organic maturation is a topic which is in the earthscience syllabus and i want to share some information or ideas

1. • Requirement for Hydrocarbon potential:
– Environment of deposition
– Volume of source rock (s), Area of extent, Thickness variation
– Type (s) of organic matter
– Source proclivity (Oil and/or Gas)
• Generation of the oil/gas from source rock:
– Typing by e.g., biomarkers
– Oil families (different source rock in a basin)
– Risk of Biodegradation/water-washing/gas flushing
• How much, when, & quality of the oil/gas in the trap?
– Charge modeling (burial, thermal regime, source quality, etc.,)
– Timing of charge relative to timing of trap formation.
Organic maturation

2. 1. EVALUATION OF THE SOURCE
This involves quantification and evaluation of the nature of
organic-rich rocks so that the type and quality of expelled
hydrocarbon in a basin can be assessed by GEOCHEMICAL
methods of analysis.
STEPS IN SOURCE ROCK ANALYSIS
 First establish likelihood of presence of organic-rich sediments
deposited in the past on the basis of studies of local
stratigraphy, paleogeography and sedimentology of the area.
 Identification and delineation of area of potential source rock.
 Determine the type of KEROGEN and state of its maturation.
 Calculation of thermal maturity and timing of maturation.
 Finally determine the likelihood of oil / gas generation in the area
and calculate the depth of oil window.
(Majority of oil generation occurs in the 60° to 120°C range. Gas
generation starts at similar temperatures, but may continue up beyond
this range, perhaps as high as 200°C.)

3. Source Rock
• A petroleum source rock is sedimentary rock that contains sufficient organic matter to generate and expel (or
has done so) economic quantities of oil and /or gas upon burial.
• A potential source rock is one that is too immature to generate petroleum in its natural setting but will form
significant quantities of petroleum when heated in the laboratory or during deep burial.
• Effective source rock is one that has already formed and expelled petroleum to a reservoir.
Source Rock Can Be:
 Marine organic shales
 Marine organic carbonates / Marls
 Lacustrine organic rich shales
 Liptinitic coals (Tertiary & some mesozoics)
 Thermal Gas comes from humic coals or Source rocks at higher maturity.
 Bacterial gas from shallow mudstones
Maturity:
• Maturity is the time-temperature integral over the thermal history of formation, in particular a
petroleum source rock during diagenesis, catagenesis and metagenesis, and changes many
physical or chemical properties of the organic matter. These properties may be considered as
indicator of maturation.
• Maturity reflects the conversion state of source rocks:
a). Immature: source rocks have not yet generated oil or gas
b)Mature: source rocks are in the generation stage and the oil window is called mature.
The oil window: the depth interval in which a petroleum source rock generates and
expels most of its oil is called the oil Window. The most of oil windows are in the
temperature range from 600C to 1600C.
c). Post mature: source rocks have generated all the hydrocarbons

5. Measurement of Maturity
Maturity of rock can be measured based on a number of different
techniques:
• Vitrinite reflectance: Vitrinite is a maceral of OM that reflects light.
The % of reflected light increases with maturity.
• Thermal Alteration Index (TAI) or Spore Colour Index (SCI):
Spores change colour from light yellow to brown to black with
increase in Temperature & Time (dependent on spore type).
• Fluorescence Colour: Organic matter changes fluorescence under
UV light from dark brown to yellow to white/blue with increasing
maturation. Reliable, but dependant on type of organic matter
• Rock Eval Tmax: Depends on OM type & Quantity; and affected by
contaminants.
• N-Alkane & isoprenoids distribution.
• Biomarkers based parameters

7. Vitrinite Reflectance (%VRo)
• Reflectance of coal macerals
measured in reflected light has long
been used to evaluate coal rank.
• A relationship has been established
between huminite-vitrinite reflectance
and other properties.
• Vitrinite forms the familiar brilliant
black bands of coal which reflects
light and the amount of reflection
increases with maturation of OM.
• A petrographic microscope is used to
measure the amount of incident light
which is reflected from a highly
polished surface of a coal block or
grains mounted on epoxy resins.
• Reflectance values are taken on a
minimum 50 vitrinite particles for one
sample.
• For oil window VRo ranges from 0.6%
to 1.3%.

8. VRo (%) and stages of oil generation
• VRo <0.5 to 0.7%: diagenesis stage, source rock is immature.
• 0.5 to 0.7% <VRo < 1.3: catagenesis stage, main zone of oil generation,
also referred to as oil window (Peak oil window)
• 1.3%<VRo < 2%: Catagenesis stage, zone of wet gas and condensate
• VRo >2%: Metagenesis stage, methane remains as the only
hydrocarbon (Dry gas zone)
• VRo 3% Top of rock metamorphism
• Limitations:
• (i) Vitrinite is absent in Kerogen Type I and moderately in type II and
abundant in the type III of the excellent purposes.
• (ii) The reflectance intervals shown above are only approximate.
• There are no sharp boundaries as different type of organic matter have
different composition and different rate of transformation in response to
an increase of temperature. This effect is stronger in the lower than in
the higher level of maturation.

9. • This histogram shows that
the reflectance increases
from liptinite particle to
vitrinite and finally to
inertinite. Both liptinite
and vitrinite reflectances
increase with thermal
evolution. However, only
huminite or vitrinite
particles are generally
used for reference to the
coalification scale,
because vitrinite is the
main maceral of humic
coals.
• Vitrinite particles with
higher reflectance are
considered to be
reworked.

10. Thermal Alteration index (TAI)
• It is a numerical scale based on thermally induced
colour changes in spores and pollen observed
under the microscope in transmitted light.
• Colour changes from light yellow to brown with
increase in time and temperature and, therefore, in
maturity as the colour is originally yellow then
comes orange or brown (diagenesis), brown
(catagenesis), finally black (metagenesis).
• Alteration of the structural features occurs mostly
during catagenesis and metagenesis.
• Correia (1967) and Staplin (1969) proposed two
scales in which both colour and structure alteration
were used; they are respectively name state of
preservation of palynomorphs, and thermal
alteration index. Five states are defined by colour
and alteration of shape and ornamentation.
• Stage 1 refers to fresh yellow material, whereas
stage 5 corresponds to barely identifiable remnants
of the zone of metamorphism.
• The color is related to the vitrinite reflectance.

11. A-H= pollen grains showing different stages of maturation; I=Terrestrial woody matter
(fusinitic) pitted structures; J,o=Terrestrial matter(partly cellular structure); L=Terrestrially
sourced resin body; M=Fungal spore and associated with amorphous matter; N,P,R,S=
Biodegraded terrestrial matter; Q=Humic facies (terrestrial biodegraded, woody and
semifusinitic components); T=mixed facies (sapropelized amorphous matter and biodegraded
humic matter); U= Amorphous matter; V=amorphous sapropelized facies; W,X,Y=structured
aqueous matter (marine)

12. Fluorescence
Fluorescence microscope technique is used as a tool
for quantitative estimation of organic matter
maturation and has been found to be quite useful in
identification, characterization and quantitative
assessment of hydrogen rich macerals.
Fluorescence of various liptinite constituents is
induced by blue or UV light. The visible light emitted
by kerogen in response to excitation may be
characterized by its intensity and colour spectrum.
Fluorescence is intense in shallow immature samples
and decreases during diagenesis and most of
catagenesis. It has completely disappeared at the end
of the oil zone.
The use of this microscope gives the emission spectra
and maximum spectral wavelength of light emitted
by the excited organic matter and indicates the value
of maturity.
Maximum range of 400-585nm is indicative of
immature facies; range of 585-590nm is depictive of
organic matter in the early phase of maturation;
maximum range of 590-600nm reflects initiation of
generation of hydrocarbons; while 600nm and more
is indicative of mature facies.

13. ROCK EVAL Pyrolysis
• Rock Eval is the most widely accepted technique of source rock evaluation or organic
matter.
• It uses the high temperature pyrolysis which mimics in the laboratory the natural
hydrocarbon generation process.
• Rock Eval instrument consists of the facility for programmed temperature heating
(25oC/min) in an inert atmosphere (nitrogen, helium) of a small amount (100mg) of
powdered rock sample using a mico pyrolysis oven.
• The pyrolysis yields three main groups of compounds equivalent to S1 (or P1), S2(or P2)
and S3 (or P3) of the method for characterizing the type of organic matter:
• 1. HC already present in the rock (S1) which are volatized by moderate heating at 200 to
2500C (i.e. any free HCs in the rock that either were present at the time of deposition or
were generated from the kerogen since deposition). Heating at 3000C simply distills
these free HCs out of rock.
• Or: Quantities of free petroleum i.e. the hydrocarbon already present in the specimen
are represented as S1. It is an indicator of the amount of petroleum generated by the
Kerogen to the present sate of maturity.
• 2. HCs and related compounds (S2) generated at high temperatures by pyrolysis of
insoluble kerogen i.e. Hydrogen index (or HI), S2/organic carbon. Between about 350
and 5500C, HCs P2 or S2 are generated by cracking the kerogen until only residual non
generating carbon remains.
• Or. S2 indicates the remaining potential of the kerogen in the rock.
• 3. Carbon dioxide (S3) and water i.e. oxygen index (OI), S3/organic carbon. These
carboxyl groups in the kerogen break off between 300 and 3900C, yielding CO2 (P3 or
S3) which is trapped and analyzed later during the cooling cycle using a thermal
conductivity detector (TCD).
• Tmax is the temperature at which the S2 (mg/g rock) peak reaches its maximum amount
of hydrocarbon generation during Rock Eval pyrolysis.

14. Hydrogen index & S2/S3 proportional to the amount of hydrogen in the
kerogen. High hydrogen indices indicate greater potential to generate oil.

15. Rock Eval: Tmax & Production Index
• This method is represented by the two
indices for evaluation of the organic
matter such as the ratio S1:(S1+ S2)
and temperature T.
• The continuous increase of this ratio as
a function of depth makes it a valuable
index of maturation.
• In addition to measuring maturation,
the values S1 & S1:(S1+ S2) can be used
for a quantitative evaluation of the HCs
generated. The result is conveniently
expressed in grams HCs per ton rock &
grams HCs per Kilogram organic matter,
respectively.
Tmax. for same organic
facies, Tmax increases with
increasing depth. Tmax is
partly determined by the
type of organic matter.
It is expressed to plot the variation of the transformation
(or productive) ratio , PI= S1:(S1+ S2) versus depth in
order to identify such accumulations by their anomalously
high values compared with the average curve.
PI = S1/(S1+S2)
Reservoir rocks show anomalously high PI values
compared to adjacent fine-grained rocks. The
beginning of significant oil generation is around a PI
of 0.1 and the end is around 0.4.
The Tmax at the beginning
of oil generation starting
around 4300C and the end
being around 4600C.

17. Source rating TOC % S2 mg HC/g rock Source proclivity
Poor <0.5 <1 Gas
Fair 0.5-1.0 2-5 Gas and oil
Good 1.0-2.0 5-10 Gas and oil
Very Good 2.0-5.0 10-20 Gas + oil
Source rock evaluation parameters for Rock-Eval/TOC (Jones and Demaison, 1982)
Source rock evaluation parameters for Rock-Eval/TOC data (Jones and Demaison, 1982)
Maturation stage Tmax (0C) HC Type
Immature <430-435 Oil
Mature 430-465 Oil
Over mature >465 Gas+Oil
Source rock evaluation parameters for Rock-Eval/TOC data (Peters, 1986)
TOC S1 mg HC/g rock S2mg HC/g rock
0-0.5 0-0.5 0-25
0.5-1 0.5-1 25-5
1-2 1-2 5-10
2-4 2-4 10-20
>4 >4 >20

18. • From this analysis, it has been found that the S1:(S1+ S2) is
fairly independent of the type of organic matter during
catagenesis and metagenesis. The temperature T is
influenced by the type of organic matter during the
diagenetic stage and at the beginning of catagenesis.
• It is lower in the terrestrial kerogen of type III and higher in
the marine or lacustrine types I and II. However, the T values
are almost equivalent for the different kerogen types in the
peak zone of oil generation and later in the gas zone.
• As generation proceeds, higher temperatures are required to
crack the remaining kerogen. This causes the peak of S2
which is called the Tmax (the maximum liberation of HCs), to
shift gradually to the right of the dashed line in the above fig.
The actual temperature increase is shown in the lower right,
with the Tmax at the beginning of oil generation starting
around 4300C and the end being around 4600C.

19. Source Rock Evaluation Criteria

20. Estimation of organic matter type and
degree of maturation (Espitalie et al.
1985).
Hydrocarbon Index (HI) vs Tmax diagram
(Delvaux et al. 1990)

21. TOC vs S2: for inferring the kerogen type and the
present day generative potential (S2).
PI vs Tmax: this plot can help in identifying
whether the rock under study produces gas or
oil. It helps to identify the type of gas whether it
is dry or wet.
S1+S2 vs TOC: helps to evaluate source rock
potential.

22. van Krevelan diagram for OM Typing & Oil Window
The atomic H/C versus O/C diagram was
given by van Krevelan, 1961 as a simple and
rapid method for following the chemical
processes that occur during coal maturation
(coalification). Since the main elements of
coal are carbon, hydrogen, and oxygen
changes in coal composition during burial
can be recognized by plotting atomic H/C
versus O/C ratios.
Moving from the right to the left across the
diagram along the coalification line
represents the loss of oxygen relative to
carbon, which occurs with the formation of
CO2 or H2O.
Moving from the top to the bottom of the
diagram represents the loss of hydrogen
relative to carbon. This occurs because of
the formation of oil and gas, which have
higher H/C ratios than either kerogen or
coal.

23. Kerogen Type I: kerogen with a high initial H/C atomic (1.5 or more) and a low initial O/C
ratio (generally smaller than 0.1). Kerogen comprising lipid material particularly aliphatic
chains). The content of polyaromatic nuclei and heteroatomic bonds is low, compared with
other type of organic matter. A small amount of ester bonds present. Lacustrine
Botryococcus and associated forms are commonly preserved. The H/C ratio is originally high
and the potential for oil and gas generation is also high. This type of kerogen is either mainly
derived from algal lipids or from organic matter enriched in lipids by microbial activity.
Kerogen Type II: this is mainly source rocks for petroleum and oil shales, with relatively high
H/C and low O/C ratios. Polyaromatic nuclei and heteroatomic ketone and carboxylic acid
groups are more important than they are in type I, but less than type III. Ester more
prominent. Kerogen is related to marine sediments derived from a mixture of phytoplankton,
zooplankton and microorganism (bacteria), has been deposited in a reducing environment.
The H/C ratio and the oil and gas potential are lower than observed for type I kerogen but
still very important.
Kerogen Type III: refers to kerogen with a relatively low initial H/C ratio (usually less than 1.0)
and a high initial O/C (as high as 0.2 or 0.3). Comprising an important proportion of
polyaromatic nuclei and heteroatomic ketone and carboxylic acid groups but no ester groups
with minor aliphatic chains. It is mainly derived from continental plants and contain much
identifiable vegetable debris. H/C is low and oil potential is only moderate, although this
kerogen may still generate abundant gas at greater depths.

26. Pyrolysis-Gas Chromatography-Mass spectrometry (Py-
GC-MS) as finger printing and Whole oil and saturates fraction
Gas Chromatography of
Oils/EOM (extractable
organic matter) (saturates)
provides
information on :
1. Source type as Marine
vs. Terrestrial
2. Carbon Number
3. Distribution
Maturity (a) Shift towards
lower HC’s with increasing
maturity (b) Primary
(Preserved) /
Biodegraded
Chem StationSoftware

27. Prolysis Gas
Chromatography-
Mass spectrometry
(GC-MS)

28. Methodology:
1. Measured sample and washed for removing
contaminant
2. Make powder and free from moisture at
oven
3. After washed glass ware, transfer the
powder to breaker
4. Pour solvent (Methol:DCM=1:9) to breaker
5. Put breaker to ultrasonic vibrate about 15
min upto ¾ times.
6. After ultrasonic, soln poured into furnace for
extraction of bitumen
7. Dry bitumen
8. Prepare column by using silica gel with
cotton.
9. Transfer bitumen to column by using
DCM solvent and dry column about ½
days.
10. Prepare saturate , aromatic and polar
11. Saturate: using n-hexane as ratio of
DVx3/8 i.e. 4x3/8ml & excess using
same volume.
12. Aromatic: prepare soln of n-
hexane:DCM (4:1) and 4xDV i.e.
4x4=16
13. Excess done by using same solvent of
above.
14. Polar: DCM:Methnol (4:1) and
quantity of DVx4ml.
15. Treatment of sulfur for saturate if
need by using copper.
16. Transfer saturate fraction, aromatic
and polar to vials by using n-hexane,
DCM and DCM for GC-MS analysis.

30. Saturate: CnH2n+2
n-Alkanes/acyclic compoound: m/z 57;
Pr/Ph (Pristane, C17/Phytane, C18)
Pr/Ph >1.2 Oxic-suboxic
Pr/Ph<1 Anoxic
Biomakers: Sesquiterpanes =m/z 123 or
193
Diterpanes=m/z 123
Hopanes=m/z 191
Steranes=m/z 217
Bicadinanes=369 This plot
shows that the
redox
condition and
source of the
organic matter.
The samples
are plotted in
the zone of
early mature,
terrigenous
oxic
depositional
environment
GC GC MS

31. •Saturates: 1. Acyclic compounds-In chemistry, a compound which is an open-
chain compound, e.g. alkanes and acyclic aliphatic compounds.
•2. Sterane: Staranes are derived from sterols that are found in most higher
plants and algae. Four principal sterol precursors (organic molecules) containing
27, 28, 29 and 30 carbon atoms have been identified in numerous photosynthetic
organism. The C27 – C29 names cholestane, ergostane and sitostane respectively
and also name of cholestane, 24methylcholestane, and 24ethylcholestane
respectively. In very immature sediments there occur some unstable compounds
that are intermediates in the transformation of sterols to steranes.
•Triterpanes (put to m/z 191)-The source organisms for most triterpanes
biomarkers are believed to be bacteria. Triterpanes can be divided into three
distinct families based on the number of rings. The most common and most
thoroughly studied triterpanes have five rings and therefore called (1)
pentacyclic. Most of these compounds contain from 27 to 35 carbon atoms,
although they have been reported upto C40.
The (2) tricyclic (sometimes C19 to C54) has only three rings and ranges about 21
to more than 40 carbon atoms but those with fewer than 25 carbon atoms are
dominant. The (3) tetracyclic is the least studied and most poorly understood
family.

32. •Tricyclic terpanes is characterized by two classes of tricyclic compounds-the
regular tricyclic terpanes (m/z 191) and novel tricyclic terpanes (m/z 123). The
regular tricyclic terpanoids may be produced by prasinophycean algae like
Tasmanites or Leiosphaeridia. The alkyl side chain in novel tricyclic terpanes may
be produced by certain bacteria.
•Pentacyclic divided into hopanoids and nonhopanoids. The partial mass
chromatogram at m/z 191 represents the pentacyclic hopanoids associated with
tetracyclic terpane and C24-26 17, 21-secohopanes (which is indicative of
thermally immature) . The most common pentacyclic triterpanes are the hopanes
(also called moretanes). The hopanes most frequently analysed contain 27 to 35
carbon atoms (Hopanoids series is characterized by C27 to C32 hopanes). The C34 –
C40 or C31-C35 Biomarkers like hopanoids are useful as maturity parameter. The
Hopanes are the precursor of hopanes and indicate the immature nature of
source rock. They may have a microbial origin and derived from higher plants.
Nonhopanoids (gammacerane and a family of compounds called Oleananes).
Oleananes comes from angiosperms (terrestrial plants).
•Terpenoids: The distribution of terpenoids is represented by the occurrence of
sequiterpenoids, diterpenoids and tritepenoid. These biomarkers are identified
by using m/z 123 partial mass fragmentation.
•The occurrence of diterpenoids suggests that conifers served as the source
material for the formation of coal.

34. • PY-GC technique which became the chemical data systems
pyrolyzer. It analyzed for the individual hydrocarbons in the
P1 (S1) and P2 (S2) peaks by GC.
• PY-GC and MS have been combined to determined both
quantity and identity of individual HCs obtained by pyrolysis.
• The procedure involves placing about 20 to 30 mg of finely
milled rock in a glass tube. The GC column is cryogenically
cooled to -50 C and the pyrolysis unit heated to 2500C. The
drives the P1 (S1) free HC into the GC capillary column.
Subsequent heating of the column transfers the P1 (S1) HC to
the FID and MS thereby providing both a gas chromatogram
and mass spectrum of the P1(S1) HCs.
• The GC is cooled again to -50C while the pyrolzer is heated to
7000C to release the cracked HCs, P2(S2). These are trapped in
the cooled capillary column and subsequently released for
the GC-MS analysis.

35. •In the first figure shows the free P1 and the cracked P2 HCs from the Austin Chalk of Texas. The
curve at 4565ft displays a small amount of free HC in the chalk (P1), but the potential to
generate HCs is considerable (P2). The curve for the sample at 9093 shows a large P1, indicating
that the chalk already has generated considerable quantities of HCs but the P2 is smaller
because an appreciable part of kerogen already has been converted to HCs. The production
index, PI (PI=S1/(S1+S2), is about 0.1 in the shallow sample and 0.7 in the deeper sample have
already taken place.
•In the second figure shows the distribution of HCs that make up the P1 peak at 12600ft and
15690ft. Note that the C11, n-paraffin is present in the highest concentration at the greater
depth. At the shallower depth, the earliest eluding C7 n-paraffin is dominant. This shift to
larger molecules at greater depths is believed to be due to the preferential migration of the
smaller HC molecules out of the source rock with increasing maturity. The quantity of specific
HC groups such as the n-alkanes in the C7-C14 range in mg/g TOC can be calculated with depth.

36. Biomarkers in Exploration
• Biomarkers are “Molecular Fossils”. Biomarkers in crude oils
can provide information about the characteristics of the source
rock from which the oil was generated.
– Particularly useful, because very often no source rocks have
been collected from a basin
• Biomarker data can provide information about:
– source rock correlates with the oil (Oil-Source correlation).
• (Type of organic matter & Maturity of Source Rock) on the basis of
(1) Presence or absence of Distinctive Biomarkers
(2) Relative abundance
- these oils are from the same source rock (Oil-Oil correlation).
– Environment of deposition of the source rock of the oil.
– the lithology of the source rock of the oil.
– the age of the source rock of the oil (MS-MS in particular)
– the basin has more than one oil family.
(i.e., multiple petroleum systems)

37. Main Biomarker groups
• Steranes : C19-C31, 3-4 ring alkanes, derived from plants and animals
• Hopanes : C27-C35 pentacyclic alkanes,derived from bacteria
(Dominant fraction in the Triterpane group)
• Diterpanes : C20 bi-tri cyclic alkanes, derived from plants
• Isoprenoids : Chain-alkanes, various sources
• Aromatic steroids: Related to Steranes.
Biomarkers are identified by the characteristic fragments that
are created in the mass spectrometer
Distinct Biomarkers:
Botryococcane: Freshwater algae, Fresh water lakes
Abeitane: Higher Land Plants esp. Gymnosperms, Delta tops
Oleanane: Higher Land Plants esp. Angiosperms (post Late Cretaceous input),
Delta tops.
Dinosteranes: Dinoflagellates, Marine
Bisnorhopane: H2S oxidizing bacteria, Marine
Relative abundance of Hopanes and Steranes gives idea about Organic Matter
input

40. • Thermal cracking of kerogen:
• Oil and gas are generated by the thermal degradation of kerogen in the source beds.
With increasing burial, the temperature in these rocks rises and, above a certain
threshold temperature, the chemically labile portion of the kerogen begins to
transform into petroleum compounds.

41. • 1. In a first stage, corresponding to diagenesis, heteroatomic bonds are broken
successively and roughly in order of ascending rupture energy, starting with
some labile carbonyl and carboxyl groups (like ketones and acids). Heteroatoms,
especially oxgyen are partly removed as volatile products: H2O, CO2. the rupture
of these bonds liberates smaller structural units made of one or several bound
nuclei and aliphatic chains. These structural fragments are the basic constituents
of bitumens.
• The larger one are structurally similar to kerogen but of lower molecular weight
and therefore soluble (MAB extract=heavy bitumen, extracted by methanol-
acetone-benzene mixture i.e. asphaltenes). The smaller ones are linear,
branched, and cyclic HCs and closely related heterocompounds such as
thiophene derivative.
• During this stage, the larger fragments containing heteroatoms, especially
oxygen (resin, asphaltenes, MAB extract), are predominant.
• 2. During catagenesis, temperature continue to increase, more bonds of various
types are broken, like esters and also some carbon-carbon bonds, within the
kerogen and within the previously generated fragments (MAB extract,
asphaltenes etc.). The new fragments generated become smaller and devoid of
oxgyen: therefore, HCs are relatively enriched. This corresponds first to the
principal phase of oil formation, then to the stage of wet gas and condensate
generation.
• At the same time, the carbon content increases in the remaining kerogen, due to
the elimination of hydrogen. Aliphatic and alicyclic groups are partly removed
from kerogen. Carbonyl and carboxyl groups are completely eliminated and most
of the remaining oxygen is included in ether bonds and possible in heterocycles.

42. • At deepest part of sedimentary basins with quite high
temperature, a general cracking of carbon-carbon bonds
(cracking) occurs, both in kerogen and bitumen already
generated from it. Aliphatic groups that were still
present in kerogen almost disappear. Corresponding, low
molecular weight compounds especially methene are
released. The remaining sulfur, when present in kerogen,
is mostly lost and H2S generation may be important. This
is the principal phase of dry gas formation which
requires that the input of thermal energy exceed certain
minimum levels (activation energy). Activation energies
vary according to the position and type of carbon-carbon
bond within the Kerogen structure. The bonds between
carbon and heteroatoms (N, S, and O) are more labile
and hence easier to break. The first products generated
by source rocks during burial with N, S, and O
compounds, carbon dioxide (CO2) and water.

43. •At higher temperature
levels, petroleum
compounds are generated
by the cracking of carbon-
carbon bonds within the
kerogen structure in such a
way that long aliphatic side
chains and saturated ring
structures are removed from
it. These reactions result in
gradual changes in the
elemental composition of
the kerogen, especially in a
decrease of its hydrogen
content. These changes are
expressed in the van
Krevelen diagram for each
Kerogen type as trend lines,
the so-called evolutionary
pathways.

44. Recommended books:
1. The biomarker guide volume 1 & 2 by Kenneth E. Peters, Clifford C. W alters, and J.
Michael Moldowan
2. Petroleum Geochemistry and Geology by John M. Hunt
3. Petroleum Formation and Occurrence by B.P. Tissot and D.H. Welte.