain), the hydrocarbon constituents needed for creating the end product will
be preserved only if the water column through which they are falling is anoxic - lacking
living organisms, fall is rapid - the particle size must not entirely be microscopic, bottom
dwelling predators are lacking, and there is a rapid sedimentation rate - rapid deposition
buries the OM below the reach of mud-feeding scavengers.

7. Once the organic material is buried within the sea floor,
transformation begins. It is a slow process that occurs to the OM. The
general process can be illustrated by the following formulas:
OM + Transformation = Kerogen + Bitumen (by product)
Kerogen + Bitumen + more Transformation = Petroleum
There are three phases in the transformation of OM into
hydrocarbons: Diagenesis, Catagenesis, and Metagenesis (Tissot,
1997). Diagenesis occurs in the shallow subsurface and begins during
initial deposition and burial. It takes place at depths from shallow to
perhaps as deep as 1,000 meters and at temperatures ranging from
near normal to less than 60oC. Biogenic decay aided by bacteria (such
asThiobacillus) and non-biogenic reactions are the principal processes
at work producing primarily CH4(Methane), CO2 (Carbon Dioxide),
H2O (Water), kerogen, a precursor to the creation of the petroleum,
and bitumen.

8. Temperature plays an important role in
the process. Ambient temperatures
increase with depth of burial which
decreases the role of bacteria in the
biogenic reactions because they die out.
However, much of the initial methane
production begins to decline because it is
the bacteria that produces the methane
as a by-product during diagenesis.
Simultaneous to the death of the bacteria
however, the increased temperatures
accelerate organic reactions.

9. The dependence of chemical reaction rates upon
temperature is commonly expressed by:
Arrhenius equation
k=A*e[-E/RT]
R =Gas constant (0.008314 KJ/mol0K)
T=absolute temperature
E=activation energy
A=frequency factor

10. The Catagenesis (meaning thermodynamic,
nonbiogenic process) phase becomes
dominant in the deeper subsurface as burial
(1,000 - 6,000 m), heating (60 - 175oC), and
deposition continues. The transformation of
kerogen into petroleum is brought about by a
rate controlled, thermocatalytic process where
the dominant agents are temperature and
pressure

11. The temperatures are of non-biological origin; heat is
derived from the burial process and the geothermal
gradient that exists within the earth's crust. The
catalysts are various surfactant materials in clays and
sulfur. Above 200o C, the catagenesis process is
destructive and all hydrocarbons are converted to
methane and graphite. And at 300o C, hydrocarbon
molecules become unstable. Thus thermal energy
(temperature) is a critical factor, but it is not the only
factor The time factor is also critical because it provides
stable conditions over long periods of time that allows
the kerogen sufficient cooking time - exposure time of
kerogen to catagenesis. Thus the Catagenesis phase
involves the maturation of the kerogen; petroleum is
the first to be released from the kerogen followed by
gas, CO2 and H2O.

13. The Third phase is referred to as Metagensis. It
occurs at very high temperatures and pressures
which border on low grade metamorphism. The
last hydrocarbons released from the kerogen is
generally only methane. The H:C ratio declines
until the residue remaining is comprised mostly
of C (carbon) in the form of graphite.

14. Preservation of Organic Matter
The biomolecules described before are reduced forms
of carbon and hydrogen. Their preservation potential
depends crucially on anoxic conditions, i.e. the
absence of oxygen that could oxidize them.
Stratified basins that prevent vertical circulation and
thus the transport of oxygen to greater depths
provide excellent conditions for this. An example is the
Black Sea, which is salinity-stratified, but many lakes
are also anoxic in their deeper waters because of
thermal stratification or abundance of nutrients and
lack of circulation.

15. Preservation of Organic Matter
Access to air (oxygen) rapidly - at geological scales - oxidizes
organic matter and converts it into CO2 and H2O.
The total carbon content in the Earth’s crust is 9·1019 kg (the
hydroand biosphere contain less than 10-5 of this). Over 80%
of this is in carbonates. Organic carbon amounts to 1.2·1019
kg and is distributed approximately as follows:
Dispersed in sedimentary rocks (~) 97.0 %
Petroleum in non-reservoir rocks 2.0 %
Coal and peat 0.13 %
Petroleum in reservoirs 0.01 %
This illustrates the low efficiency of the preservation process.

16. Total Organic Carbon (TOC)
If a rock contains significant amounts of organic carbon, it is
a possible source rock for petroleum or gas. The TOC content
is a measure of the source rock potential and is measured
with total pyrolysis.
The table below shows how TOC (in weight percent) relates
to the source rock quality.
TOC Quality
0.0-0.5 poor
0.5-1.0 fair
1.0-2.0 good
2.0-4.0 very good
>4.0 excellent

17. TOC Types
TOC in sedimentary rocks can be divided into two types:
• Bitumen, the fraction that is soluble in organic solvents such
as chloroform
• Kerogen, (κεροσ = wax) the insoluble, nonextractable
residue that forms in the transformation from OM Kerogen is an
intermediate product formed during diagenesis and is the
principal source of hydrocarbon generation. It is a complex
mixture of high-weight organic molecules with the general
composition of (C12H12ON0.16)x

18. Conversion of OM to HC
The principal condition is that this conversion take place in an
essentially oxygen-free environment from the very beginning of
the process. Anaerobic bacteria may help extract sulfur to form
H2S and N, in addition to the earlier formation of CO2 and H2O.
This explains the low sulfate content of many formation waters.
On burial, kerogen is first formed. This is then gradually cracked
to form smaller HC, with formation of CO2 and H2O. At higher
temperatures, methane is formed and HCs from C13 to C30.
Consequently, the carbon content of kerogen increases with
increasing temperatures. Simultaneously, fluid products high in
hydrogen are formed and oxygen is eliminated.

19. Dehydrogenization and Carbonization
The dehydrogenation and carbonization of organic
source n be illustrated with the H:C ratio during the
formation of coals:
Source material H:C ratio
Wood 1.5
Peat 1.3
Lignite 1.0
Bit. coal 0.8
Anthracite 0.3-0.0
Average, in weight %

20. Deoxygenization and Carbonization
The deoxygenation and carbonization of the
source material is illustrated with the
formation of petroleum:
Source material O:C ratio
Organisms 0.35-0.6
Pyrobitumen (kerogen) 0.1-0.2
Petroleum (average) 0.004
Average, in weight %

21. Source Rock Quality
The primary factor determining source rock
quality is the level of TOC.
Additionally, the quality of the source rock is
better for higher H:C ratios before thermal
maturation.
As thermal maturation proceeds and HCs are
formed, the kerogen will continuously
deteriorate as a source for HC formation.

22. Sapropelic
kerogen (algae)
Lipid-rich kerogen
(phyto- and
zooplankton)
Humic kerogen
(land plants)
“Van Krevelen diagram” TAI, VR: Maturation indicators

23. Transformations with Depth
Source: North, F.K. (1985) Petroleum Geology, Allen & Unwin
LOM = level of organic metamorphism; BTU = British
Thermal Unit; VM = volatile matter

24. Rate of Maturation
Source: North, F.K. (1985) Petroleum Geology, Allen & Unwin
Temperature is the single most important factor in thermal maturation.

25. Rate of Maturation ctd.
Source: North, F.K. (1985) Petroleum Geology, Allen & Unwin
Time is the second most important factor in thermal maturation

26. Purposes of Maturation Indicators
• To recognize and evaluate potential source rocks for oil and
gas by measuring their contents in organic carbon and their
thermal maturities
• To correlate oil types with probable source beds through their
geochemical characteristics and the optical properties of
kerogen in the source beds
• To determine the time of hydrocarbon generation, migration
and accumulation
• To estimate the volumes of hydrocarbons generated and thus
to assess possible reserves and losses of hydrocarbons in the
system.

27. Lopatin’s TTI Index
V. Lopatin (1971) recognized the
dependence of thermal maturation
from temperature and time. He
developed a method where in the
temperatures are weighted with the
residence time the rock spent at this
temperature. Periods of erosion and
uplift are also taken into account. This
so-called time-temperature index TTI
is still in use, although in variations.
The plot on the right shows a simple
depiction of it. Rock of age A enters the
oil-generating window at time y, while
the older rock B has been at that time
already in the gas-generating window
and will stay there until the present.
Source: North, F.K. (1985) Petroleum Geology, Allen & Unwin

28. Other Maturation Indicators
Several approaches to quantify the degree of maturation have been
proposed aside from the TTI. Most of them are sensitive to
temperature and time.
• Vitrinite Reflectance (Ro) measures the reflectance of vitrinite (see
Kerogen maturation diagram) in oil, expressed as a percentage. It
correlates with fixed carbon and ranges between 0.5 and 1.3 for the
oil window. Laborious but widely used.
• Thermal Alteration Index (TAI) measures the color of finely
dispersed organic matter on a scale from 1 (pale yellow) to 5
(black). This index has a poor sensitivity within the oil window (TAI
around 2.5 to 3.0) and is not generally used.
• Level of Organic Maturation (LOM) is based on coal ranks and is
adjusted to give a linear scale.

29. Correlation of TTI, Ro, and TAI

30. The Oil and Gas Windows
The Oil and Gas Windows
A similar slide as before. It
shows clearly at what
temperatures oil generation
peaks.
Gas generation diminishes
above ~180°C
Source: North, F.K. (1985) Petroleum Geology,
Allen & Unwin

31. Oil Source Rock Criteria
The criteria for a sedimentary rock to be an effective oil source
can be quantitatively described. They are as follows:
• The TOC should be 0.4% or more
• Elemental C should be between 75% and 90% (in weight)
• The ratio of bitumen to TOC should exceed 0.05
• The kerogen type should be I or II (from lipids)
• Vitrinite reflectance should be between 0.6 and 1.3%

32. Summary: Origin and Maturation
This diagram shows the
development of biomolecules
into petroleum and, with
further maturation, into gas
(left branch at bottom) which
causes the residues to
become increasingly more
carbon-rich (right branch at
bottom)
Source: Hunt, J.M. (1995) Petroleum Geochemistry and
Geology, 2nd edition. W.H. Freeman & Co