petroleum geology pt. 1

1. PETROLEUM GEOLOGY PART 1
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2. Fundamentals Petroleum Geology
• It is an applied geoscience discipline that involves
the study of:
– The origin
– Generation
– Migration
– Accumulation; of HC’s with a view to defining ways to
successfully explore and produce it
• In involves the integration of basic geology
disciplines such as: sedimentology, stratigraphy,
structural geology, mineralogy, geochemistry,
geophysics, palynology, paleontology, etc., and
other pure science disciplines
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3. Relationship between Petroleum geology, basic geology disciplines and pure
science disciplines (Selley, 1998) 3

4. What is petroleum
Latin; Petra: ROCK; Oleum: OIL
Petra + oleum = rock oil
‘’A naturally occurring liquid, gas, semi-solid or solid
mixture of hydrocarbon and non-hydrocarbon
molecules.”
• The aforementioned suggests petroleum can occur
as:
– liquid (crude oil)
– Gas (natural gas)
– Solid and semi-solid (Asphalt, Bitumen, Tar, etc.)
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5. Natural Gas
• Petroleum in gaseous state; ‘it is the lightest form of
petroleum with little complexity’
• It can also be defined as petroleum that is non-
condensable at ≥ 200
C and atmospheric conditions (i.e.
at ῀ surface conditions)
• Can be classified as dissolved gas (if it occurs in solution
in crude oils); associated gas or ‘gas cap’ (above oil
column in HC reservoirs); or non associated gas or ‘NAG’
(no associated oil column)
• Natural gas can also be termed as Wet gas (methane +
ethane, propane and butane and other higher HC’s ) if
it contains liquid oil vapour (i.e. ≥ 0.3 gal/1000ft3 of
condensate) or Dry gas (mainly methane) if it contains
very little or no liquid oil vapour (i.e. ≤ 0.1 gal/1000ft3 of
condensate) 5

6. • Natural gas may exist as liquids (Natural gas liquids) at
the surface under certain conditions. These are termed:
– Liquefied Petroleum gas (LPG): natural gas (that
exist normally in gaseous state at STP) liquefied and
condensed at very low T and High T. chemically,
LPG is mainly propane and butane
– Condensate: wet gas; liquid at STP. Chemically,
condensate comprises of Pentanes and other higher
HC’s
• Occurrences of non HC gases like He, H2S, Ar, H,
Co2,O2, etc have been reported in association with
Natural gas.
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7. Crude oil
• Liquid HC at reservoir and surface conditions
• Vary in colour from yellow through green to brown
and black
• It is insoluble in water; its solubility is inversely
proportional to its carbon number
• Commonly has some dissolved gas in it
• Crude oil is said to be saturated when no more gas
can be taken into solution i.e. an associated gas
column (gas cap) exist with it at reservoir levels
• It is unsaturated if there’s still potential for natural gas
to be dissolved in it
• It has a wide range of SG (i.e from heavy oil to light
oil) and viscosity
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8. Petroleum chemistry
Molecular groups of hydrocarbons:
• Paraffins = alkanes (aliphatics)
• Napthenes (cycloparaffins) = cycloalkanes
• Aromatics = arenes
• Naphtheno-aromatics (complex polycyclic molecules)
• Olefins = alkenes
Paraffins = alkanes (aliphatics)
• Can occur as straight chain (normal alkanes) or branched
chain (iso-alkanes)
• Saturated HC’s (all carbon bonds (C-C) are saturated with
hydrogen) with a general formula of CnH2n + 2, where n =
1– >60
• n = 1–4 are gases (1-3 dominate natural gas); n = 5–17 are
mainly liquids (dominate gasoline fraction of petroleum) n =
> 17 are waxes, semi-solids, asphalts, etc.
• They form a homologous series (each successive member
differ by CH2)
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9. Isoparaffins
• Isomers of n-paraffins
•same composition and same general formula with n-paraffins
• differ in structure, physical and molecular properties
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10. Naphthenes (cycloparaffins)
• Saturated; general formula of : CnH2n
• C-atoms are joined in a ring
• Dominant structures in petroleum
• Form a homologous series up to n=30
• Generally, rings greater than C7 are unstable and become
strained in crude oil
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11. Aromatics (arenes)
• Unsaturated; General formula: CnH2n-6r (where r =
number of benzene rings)
• They react to add CH3 (akyl benzene) to the ring (i.e. H
atoms can be substituted; soluble in water)
• mainly found in heavy fractions.
• Toluene is most common form.
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12. Naphtheno-aromatics (cycloalkyloaromatics,
alkyloaromatics, etc.
• ‘’Complex, condensed polycyclic hydrocarbons
produced by joining of rings, chains, etc.’’
• Common in heavy fractions of petroleum. Some are
carcinogenic.
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13. Olefins (alkenes)
• Unsaturated with General formula of CnH2n
• Form homologous series
• Rare in natural crudes—normally reduced to paraffins
• highly reactive
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14. Asphaltics – NSO Compounds
• Non HC’s; abundant in the heavy solid to semi solid
residual fractions of petroleum; may also be present
in lighter HC’s (crude oil).
• As a rule of thumb, the lower the °API Gravity
(higher the viscosity), the higher the proportion of
NSO-compounds
Examples include:
Nitrogen Compounds
• Pyridines
• Quinolenes
• Indoles
Range in natural crude oils: < 0.25–0.8% N
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15. Sulphur compounds
• most abundant after HC’s
• Thiols (mercaptans)
• Thoiophenes
• Free S and H2S
Range in natural crude oils: < 0.5–5% S
Oxygen Compounds
• Common in Immature oils
• Organic acids
• Alcohols, phenols, esters
• Indoles
Range in natural crude oils: < 0.1–2.0% O
Other components include various metallic
compounds, spores, altered microfossil remains
(geochemical fossils or biomarkers), etc.
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16. Proposed classification of crude oils on the basis of relative
proportions of alkanes, cyclo-alkanes, and aromatics plus
NSO compounds (Tissot and Welte, 1984) 16

17. Properties of Petroleum
Specific Gravity
• A measure of the weight of a given material compared to the
weight of an equal volume of water at standard temp & pressure
• The degree API or API is the Industry standard scale of measure.
It relates to SG by: “API0
= (141.5/S.G) - 131.5’’
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18. • API is used as an index to adjust the prices of crude
Viscosity
• Measures the fluid’s resistance to flow. It is affected by the
amount of dissolved gases in the oil at a definite T &P.
Viscosity is measured in centipoises (cp = 10-2
poise).
Pour Point
• Measures the lowest temperature at which a crude oil will
flow, just before it starts to turn into a solid. Generally, the
higher the paraffin content of a crude the higher will be the
pour point. Lower pour point is preferred to higher pour point.
Colour
• Crude oils vary in colour from colourless (very light crudes)
to greenish-yellow to reddish to black (generally the heavy
crudes).
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19. Odour
• Various crudes oils smell like gasoline (sweet crudes) or
rotten eggs (sour crudes) or have a sickly fruity smell
(aromatic crudes).
Cloud point
• Defined as “ the Temp. at which the first cloud appears during
cooling of heated oil”. It is a consequence of settling out of
paraffin waxes. N/B: non waxy crudes show no cloud point
Flash point & Burning point
• Flash point is ‘the temperature at which vapour rising from a
heated oil will be ignited with a flash of short duration when a
flame is passed over it
• Burning point is the lowest temperature at which a heated oil
will ignite and burn with a steady flame
(these props. are useful when evaluating potential hazards
of handling and storing crude oil)
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20. Fluorescence
• Hydrocarbons usually emit visible light (fluoresces) when
exposed to UV light. Colour ranges from green to light blue. This
fluorescence property comes in handy when looking for evidence
of oil shows in cuttings or cores
Refractive index
• This is a function of the density and temperature of crude oils. It
is the ratio of the velocity of light through the crude to the
velocity of light through a vacuum. It ranges from 1.39 – 1.50
Optical activity
• Certain compounds possess the ability to rotate the plane of
polarization either to the right or left. This property is called
optical activity. It is a property exhibited by petroleum as a result
of its porphyrin content. Porphyrins are the organometallic
derivatives of haemin (animal) and chlorophyll (plants)
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21. Origin of Petroleum
Inorganic hypothesis
• This hypothesis is mainly supported by the Russians
• Evidences to support this hypothesis are:
– Discovery of methane in atmospheres of Jupiter, Saturn and
Neptune; occurrence of carbonaceous chondrites
– Occurrence of petroleum in basement rocks
– Production of methane during volcanism
– Petroleum synthesized in the Laboratory:
FeC2 + 2H2O = C2H2 [acetylene] + Fe(OH)2
Al4C3 + 12H2O = 3CH4 + 4Al(OH)3
Fischer-Tropsch reaction:
CO2 + H2 = CO + H2O, then CO + 3H2 = CH4 + H2O
The inorganic hypothesis has now been laid to rest. A lot
of scientists now favour an organic origin for petroleum
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22. • The inorganic hypothesis has now been laid to rest.
• A lot of scientists now favour an organic origin for petroleum
Some Organic evidences for petroleum
• Poor correlation between petroleum and volcanism
• Paucity of Precambrian oil
• Isotopic evidence favours organic origin
• Petroleum is “optically active” – linked to organic origin
• Presence of homologous series
• Geological association of petroleum with sedimentary
basins
• Presence of organic debris (foram test, lignite) and
geochemical fossils (biomarkers)
• Petroleum are very complex hydrocarbons. This reflects the
great variability of the primary source material and
differences in the physio-chemical conditions that persisted
during petroleum generation and migration
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23. Petroleum occurrence
• Petroleum occurs either on the surface or in the sub-surface
Surface occurrences
• Are mainly petroleum springs and seepages found on land
and underwater;
• flow of petroleum in seeps is usually very slow (sluggish),
whereas in Springs, it is reasonably rapid are
• Their occurrence is indicative of a working petroleum system
(effective SR), and potential subsurface accumulations
• Usually, evaporation/biodegration of petroleum (crude oil) in
these occurrences leave behind viscous residues (Tar,
bitumen, asphalt, etc)
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24. Subsurface occurrences
• These are the main targets of any exploration campaign
• Subsurface accumulations can only take place if all the
geologic conditions are met (timing of elements and
processes of a petroleum system)
• they can be minor showings or major (commercial) finds.
• petroleum usually accumulates in Pools within the Fields in
a Petroleum Province.
• A pool of oil is the simplest unit of commercial occurrence. ‘It
is an accumulation of petroleum in the same reservoir
and trap, under the same pressure and temperature
conditions
• Several petroleum pools related to same geological
features is a field
• These geologic features can be structural or
stratigraphic.
• Very large fields are called Giants.
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25. Petroleum generation
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26. • By now we know Petroleum is generated in organic rich
fine grained sedimentary rocks
• Tissot (1977) outlined 3 stages in conversion of OM in the
source rocks to Petroleum
• These stages are:
Diagenetic stage
• This takes place at temp. up to ῀650C & low pressures;
depths from sediment interface up to 2km (depending of
the geothermal gradient of the basin)
• OM matter is broken down firstly biogenically (aided by
aerobic bacteria using up all trapped oxygen in the fine
grained sediments)
• And thereafter abiogenically (aided by anaerobic bacteria),
reducing nitrates, nitrites and sulphates
• The end product is Kerogen (a complex insoluble HC),
swamp gas/biogenic gas/dry gas (methane) + CO2 and
H2O (other non HC gases include NH3, H2S and P2O5
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27. Nitrate Reduction
• After oxygen is depleted, NO3
- is used as an energy source:
6CH2O + 4NO3
- = 6CO2 + 6H2O + 2N2
When nitrate is exhausted:
Sulphate Reduction
2CH2O + SO4
2- = H2S + 2HCO3
-
SO4
2- = S + 2O2 (mainly by Desulfovibrio bacteria)
• If iron is available in the sediment, H2S may combine with
Fe to form pyrite.
• Consequently, pyrite is so commonly associated with in
black, organic-rich shales and coal
• In the absence Fe or other metallic ions, the H2S may or
may combine with organic molecules.
• This can result in an S-rich crude oil
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28. Methanogenesis (fermentation)
• Methanogens produce CH4 from the residue of the overlying
zones:
CH3COOH (cellulose) = CH4 + CO2 (acetate fermentation)
CO2 + 8H+ = CH4 + 2H2O (CO2 reduction)
The early microbial reactions:
• remove much of the N, S, O, and P
• Enrichment of C and H in the residue (it is a reduction
process)
• Residual organic products (lipids and lignin) following
anaerobic diagenesis continue to transform into kerogen
• Kerogen is formed by partial destruction and rebuilding of the
organic building blocks (polycondensation)
• Humic substances which gives fine grained sediments a dark
colour (humic and fulvic acid) is formed in this process
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29. • Humic substances are partially soluble in water.
• Further condensation (insolubilization) eliminates much of the
remaining nitrogen and converts the humic substances into
insoluble kerogen.
Catagenetic stage
• With increasing burial, T and P increases
• The OM (Kerogen) is thermally cracked; complex
kerogen structure is broken (i.e. maturation takes place)
• Petroleum is generated: mainly oil in the early stage,
then oil and gas (wet gas) at the late stage
• This leads to a reduction in H:C ratio, TOC and S2
• S1 increases
• All these occurs between T ῀ 65oC - ῀175oC and depths of
between ῀1km – ῀3.5km (depending of the geothermal
gradient of the basin)
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30. Metagenetic stage
• At greater T, P and depth of burial, C-C bonds are further
broken down (cracked)
• H:C ratio, TOC and S2 decreases further
• S1 increases
• The only HC produced at this stage is methane (dry gas)
• Source rocks lose their HC generation potential and
overmature
• The end result is a carbon residue - graphite
There are several ‘paleothermometers’ that can be used to
track the stages of conversion of OM through Petroleum to
graphite (carbon residue). Some of which have been
previously mentioned. Others include:
• Clay diagenesis
• TR
• Fluid inclusion
• Gas chromatography (evolution of n-alkanes), etc.
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31. HC generation at different
depths conditioned by
geothermal gradient (Pusey,
1973)
Stages in thermal maturation
(Tissot and Welte, 1978)
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32. Correlation of HC generation with some paleothermometers
(Selley, 1998) 32

33. GC paleothermometer
(Selley, 1998)
Changing TR with
maturation (Espitalie et
al., 1977) 33

34. Petroleum alteration
• The initial physico-chemical properties of petroleum
is often times adjusted (altered) by new set of
conditions encountered in petroleum reservoirs
(Blanc and Connan, 1993)
• The processes bringing about these changes can
be summed into the underlisted:
• Changes due to biodegradation/water washing
• Changes due to thermal alteration
• Changes due to de-asphalting
• Phase segregation
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35. 35
Effects on the API gravity by differences in source rock-types and by
alteration processes post-dating the accumulation
(Tissot and Welte, 1984)

36. Phases of Biodegradation from GC (adapted from Wenger et al., 2002)36

37. Sequence of Biodegradation (adapted from Wenger et al., 2002)
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38. Case study: Niger Delta
• Evamy et al.(1978) found out that in some fields,
heavy oil was found in shallow reservoirs while
lighter oil was found at deeper levels
• Dickey et al. (1987) studied Imo River, Odidi,
Afiesere, Batan, Oroni and Agbada fields and
observed the change from heavy to light crude
is often dramatic
• Furthermore, heavy oil was found to be
associated with meteoric, connate and mixed
water (no link with meteoric water and heavy oil)
• Undegraded oil was found with meteoric water.
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39. END OF SLIDE SHOW
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