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Petroleum Geochemistry for Source
Rock Evaluation
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D.K. Asiedu
Petroleum source rocks
 Every oil or gas play originates from source rocks
 The viability of each play depends on its source rock
 Without this source of petroleum, all other components and
processes needed to exploit a play become irrelevant
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Petroleum source rocks
 Source rock is as any fine-grained, organic-rich rock that is
capable of generating petroleum
 Its petroleum-generating potential is directly related to its
volume, organic richness and thermal maturity
 Organic richness refers to the amount and type of organic
matter contained within the rock
 Thermal maturity refers to a source rock’s exposure to heat
over time
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Petroleum generating potential of
source rocks
 When assessing source-rock potential, four questions must
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be answered :
First, does the rock have sufficient organic matter?
Second, is the organic matter capable of generating
petroleum and, if so, is it oil prone or gas prone?
Third, is the organic matter thermally mature?
Fourth, have generated hydrocarbons been expelled from the
rock?
D.K. Asiedu
 The question of whether or not the rock has sufficient organic
matter to be considered a source rock can be answered on the
basis of total organic carbon (TOC) measurements.
 Rocks that have insufficient TOC content can be rule out as
possible source rocks.
 The TOC content needed for petroleum generation is thought to
be greater in siliciclastic shales than in carbonate source rocks.
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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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Maturation of kerogen
 During the phase of catagenesis, kerogen matures and gives off
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oil and gas
Establishing the level of maturation of kerogen in the source
rocks of an area subject to petroleum exploration is vital
When kerogen is immature, no petroleum has been generated
With increasing maturity, first oil and then gas are expelled
When kerogen is overmature, neither oil nor gas remains
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Correlation between hydrocarbon generation and temperature
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Fundamentals of source rocks
 Although many organic-rich source rocks are argillaceous,
carbonates (typically marls) can also make excellent source rocks
 Generally, the quality of the source rocks share a common
characteristics:
 They form in anoxic or highly reducing environments
 Are generally laminated
 Have moderate to high TOC
 Contain organic matter with atomic hydrogen/carbon ratios
exceeding 1.2
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Kerogen types
Kerogen
type
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Source material
General
environment
of deposition
Hydrocarbons
generated
H/C
O/C
I
Mainly algae
Lacustrine
setting
Oil, Gas
≥ 1.5
< 0.1
II
Mainly plankton,
some contribution
from algae
Marine setting
Oil, Gas
1.0 to
1.4
0.09 to
1.5
III
Mainly higher plants
Terrestrial
setting
Dry gas
< 1.0
0.2 to
0.3
IV
Reworked , oxidized
material
Varied settings
None
Poor in H, high in C
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 The second question asks what type of organic matter is present within
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the rock.
The type of organic matter, if present in sufficient quantity, will
determine if a source rock will produce principally oil or principally gas
upon maturation.
Algal, herbaceous (plantonic), and much amorphous kerogen (kerogen
types I and II) will generate oil and associated gas upon maturation
Woody kerogen (kerogen type III) and some amorphous kerogen will
generate gas and possibly a minor amount of oil or condensate upon
maturation.
Inertinites are type IV kerogens that have extremely low hydrogen
contents and are incapable of generating significant amounts of
hydrocarbons.
It is possible to differentiate kerogen types I, II, and III using Rock-Eval
pyrolysis
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Paleothermometers
 Paleothermometers measures the maximum temperatures to
which the source rock was ever subjected.
 This is important because they let us know whether or not the
source rocks have matured sufficiently to generate petroleum or
whether they are supermature and barren
 Broadly, two major groups of techniques are used for measuring
the maximum paleotemperature to which a rock has been
subjected:
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 Chemical paleothermometers:
 Carbon ratio
 Electron spin resonance (ESR)
 Pyrolysis
 Gas chromatography
 Biological paleothermometers
 Pollen and spore coloration (visual kerogen analysis)
 Vitrinite reflectance
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Pyrolysis
 Pyrolysis is the heating of kerogen or source rock
 Pyrolysis is carried with a flame ionization detector
 Analysis can be carried out on whole rock samples using the
Rock-Eval pyroanalyzer
 Sample is heated and the expelled hydrocarbon gases recorded
with a hydrogen flame ionization detector
 At low temperatures (200 – 300ºC) any free hydrocarbons in
the sample are measured (S1).
 At increasing temperatures hydrocarbons are expelled from the
kerogen itself; the maximum amount of hydrocarbon is
measured (S2)
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 With further heating to some 390ºC, carbon dioxide is expelled
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to generate a third peak (S3)
The temperature at which the S2 peak occurs is termed Tmax
The three readings can be used to determine the maturation level
of the source rock
Where migration has not occurred, the ratio S1/(S1 + S2) shows
the amount of petroleum generated compared with the amount
capable of being generated. The ratio is referred to as the
production index (PI)
S2/TOC (Hydrogen Index) relates directly to the potential of
rock to generate oil rather than gas. The higher the HI, the higher
the potential to generate oil
S2/S3 ratio is an indicator of hydrogen richness in the kerogen
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Rock-Eval pyroanalyzer
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Cross plots
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Cross plots
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Cross plots
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Visual kerogen analysis
 The level of thermal maturity can be evaluated using visual kerogen
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analyses.
Kerogen has many colours and shades, which are dependent on both
maturation and composition
Spores and pollen begins life essentially colourless
As they are gradually heated they changes to yellow, orange, brown
(light to dark), then to black
Based on calibrated colour charts, the sample is assigned a numerical
value (Thermal Alteration Index or TAI), which ranges from 1.0
(immature) to 5.0 (metamorphosed).
This is a standard method of determining thermal maturity that can be
used to confirm measurements made with other techniques
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Vitrinite reflectance
 The shininess (or reflectance) of vitrain, a coal maceral, increases with
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maturity and can be measured optically
Vitrain occurs widely throughout sedimentary rocks
Kerogen, which includes vitrain, is separated from sample by solution
in HCl.
The residue is mounted on slide and then polished
A reflecting-light microscope is then used to measure the degree of
reflectivity, termed Ro
An empirical relationship has been noted between vitrinite reflectance
and hydrocarbon generation
Crude oil generation occurs for Ro values between 0.6 and 1.5; Gas
generation takes place between 1.5 and 3.0.
Above 3.0 the rocks are graphitic and devoid of hydrocarbons
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Vitrinite reflectance equipment
QDI CoalPro™ Vitrinite Reflectance Measurement System
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Correlation between hydrocarbon generation, temperature and some
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paleothermometers
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Transmitted light optical techniques
 Microscopic study is very useful for kerogen identifiction and
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characterization
This method of kerogen characterization may complement data
obtained by chemical analysis or pyrolysis
Some type III kerogens may be confused with other types of
kerogens and result in misleading characterization of kerogen
types when using pyrolysis
Oxidation of kerogen may also alter its Rock-Eval character.
Furthermore, pyrolysis cannot discern the different varieties of
kerogens present in samples with mixed kerogen assemblages.
For these reasons, Rock- Eval pyrolysis should be used only as
reinforcement for petrographically determined kerogen
identification
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photomicrographs of kerogens
(in transmitted light)
(a) structured amorphous
(b) woody fragments,
(c) unstructured amorphous,
(d) spore and pollen,
(e and f) dinoflagellates
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Vitrinite – woody tissues
Inertinite – burned organic tissue
Alginite – algae, plankton
Exinite – higher plant protective tissues,
spores, pollen, resins
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 The fourth and final question concerns the expulsion of
generated hydrocarbons from the source rock. This question
is more difficult to answer than the other three questions.
 For the most part, studies of thermal maturity of a source
rock are empirically correlated with the presence of oil in
associated reservoirs.
 It is generally assumed that once a sufficient volume of
hydrocarbons have been generated in a source rock, they will
be expelled and migrate into reservoirs.
 Source rocks that are thinly interbedded with reservoirs will
expel hydrocarbons at lower levels of thermal maturity than
thick source rocks that contain few or no interbedded
reservoirs.
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