Dr. TabariDr. Tabari1

Introduction to PetroleumGeochemistry

Introduction to PetroleumGeochemistry

Unit ONEUnit ONE

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Preservation or Productivity ? 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.

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4

5

6

7

8

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Depositional Environment of Organic Rich Source Rocks

Depositional Environment of Organic Rich Source Rocks

Large anoxic lakes.Anoxic silled basins.Areas of coastal upwelling.Anoxic zones of the open oceans.

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Origin of Fossil Fuels 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.

IMMATURE

OIL

WET GAS

DRY GAS

DIAGENESIS

CATAGENESIS

METAGENESIS

~~~~~~~

Land Plants

Aquatic PlantsOxic

Anoxic

Potential

Source Bed

Burial and HeatOil Trap

Oil and GasMigration

Oil

OilSource

Bed

Heat

Evolution of Organic Matter in Sedimentary Rocks

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

0

1

2

3

4

5

Dep

th (

km)

Products

Biogenic Gas

Thermogenic Gas

Oil

Bio

mar

kers

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Origin of Fossil FuelsOrigin of Fossil FuelsThe 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.

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Origin of Fossil FuelsOrigin of Fossil Fuels

An even smaller fraction of this trapped organic carbon may ultimately become part of some type of fossil fuel.

Atmospheric CO2

PhotosynthesisPlants-Bacteria

Dead & DecayingPlants, Animals,

Bacteria

SedimentaryOrganic Material

Incorporation

DAYS-YEARS~3.0X1012 Tone Organic Carbon

MAJOR CYCLE

CO2

Combustion ofFossil Fuels

Oil, Gas

Bitumen

Shale, CoalKerogen

MILLION YEARS6.5X1015 Tone Organic Carbon

MINOR CYCLE

Leakage(0.01-0.1%

Total OrganicCarbon)

Oxidation

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Composition of Organic Matter 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.

SubstanceElemental Composition in Weight Percent

C H S N OCarbohydrates 44 6 - - 50

Lignin 63 5 0.1 0.3 31.6Proteins 53 7 1 17 22Lipids 76 12 - - 12

Petroleum 85 13 1 0.5 0.5

DECREASINGHYDROGEN

Relative hydrogen contents[ example: 6-carbon compounds ]

Average Organic Matter in Rocks

MINERAL MATTER ~99%ORGANIC MATTER ~1%

KEROGEN ~90% BITUMEN ~10%

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Evaluation of source Rock:

TOCQuality of organic matterThermal maturity

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Methods of quantifying maturity in source Rock:

Vitrinite reflection (Ro)Thermal alteration Index (TAI)C.P.IRatio of hydrogen to carbon (H/C)Elemental compositionTmaxTime temptation index (TTI)Spore & pollenRock Eval pyrolysisBiomarker Guide

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Composition of Organic MatterComposition of Organic Matter

H

H

24-Ethytcholeatane

H 10.4 ppm

49 ppm

C28TA-Steroid

H

H

H 145 ppm17(H)-Hopane

3.8 ppm

C29MA-Steroid

Triaromatic Steroids

Monoaromatic Steroids

17(H)-Hopanes

Steranes

Saturates

Aromatics

Whole Oil

The compounds derived from the lipid precursors and occurring in crude oils, typically known as biomarkers.

29%

24%

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Chemical Structures and Nomenclature 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

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BiomarkersBiomarkers

H

H

24-Ethytcholeatane

H 10.4 ppm

49 ppm

C28TA-Steroid

H

H

H 145 ppm

17(H)-Hopane

3.8 ppm

C29MA-Steroid

Triaromatic Steroids

Monoaromatic Steroids

17(H)-Hopanes

Steranes

Saturates

Aromatics

Whole Oil

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.

They are very useful in determining the source and diagenesis, and the maturity of organic matter as well as the reservoir transformations.

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Examples of BiomarkersExamples 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

CD

E

A-Tricyclic Terpane

De-A-Lupane Hopane

Gammacerane

TRITERPANE

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Numbering System Numbering System

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.

1

STERANES

2

34

5

19

910 8

67

1211

1713 16

1514

18

21

2022

2324

25

26

29

28

TRITERPANES1

2

34

5

25

910 8

67

1211

2813 16

1514

18

26

19 20

21 34

3522

27

30

32

33

29

31

19

23 24

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Nomenclature of BiomarkersNomenclature 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.

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StereochemistryStereochemistry

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.

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A

StereochemistryStereochemistry

Alpha (α) position:Alpha (α) position:refers to substituents that attachedrefers to substituents that attachedbelow the plane of the ring structure.below the plane of the ring structure.Beta (ß) position:Beta (ß) position:refers to substituents that attachedrefers to substituents that attachedabove the plane of the ring structure.above the plane of the ring structure.

Two Systems of PresentationTwo Systems of PresentationWedgesWedges Solid wedges indicate Beta (ß) position.Solid wedges indicate Beta (ß) position. Dashed or dotted wedges indicate Alpha (α) position.Dashed or dotted wedges indicate Alpha (α) position.

CirclesCircles Solid (dark) circles indicate Beta (ß) position.Solid (dark) circles indicate Beta (ß) position. Open circles indicate Alpha (α) position.Open circles indicate Alpha (α) position.

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StereochemistryStereochemistryChirality 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

ab

d

c

ba

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StereochemistryStereochemistryR (right or rectus) epimerIt indicates that the rank of the different four groups bonded to an asymmetric carbon atom decreases in the clockwise direction.S (left or sinister) epimerIt indicates that the rank of the different four groups bonded to an asymmetric carbon atom decreases in the counterclockwise direction.

X

H22

22R

X

H22

22S

H

CC

I-C3H7 C2H5

n-C4H9

C20H35