- Petroleum is formed from the remains of ancient organisms that are transformed into kerogen and eventually hydrocarbons through geological processes over millions of years. - Key factors that influence petroleum formation include the type and amount of organic matter deposited, the environmental conditions that allow for preservation of organic matter, and the rate and depth of burial which determines the temperature and pressure the organic matter experiences. - As organic matter is buried deeper in sediments, the increasing heat and pressure causes kerogen to crack and thermally degrade first into liquid and gaseous hydrocarbons through catagenesis. Further heating in metagenesis converts more kerogen into methane and a carbon residue.

1. Transformation of organic
matter to Kerogen

2. What is Petroleum
• Note: petroleum=latin petra =rock & oleum=oil i.e.
rock oil or mineral oil.
• Natural hydrocarbons= the chemical compounds of
hydrogen and carbon found in rocks in gaseous, liquid
or solid states are known as petroleum.
• On the other hand, in common usage natural liquid
oil occuring below the surface of earth is known as
petroleum.
• Gas may be associated with petroleum, or found
separately under the surface of the earth is know as
natural gas.
• The semi-solid or solid part is known as asphalt or tar.
• Mitti ka tel, mineral oil with drilling.
• The oil that comes out from inside the earth or that is
recovered by digging well, is known as unrefined oil
or Crude oil.

3. Uses of petroleum
• Fuel energy-modern transport systems
• Industrial power
• Synthetic materials
• Fertilizers
• Room heating, kerosene lamp
• Cooking gas (LPG-liquified petroleum Gas) or
kerosene
• Asphalt as heavier residual substance of crude oil
for roads surface, roofing and water proofing
purpose;, wax.

4. Early history
• Seepages and burning with atmospheric oxygen
and worshipped by man.
• Crude oil hardens on reaching surface to form
bitumen and used in Southern Persia (Iran) as far
as back 6000 B.C., as mortar in buildings and as a
kind of glue for other uses.
• Dark and evil smelling has been commented by
Herodatus during 5th century in Persian empire.
• Eternal fires of the Persian worshippers and used
as welfare, arrow tipped with burning pitch.
• Petroleum was known in India, Burma and the
East Indies and using binding, waterproofing and
source of oil lamp.

5. • PETROLEUM GEOLOGY REFERS TO THE SPECIFIC SET OF GEOLOGICAL DISCIPLINES THAT ARE
APPLIED TO THE SEARCH FOR HYDROCARBONS DURING OIL EXPLORATION.
• PETROLEUM GEOLOGY IS PRINCIPALLY CONCERNED WITH THE EVALUATION OF SEVEN KEY
ELEMENTS IN A SEDIMENTARY BASIN TO OBTAIN AN IDEA OF THE SUBSURFACE AND OVERALL
PETROLEUM SYSTEM.
• Petroleum principally comprises of Hydrocarbons and exists in the gaseous or liquid state
in natural underground reservoirs.
• The principal forms of petroleum are:
NATURAL GAS, which does not condense at standard temperature and pressure
(760mm Hg, 60oF or 15.6oC); CONDENSATE, which is gaseous in the ground but
condenses at the surface; and CRUDE OIL, the liquid part of petroleum.
• Hydrocarbons: Because of their low susceptibility to microbial degradation relative to other types
of organic matter, saturated hydrocarbons are generally robust recorders of the origins of organic
matter in sediments. The hydrocarbon compositions of many aquatic algae and photosynthetic
bacteria are dominated by the C17 n-alkane (Cranwell et al., 1987).
 Source
 Reservoir
 Seal
 Trap
 Timing
 Maturation
 Migration

6. Organic matter in average rocks
Mineral matter ~99%
Organic matter ~1%
Kerogen ~90% Bitumen ~10%
The concentration of kerogen in sedimentary rocks depends on the
amount of OM deposited, its preservation and the rate of sedimentation.

8. Composition of the Biomass
• Organic matter deposited and incorporated in sediments
is originally sourced by the living organisms, mainly from
the plant kingdom of photosynthetic plants,bacteria and
archea.
• This organic matter in due course of geologic time and
after undergoing certain chemical reactions generates
petroleum.
• All organisms are basically composed of the same
chemical constituents: viz. proteins, fats, fatty acids,
carbohydrates, lipids and lignin in higher plants.
• Lipids encompass fat substances, oils, waxes are compounds of
fatty acids and lipid like components such as oil-soluble (in
ether, chloroform and benzene) pigments, terpenoids, steroids
and many complex fats. Fats also are used for body insulation
and for controlling buoyancy in marine animals.
• With respect to formation of petroleum, lipids are most
important. The lipids, essential oils and plant pigments are
similar to petroleum in composition.
• In fact , many chemical structures in these groups can be
recognized in crude oil.
• Proteins (amino acids) are giant molecules that make up the
solid constituents of animal tissues and plant cells. They
contain about 50-55% of carbon, 7% H and 19-24% O,15-19%
N, 0.3-2.4% S and a small amount of Phosphorus.
• Carbohydrates (sugar, starches, cellulose), the fundamental
constituent of plant tissues are built up of molecules like that
of glucose. Carbohydrates lack phosphorus, nitrogen and
sulfur. They have the general formula C(H2O)n, with n equal to
or greater than 4.
• Most zooplankton are rich in both protein and fats (lipids), but
poor in lignin and in carbohydrates. Fats and proteins are
richer in carbon than carbohydrates, but much lower in O % .
So it concluded that fats and proteins are the important
biomolecular precursors of petroleum.

10.  1. Inorganic origin: originated due to chemical reactions of inorganic
substances e.g. during volcanic eruptions different chemical reactions
may give rise to petroleum.
 Organic Origin: Petroleum is derived from organic matter once
trapped in sediments
 Petroleum is derived from the remains of living things which contains a
material called kerogen.
 Organic matter becomes petroleum with time, the kerogen matures into an
assortment of hydrocarbon molecules of all sizes and weights.
 The lightest (small) hydrocarbon molecules as natural gas, and the heavier
ones make up an oily liquid.
 Petroleum source rocks are of terrestrial and marine origin.
 Terrestrial source rocks are deposited in lakes, delta and river basins having
woody plant matter, algae etc.
 Marine source rocks contain dead planktons, algae, organic remains etc.
phytoplankton are the original source of nearly all organic matter in
sediments, very little of it comes from the land.
 In both the settings, the mixture is buried under conditions of no oxygen.
The Kerogen are classified as type I,II, III & IV as per their origin and are
capable of producing oil or gas or both.
ORIGIN OF PETROLEUM FORMS

11.  Under the anaerobic conditions, the kerogen is transformed into a
flammable substance called bitumen by the action of heat and
anaerobic microbes in the sediment and natural catalysts.
 Most of the bitumen is eventually cooked into tarry asphalt
releasing hydrocarbon molecules (as well as water and carbon
dioxide) out of the source rock as it heats.
 Heavy oils form first, then light oils. As temperatures rise to and
above 100° C, source rocks produce more gas.
 Being lighter than rocks, petroleum tends to rise upward through
fractures and the pores of coarse sandstone beds.
 A small fraction of that leakage, perhaps 2% is preserved in large
pools having an impermeable cap / seal over it.

12. Preservation of OM
• Environment of the transformation:
• Preservation depends on the interplay of three factors:
I. Rate of supply of organic matter
II. Environment of deposition (oxic/ anoxic)
III. Rate of burial
 Once the OM is deposited, the process of its chemical alteration and degradation begins.
 The elemental attributes of organic matter favourable to its hydrogen source potential are
of course its initial carbon content, its capacity to retain or acquire hydrogen, and as low an
oxygen content as possible.
 All fatty acids are monobasic i.e. presence of both animal and plant. Formic acid occurs in
ants, stinging insects, and stinging plants and acetic acid which gives vineger its taste. The
fatty acids are the most stable type of organic acids, and source of long chain molecules,
which are almost certainly the fundamental molecules of petroleum.
 The transformation of OM to Kerogen proceeds from shallow depths of burial to depths of
perhaps 1000m, with temperatures up to about 500C. On further burial and heating, the
large molecules crack to form smaller lower molecular weight hydrocarbons
(geomonomers), in the depth and temperature ranges of 1000 to 6000 m and 50-1760C
requiring millions of years.
 Initial products are mostly H2O and CO2, at higher temperatures becoming divided
between volatile products (hydrogen and methane) and liquid products (from C13 to C30).
Oxygen is lost most rapidly, by dehydration and decarboxylation (loss of CO2 from fatty
acids), carbon and nitrogen are lost least rapidly. Consequently the carbon content of the
kerogen residue increases and H:C ratio decreases with increasing temperature.
 During this post depositional alteration, the organic constituents of the sediments are
progressively transformed by thermal processes into two fractions:
 (1) a fluid product high in hydrogen, eventually petroleum and natural gas.
 (2) a residue high in carbon, such as bituminous coal.

13. • The conversion of lipids (fats), proteins, &
carbohydrates of living material into the organic
matter and Kerogen of sedimentary rocks. When this
kerogen is buried deeper at higher temperatures, it
cracks to form a bitumen that breaks down further to
form petroleum.
• Some HC also form directly from kerogen. Due to
increasing temperature, it changes into smaller H rich
molecules (methane line) and larger H-deficient
molecules (graphite line). The end products are
methane and graphite.
• Organic diagenesis is the biological, physical and
chemical alterations of the organic debris before a
pronounced effect of temperature. It covers the
temp. range up to appr. 500C and is represented by
unshaded area.
• The stage in which increasing temp. cause kerogen to
thermally decompose to bitumen and bitumen to oil,
condensate and gas is called catagenesis. The
catagenesis range is from 500C to 2000C.
• The zone of higher temp. from 2000C to 2500C in
which small amount of methane continue to be
formed and the remaining organic matter is
converted to graphite residues is called metagenesis.
The end of metagenesis is appr. the beginning of
metamorphism, in which kaolinite is converted to
muscovite and the greenschist facies begins to
appear. Biomarkers are severely diminished in
concentration or completely destroyed because of
their instability under these conditions.
Origin and maturation of petroleum

14. Transformation of organic matter:
There are three major transformation processes:
1. Diagenesis: Low temperature process which leads to the
formation of Kerogen. Biogenic methane may form.
2. Catagenesis: Thermal degradation of kerogen as a result of an
increase in temperature during burial in sedimentary basins.
This process is responsible for the generation (commences
from ~ 50oC upto 200oC) of most of the hydrocarbons, i.e. oil
and gas. Three stages of catagenesis are recognized:
• Initial Oil Generation
• Peak oil generation
• Wet gas stage
3. Metagenesis : This process takes place at great depth.
• At this stage, the kerogen is converted into a carbon residue and
the produced petroleum into methane and pyrobitumen. Capacity
of kerogen is to produce only a limited amount of methane.
• It usually takes place at temperature above 200oC.

15. Main field
of kerogen
occurrence

17. Type of Kerogen
• The TOC (total organic carbon) in sedimentary rock is separately into two fractions:
(a) BITUMEN: The fraction of the OM in rocks that is soluble in organic solvents (such as CS2
and chloroform) is bituminous, and loosely called bitumen. Bitumen consists of hydrocarbons,
asphaltenes and resins.
• (b) KEROGEN: the insoluble (insoluble in acids, bases and organic solvents), non extractable
residue from the initial transformation of OM is called Kerogen (i.e., wax). In ancient sedimentary
rocks, particularly in shales, kerogen normally constitutes 80-95% of the total OM.
Visual Microscopy facilitates in OM classification based on
• Morphology
• Fluorescence characteristics
• Macerals (Relative Proportion)
• Structured vs. Amorphous - Sapropelic
Based on the microscopic examination and
chemical analyses, kerogen has been classified
into three types:
Type I: Alginitic
Type II: Liptinitic – Exinitic and
Type III: Vitrinitic – Inertinitic and
Type IV Inertinitic
• Type I: This type of kerogen is relatively rare and occurs most often in lacustrine environments or
in a closed basins where planktonic algae are the main contributors of OM. H/C ratio is 1.4 to 1.8
and it is oil prone (68% conversion by weight).
• Type II: It is found commonly marginal marine (marine transgression) with admixture of
continental and aquatic (especially planktonic) OM. Algal tissue, pollen, and spores are important
contributors. This type includes the principal source sediments for oil, and some ancient oil shales
of marine origin. H:C ratio about 1:4.
• Type III: It occurs primarily humic OM, of terrestrial, woody origin, equivalent to the vitrinite of
coals. The material was deposited at the oxic sediment/water interface; it is gas-prone. H:C ratio
1.0 or less i.e. there are more carbon atoms than hydrogen atoms.
• Type IV: OM may have come from any source, but it has oxidized, recycled, or altered during
some earlier thermal events. The inert carbonaceous material is now deficient in H (H:C ratio
about 0.4 or less), has no evolutionary path left for it, and yields negligible or no HC.

18. Petroleum System

19. Generation Migration Entrapment
Critical
Temperature
60-120 0C