Petroleum is formed from organic materials deposited in sedimentary basins millions of years ago. As these materials are buried deeper over time, the increasing heat and pressure causes them to thermally degrade into liquid and gaseous hydrocarbons. This process occurs within organic-rich source rock. The hydrocarbons then migrate upward through permeable layers until becoming trapped beneath an impermeable cap rock, accumulating in porous and permeable reservoir rock. The world's most productive sedimentary basins containing major petroleum deposits are located in Saudi Arabia, Kuwait, Alaska, Texas-Louisiana, Iraq, Iran, Mexico, Venezuela, Nigeria, and basins in Sri Lanka include Mannar, Tabbowa, and Andig

1. Petroleum and Its Formation
Mineral Resources and Technology Degree Programme
Group No : 02
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2. What is Petroleum?
• Petroleum or ‘Rock oil,' is a general term for crude oil and natural gas.
• Petroleum is a complex mixture of hydrocarbons (Hydrogen + Carbon)
and paraffins in some areas, and aromatics and cyclo-paraffins in
other areas.
• It contain some quantities of S,O,N containing compounds and some
metals.
 High Sulphur (Poor Quality) – Mostly in Middle East countries
 Less Sulphur (High Quality) – USA, Japan, Europe countries
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3. Uses of
Petroleum
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4. World’s Most Productive
Sedimentary Basins
 Saudi Arabia
 Kuwait
 Alaska – North slope
 Texas – Louisiana
 Iraq and Iran
 Mexico
 Venezuela
 Nigeria
Basins in Sri Lanka
 Mannar
 Tabbowa
 Andigama
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5. How is Petroleum Created?
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6. How is Petroleum Created Cont….
• In the sea as on land, all organic matter is originally formed by
photosynthesis.
• Photosynthesizers in the sea are pelagic phytoplankton and benthic
algae.
• The biological productivity of these plants or the abundance of
organic matter is related to both chemical and physical parameters.
• Physical Parameters :
The amount of light
Water depth
Latitude
Water temperature
Turbidity of water
• Chemical Parameters:
Abundance of nutrients such as Phosphates and Nitrates
• Conditions which favour the preservation of organic matter
Anaerobic conditions
Rapid sedimentation by fine grained material 6

7. 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.
There are three major phases in the evolution of the organic matter in
response to burial.
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Diagenesis :
• Occurs in shallow
subsurface at near normal
temperature & pressures.
This includes all the
chemical, physical &
biological changes during
the conversion of
sediments into
sedimentary rocks. It
doesn’t include process of
weathering or other
surface changes.
Catagenesis :
• Occurs in deeper
subsurface as burial
continues and temperature,
pressure increase.
Petroleum is released from
kerogen during this-first oil
and later gas.
Metagenesis :
• Occurs at high
temperatures and pressures
verging on metamorphism.
This phase is not important
for petroleum formation.

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9. Types of Kerogen
Type I - Algal - (oil prone) – Sapropelic
Type II - Mixed -(oil & gas)
Type III - Woody - (gas prone) – humic
Algae = Hydrogen rich = oil prone
Wood = Hydrogen poor = gas prone
Oil window – Area where it
generate oil;
100-150°C
2.5-4.5 km
Gas window- Area where it
generate gas;
150-230°C
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10. Petroleum System
Element
Source Rock
Migration Route
Reservoir Rock
Seal Rock (Trap)
Process
Generation
Migration
Accumulation
Preservation
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12. Source Rock
• The source rock host the processes that involve in the formation oil and
gas until they start to immigrate toward the upper or nearer rock(s)
named reservoir due to the fluidity of oil and gas.
• Particularly the source rock is a subsurface fine grained sedimentary
rock unit(s) which is made of shale or limestone.
• It contains the precursors of hydrocarbon formation, organic matters in
sufficient amount which were subjected to high temperature for
longtime.
• High concentrations of organic matter occur in areas of high productive
& stagnant water.
• TOC (Total Organic Carbon) is the fundamental parameter to evaluate
source rock. Generally 1wt% is considered as standard value for good
SR.
• Quality of source rock depends on abundance of organic matter & its
preservation potential.
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13. Maturity of the source rock
• The evaluation of maturity level of source rock has an importance for
commercial hydrocarbon exploration.
• Maturity usually increase with the depth due to an increase in
temperature and age of source rock.
• Age of the source rock should be atleast greater than 20Ma
12-15Ma – Indonesia
15-18Ma - Japan
• Thermal maturity is the primary factor that determines whether a source
rock can produce oil, gas, or condensate. Rock–Eval pyrolysis method is
used in determining the thermal maturation of kerogen.
• Vitrinite Reflector (Ro) also determines whether it’s mature or not.
>0.55(mature)
<0.55(immature) 13

14. Migration Route
• Migration takes place after maturation. The hydrocarbons from the
impermeable source rock move to the porous reservoir rock.
• Migration is the process of moving oil and gas from the source rock to the
reservoir pores when it is trapped after its generation.
• The main factors of the oil and gas migration are compression, buoyancy,
chemical potential; thermal expansion, topography, maturation (increase in
volume with time), and gravitational separation of hydrocarbons and water
from each other.
• In order to facilitate migration permeability should be high.
• There are two routes
1. Primary migration : The release of hydrocarbons & their transport
through the capillaries & narrow pores of a fine grained source rock.
2. Secondary migration : The movement of hydrocarbons beyond the
source rock through more permeable & wider porous carrier and
reservoir rocks.
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15. Timing of Petroleum Migration
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 A key factor in assessing the petroleum resources is the timing of
oil and gas migration relative to the trap formation.
 If migration of petroleum occurs before deposition of a suitable
reservoir or seal combination, then the petroleum will not be
trapped.
 The tightest constraints on the timing of oil and gas migration
result from combining the mode of occurrence of fluid inclusions
and the temperature measurements.
 The occurrence of oil and gas inclusions in crack-seal
textures within vein-filling cements indicates that hydrocarbons
were
migrating during an episode of continuous deformation that
produced the crack-seal texture.

16. Reservoir Rock
• The natural subsurface reservoir is a container of oil, gas, and water where
they can move, and its shape is determined by the relationship between the
reservoir rock and its surrounding poorly permeable rocks.
• This element is a kind of porous or permeable lithological unit(s) which retains
the immigrating oil and gas from source rock.
• petroleum accumulations occupy by the pore spaces of sandstone or porous
carbonates.
• Oil and gas usually accumulate on the top of water and they are always there
relatively to their difference of densities.
• The following features are used in describing a reservoir:
1) Type of the reservoir rock comprising the reservoir.
2) Relationship between the reservoir and the
surrounding impermeable rocks.
3) Reservoir capacity.
4) Depositional environment.
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17. Cap Rock/ Trap (Seal)
• It is a lithological unit(s) with low permeability which restricts
hydrocarbons to escape from the reservoir.
• It is made of chalks, shale or evaporites.
• Sandstones, are described as permeable and tend to have many
large, well-connected pores. Impermeable formations, such as
evaporites, shales and siltstones, tend to be finer grained or of a
mixed grain size, with smaller, fewer, or less interconnected pores.
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18. Cap Rock/ Trap (Seal) cont…
• Its analysis based on reaching the extent and thickness to know
how much cap rock is efficient to oil and gas retention.
• In the absence of seals, petroleum continue to rise until it
reaches the earth’s surface. (Ex: The La Brea tar pit)
• Therefore, surface chemical processes including bacterial
activities destroy the petroleum.
• Various approaches have been used for trap classification.
• Famous classification is Crude classification published by the
American Association of Petroleum Geologists.
I. Stratigraphic Traps
II. Structural Traps and/ or
III. Combine Traps
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19. Petroleum traps
• A trap is a geologic structure
or a stratigraphic feature
capable of retaining
hydrocarbons.
• Three types
 Hydrocarbon traps that result
from changes in rock type or
pinch-outs, unconformities, or
other sedimentary features such
as reefs or buildups are
called stratigraphic traps.
 Hydrocarbon traps that form in
geologic structures such as
folds and faults are
called structural traps.
 Any mixture of structural and
stratigraphic elements is called
a combination traps.
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Group Members (Group No: 02)
1) Dilshan W.M.L.I. UWU/MRT/15/007
2) Kalhari M.P.R. UWU/MRT/15/011
3) Karunathilake S.S.S. UWU/MRT/15/012
4) Kumarasinghe D.C.J. UWU/MRT/15/014
5) Kuruppu K.A.D.D.N. UWU/MRT/15/015

22. Permeability : The measure of degree to which fluid can be transmitted or the
measure of the soil’s ability to permit water to flow through its pores or voids.
Unit – Darcy(D)
Typically permeability of reservoir is 10D or less.
Gas may produced at 0.1mD and oil reservoirs 10 × or 100 × more permeable.
Porosity : Porosity or the void fraction is the void space in the rock reported
either as a fraction of 1 or as a %. A porous rock has the capability to hold
fluid. Ex:-Reservoir rock
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