Updated Lecture-part 1_0974cde6f860831970d41e9995e4a7d0.pdf

Petroleum Geology
2023-2024
Assistant Prof. Dr. Namam M. Salih
Post-Graduate Course
Soran University
Faculty of Engineering
Department of Petroleum & Mining Engineering

Introduction
´ Petroleum geology is the application of geology (the study of rocks) to the
exploration for and production of oil and gas.
´ Geology itself is firmly based on chemistry, physics, and biology.
´ Petroleum is the name given to fluid hydrocarbons, both the gases and the liquid
crude oil.
´ It is commonly noted that petroleum occurs almost exclusively within sedimentary
rocks (sandstones, limestones, and claystones).
´ Petroleum is seldom found in igneous or metamorphic rocks.

Oil and Natural Gas Deposits
´ Oil and natural gas lie in earth at different depths, between rock particles. These particles are tiny
and narrow, but all together form a huge volume nearly 30% or more of the overall volume of
rock structure.
´ All Sedimentary Rocks, like Sandstone, Dolomite and Limestone have narrow pores among them
which help to contain liquids and gasses unlike other igneous rocks and metamorphic rocks.
´ Rocks where crude oil or natural gas gather are called oil reservoirs, and these are actually a mass
of rocks which gather in its permeable pores and contain inside them both crude oil and natural
gas.
´ Huge and limitless amounts of these two constituents (crude oil and natural gas) gather inside
the rock pores, which have the ability to easily release the liquids inside them making the flow of
crude oil and natural gas free and natural after piercing these layers of rocks, i.e. digging in order
to reach these rocks.

Introduction to petroleum Geology
Petroleum geology comprises those geological disciplines which are of greatest significance for
the finding and recovery of oil and gas.
Since most of the obvious and “easy to find” petroleum already has been discovered it is necessary to
use sophisticated methods in the exploration of economic reservoir sedimentary basins.
These include:
1. advanced geochemical techniques;
2. Advanced geophysical techniques;
3. and basin modelling.
There is also much more emphasis now on enhanced
recovery from the producing fields. Petroleum technology
has made great progress and many new tools and modelling
programs have been developed, both in exploration and
production.

Petroleum geology
´ It is however important to understand the geological processes which determine the distribution of different
sedimentary rocks and their physical properties.
This
´ knowledge is fundamental to being able to successfully
apply the methods now available.
NOTE:
It is difficult to know where to start when teaching
petroleum geology because nearly all the different
disciplines build on each other.

´ Since practically all petroleum occurs in sedimentary rocks, sedimentary geology forms one of the main
foundations of petroleum geology.
Sedimentological models are used to predict the location of different economic facies in the sedimentary basins.
´ Therefore likely presence of source rocks with a high content of organic matter, reservoir rocks and cap rocks
´ The distribution and geometry of potential sandstones or carbonate reservoirs requires detailed
sedimentological models, and sequence stratigraphy has been a useful tool in such reconstructions.
NOTE:
For one whom attached to Petroleum industry
should have a detailed information about
sedimentary rocks

methods in the exploration of economic reservoir
sedimentary basins:
´ Biostratigraphy is the correlation of stratigraphic
units based on fossil content, either through the
use of index (guide) fossils or similarities in fossil
assemblages. An index fossil is a useful guide for
correlation if it possesses the following
characteristics: It has widespread geographical
distribution.
1. The biostratigraphic correlation of strata
encountered in exploration wells is achieved by
micropalaeontology
(including palynology), a field developed very
largely by the oil industry.
´ Reservoir rocks are mostly sandstones and
carbonates which are sufficiently porous to hold
significant amounts of petroleum.
´ The composition and properties of other rock
types such as shales and salt are also important.
NOTE: Due to the small size of the samples obtained during drilling operations one cannot rely on macrofossils;
even in core samples the chance of finding good macrofossils is poor. By contrast a few grams of rock from the drill
cuttings may contain several hundred microfossils or palynomorphs. These also usually provide better stratigraphic
resolution than macrofossils.

The sedimentary environments
(sedimentary facies) determine the
distribution of reservoir rocks and their
primary composition.
´ Sediments do, however, alter their
properties with increasing overburden
due to diagenesis during burial.
Diagenetic processes determine the porosity, permeability and other physical properties such as velocity,
in both sandstone and limestone reservoirs.
Chemical processes controlling mineral reactions are important.

2. Organic geochemistry, which includes the study
of organic matter in sediments and its transformation
into hydrocarbons, has become another vital part of
petroleum geology.
3. Tectonics and structural geology provide an understanding of the subsidence, folding and uplift
responsible for the creation and dynamic history of a basin.
´ The timing of the folding and faulting that forms structural traps is very important in relation to the
migration of hydrocarbons.
4. Geophysical measurements may include gravimetry and magnetometry; electromagnetic methods that
were used mostly in ore exploration have also been applied to oil exploration.
5. Seismic methods have become the main tool for mapping sedimentary facies, stratigraphy, sequence
stratigraphy and tectonic development.
• Marine seismics recorded from ships have become very efficient and seismic lines are shot at only a
few 100 m spacing or less.

Accumulations of Organic Matter
´ Most of the organic materials which occur in source rocks for petroleum are algae, formed by photosynthesis.
´ The zooplankton and higher organisms that are also represented grazed the algae and were thus indirectly
dependent on photosynthesis too.
´ Since petroleum is derived from organic matter, it is important to understand how and where sediments with a
high content of organic matter are deposited.
• Biological production is greatest in the
uppermost 20–30 m of the ocean and most of
the phytoplankton growth takes place in this
zone.
• In clear water, sunlight penetrates much deeper
than in turbid water, but in clear water there is
usually little nutrient supply.
• At about 100–150 m depth, sunlight is too weak
for photosynthesis even in very clear water.

´ Phytoplankton provides nutrition for all other marine life in the oceans.
´ Basins with restricted water circulation will preserve more organic matter and produce good source rocks
which may mature to generate oil and gas

Upwelling of water rich in nutrients on a continental margin with deposition of organic-rich mud which may
mature to generate oil and gas

Breakdown of Organic Matter
Almost all (>99%) of the organic matter which is produced on land and in
the oceans is broken down through direct oxidation or by means of
microbiological processes.
´If oxygen is present, organic matter willbe broken down in the following
manner:
CH2O + O2 → CO2 + H2O
´If water circulation is restricted due to density stratification of the
water column, the oxygen supply will be exhausted. Instead, the
bound oxygen in sulphates or nitrates is used by sulphate-reducing
and denitrifying bacteria which decompose organic material in an
anoxic environment.

Formation of Source Rocks/ The Origin and Habitat of Petroleum
´ On the bottom, organic matter will be subjected to breakdown by micro-organisms (bacteria). It will also be
eaten by burrowing organisms which live in the top portion of the sediments.
´ Many hypotheses concerning the origin of petroleum have been advanced over the last years. Currently, the
most favoured one is that oil and gas are formed from marine phytoplankton (microscopic floating plants)
and to a lesser degree from algae and foraminifera.

Formation of Source Rocks/ The Origin and Habitat of Petroleum
´ In the ocean, phytoplankton and bacteria are the principal of organic matter buried in sediment. Most of
organic matter is trapped in clay mud that is slowly converted into shale under burial. During this conversion,
the organic compounds are transformed (mainly by the geothermal heat) into petroleum, defined as gaseous,
liquid or semisolid natural substances that consist mainly of hydrocarbons.

The Origin and Habitat of Petroleum
• and to a lesser degree from algae and foraminifera.

Marine
Sedimentary
Basins
The mechanism and formation of petroleum

´ Local large concentrations of organic matter in sedimentary rocks, in the
form of coal, oil or natural gas are called the fossil fuels.
´ A rock rich in primary organic matter is called a source rock, because it is
capable of releasing large amounts of hydrocarbons in natural burial
conditions.
The mechanism and formation of petroleum (Ocean)

´ Usually this is a shale or mud rock which itself is a very common rock type,
consisting about 80% of the world’s sedimentary rock volume. Organic carbon-
rich shale and mud rock are characteristically black or dark greyish in color,
which indicates a non-oxidised primary organic matter.
The mechanism and formation of petroleum (Ocean)

Chemical Composition Of Organic Matter
• The composition and type of organic matter to be deposited and incorporated in sediments depend
on the natural association of phytoplankton, zooplankton, higher plants and bacteria in the
depositional environment.
• As far as their soft parts are concerned, basically all organisms are composed of the same chemical
constituents:
Ø i.e., proteins, lipids, carbohydrates and, in higher plants, lignins.
Ø There is a fundamental difference between the chemical composition of marine planktonic algae
and terrestrial higher plants.
Ø The organic matter of marine plankton is mainly (50% and more) composed of proteins, a variable
amount of lipids (5 to 25%) and generally not more than 40% carbohydrates.
Ø Higher terrestrial plants are largely composed of cellulose (30 to 50%) and lignin (15 to 25%).
Organic material mainly derived from marine plankton is more of an aliphatic or alicyclic nature
and reaches hydrogen to carbon ratios of around 1.7 to 1.9.
Ø Predominantly land-derived organic matter with high contents of lignin and carbohydrates is more
aromatic and has hydrogen to carbon ratios of around 1.0to 1.5.

Kerogen is a collective name for organic material that is insoluble in organic solvents, water
or oxidizing acids. The portion of the organic material soluble in organic solvents is called bitumen,
which is essentially oil in a solid state.
´ As organic material becomes buried by the accumulation of overlying sediments, water is
gradually expelled during compaction.
´ Complex organic compounds like proteins are broken down into amino acids, and
carbohydrates into simpler sugar compounds.
´ These are able to recombine to make larger compounds, for example by amino acids
reacting with carbohydrates (melanoid reaction).
´ As this type of polymerisation proceeds, the proportion of simpler soluble organic
compounds diminishes at depths of a few tens of metres down in the sediment.
´ It is these newly-formed complex organic structures which are called kerogen.

What is kerogen
´ Kerogen : is formed during sedimentary diagenesis
from the degradation of living matter. The
original organic matter can comprise lacustrine and
marine algae and plankton and terrestrial higher-order
plants. or is a solid waxy mixture of organic matter in
sediments from which petroleum is released.
´ Many organic carbon-rich marine and lake shales never
reach the burial temperature level at which the original
organic molecules are converted into hydrocarbons
forming oil and natural gas.
´ Instead, the alteration process is limited to certain
wax-like substances with large molecules. This
material, which remains solid, is called kerogen, and is
the organic substance of so-called oil shales.
´ Kerogen can be converted into oil and gas by
further burial through mining the shale and subjecting it
to heat it in a retort.

Updated Lecture-part 1_0974cde6f860831970d41e9995e4a7d0.pdf

´ Petroleum is generated when the kerogen is subjected to a sufficient high
temperature in the process of the sediment burial.
´ The alteration of kerogen to petroleum is similar to other thermal-cracking
reactions, which usually require temperatures greater than 60oC.
´ At lower temperatures, during the early diagenesis, a natural biogenic methane
called marsh gas, is generated through the action of microorganisms that live
near the ground surface.
Source Rock

´ A temperature range between about 60oC and 175oC is most
favourable for the generation of hydrocarbons, and is commonly called
the oil window.
´ At temperatures much above 175oC, the generation of liquid
petroleum ceases and the formation of gas becomes dominant.
´ When the formation rock temperature exceeds 225oC, most of the
kerogen will have lost its petroleum-generating capacity
Generation of Hydrocarbons

Generation of Hydrocarbons
Overburden weight
Or OV. Depth
Case from NE-Iraq-
Kurdistan; Canada;
USA

´ The long and complex chain of chemical reactions involved in the conversion of
raw organic matter into crude petroleum is called maturation.
´ Additional chemical changes may occur in the oil and gas even after these have
been generated or accumulated.
´ This explains, for example, why the petroleum taken from different oil fields has
different properties, despite a common source rock. Likewise, primary
differences in the source composition may be reflected in the chemistry of the
petroleum.
Source Rock and Maturation of Petroleum

The generation and preservation of organic matter at the earth’s surface, it is
appropriate to consider what happens to this organic matter when buried in a
steadily subsiding sedimentary basin. As time passes, burial depth increases,
exposing the sediment to increased temperature and pressure,
defined three major phases in the evolution of organic matter in response to burial:
1- Diagenesis:
2- Catagenesis:
3- Metagenesis
Generation of Hydrocarbons from source rocks

1. Diagenesis: This phase occurs in the shallow subsurface at near normal
temperatures and pressures. It includes both biogenic decay, aided by bacteria,
and abiogenic reactions. Methane, carbon dioxide, and water are given off by the
organic matter, leaving a complex hydrocarbon termed kerogen.
2. Catagenesis: This phase occurs in the deeper subsurface as burial continues and
temperature and pressure increase. Petroleum is released from kerogen during
catagenesisd first oil and later gas.
3. Metagenesis: This third phase occurs at high temperatures and pressures
verging on metamorphism. The last hydrocarbons, generally only methane
expelled. Porosity and permeability are now negligible.

1. Diagenesis: Sediments deposited in a sedimentary basin in subaquatic environments contain
large amounts of water, minerals, dead organic matter and numerous Sediments deposited in
a sedimentary basin in subaquatic environments, living micro-organisms.
• During diagenesis this system tends to approach equilibrium under conditions of shallow
burial, and the sediments normally become consolidated. The depth interval concerned is
in the order of a few hundred meters, occasionally to a few thousand meters.
• During early diagenesis, one of the main agents of transformation of the organic matter is
microbial activity.
• Chemical rearrangements, such as polycondensation and insolubilization, then occur at
shallow depths.
• Diagenesis of organic matter leads from biopolymers (proteins, lipids, carbohydrates and
lignins as synthesized by plants and animals) to geopolymers collectively called kerogen

Kerogen composition
Chemistry of Kerogen
Kerogen is the term applied to disseminated organic matter in sediments that is insoluble in normal
petroleum solvents, such as carbon bisulfide. This insolubility distinguishes it from bitumen.
Chemically, kerogen consists of carbon, hydrogen, and oxygen, with minor amounts of nitrogen and
sulfur.

Kerogen classification
q Type I kerogen.
´ This type is either mainly derived from algal lipids or from organic matter enriched in
´ lipids by microbial activity. The hydrogen to carbon ratio is originally high, and the
´ potential for oil and gas generation is also high.
q Type II kerogen.
´ This type is usually related to marine organic matter deposited in a reducing environment with
´ medium to high sulfur content. The hydrogen to carbon ratio and the oil and gas potential are lower
than observed for type I kerogen but still very important.
q Type III kerogen.
´ The organic matter is mostly derived from terrestrial higher plants. The hydrogen to carbon ratio is
low, and oil potential is only moderate. This kerogen may still generate abundant gas at greater
depths. The oxygen to carbon ratio is comparatively higher than in the other two types of kerogen.

Residual kerogen
Residual kerogen
´ is one form of ‘dead carbon’ and has no potential for oil and gas. Besides kerogen, at
the end of diagenesis organic matter comprises a minor amount of free hydrocarbons
and related compounds, as synthesized by living organisms and preserved with minor
alteration. They can be considered as geochemical fossil

Because these three kerogen types generate different hydrocarbons their distinction and
recognition are important.
Type I kerogen is essentially algal in origin .It has a higher proportion of hydrogen
relative to oxygen than the other types of kerogen have (H:O ratio is about 1.2e1.7). The
H:C ratio is about 1.65 ( See Table ). Lipids are the dominant compounds in this kerogen,
with derivates of oils, fats, and waxes.

Type II, or liptinitic, kerogen is of intermediate composition (Plate 5.6). Like algal kerogen, it
is rich in aliphatic compounds, and it has an H:C ratio of >1. The original organic matter of
type II kerogen consisted of algal detritus, and also contained material derived from
zooplankton and phytoplankton.

Type III, or humic, kerogen has a much lower H:C ratio (<0.84). Chemically, it is low in
aliphatic compounds, but rich in aromatic ones. Humic kerogen is produced from the lignin of the
higher woody plants, which grow on land. It is this humic material that, if buried as peat, undergoes
diagenesis to coal. Type III kerogen tends to generate largely gas and little, if any, oil.
Nonmarinebasins were once thought to be gas prone because of an abundance of humic
kerogen,whereas marine basins were thought to be oil provinces because of a higher proportion of
algal kerogen. This type of generalization is not valid. Many continental basins contan ilacustrine
shales rich in algal kerogen.

This review of the three basic types of kerogen shows the importance of identifying the nature
of the organic matter in a source rock so as to assess accurately its potential for generating
hydrocarbons. A second important factor to consider is not only the quality of kerogen but also
the quantity necessary to generate significant amounts of oil and gas suitable for commercial
production. Several separate items are to be considered here, including the
average amount of kerogen in the source bed, the bulk volume of the source bed, and the
ratio of emigrated to residual hydrocarbons. The total organic matter in sediments varies
from 0% in many Precambrian and continental shale to nearly 100% in certain coals. A figure
of 1500 ppm TOC is sometimes taken as the minimum requirement for further exploration of
a source rock

Kerogen environment
ASSi. 2 Explain the figure below according to the environment of petroleum
maturation, What cause these kerogen types distributed in a specific location?

Maturation of Kerogen
• During the phase of catagenesis, kerogen matures and
gives off 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 the kerogen is overmature, neither oil nor gas
remains.

43
• Significant oil generation occurs between 60
and 120 C, and significant gas generation
between 120 and 225 C.
• Above 225 C, the kerogen is inert, having
expelled all hydrocarbons; only carbon remains
as graphite.
Maturation of Kerogen

Catagenesis
• During the continued burial of sediments, the increase in temperature results in the
thermal degradation of kerogen, which eliminates hydrocarbon chains and cycles.
• Most of the newly formed hydrocarbons are of medium to low molecular weight. These
hydrocarbons are the source of the bulk of crude oils. Catagenesis is the principal stage
of oil formation.
• The corresponding depth range is also referred to as oil window. In addition, catagenesis
also corresponds to the beginning of the cracking stage (i.e. cracking of oil to gas;
cracking = breaking of carbon-carbon bonds), which produces wet gas with a rapidly
increasing proportion of dry gas.
• In terms of hydrocarbon exploration, source rocks are considered as being mature during
catagenesis.

Metagenesis
´ This last stage of evolution of organic matter is reached only at great depths. During
metagenesis no significant amounts of hydrocarbons are generated from kerogen,
except for some methane.
´ However, large amounts of methane may result from the cracking of previously
generated liquid hydrocarbons.
´ The residual kerogen usually consists of two or more carbon atoms per three atoms
(hydrogen to carbon ratio less than 0.5). In terms of hydrocarbon exploration, the
stage of metagenesis corresponds t o the dry gas zone.

The evaporation causes the surface waters to have
higher salinity and higher density. This denser water
eventually sinks to mix and oxygenate the water
column. An anoxic silled basin, like the Black Seas,
has a positive water balance (Fig. 2.), where fluvial
water input exceeds evaporation. These fresher, less
dense fluvial waters remain in the surface layers and
contributing to the formation of a stratified water
column. Lack of mixing eventually leads to the
development of anoxic bottom waters conducive to
organic matter preservation.
FIGURE Potential depositional settings for source
rock formation in the marine environment. The
numbers II, III, and IV refer to the chemical kerogen
type expected. (A) Ventilated open ocean. (B) Silled
basin.
After Demaison, G.J., Hoick, A.J.J., Jones, R.W., Moore, G.T., 1983.
Predictive source bed stratigraphy; a guide to regional petroleum
occurrence. Proceedings of the 11th World Petroleum Congress, vol.
2. John Wiley & Sons, Ltd., London, p. 17.
47

After Demaison, G.J., Moore, G.T., 1980. Anoxic environments and oil
source bed genesis. American Association of Petroleum Geologists
Bulletin 64, 1179–1209.
• As shown in Fig. , a negative water balance leads to an oxic
water column, usually in arid regions, when evaporation
exceeds fluvial water input, such as the Mediterranean Sea.
• The evaporation causes the surface waters to have higher
salinity and higher density. This denser water eventually
sinks to mix and oxygenate the water column.
• An anoxic silled basin, like the Black Seas, has a positive
water balance where fluvial water input exceeds evaporation.
• These fresher, less dense fluvial waters remain in the surface
layers and contributing to the formation of a stratified water
column.
• Lack of mixing eventually leads to the development of anoxic
bottom waters conducive to organic matter preservation.
• These same conditions can lead to anoxic bottoms waters in
epicontinental seas and lagoonal settings.
• Silled basins can also form on continental shelves and slopes,
as observed in north central Gulf of Mexico.
• Salt movement there has resulted in the formation of
numerous intra slope “mini-basin.” If these basins are large
and deep enough with bathymetric conditions around the
margins restricting or preventing water circulation into the
basin, anoxic conditions can develop (Williams and Lerche,
1987 ).
48

Formation of Source Rocks
INCORPORATING ORGANIC MATTER INTO SEDIMENTS
A schematic of transport mechanisms introducing organic matter to a marine deposition
environment
(Gagosian, 1983).
49

50
After Tissot and Welte 1984: Petroleum Formation and Occurrence

Migration of Petroleum
´ Petroleum migrates from low permeability
source rocks into high permeability reservoir
rocks from which the petroleum can be
produced (Fig. 1.2b).
´ The main driving force for petroleum
migration is buoyancy because it is less dense
than water. The forces acting against migration
are the capillary forces and the resistance to
flow though rocks with low permeabilities
´ Migration of oil and gas will therefore nearly
always have an upwards component.
´ We distinguish between primary migration,
which is the flow of petroleum out of the
source rock and secondary migration, which is
the continued flow from thesource rock to the
reservoir rock or up to the surface

´ Oil and gas may also migrate (leak) from the reservoir to a higher
trap or to the surface.
´ Hydrocarbons are relatively insoluble in water and will therefore
migrate as a separate phase.
´ Solubility varies from as little as 24 ppm for methane to 1,800
ppm for benzene. Othercompounds, such as pentane, are even less
soluble (2–3 ppm). However, solubility increases markedly with
pressure. Many hydrocarbons have solubilities of less than 1 ppm
in water
´ Gas, in particularly methane, has a fairly high solubility in water,
especially under high pressure. If methane-saturated water rises to
lower pressures, large quantities of methane can bubble out of a
solution.
´ It is therefore necessary to assume that oil is mostly transported as
a separate phase.
´ Oil is lighter than water, and oil droplets would be able to move
through the pores in the rocks but the caplliary restance is high for
separate oil drops in a water-wet rock .

´ In order to pass through the narrow passage between pores (pore throat), the oil droplets must overcome the
capillary forces.
´ When the pores are sufficiently small in a fine-grained sediment, these forces will act as a barrier to further
migration of oil. The small
gas molecules, however, can diffuse through extremely
small pores and thus escape from shales which form
tight seals for oil.
Oil can therefore not migrate as small discrete
droplets, but moves as a continuous string of oil where
most of the pores are filled with oil rather than water
(highly oil-saturated). The pressure in the oil phase
at the top is then a function of the height of the oilsaturated
column (string) and the density difference
between oil and water.
The rate of migration is a function of the rate

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