1. Introduction to Petroleum Geochemistry
Unit ONE
Introduction to Petroleum Geochemistry
Dr. Tabari

2. Preservation or Productivity ?
The quantity and quality of organic matter preserved during digenesis is controlled by a number of factors, notably the oxygen content of the water column and sediments, water circulation, organic matter productivity, and sedimentation.
Preservation: anoxia (reducing environment) is the main cause for enhanced preservation of organic matter leading to the formation of organic rich source rocks. Under oxic conditions, organic matter is rapidly degraded even though the rate of productivity may be high, leading to sediments with low organic carbon contents.
Productivity: it is the driving force behind the accumulation of organic rich rocks rather than preservation. Their argument is based on laboratory experiments that show rate of destruction is similar under oxic and anoxic conditions, and the fact that anoxic Black Sea sediments show not high organic matter content.

9. Depositional Environment of Organic Rich Source Rocks
Large anoxic lakes.
Anoxic silled basins.
Areas of coastal upwelling.
Anoxic zones of the open oceans.

10. Evolution of Organic Matter in Sedimentary Rocks
Origin of Fossil Fuels
Fossil fuels result from the accumulation, preservation, and burial of organic matter in various types of sedimentary environment.
Over a long period of time and with increasing temperature, this organic matter will be subsequently degraded by diagenetic reactions and later by thermal reactions, which ultimately lead to the production of liquid hydrocarbons. ~
Land Plants
Aquatic Plants
Oxic
Anoxic
Source Bed
Potential
Burial and Heat
Oil Trap
Oil and Gas
Migration
Oil
Source
Bed
Heat - ~ 1 2 3 4 5
Depth (km)
Products
Biogenic Gas
Thermogenic Gas
Oil
Biomarkers
IMMATURE
OIL
WET GAS
DRY GAS
DIAGENESIS
CATAGENESIS
METAGENESIS
Evolution of Organic Matter in Sedimentary Rocks

11. Origin of Fossil Fuels
The starting point for the production of organic matter is the photosynthetic cycle.
Higher plants on land and phytoplankton in the oceans are the major primary producers.
They assimilate CO2 from the atmosphere incorporating carbon into compounds that become part of these living systems.
Carbohydrate is the main form in which organic matter is stored in living cells.
The reverse process of photosynthesis is respiration, which typically occurs at night in the absence of sunlight.
The majority of the photosynthetically fixed organic carbon (99.9%) is relatively rapidly recycled back into the atmosphere with only a small amount (0.1%) escaping from the photosynthetic cycle and being incorporated into the sedimentary record.

12. Origin of Fossil Fuels
An even smaller fraction of this trapped organic carbon may ultimately become part of some type of fossil fuel.
MAJOR CYCLE
MINOR CYCLE
Atmospheric CO2
CO2
Combustion of
Fossil Fuels
Oil, Gas
Incorporation
Oxidation
Photosynthesis
Plants-Bacteria
Sedimentary
Organic Material
Bitumen
Leakage
( %
Total Organic
Carbon)
Dead & Decaying
Plants, Animals,
Bacteria
Shale, Coal
Kerogen
DAYS-YEARS
~3.0X1012 Tone Organic Carbon
MILLION YEARS
6.5X1015 Tone Organic Carbon

13. Composition of Organic Matter
Naturally occurring organic matter is composed of varying proportions of six major classes of organic compounds namely proteins, carbohydrates, essential oils, resins, lipids and pigments.
Lipids constitute the precursors of many classes of compounds found in crude oils and source rock extracts.
Substance
Elemental Composition in Weight Percent C H S N O
Carbohydrates 44 6 - 50
Lignin 63 5
0.1
0.3
31.6
Proteins 53 7 1 17 22
Lipids 76 12
Petroleum 85 13
0.5

14. Relative hydrogen contents [ example: 6-carbon compounds ]
DECREASING HYDROGEN

15. Average Organic Matter in Rocks
MINERAL MATTER ~99%
ORGANIC MATTER ~1%
KEROGEN ~90%
BITUMEN ~10%

16. Evaluation of source Rock:
TOC
Quality of organic matter
Thermal maturity

17. Methods of quantifying maturity in source Rock:
Vitrinite reflection (Ro)
Thermal alteration Index (TAI)
C.P.I
Ratio of hydrogen to carbon (H/C)
Elemental composition
Tmax
Time temptation index (TTI)
Spore & pollen
Rock Eval pyrolysis
Biomarker Guide

18. Composition of Organic MatterH
24-Ethytcholeatane
10.4 ppm
49 ppm
C28TA-Steroid
145 ppm
17a(H)-Hopane
3.8 ppm
C29MA-Steroid
Triaromatic Steroids
Monoaromatic Steroids
17a(H)-Hopanes
Steranes
The compounds derived from the lipid precursors and occurring in crude oils, typically known as biomarkers.
Saturates
Aromatics
29%
Whole Oil
24%

19. Chemical Structures and Nomenclature
It is the building block of biomarkers.
The end closer to the methyl branch (-CH3) is called the “head”, and the other end is the “tail”.
Monoterpane: two isoprene units joined head- to- tail.
Diterpane: four isoprene units or two monoterpanes.
Triterpane: six isoprene units or three monoterpanes.
Monoterpane
Isoprene

20. Biomarkers
Complex organic compounds composed of carbon, hydrogen, and other elements which are found in oil, bitumen, rocks, and show little or no change in structure from their parent organic molecules in living organisms. H
24-Ethytcholeatane
10.4 ppm
49 ppm
C28TA-Steroid
145 ppm
17a(H)-Hopane
3.8 ppm
C29MA-Steroid
Triaromatic Steroids
Monoaromatic Steroids
17a(H)-Hopanes
Steranes
Saturates
Aromatics
Whole Oil
They are very useful in determining the source and diagenesis, and the maturity of organic matter as well as the reservoir transformations.

21. Examples of Biomarkers
Steranes
Four rings A, B, C and D.
D-ring contains only “5C” atoms.
Triterpanes
3-6 rings.
5-rings are the most common (hopanes) in which E-ring contains only “5C atoms”.
Gammacerane: 5-rings with “6 C atoms” in E-ring. A B C D
Cholestane
Diacholestane
STERANES A B C D E
A-Tricyclic Terpane
De-A-Lupane
Hopane
Gammacerane
TRITERPANE

22. Numbering System The precise number of carbon atoms depends on:1
STERANES 2 3 4 5 19 9 10 8 6 7 12 11 17 13 16 15 14 18 21 20 22 23 24 25 26 29 28
The precise number of carbon atoms depends on:
Source material.
Diagenesis.
Thermal maturity.
Biodegradation.
The numbering system indicates where side chains are attached to the ring system.
TRITERPANES 1 2 3 4 5 25 9 10 8 6 7 12 11 28 13 16 15 14 18 26 19 20 21 34 35 22 27 30 32 33 29 31 23 24

23. Nomenclature of Biomarkers
Nor-
Prefix meaning without, indicating that a methyl group has been lost from the base compound (parent molecule) of the biomarker at the position indicated by the preceding number. Thus, 28,30-bisnorhopanes lack the C28 and C30 methyl groups found in hopanes.
Homo-
Prefix referring to additional carbon atom on the structure of the parent molecule.

24. Stereochemistry
It refers to the spatial relationship of atoms in a molecule.
Whenever two rings are joined, each of the atoms at the junction is attached to three other carbon atoms in the ring structure.
Its fourth bond (H2 or CH3-) can point either up or down with respect to the plane of the ring.

25. Stereochemistry Alpha (α) position:
refers to substituents that attached
below the plane of the ring structure.
Beta (ß) position:
above the plane of the ring structure.
Two Systems of Presentation
Wedges
Solid wedges indicate Beta (ß) position.
Dashed or dotted wedges indicate Alpha (α) position.
Circles
Solid (dark) circles indicate Beta (ß) position.
Open circles indicate Alpha (α) position. A

26. Stereochemistry Chirality or Asym.carbon atoms:
If four different substituents are attached to a particular carbon atom (e.g. C22 in hopanes or C20 in steranes), the atom is called an asymmetric or chiral carbon atom.
In a molecule containing more than one asymmetric center, inversion of all the centers leads to the enantiomer (mirror image). Inversion of only one center yields an epimer. Inversion of more than one or less than all yields a diastromer. d c a b d c b a

27. Stereochemistry R (right or rectus) epimer
It indicates that the rank of the different four groups bonded to an asymmetric carbon atom decreases in the clockwise direction.
S (left or sinister) epimer
It indicates that the rank of the different four groups bonded to an asymmetric carbon atom decreases in the counterclockwise direction. H C
I-C3H7
C2H5
n-C4H9
C20H35 X H 22
22R
22S