Tiago Miguel dos Santos Pedrosa Caracol - ULisboa · Tiago Miguel dos Santos Pedrosa Caracol Thesis...

Thermal Analysis for the characterization of drill cuttings

Tiago Miguel dos Santos Pedrosa Caracol

Thesis to obtain the Master of Science Degree in

Chemical Engineering

Supervisors: Prof. Maria Amélia Nortadas Duarte de Almeida Lemos (IST)

Bruno Alexandre de Oliveira e Melo (Geolog)

Examination Committee

Chairperson: Prof. Carlos Manuel Faria de Barros Henriques

Supervisor: Eng. Bruno Alexandre de Oliveira e Melo

Members of the Committee: Amílcar de Oliveira Soares

June 2018

i

“It always seems impossible until it´s done” Nelson Mandela

ii

Acknowledgements

First of all, I would like to acknowledge Professor Francisco Lemos and Maria Amélia Lemos for

giving me the opportunity to participate in this research work. I want to acknowledge the support they

gave me, back in February 2017. I also want to give a word of recognition to all the time they spent with

me, all the meetings, all the e-mails, for all the improvements suggested, not only for the work but also

for myself, since they convinced me to write the thesis in English, arguing that it would be so rewarding

as it turned out to be.

Likewise, I also have to acknowledge my Co-supervisor, Bruno Melo, for the indispensable guidance

he gave me throughout this work.

To Professor Ângela Pereira I have to thank not only for providing the rock, essential to develop this

work but also for her friendly wishing for a good work.

I want to thank my father and my brother because without them reaching this stage would be

impossible. To my mother, who has been the most supportive person to me, helping me through the

most difficult parts and always having a kind word to give me, my special thanks. Special thanks to my

brother by the incentive and friendship.

To Everton Santos I would like to acknowledge his availability to help me, all the scientific talks we

shared, and all the advice, but above all, I would like to thank him for his friendship. I also want to thank

Hugo Pinto for assisting me in the experimental part and the initial part of data treatment.

For all these years, for being the friend that he is, for all the “all night long” moments that we shared,

which help me to complete this stage of my path, and for the well-spent afternoons in Arco do Cego, I

want to thank Ruben Santos.

For being such good companions from the day one, I have to thank Pedro Gomes, Eduardo Ferreira,

and Marcelo Ameixa.

A word of recognition to Mónica Catarino for the all the help she gave me and for the friendship.

Last but not least, I want to thank the friends that Tecnico brought to my life as Sofia Capelo, João

Pedro Silva, Miguel Marques, who have always been there for me.

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Resumo

O objetivo deste trabalho foi criar um modelo matemático que permita caracterizar e quantificar as

diferentes moléculas de hidrocarbonetos presentes no petróleo que está no interior de uma rocha. A

calibração do modelo foi realizada utilizando carbonato impregnado com diferentes fluidos sintéticos.

O trabalho inicial consistiu no desenvolvimento de uma descrição para a evaporação, em condições

de análise térmica similares ao processo utilizado no Rock-Eval de componentes puros e de misturas

binárias impregnadas nas rochas, obtendo parâmetros cinéticos para cada um dos componentes.

Partindo de uma lei do tipo Arrhenius de primeira ordem, obteve-se uma descrição aceitáveis com

valores de coeficiente de correlação r-quadrado acima 0,99 para um hidrocarboneto impregnado e

acima de 0,95 para uma mistura de dois hidrocarbonetos impregnados.

O modelo geral foi então testado na estimativa da composição de misturas com três, quatro e cinco

hidrocarbonetos diferentes. Verificou-se um ajuste de muito boa qualidade para misturas com três e

quatro componentes. Para a mistura com cinco componentes, em que o quinto componente era o

esqualano, os ajustes não foram de tão boa qualidade, mas ainda assim aceitáveis. No geral os erros

obtidos foram inferiores a 10% para a composição de cada um dos componentes.

Em conclusão, o modelo matemático utilizado é fiável para o cálculo de percentagens para misturas

de hidrocarbonetos lineares insaturados tendo em conta os resultados obtidos.

Palavras chave: Rock-Eval, Petróleo, Analise térmica, Rocha

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vi

Abstract

The objective of this work was to create a mathematical model that allows the identification and

quantifification of the different hydrocarbon molecules present in the oil within drill cuttings. The

calibration of the model was performed using a carbonate rock impregnated with different synthetic

fluids.

The initial work consisted of the development of a description for the evaporation, under conditions

of thermal analysis similar to the process used in Rock-Eval, of pure components and binary mixes

impregnated in the rocks, getting kinetic parameters for each of the components. Using a first order

Arrhenius type kinetic law, an acceptable description was obtained with r-square correlation coefficient

values above 0.99 for an impregnated hydrocarbon and above 0.95 for a mixture of two impregnated

hydrocarbons.

The model was then tested in the estimation of the composition of mixtures with three, four and five

different hydrocarbons. There was very good quality description for mixtures with three and four

components. For the mixture of five components, in which the fifth component was Squalane, the

description was not of such good quality, but still acceptable and the differences observed were mainly

due to the high boiling-point of Squalane. In general, the errors obtained were less than 10% for the

composition of each of the components tested.

In conclusion, work can be seen as a proof of concept for this mathematical model to be used as a

reliable way for the estimation of composition for oils in the rocks, in particular of mixtures of linear

unsaturated hydrocarbons.

Keywords: Rock-Eval, Petroleum, Thermal analysis, Drill cuttings.

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viii

Table of contents

Acknowledgements ............................................................................................................................ ii

Resumo ............................................................................................................................................. iv

Abstract ............................................................................................................................................. vi

List of Figures .....................................................................................................................................x

List of Tables .................................................................................................................................... xii

List of Abbreviations ........................................................................................................................ xiv

1. Introduction .............................................................................................................................. 1

1.1. Scope of thesis .................................................................................................................... 1

1.2. Objectives ............................................................................................................................ 2

1.3. Structure of thesis ................................................................................................................ 3

2. State of art ............................................................................................................................... 5

2.1. Chemical composition of biomass ....................................................................................... 5

2.1.1. Aquatic biomass ......................................................................................................... 5

2.1.2. Land biomass ............................................................................................................. 6

2.1.3. The chemical composition of aquatic biomass and land biomass ............................. 6

2.2. Kerogen and van Klevelen diagram .................................................................................... 8

2.3. Diagenesis, Catagenesis and metanogenesis .................................................................. 10

2.3.1. Diagenesis ................................................................................................................. 10

2.3.2. Catagenesis ............................................................................................................... 11

2.3.3. Methanogenesis ........................................................................................................ 14

2.3.4. Conclusion ................................................................................................................. 14

2.4. Biomarkers ......................................................................................................................... 15

2.5. Rock-Eval .......................................................................................................................... 16

2.5.1. Pyrolysis .................................................................................................................... 16

2.5.2. Evolution of Rock-Eval .............................................................................................. 16

2.5.3. Rock-Eval 6 ............................................................................................................... 17

2.5.3.1. Description of Rock-Eval 6 ................................................................................... 17

2.5.3.2. Rock-Eval 6 Apparatus ......................................................................................... 21

2.5.3.3. New Applications of Rock-Eval 6 ......................................................................... 22

ix

2.5.4. Objective of this work ................................................................................................ 22

3. Experimental part .................................................................................................................. 23

3.1. Hydrocarbons .................................................................................................................... 23

3.2. Rock ................................................................................................................................... 23

3.3. Methodology ...................................................................................................................... 24

3.3.1. Saturation of Limestone with synthetic oils ............................................................... 24

3.3.2. Thermogravimetric analysis ....................................................................................... 25

3.3.3. Gas chromatography ................................................................................................. 27

4. Data treatment and discussion .............................................................................................. 29

4.1. Rock characterization ........................................................................................................ 29

4.2. Hydrocarbon impregnated in rock ..................................................................................... 30

4.2.1. Results of the thermogravimetric analysis ................................................................. 30

4.2.2. Mathematical model for the evaporation of one hydrocarbon ................................... 32

4.3. Mixtures of two hydrocarbons impregnated in rock ........................................................... 35

4.3.1. Thermogravimetric analysis for two hydrocarbons impregnated in rock ................... 35

4.3.2. Mathematical model for a mixture of two hydrocarbons ............................................ 36

4.4. Estimation of percentages for a mixture of three Hydrocarbon ......................................... 39

4.5. Estimation of percentages for a mixture of four Hydrocarbons ......................................... 41

4.6. Estimation of percentages for a mixture of five Hydrocarbon ........................................... 43

5. Conclusion ............................................................................................................................. 45

6. Future Work ........................................................................................................................... 47

7. References ............................................................................................................................ 49

Appendix A ........................................................................................................................................ a

Formulas of calculated parameters of Rock-Eval 6 ....................................................................... a

Appendix B ........................................................................................................................................ b

Specifications of hydrocarbons ...................................................................................................... b

Appendix C .........................................................................................................................................c

Chromatogram of mixture Decane. Dodecane. Hexadecane and Eicosane ..................................c

Report of the chromatographic analysis of the mixture Decane, Dodecane, Hexadecane and

Eicosane. ............................................................................................................................................. d

x

List of Figures

Figure 1 - World energy demand, in Mtoe, by fuel and scenario and fossil-fuel share for each

scenario. New Policies Scenario (NPS), Current Policies Scenario (CPS) and Sustainable Development

Scenario [1]. ............................................................................................................................................ 1

Figure 2 – The biological pump [14]. ................................................................................................. 5

Figure 3 – Peptide Bond in amino acids [11]. ................................................................................... 7

Figure 4 - Molecule of Aspartic acid wherein the carboxylic acid group is with a black circle and

amine group with a red circle [11]. .......................................................................................................... 7

Figure 5 - Molecule of cellulose [11]. ................................................................................................ 7

Figure 6 - Molecule of Palmitic Acid which is a fatty acid [11]. ......................................................... 7

Figure 7 – Part of lignin molecule [11]. .............................................................................................. 8

Figure 8 - Van Klevelen diagram [20]. ............................................................................................... 9

Figure 9 – Oil and gas window. Oil window starts at 170ºC until 220ºC and gas window stars at

170ºC and finishes at 225ºC [2]. ........................................................................................................... 12

Figure 10 - Formation of the two sets of products, Hydrogen-rich (methane) and carbon-rich

(graphite) [7]. ......................................................................................................................................... 13

Figure 11 – Scheme of cracking of Kerogen [2]. ............................................................................. 13

Figure 12 – Oil and Gas window, and hydrocarbons formed for a function of depth [2]. ................ 14

Figure 13 – Van Klevelen diagram with the stages of thermal maturation [22]. ............................. 15

Figure 14 – β-carotene (biomolecule), a carotenoid pigment present in carrots and β-carotane

(biomarker) after diagenesis [24]. .......................................................................................................... 15

Figure 15 - Schematic cross section of the first commercial Rock-Eval [25] [26]. .......................... 16

Figure 16 - A General diagram showing the different fractions of the total organic matter of analysed

rocks, the corresponding parameters and their recordings [28]. ........................................................... 18

Figure 17 – Kerogen Quality as a function of the percentage of TOC [29]. .................................... 19

Figure 18 - Influence of depth on the value of tmax [2]. .................................................................... 20

Figure 19 -Production index as a function of burial in the Early Cretaceous sediments of the Camanu-

Almada basin, Brazil [2]. ........................................................................................................................ 20

Figure 20 - Tmax versus PI to characterize the samples [31]. ........................................................ 21

Figure 21 - Different Rock-Eval 6 versions commercialized by Vinci Technologies® [27]. ............. 21

Figure 22 - Comparison of reservoir rock types around the world in 1956 [34]. ............................. 23

Figure 23 - Carbonate sample......................................................................................................... 23

Figure 24 - Carbonate after being crushed ..................................................................................... 24

Figure 25 - Experimental installation for saturation. 1- Schlenk vessel, 2- Suba, 3- filter (glass tube),

4- Vacuum pump ................................................................................................................................... 25

Figure 26 – Temperature program for an inert atmosphere. ........................................................... 26

Figure 27 – Temperature program for oxidative atmosphere. ........................................................ 26

Figure 28 - Simultaneous Thermal Analyzer STA 6000 during the experiment. ............................. 27

xi

Figure 29 – Percentage of the initial mass as a function of the temperature of rock used in

experimental part in an inert atmosphere. ............................................................................................. 29

Figure 30 - Percentage of the initial mass as a function of the temperature of rock used in

experimental part at oxidative atmosphere. .......................................................................................... 30

Figure 31 – mg HC in rock/mg rock as a function of temperature for assay 1 of each hydrocarbon

............................................................................................................................................................... 31

Figure 32 - mg HC in rock/mg rock as a function of time for assay 1 of each hydrocarbon ........... 31

Figure 33 - mg HC in rock/mg rock as a function of temperature for assay 2 of each hydrocarbon

............................................................................................................................................................... 31

Figure 34 - mg HC in rock/mg rock as a function of time for assay 1 of each hydrocarbon ........... 32

Figure 35 – Normalized mass (in relation to the hydrocarbon) as a function of temperature for two

assays of Decane. ................................................................................................................................. 32

Figure 36 - Experimental mass loss and model mass loss to assay 1 of hexadecane................... 34

Figure 37 - Thermogravimetric analysis of mixture Decane + Dodecane, Dodecane + Hexadecane

and Decane + Hexadecane wherein percentage of initial mass as a function of Temperature. All the

mixtures are 50:50. ................................................................................................................................ 36

Figure 38 - Thermogravimetric analysis for Decane, Dodecane and mixture of Decane + Dodecane.

............................................................................................................................................................... 36

Figure 39 - Experimental mass loss and model mass loss to assay 1 of a mixture of Decane +

Dodecane. ............................................................................................................................................. 38

Figure 40 - Energy activation as a function logarithmic of a kinetic constant to show the correlation

between these two parameters for the different components under study. .......................................... 39

Figure 41 - Experimental and model mass loss and to assay 1 of a mixture of Decane + Dodecane

+ Hexadecane. ...................................................................................................................................... 40


+ Hexadecane. ...................................................................................................................................... 40


+ Hexadecane + Eicosane. ................................................................................................................... 41


+ Hexadecane + Eicosane. ................................................................................................................... 42


+ Hexadecane + Eicosane + Squalane. ................................................................................................ 43


+ Hexadecane + Eicosane + Squalane. ................................................................................................ 43

Figure 47 - Chromatogram of mixture Decane. Dodecane. Hexadecane and Eicosane ...................c

xii

List of Tables

Table 1 – Summary Table with preservation ability and in what element is rich/poor [2]. ................ 8

Table 2 -Acquisition parameters of Rock-Eval 6 [27]. ..................................................................... 18

Table 3 - Calculated parameters from the acquisition parameters [27]. ......................................... 19

Table 4 - Kinetic constant and the activation energy hydrocarbon impregnated in rock. ............... 35

Table 5 - Enthalpy of vaporization for each compound [36]. ........................................................... 35

Table 6 - Kinetic constant and the activation energy for each hydrocarbon that forming mixtures

studied. .................................................................................................................................................. 38

Table 7 – Real composition, the estimated composition of the model and relative error of each

hydrocarbon for each assay. ................................................................................................................. 41

Table 8 - Real composition, the estimated composition of the model and relative error of each

hydrocarbon for each assay. ................................................................................................................. 42

Table 9 – Formulas of calculated parameters of Rock-Eval 6 .......................................................... a

Table 10 - specifications of the Hydrocarbons .................................................................................. b

Table 11 – Peak of Chromatographic analysis of the mixture Decane, Dodecane, Hexadecane and

Eicosane. ................................................................................................................................................. d

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xiv

List of Abbreviations

IEA- International Energy Agency

Mtoe - Million Tonnes of Oil Equivalent

TGA- Thermogravimetric Analyzer

TOC - Total organic carbon

Tmax - Value of tmax of peak S2

PI - Production Index

FID - Flame ionization detector

TG - Thermogravimetric

CO - Carbon monoxide

CO2 - Carbon dioxide

HC - Hydrocarbon

kTref- Kinetic constant for the reference temperature

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1

1. Introduction

1.1. Scope of thesis

Energy is something indispensable for human life that can be used for power, industry, transport

and buildings. Its increase in demand is related to the increase of the world population, the improvement

in the quality of living conditions and the emergence of new economies, mainly India, China, and

Southeast Asia and on a smaller scale the Middle East, Latin America, and some parts of Africa [1]

According to the latest world energy Outlook of IEA, fossil fuels will continue to be the most used

energy source in the next two decades, with a share of 75-80%. Gas will overtake coal, and this is due

to the objective of reducing emissions of CO2 and the growing need for energy cannot be satisfied just

with renewable energy/non-fossil fuels whereby oil will continue to be one of the main primary energy

sources for the next two decades, always having a share of around 30% for any scenario [1].

These conclusions are clearly depicted in Figure 1.

Figure 1 - World energy demand, in Mtoe, by fuel and scenario and fossil-fuel share for each scenario. New Policies Scenario (NPS), Current Policies Scenario (CPS) and Sustainable Development Scenario [1].

In nature, oil is found in the lithosphere, in particular in reservoir rock rocks that have the ability to

store fluids (water, oil, gas) inside their pores, possessing good porosity and permeability to allow the

accumulation and draining of the oil in economical amounts [2] [3].

In the exploration of oil, the exploring companies (drilling/services companies) continuously invest

the improvement of methods (geological, geophysical and geochemical) ) that aid the exploration and

provide an analysis, as detailed as possible, of the hydrocarbons that are in the reservoir [4].

The geochemical methods performed at the reservoir and source rock are aimed at characterizing

the organic matter present in the rocks and to analyse the petroleum potential, maturity and origin [5].

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13760 15182 17584 15690 19299 13921 1408410035

Mtoe

2

In the petroleum industry, Geolog provides the GeoSource service where it provides data on TOC and

Pyrolysis in near real-time which allows for faster drilling planning so there are no unnecessary delays

and thus save costs [5] [6].

The most widely used pyrolysis process is Rock-Eval; it simulates in a short time the geological

process that organic matter is subjected to when it is buried. The Increasing depth causes an increase

of the temperature and pressure and will result the thermal cracking, this process takes thousands of

years. In the Rock-Eval, the sample only suffer an increase of temperature and this process takes one

hour .Although the differences in time and in conditions of pressure and temperature, pyrolysis can be

used because the cracking reactions are similar [2] [7].

The Thermogravimetric Analyzer (TGA) is an essential laboratory tool used for material

characterization. The areas where it is most used are environmental, food, pharmaceutical and

petrochemical. Thermogravimetric analysis is a technique in which the mass of a sample is monitored

as a function of the temperature or the time of analysis, as the temperature is subject to a pre-determined

programme. The thermogravimetric analysis is carried or pan out in a crucible that is place in a precision

balance. This crucible during the thermogravimetric analysis is in a furnace in which the sample is

heated or cooled during the experiment and the mass of the crucible and sample is monitored.

The environment is controlled by the purge gas and an inert gas, or a reactive gas flow may be used

[8].

Another relevant parameter when drilling is the characterization of the hydrocarbons present in

drill cutting. In the oil industry the technique used is thermal extraction gas chromatography, specifically

the company Geolog Surface Logging with service G9+, and thus it is possible to have a characterization

of the C9 until C27. The characterization of hydrocarbons is applied to detect and characterize oil and, in

the source, rocks to define source type and properties. Cuttings newly acquired in the well can be

analysed avoiding the evaporation of the lighter hydrocarbons and thus reducing the error of the analysis

due to the short analysis time about 40 minutes [9] [10].

1.2. Objectives

The scope of this project is to use the thermo gravimetric analysis (TGA) to identify and quantify the

different hydrocarbon molecules present in the oil within the drill cuttings.

One of the main concerns when creating a new model is the underlying calibration. The calibration will

be performed using carbonate impregnated with different synthetic fluids.

The objective is to recognize consistency/correlations, and measure the data uncertainty in the analysis,

in order to validate the model.

This project aims to give a positive input to the formation evaluation field in the oil and gas industry.

The main workflow branch will be related to calibration and quality control of the data, as well as, to data

treatment and uncertainty evaluation. The sample is subjected to a programmed temperature program

previously set in a controlled atmosphere (usually nitrogen atmosphere).

3

1.3. Structure of thesis

The thesis is composed of six chapters. The introduction is the first chapter where the scope of the

thesis (World energetic overview of the next two decades and some methods used in oil exploration)

and the objectives of the thesis are approached. The second chapter covers the state of art review in

the petroleum geochemistry, covering chemical composition of biomass and kerogen and the Van

Klevelen diagram, oil and gas formation (diagenesis, catagenesis and methanogenesis) and

biomarkers. In the second chapter is also described the Rock-Eval technique is also described as well

as the evolution of the apparatus. The third chapter cover the experimental details; it describes the

hydrocarbons and rock used in this study, the rock saturation procedure, the temperature program used

in the thermogravimetric analysis as well as the TGA and chromatography apparatus used in this work.

In the fourth chapter are presented the results obtained as well as the data processing used and the

corresponding discussion, based on the results obtained. In this chapter the general aspects of the

model are also described with an emphasis on how the calibration is done. The fifth chapter presents

the conclusion, where the main considerations drawn from the work are described. The sixth chapter is

focused on the future development work. Lastly, the seventh chapter consists of the references used in

this thesis.

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5

2. State of art

2.1. Chemical composition of biomass

Organic matter is constituted by organic molecules that are composed mainly of carbon, hydrogen,

oxygen, nitrogen, and sulfur. The organic matter that will undergo degradation and subsequent

sedimentation derives from living organism, and the products of their metabolism and an example of a

metabolism reaction is the reaction of photosynthesis, Equation 1, that occurs in organisms with

chloroplasts. The process of photosynthesis converts the energy from light (ℎ. 𝜐) into chemical energy

by synthesising hydrocarbons out of carbon dioxide and water. The mechanism involves basically a

transfer of hydrogen from water to carbon dioxide to form glucose (organic matter) [11] [12].

6 CO2 + 6 H2Oh. υ⇄

674kcalC6H12O6 + 6 O2 (Eq. 1)

The autotrophic organisms (synthesize the organic matter) from the glucose (𝐶6𝐻12𝑂6) will be able to

metabolize polysaccharides and other constituents necessary for their subsistence [11].

2.1.1. Aquatic biomass

The main source of aquatic organic matter is phytoplankton and is constituted by microalgae and

these are similar to terrestrial plants because they contain chlorophyll (present in chloroplasts) and

require sunlight to live and grow. The majority of the phytoplankton is in the upper layer of the oceans

and seas (euphotic zone) where the penetration of carbon dioxide and sunlight occurs (first step of

biological pump) and it is in this layer that the phytoplankton perform the photosynthesis as can be seen

in Figure 2 [11] [12] [13] [14].

Figure 2 – The biological pump [14].

6

Phytoplankton also needs inorganic nutrients such as nitrates and phosphates (in the form of dissolved

salts) for their growth, these are converted into proteins, lipids, and carbohydrates. In the euphotic zone,

there is also zooplankton that feeds on phytoplankton and this is a set of organisms of very small size

[2] [11] [12].

There are two main types of environments with different aquatic biomass, lacustrine and marine

environment. Most of the biomass of the lacustrine environment is phytoplankton. As this is a very

sensitive system to the seasonal changes and if there is an increase in the supply of nutrients it is

promoted the growth of primitive photosynthetic bacteria (e.g., Cyanobacteria), these organisms are

devoid of the cellulosic cell wall and thus when in abundance promote a change in the chemical

composition of the biomass, because they have a different constitution of phytoplankton [2].

The microorganisms in marine environment biomass is essentially constituted by phytoplankton and

zooplankton [2] [11] [12].

2.1.2. Land biomass

The land biomass is of plant origin and is stored in soils with high moisture content. The majority of

dead organic matter is degraded or recycled, and little will suffer in situ fossilization. The precursors of

this type of biomass have in common with the phytoplankton that they carry-out photosynthesis [2].

High rainfall and high temperatures during the growth period promote high primary production and

thus increasing the matter that can give rise to organic matter [2].

2.1.3. The chemical composition of aquatic biomass and land biomass

As previously described the constitution of aquatic biomass (marine and lacustrine) and land

biomass are different so the sedimentary organic matter that they will give rise to will also have a different

chemical composition. The aquatic biomass is composed chemically by proteins, lipids and,

carbohydrates while the land biomass is composed mainly by cellulose (carbohydrate polymers), lignin,

proteins, lipids [11] [12].

As it has been said in previous subchapters the lacustrine and marine environment biomass is mainly

phytoplankton and less quantity zooplankton while the land biomass is essentially of plant origin and the

overall chemical composition of these two different types of biomass can be described as follows [12].

Plankton organic matter Vegetal organic matter

• Proteins – 30% • Cellulose – 45%

• Lipids – 50% • Lignin – 30%

•Carbohydrates – 20% • Proteins/ lipids – 5%

• Carbohydrates – 20%

The values presented are average because there is a lot of diversity and the environment also plays a

7

fundamental role. For example, the lipid content of the planktonic species tends to increase from the

equatorial regions to the more temperate and cold regions [11] [12].

Proteins are polymers which are composed by amino acids (monomers) which are bound by peptide

bonds [11].

Figure 3 – Peptide Bond in amino acids [11].

Amino acids are composed by functional groups such as amine and carboxylic acid.

Figure 4 - Molecule of Aspartic acid wherein the carboxylic acid group is with a black circle and amine group with a red circle [11].

Carbohydrate is the name given to simple sugars and their polymers and has this name because of

its empirical chemical formula Cn(H2O)n which alludes to the fact that carbon is hydrated. In the group

of carbohydrates, we have molecules like cellulose. Cellulose is one of nature's most abundant

polysaccharides since 40-60% of the wood's constitution is cellulose, thus cellulose is the probably the

most important molecule for the chemical composition of land biomass [11].

Figure 5 - Molecule of cellulose [11].

The term lipids was proposed by Bergmann in 1963 who defined them as insoluble substances in

water but extractable with fatty solvents [15].

Lipids are biomolecules that encompass various types of molecules such as fatty acid, waxes, sterols

and, fat-soluble vitamins (A, D, and E) and are found in cell membranes, protective tissues and organic-

walled phytoplankton. Fatty acids are carboxylic acids with long chains that can be saturated or

unsaturated [11] [12].

Figure 6 - Molecule of Palmitic Acid which is a fatty acid [11].

8

Lignin is characterized by having an aromatic (phenolic) structures. Aromatic compounds are not

synthesized by animals but are very common in the tissues of plant organisms. Lignin is a complex

polymer and is found in the cell walls of plant organisms and some algae, and its function is to grant

mechanical force to the cell walls and to aid in the transport of the sap [2] [11] [12].

Figure 7 – Part of lignin molecule [11].

Finally, with the description made throughout this chapter of the main molecules present in biomass,

it is possible to construct a summary table (see Table 1) where information about its preservation and

which chemical elements are more relevant has also been added [2].

Table 1 – Summary Table with preservation ability and in what element is rich/poor [2].

Type of compound Preservation ability Element

Proteins Very low -

Carbohydrate Very low -

Cellulose High Rich in oxygen

Lipids Very high Rich in Hydrogen

Lignin Very high Poor in Hydrogen

In Table 1 there is no information regarding Cyanobacteria’s but with it was said in subchapter 2.1.1. its

abundance will imply changes in the chemical composition of the biomass of the lacustrine environment,

thus increasing the H/C ratio and decreasing the O/C ratio [2].

2.2. Kerogen and van Klevelen diagram

The term kerogen was designated for the first time by Crum Brown in 1912 as the insoluble organic

matter found in oil shale [16].

In 1961 Breger defined kerogen as the organic constituent of sedimentary rocks that is not solvable in

aqueous alkaline solvents nor in common organic solvents [17].

Later Vanderbroucke defined kerogen as organic matter that has resisted the decomposition process

and can generate oil and gas, which is the most commonly used definition today [18].

9

The kerogen originates from the land and aquatic biomass that sedimented and underwent chemical

transformations and is probably the most abundant form of organic matter on earth; it is believed to also

1000 times more abundant than all coal and petroleum reservoirs [11] [18].

In a sedimentary rock, the percentage of organic matter varies, between 1 and 2% is classified as being

of good quality if it presents values higher than 4% it is classified as excellent quality and in this organic

matter about 90% is kerogen and 10% is bitumen [7].

By the end of taphonomy stage (the process of burial, decay and preservation of organic matter

and became fossilized) the kerogen has characteristics that determine its potential for generating oil and

gas and reactivity to further thermal transformation [2].

The characteristics of kerogen depend on organic matter from which come, the chemical processes and

biochemical processes that occurred during the taphonomy stage [2] [11].

Kerogen is a complex and heterogeneous mixture that contains the organic matter that remains at the

end of the taphonomy stage, mainly the tissues of greater resistance and amorphous organic

substances that derive from the condensation and polymerization of functionalized or unsaturated

organics moieties (functionals groups) [2]. Consequently, the kerogen will have a chemical composition

similar to the composition of the biomass that was at its origin. Table 1 indicates how, depending on the

constituents of organic matter, the ratios of H/C and O/C will be different.

Since the atomic composition of the three main elements (carbon, oxygen and hydrogen) varies

depending on the origin of the biomass D.W. van Klevelen propose a diagram, which is known as the

van Klevelen diagram, wherein represents the organic matter composition depending on their H/C and

O/C ratios and that can be used to represent the different types of kerogen [19].

Figure 8 - Van Klevelen diagram [20].

Van Klevelen divided the diagram into four types of kerogen, type I, II, III and IV, the latter is not

represented in Figure 8.

10

This classification of kerogen into types provides information on the potential for generating oil and

gas [2].

Type I ≈ 60-70%

Type II ≈ 40-60%

Type III ≈ 15-30%

Type I kerogen has a high H/C ratio (1.5 or more). This type of kerogen has a lot of lipid material

mainly aliphatic chains. It has a small amount of oxygen (ratio usually less than 0.1) which comes mainly

from ester bonds. The large amount of lipids is due to the accumulation of algae of a lacustrine

environment. As it was said in subchapter 2.1.1 large amounts of cyanobacteria increases the ratio of

H/C. This type of kerogen is from lacustrine provenance [2] [11]. This type of kerogen generates large

amounts of oil and gas. [2] [11].

Type II kerogen is very common in rocks that generate oil and gas. It presents a H/C ratio smaller

than type I (1.2 <H/C <1.6), although still high, and also a higher O/C ratio (0.1 <O/C <0.2). This type of

kerogen presents an O/C ratio because of the existence of more ester bonds and the presence of more

ketones and carboxylic acids. It is a material that contains abundant aliphatic chains and some

naphthenic rings. This kerogen is associated with the marine organic matter that is deposited in reducing

environments, when there is little or no oxygen. The presence of sulphate in these media can incorporate

sulphur into the organic matter [2] [11].

Type III kerogen is the kerogen with lower H/C ratio, of the three represented in Figure 8 (H / C <1.2)

and higher O/C ratio (O/C> 0.2). This kerogen doesn´t have many ester bonds but features the highest

amount of ketones and carboxylic acids and contains few amounts of aliphatic groups. It is the least

favourable kerogen for the generation of oil and gas. It derives from land-based organic matter which is

mostly composed of cellulose and lignin, which are deficient in hydrogen [2] [11].

Kerogen type IV (which is not shown in Figure 8) is an immature kerogen, this is due to the

occurrence of a carbonization (combustion) of the organic matter before the deposition and thus this

kerogen is very poor in hydrogen [11].

2.3. Diagenesis, Catagenesis and metanogenesis

The organic matter undergoes physicochemical transformations since it is on the surface until it

turns into kerogen inside sedimentary rocks. Since the initial burial, the organic matter begins to undergo

compositional changes, first because of biological activity and as it is goes to greater depth the actions

of pressure and temperature are felt with more intensity. These processes are known as thermal

maturation and are divided into 3 stages: diagenesis, catagenesis and, methanogenesis. Each is

characterized by different types of chemical processes [2] [11].

2.3.1. Diagenesis

Diagenesis is defined as the set of physicochemical and microbiological changes that act during

deposition and down to a few meters of depth and at relatively low temperatures (<65ºC) and low applied

11

pressures. Diagenesis is the initial stage changes/degradation of organic matter through biochemical

processes at low temperature [11] [12].

Diagenesis is a process in which the system tends to approach equilibrium under conditions of

shallow burial and where the sediment becomes consolidated. The depth interval where diagenesis

occurs is in the range of a few meters up to a few hundred meters where the increase in pressure and

temperature are considered mild. The biopolymers (proteins and carbohydrates) are degraded by

microbial activity during sedimentation and diagenesis and its constituents form new polycondensed

structures (geopolymers) which are the precursors of kerogen [11].

At the end of diagenesis, the organic matter is mainly kerogen, in terms of oil exploration these rocky

matrices containing organic matter are considered immature [11].

Since there is a lot of microbial activity under anaerobic conditions, the most important hydrocarbon

formed at this stage is methane, which is called biogenic methane because it is formed due to

biochemical decomposition performed in this state of maturation [2] [11].

The formation of kerogen represents a "halfway point" between the original organic matter and

fossils fuels [2].

Due to the transformations/degradations that occurred at the end of this stage, an equilibrium is

obtained [11].

Besides the kerogen, there are also hydrocarbons that are synthesized by the living organisms in

the rocks that can be used as biomarkers, or chemical fossils, that provide relevant information such as

the environment in which sediment deposition occurred [11] [12].

2.3.2. Catagenesis

The continuous deposition of sediments results in the continued burial of the previously mentioned

beds that can go down to a few kilometres from the subsidence of the basin. This means a considerable

increase in temperature and pressure and tectonics may also contribute to this increase [11].

The temperature attained by the rocks can vary between 50°C and 150°C and the geostatic pressure

between 300bar and 1500bar. These are the reaction conditions that will drive the thermal maturation

of the kerogen and the reaction time of catagenesis varies between a few thousands to millions of years

[7] [11].

These changes in temperature and pressure cause the system to become out-of-balance and therefore

will result in further changes [11]. When this happens, the stage of catagenesis is entered, and this term

was used the first time by Vassoevich. The stage of catagenesis is defined as transformations that occur

due to the thermal process in which kerogen is converted into hydrocarbons [11] [21].

The main inorganic modification that occurs is the rock compaction and the water contained in the

rock is expelled since its porosity and permeability decrease considerably. At the organic level, the

kerogen will undergo thermal pyrolysis which produces first oil (oil window) and later on (at higher

temperature and deeper burial) gas (gas window). This a continuous process in which, after the

maximum production of oil occurs, the rock starts to produce wet gas and, as the depth and temperature

increase, the production of gas increases and oil decreases until only gas is produced (gas window).

12

This process is due to the thermal cracking that happens around 60-120ºC and this is the process

responsible for forming oil and gas in the rock source [2] [7] [11].

Figure 9 – Oil and gas window. Oil window starts at 170ºC until 220ºC and gas window stars at 170ºC and finishes at 225ºC [2].

The transformation of kerogen into fossil fuels is induced primarily by temperature, there is a

temperature rise, depending on the location the temperature will increase by 10-30ºC / km. This increase

in temperature is mainly due to the heat released from the radioactive decay of the elements like 40K,

232Th, 235U and, 238U, although, as previously said, tectonics may also influence this process. During

catagenesis, internal hydrogen transfer reactions occur, which occurs manly by a radical mechanism

and tend to produce products with a higher H / C ratio along with more carbonaceous residues, see

Equation 2 and 3 [7].

C6H5CH3 → 2 CH4 + 5 C (Eq. 2)

C7H16 → 4 CH4 + 3 C (Eq. 3)

As a consequence, with the production of High H/C ratio products, also produces graphite-like

residue, a product composed almost solely of carbon atoms. The production of compounds with a higher

H/C ratio other compounds will have to supply hydrogen, as depicted in Figure 10 [7].

13

Figure 10 - Formation of the two sets of products, Hydrogen-rich (methane) and carbon-rich (graphite) [7].

The internal hydrogen transfer reaction happens before the thermal cracking. Thermal cracking

occurs mainly in molecules with more than sixteen carbon atoms and in branched linear chains or cyclic

and it is a radical mechanism [7].

The thermal cracking has two stages, primary cracking and secondary cracking. Primary cracking

is the kerogen cracking that forms oil and secondary cracking is the cracking of the primary cracking

products into products with less carbon atoms and can occur in source rock itself (in-situ) or in the

reservoir rock, in case the compounds were expelled from the source rock and passed through the

carrier system into a trap where it was housed [7].

Figure 11 – Scheme of cracking of Kerogen [2].

In conclusion, at the beginning of the catagenesis there is kerogen in the rock and, with increasing

depth, due to thermal cracking the kerogen will form oil and with increased depth originates oil and wet

gas. Oil production essentially ends at the end of the catagenesis stage as depicted in Figure 12 [2].

14

Figure 12 – Oil and Gas window, and hydrocarbons formed for a function of depth [2].

2.3.3. Methanogenesis

The final stage of thermal maturation is methanogenesis it occurs at temperatures above 225°C, at

greater depths. The organic matter present in the rock at this stage is constituted by compounds with a

high carbon content. In methanogenesis, the hydrocarbons that is formed is called dry gas that contains

essentially methane, ethane, propane, and butane, and finally methane alone. There may still be some

cracking of kerogen or previously formed liquid hydrocarbons that are converted into methane. If the

conditions are met, methane, carbon dioxide and graphite may be formed, and this occurrence is called

metamorphism. At this stage, the rocky matrix is considered super mature [7] [11] [12].

2.3.4. Conclusion

In conclusion, the processes of thermal maturation are diagenesis, catagenesis and

methanogenesis. In diagenesis, the organic matter is converted into kerogen (geopolymer). In the

catagenesis, due to the increase of temperature and pressure, thermal cracking occurs and sequentially

oil is formed, followed by wet gas and, at the end of the stage only wet gas. Finally, in the

methanogenesis, there is formation of methane and thus the thermal maturation is terminated. These

stages of thermal maturation can be repreented in a van Klevelen diagram as it can be seen in Figure

13 [22].

15

Figure 13 – Van Klevelen diagram with the stages of thermal maturation [22].

2.4. Biomarkers

The term "biomarkers" is the widely used today and was introduced by Seifert & Moldowan in 1986

although throughout the 20th century there were several definitions of chemical fossils, Egliton & Calvin

in 1967 where the first, to describe organic compounds in the geosphere that the carbon skeleton

suggested a direct link to a known natural product (see Figure 14). The first clear association of a

biomarker with a precursor that was described was a porphyrin that had chlorophyll as its precursor [11]

[12].

Usually, lipids, pigments and biomembranes have a greater resistance to degradation and, thus,

they have a better capacity to be preserved in longer geological times. Biomarkers cannot be

synthesized by abiological processes and each precursor generally is altered by diagenetic and

catagenetic processes to a sequential series of derivative products [23].

Biomarkers can be used to determine the origin of organic components, to determine

paleoenvironments of deposition and the maturity of organic matter [11] [12].

Figure 14 – β-carotene (biomolecule), a carotenoid pigment present in carrots and β-carotane (biomarker) after diagenesis [24].

16

2.5. Rock-Eval

2.5.1. Pyrolysis

Pyrolysis is a widely used degradation technique that allows cracking of complex compounds by

heating under an inert gas (nitrogen or helium) and cracking products are easily analysed and quantified.

Pyrolysis can be applied for geochemical analysis, to simulate kerogen cracking. In nature, this process

takes long time intervals since it occurs at relatively low temperatures. At the laboratory level, high

temperatures are used, and the cracking duration is much shorter. Although there are large differences

between what happens in nature and at the laboratory level, pyrolysis can be used to characterize the

organic material present in rocks because the cracking reactions are similar [25].

2.5.2. Evolution of Rock-Eval

Rock-Eval is one of the most commonly used techniques for the evaluation of hydrocarbons in

source and reservoir rocks.

Rock-Eval 1 was the first Rock-Eval apparatus and was marketed for the first time in 1977 and had

only one pyrolysis oven. After preliminary heating at 300°C at 3 minutes, the analysis begins increasing

the temperature up to 600°C. With this temperature program, it was possible to characterize the thermal

maturity of the kerogen and oil potential of the sediment. To detect the hydrocarbons that were

generated during the pyrolysis procedure an FID is required at the exit of the pyrolysis furnace as is can

see in Figure 15. The limitation of the analysis to a maximum temperature of 600ºC, was a problem

because, in order to have a complete analysis of type III kerogen, it was necessary to analyse at higher

temperatures. Helium was used as the inert gas used in this equipment [25] [26]. As only pyrolysis was

carried-out there was no possibility to analyse the presence of fixed carbon.

Figure 15 - Schematic cross section of the first commercial Rock-Eval [25] [26].

17

Rock-Eval 2 was commercialized in 1979, having already two different ovens one oven for oxidation

and another for pyrolysis. This equipment could also have an organic carbon analysis module which is

used to determine the total organic carbon (TOC), anteriorly to calculate TOC was used the LECO

method [26].

The Rock-Eval 3 Oil Show Analyzer OSA is an improved version of Rock-Eval version 2 which was

easier to use and can be used on-site at drilling sites. In this version, the pyrolysis temperature begun

at 180ºC and ended at 600ºC. The lower starting temperature used with this version, made it already

possible to quantify some free hydrocarbons present in the samples. This version was further automated

in relation to the previous version, with a microprocessor and an apparatus which transfers the analysed

sample from pyrolysis oven to the oxidation oven [26].

The latest version of Rock-Eval, Rock-Eval 6, was launched on the market in 1996 by Vinci

Technologies. The temperature program of this version starts at 100°C and goes up to 850°C (100°C <

Pyrolysis Temperature<650°C and 100°C < Oxidation Temperature <850°C). With this temperature

program the analysis of light hydrocarbons was possible, along with heavy oils and complete analysis

of type III kerogen as well as mineral carbon [27].

2.5.3. Rock-Eval 6

2.5.3.1. Description of Rock-Eval 6

The characterization of the organic matter present in the sedimentary rocks is one of the main

objectives of the use of Rock-Eval for geochemistry analysis and, in the present days, it is a critical step

for evaluating the potential of a prospection. In the last 50 years, several authors/researchers (e.g.,

Barker, 1974; Claypool and Reed, 1976; Espitalié et al., 1977 and 1984; Clementz et al., 1979; Larter

and Douglas, 1980; Horsfield, 1985; Peters and Simoneit,1982; Peters, 1986) have used pyrolysis

methods to obtain data on the petroleum potential, thermal maturity, type of the source rocks in different

sedimentary basins. Among the techniques used by the various authors mentioned previously, Rock-

Eval was the most widely used, becoming a standard method in the industry of oil exploration. As

described in section 2.5.2. the Rock-Eval Technique has undergone several changes over the years in

order to improve the technique and allow it to obtain more parameters with its analysis, in order to have

a more complete report and thus better assist in drilling planning [28].

The Rock-Eval technique is based on using a programmed temperature heating of a rock sample

(about 100 milligrams) under an inert atmosphere (helium or nitrogen) to determine the amount of

hydrocarbons, present in the sample that were generated by natural processes, thermo-vaporization

(peak S1), generated by thermal cracking of the kerogen present in the sample (peak S2) and CO2

released due to thermal cracking of kerogen containing oxygen atoms (peak S3) [28].

18

Table 2 -Acquisition parameters of Rock-Eval 6 [27].

Acquisition parameters Unit Name

S1 mg HC/g rock Free Hydrocarbons

S2 mg HC/g rock Oil potential

TpS2 °C Temperature of peak S2 maximum

S3 mg CO2/g rock CO2 mineral source

S3` mg CO2/g rock CO2 organic source

TpS3` °C Temperature of peak S3` maximum

S3CO mg CO/g rock CO2 organic source

TpS3CO °C Temperature of peak S3CO maximum

S3`CO mg CO/g rock CO organic and mineral source

S4CO2 mg CO2/g rock CO2 organic source

S5 mg CO2/g rock CO2 mineral source

TpS5 °C Temperature of peak S5 maximum

S4CO mg CO/g rock CO organic source

The organic matter detected during the pyrolysis process is divided into free hydrocarbons and

kerogen. When the thermo-vaporization of hydrocarbons in the rock occurs, peak S1(oil and gas) is

obtained. Subsequently, with the increase of temperature, thermal cracking of kerogen occurs (peak

S2); in the case of kerogen containing oxygen a CO or CO2 peak is also detected (peak S3) besides

the peaks that were mentioned before. From the results it is also possible to determine the TOC of the

rock using the pyrolysis oven in combination with a second oven, in this case, for oxidation. The oxidation

oven will allow the determination of the residual organic carbon left by the pyrolysis (peak S4) [7], see

Figure 16 [28] .

Figure 16 - A General diagram showing the different fractions of the total organic matter of analysed rocks, the corresponding parameters and their recordings [28].

With the parameters acquired with Rock-Eval, peaks and the measurement of Tmax (TpS2 in Table

2), the temperature for the maximum rate of thermal decomposition of kerogen (corresponding to S2),

several other parameters of extreme importance can be obtained for a more detailed analysis (see Table

3) [27]. The formulas to calculate the parameters presented in table 3 can be consulted in appendix A

19

Table 3 - Calculated parameters from the acquisition parameters [27].

Calculated parameters Unit Name

Tmax °C Tmax

PI

Production index

PC Wt% Pyrolysable org. carbon

RC CO Wt% Residual org. carbon (CO)

RC CO2 Wt% Residual org. carbon (CO2)

RC Wt% Residual org. carbon

TOC Wt% Total organic carbon

S1/TOC mg HC/g TOC -

HI mg HC/g TOC Hydrogen index

OI mg CO2/g TOC Oxygen index

OI CO mg CO2/g TOC Oxygen index (CO)

PyroMinC Wt% Pyrolysis mineral carbon

OxiMinC Wt% Oxidation mineral carbon

MinC Wt% Mineral Carbon

From the parameters presented in Table 2 and 3 Tmax(TpS2), PI (Production index) and TOC (Total

organic carbon) stand out.

The TOC calculation provides the information on the quality of the organic matter in the source rock

and is represented by weight percent and the TOC is the sum of pyrolysable organic carbon and residual

organic carbon. The pyrolysable organic carbon is the carbon released at peak S1, 83% of the contents

of the peak S2 and CO and CO2 released in peak S3. These peaks are obtained in an inert atmosphere.

The residual carbon is the carbon leaving in the form of CO and CO2 in the S4 peak, this peak is in the

oxidative atmosphere [28] [29].

Figure 17 – Kerogen Quality as a function of the percentage of TOC [29].

Tmax is the temperature at which the maximum rate of hydrocarbon generation occurs in

a kerogen sample during pyrolysis analysis. The peak S2 represents the rate of hydrocarbon generation

of the kerogen undergoing a thermal cracking [2] [30].

http://wiki.aapg.org/Petroleum_generation

http://wiki.aapg.org/Kerogen

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The value of Tmax can be used as a measurement of the thermal maturity of organic matter inside

the rock. The higher the value of Tmax the higher the degree of thermal maturity. With the increase of

the depth the rocks pass from the immature to the oil window and then to the gas window and with this

increase of depth Tmax and the area of the peak S1 increase while that of the peak S2 decreases, this

is due to the advancement in the stage of thermal maturation and thus the rock having more and more

free hydrocarbons and less pyrolysable kerogen as schematized in Figure 18 [2].

Figure 18 - Influence of depth on the value of tmax [2].

The production index (PI) represents the amount of hydrocarbons that have been produced from

the original organic matter and can be calculated through 𝑆1

𝑆1+𝑆2 for a closed system in other words if

there is no expulsion of hydrocarbons from the rock, with subsequent migration. The sum of 𝑆1 + 𝑆2

represents theoretical total petroleum potential. PI will be higher the larger the area of the S1 peak. This

parameter is highly correlated with Tmax because the higher Tmax correspond to higher maturity of the

rock, indicating that more hydrocarbons have been generated by pyrolysis and, thus, the larger would

be the area of the S1 peak and the larger the value of PI. Thus, PI increases with increasing depth as it

is possible to observe in Figure 19 [2].

Figure 19 -Production index as a function of burial in the Early Cretaceous sediments of the Camanu-Almada basin, Brazil [2].

21

The combination of Tmax and PI provide a very good overall analysis of thermal maturity as can be

seen in Figure 20 [31]. Note, however, that PI is not always easy to determine as the source rock

systems cannot usually be considered as closed systems [31].

Figure 20 - Tmax versus PI to characterize the samples [31].

The Rock-Eval version 6 can also determine the mineral carbon content of drill cutting (sample).

The amount of mineral carbon is calculated by combination of the CO2 released during pyrolysis above

400°C and the CO2 from carbonate decomposition during the oxidation phase from 650°C to 850°C.

Magnesite, bicarbonate and siderite begin to decompose when the pyrolysis temperature approaches

400ºC and dolomite and calcite (the most important minerals in carbonate sequences) decompose

during oxidation [28]. During pyrolysis, the siderite can undergo two reactions, the first produces CO2

and FeO and the second departs from the products of the first reaction to produce CO and Fe3O4 see

Equation 4 and 5.

FeCO3 → FeO + CO2 (Eq. 4)

3 FeO + CO2 → Fe3O4 + CO (Eq. 5)

2.5.3.2. Rock-Eval 6 Apparatus

There are three versions of the Rock-Eval apparatus device marketed by Vinci technologies® [27].

Figure 21 - Different Rock-Eval 6 versions commercialized by Vinci Technologies® [27].

22

The simplest version, Classic Rock-Eval 6, has just a pyrolysis oven which that when heating the

sample will release hydrocarbons, free hydrocarbons and cracking of kerogen (S1 and S2 peaks), and

this release will be monitored by an FID (Flame ionization detector). the CO and CO2 released during

pyrolysis will be monitored by an infrared cell [27].

There are two more complete versions, standard Rock-Eval (serial process) and turbo Rock-Eval

(Parallel process). These two versions have the same constituents as the classic version, but they have

one more oxidation oven, to account for the CO and CO2 released during the oxidation with an infrared

cell measurement. The major difference between the versions it's the time of analysis, the parallel

process is faster than the serial process because the pyrolysis and oxidation are done simultaneously

while in the serial process the same infrared cell is used to account for the CO and CO2 released in the

pyrolysis and oxidation using switching valve to select the oven outlet [27].

2.5.3.3. New Applications of Rock-Eval 6

The Rock-Eval apparatus, with the technological advances that were added along time, started to

have new applications, namely reservoir geochemistry and soils contamination studies.

Geochemistry of reservoirs is an area of growing interest with remarkable economic importance

because it can be used to evaluate reservoir continuity during field appraisal, to identify non-productive

reservoir zones, and to analyse commingled oils for production allocation calculations [32]. The Rock-

Eval method has been used successfully in reservoir geochemistry for predicting the oil API( American

Petroleum Institute gravity) and for detecting tar-mats [28].

For the study of Soils contamination, the Rock-Eval player is called Pollut-Eval. For these types of

studies, the apparatus is equipped with a cooled autosampler that reduces the loss of light compounds.

The analysis performed provides parameters necessary to characterize a contaminated site: what are

the pollutants, how much and where. Additionally it provides a short analysis time, about 30 minutes

and so the time to assess the extent of a contaminated site is drastically reduced when compared with

the other techniques used, because the other types of analyses, chromatography, infrared spectroscopy

or chromatography-mass spectrometry, require that the pollutant is extracted [28] [33].

2.5.4. Objective of this work

The objective of the current thesis is to provide a computational way to analyse data from the S1

peak from Rock-Eval so as to allow the estimation of the hydrocarbons present without adding additional

time to the analysis time.

In fact, the S1 peak corresponds to the hydrocarbons that have already been generated in source

rocks or, more directly, to the oil present in a reservoir rock. Although it provides quantification of these

hydrocarbons it does not provide information on it these hydrocarbons themselves, something that

requires additional analysis after extraction of these hydrocarbons. In this work the aim is to provide a

fast, on-line estimation of some of the properties of these hydrocarbons.

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3. Experimental part

This chapter describes the hydrocarbons and rock used as well as the methodology used in this

thesis work

3.1. Hydrocarbons

The hydrocarbons used in this thesis work were Decane(C10H22), Dodecane (C12H26), Hexadecane

(C16H34), Eicosano (C20H42) e Squalane (C30H62).

The hydrocarbons used are paraffins which are saturated aliphatic hydrocarbons with straight or

branched chain. Squalane has a particularity of being a biomarker or molecular fossils, as previously

described in subchapter 2.4. The specifications of the hydrocarbons used are present in appendix B

3.2. Rock

As it is desired to simulate a sample taken from a reservoir it is necessary to choose a type of rock

that has representativeness as a reservoir rock. As depicted in Figure 22, reservoirs are made up mainly

by sandstone and carbonate. For this work a carbonate was chosen.

Figure 22 - Comparison of reservoir rock types around the world in 1956 [34].

The carbonate sample used in this work is shown in Figure 23, then crushed, Figure 24, to be

saturated

Figure 23 - Carbonate sample

24

Figure 24 - Carbonate after being crushed

The sample was collected from the quarry of Pedramoca located in Serra de Aire e Candeeiros

ownership of Mocapor. The sample is an oolitic limestone (oolitic is the designation given to round sand-

sized grains, 0.25 to 2mm, formed by inorganic chemical precipitation of calcium carbonate in agitated

waters and with little deposition of clastic material), that belongs to the formation of Santo António which

dates from the Middle Jurassic [35].

3.3. Methodology

The methodology used in the experimental part of this work consists in three steps, saturation of the

Limestone with synthetic oils, thermogravimetry analysis and subsequent data processing. Additionally,

gas chromatographic analysis was also used to check the composition of the hydrocarbon mixtures that

were used.

3.3.1. Saturation of Limestone with synthetic oils

The limestone used was clean and did not possess any hydrocarbons to start with. To carry out the

study the limestones were saturated with oils of know composition. The saturation of limestone with

synthetic oils is done to simulate one drill cutting in this limestone case, which contain hydrocarbons

inside.

The saturation was carried-out by puttin 300-400mg of limestone inside a Schlenk vessel sealed

with a Suba and degassing it thoroughly by creating vacuum inside the Schlenk using a controlled

atmosphere apparatus with a vacuum pump. A filter was used to prevent that when evacuating the

Schlenk, lighter sample pieces of carbonate would go into the vacuum pump; this filter also increase the

pressure loss and thus the vacuum is introduced into the Schlenk in a more gradual manner. All the

connection between pieces is made with special hoses to be able to produce vacuum as shown in Figure

25.

25

Figure 25 - Experimental installation for saturation. 1- Schlenk vessel, 2- Suba, 3- filter (glass tube), 4- Vacuum pump

3.3.2. Thermogravimetric analysis

As the purpose of this thesis is to add a complement to Rock-Eval method the same temperature

program of the method was used, either for the inert atmosphere part or for the oxidative atmosphere

part. The temperature program with an inert atmosphere starts at 30°C and increases at 25°C/min up

to 300°C, reproduce peak S1, as there is no kerogen in samples only hydrocarbons, this will be the only

peak characterized in this thesis. After this first ramp there is an isothermal section at 300°C during 10

minutes. The second heating ramp starts at 300°C and ends at 650ºC; this is the ramp that, in Rock-

Eval, is used to characterize the S2 and S3 but as the samples used do not have any kerogen there is

no data to be treated. When reaching the 650ºC another isothermal ramp to stabilize the temperature.

At the end proceeds to a cooling of 650°C until 30°C at a 25°C/min rate.

The oxidation program begins at 30°C and ends at 850°C and has a heating rate of 25°C/min at the

end there is an isothermal ramp at 850ºC and at the end, the sample is cooled.

The description of the temperature programs above can be seen in Figures 26 and 27.

.

26

Figure 26 – Temperature program for an inert atmosphere.

Figure 27 – Temperature program for oxidative atmosphere.

The apparatus used to perform Thermogravimetric analysis was Simultaneous Thermal Analyzer

STA 6000 of brand PerkinElmer® and the software used to control de apparatus and collect the data

was the Pyris software.

0

100

200

300

400

500

600

700

0 10 20 30 40 50 60 70 80 90

Pro

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Tem

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Hold for 10 min at 300°C Heat from 300°C to 650°C at 25°C/min

Hold for 10 min at 650°C Cold from 650°C to 30°C at 25°C/min

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400

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0 10 20 30 40 50 60 70 80 90

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Hold for 10 min at 30ºC Heat from 30ºC to 850ºC at 25ºC/min

Hold for 10 min at 850ºC Cold from 850ºC to 30ºC at 25ºC/min

27

Figure 28 - Simultaneous Thermal Analyzer STA 6000 during the experiment.

3.3.3. Gas chromatography

In order to know the exact compositions of the oil mixtures that were saturated into the rocks, gas

chromatography was used. The apparatus used was a Clarus®680 Gas Chromatograph of brand

PerkinElmer® that has a BP1 column (in which the stationary phase is 100% dimethylpolysiloxane that

is non-polar and is recommended for hydrocarbons) in whose dimensions are 30m * 250μm working at

a pressure of 2bar with the following temperature program: hold for 2min at 60ºC, heat from 60ºC to

190ºC at 10ºC/min, hold for 5 min at 190ºC and heat from 190ºC to 200ºC at 10ºC/min ; the injector

used is of split-type with a split ratio of 1:50 and the injection temperature was 250°C, and the sample

volume is 0.1μL. The carrier gases are Nitrogen and helium.

28


29

4. Data treatment and discussion

In this chapter is presented all the analyzes that were carried-out, thermogravimetric analysis to

saturated samples and gas chromatography to the hydrocarbon mixtures.

4.1. Rock characterization

First, some analysis were carried-out on the rock used in this work, to identify the minerals present

in the rock and see if their mass losses during the thermochemical process they were being subjected

to could affect the interpretation of results after the samples were saturated with hydrocarbons. The first

experiments were carried-out in an inert atmosphere and the results of the thermogravimetric analysis

are shown in Figure 29.

Figure 29 – Percentage of the initial mass as a function of the temperature of rock used in experimental part in an inert atmosphere.

As it can be observed, in the inert atmosphere, the mass loss up to 650ºC is insignificant, less than

1%. Based on the reference [28] it can be suggested that the sample has siderite.

Next the thermal analysis was carried-out under an oxidative atmosphere (air) and

thermogravimetric analysis is shown in Figure 30.

99

99,1

99,2

99,3

99,4

99,5

99,6

99,7

99,8

99,9

100

100,1

130 180 230 280 330 380 430 480 530 580 630 680

Per

cen

tage

of

the

init

ial m

ass

Temperature (°C)

Assay 1

Assay 2

30

Figure 30 - Percentage of the initial mass as a function of the temperature of rock used in experimental part at oxidative atmosphere.

In the oxidative atmosphere, the results of the mass loss are similar to that under nitrogen up to

650ºC. However, in this case, the temperature was raised up to 850ºC and a total mass loss is about

45% of initial mass. The mass loss starts at around 700ºC and based on the reference [28] can be

suggested that this mass loss corresponds to calcite present on a rock. Through of Thermogravimetric

analysis, it may be suggested that the stone used in this work had calcite and siderite in its composition.

The first mass loss happens at approximately 400°C, there will be no problem because there are only

hydrocarbons in the rock after saturation and these evaporate before 400°C

4.2. Hydrocarbon impregnated in rock

4.2.1. Results of the thermogravimetric analysis

The objective of this work is to analyse the thermogravimetric behavior of the rock containing

complex mixtures of hydrocarbons. The work proceeded in a stepwise fashion, starting with rocks

saturated with a pure component before moving to the analysis of the behavior of samples saturated

with mixtures of increasing complexity.

Consequently, after the rock characterization the work proceeded with the analysis of samples

saturated with a single hydrocarbon. The hydrocarbons used were Decane, Dodecane, Hexadecane,

Eicosane and Squalane.

The results of thermogravimetric analysis realized to the samples (rock saturated with one

hydrocarbon) are shown in Figures 31 and 32(assay 1 to each hydrocarbon) and 33 and 34 (assay 2 to

each hydrocarbon), the results are represented as mg Hydrocarbon/mg rock as a function of

temperature and time.

50

60

70

80

90

100

0 100 200 300 400 500 600 700 800 900

Per

cen

tage

of

inic

ial m

ass

Temperature (°C)

Assay 1

Assay 2

31

Figure 31 – mg HC in rock/mg rock as a function of temperature for assay 1 of each hydrocarbon

Figure 32 - mg HC in rock/mg rock as a function of time for assay 1 of each hydrocarbon

Figure 33 - mg HC in rock/mg rock as a function of temperature for assay 2 of each hydrocarbon

0

0,05

0,1

0,15

0,2

0,25

0,3

0,35

0,4

30 80 130 180 230 280 330 380

mg

HC

in r

ock

/mg

rock

Temperature (°C)

Decane

Dodecane

Hexadecane

Eicosane

Squalane

0

0,05

0,1

0,15

0,2

0,25

0,3

0,35

0,4

0 5 10 15 20 25 30 35 40

mg

HC

in r

ock

/mg

rock

Time (min)

Decane

Dodecane

Hexadecane

Eicosane

Squalane

0

0,05

0,1

0,15

0,2

0,25

0,3

0,35

0,4

30 80 130 180 230 280 330 380

mg

HC

in r

ock

/mg

rock

Temperature (°C)

Decano

Dodecano

Hexadecano

Eicosane

Squalane

32

Figure 34 - mg HC in rock/mg rock as a function of time for assay 1 of each hydrocarbon

From the observation of Figures 31 to 34 it is possible to conclude that for longer carbon chains

more time is needed to finish the thermal vaporization. The Squalane curve has a different behaviour

because in the Rock-Eval temperature program there is an isothermal section at 300ºC, and at this

temperature, Squalane is still in the process of evaporating from the rock. There is only a small difference

of the weight of HC in rock/mg rock for the same set (hydrocarbon + rock) which can be due to the

variability of the rock samples themselves. Nevertheless, although the samples have slightly different

hydrocarbon content their behaviour is the same as it is possible to see in Figure 35, for the case of

Decane.

Figure 35 – Normalized mass (in relation to the hydrocarbon) as a function of temperature for two assays of Decane.

4.2.2. Mathematical model for the evaporation of one hydrocarbon

The objective is to be able to describe the evaporation kinetics of the hydrocarbons. To do this we

will assume that the evaporation can be described by an Arrhenius-type law with a first order kinetics

and we will use the experimental data to determine the activation energy and kinetic constant for each

0

0,05

0,1

0,15

0,2

0,25

0,3

0,35

0,4

0 5 10 15 20 25 30 35 40

mg

HC

in r

ock

/mg

rock

Time (min)

Decane

Dodecane

Hexadecane

Eicosane

Squalane

0

0,2

0,4

0,6

0,8

1

0 50 100 150 200 250 300

mas

s n

orm

aliz

ed

Temperature (°C)

Assay 1

Assay 2

33

hydrocarbon. To reduce the correlation in the estimation of the two kinetic parameters that are required

in Arrhenius law, this will be written in terms of a reference temperature, using Equations 6 and 7.

k(T) = k0e(− EaRT

) (Eq. 6)

kTref= k0e

(− Ea

RTref) (Eq. 7)

Where k(T) Kinetic constant for temperature T

k0 Pre-exponential factor

Ea Activation energy

R Universal gas constant

T Temperature of samples in the absolute temperature

kTref Kinetic constant for the reference temperature

Tref Reference temperature in absolute temperature

Dividing Equation 6 by Equation 7 obtains the Equation 8 can be written as Equation 9.

kT

kTref

= e− Ea

R(

1T

−1

Tref) (Eq. 8)

k(T) = kTrefe

− EaR

(1T

−1

Tref) (Eq. 9)

In the following the reference temperature chosen was 298.15 K.

Equation 10 is of the massa balance describing the mass loss of the hydrocarbon from the sample

and, insertin Equation 9 in Equation 10, it is possible to arrive at Equation 11.

−dw𝐻𝐶(𝑡)

dt= k(T) ∗ wHC(𝑡) (𝐸𝑞. 10)

Where dw𝐻𝐶(𝑡)

dt Derived of the mass loss of the hydrocarbon at time

k(T) Kinetic constant for temperature T

wHC(𝑡) Mass of hydrocarbon at time t

−dwHc(t)

dt= kTref

e− Ea

R(

1T

−1

Tref)

∗ wHC(t) (Eq. 11)

To calculate the hydrocarbon mass at each instant Equation 12 was used.

.

34

wHc(t) = wHc(tt−1) +dwHc(tt−1)

dt∗ (tt−tt−1) (Eq. 12)

Where wHc(t) Mass of hydrocarbon at time t

wHc(𝑡𝑡−1) Mass of hydrocarbon at time t-1

dwHc(𝑡𝑡−1)

dt Derived of the mass loss of the hydrocarbon at time t-1 in order of time

tt Time t

tt−1 Time t-1

Note that, in order to improve the accuracy of the calculations, the actual temperature profile was

used in the calculation and the time step used was the one corresponding to the data sampling rate.

Finally, to calculate the activation energy and the kinetic constant these values were estimated by a

least-squares approach, aiming at obtaining the smallest sum of the squares of the residuals possible.

In Figure 36 it is possible to see the comparison between the mass loss computed by the model for the

first assay of hexadecane in comparison with the experimental mass loss.

Figure 36 - Experimental mass loss and model mass loss to assay 1 of hexadecane

In Figure 36 is possible observed that model mass loss (first-order of Arrhenius law) represents with

a good degree of approximation to the experimental mass loss. The results for the calculated parameters

for the different hydrocarbons are present in Table 4.

0

0,05

0,1

0,15

0,2

0,25

0 5 10 15 20 25 30 35

mg

HC

in r

ock

/mg

rock

Time (min)

Experimental mass loss

Model mass loss

35

Table 4 - Kinetic constant and the activation energy hydrocarbon impregnated in rock.

Assay 𝑘𝑇𝑟𝑒𝑓 (s-1) Ea (J/mol)

Decane 1 3.5E-03 51769.7

2 3.8E-03 51164.5

Dodecane 1 2.4E-04 63253.5

2 2.7E-04 60703.2

Hexadecane 1 1.6E-06 81743.9

2 1.6E-06 81658.6

Eicosane 1 5.2E-10 117349.3

2 5.8E-10 117549.3

Squalane 1 8.7E-15 160899.8

2 8.7E-15 160912.5

Analysing the data of Table 4 and observing the Figures 31,32,33, and 34 one can conclude that

the longer the chain of carbon atoms or molecular weight, the higher the activation energy. This would

be expected because the larger the carbon chain length the heavier are the molecules to evaporate and

more energy will be required per molecule, see Table 5.

Table 5 - Enthalpy of vaporization for each compound [36].

Compound Enthalpy of vaporization (KJ/mol)

Decane (C10H22) 38.0

Dodecane (C12H26) 43.4

Hexadecane (C16H34) 50.5

Eicosane (C20H42) 56.4

Squalane (C30H62) 65.8

4.3. Mixtures of two hydrocarbons impregnated in rock

After in the previous subchapter in which the kinetic parameters for the evaporation of a single

hydrocarbon impregnated in rock were estimated, the next step was to use mixtures of two

hydrocarbons.

4.3.1. Thermogravimetric analysis for two hydrocarbons impregnated in rock

After the impregnation with a known mixture of two hydrocarbons in the rock the samples were,

again subjected to the thermogravimetric analysis. The results for mixtures of Decane + Dodecane,

Dodecane + Hexadecane and Decane + Hexadecane are present in Figure 37.

36

Figure 37 - Thermogravimetric analysis of mixture Decane + Dodecane, Dodecane + Hexadecane and Decane + Hexadecane wherein percentage of initial mass as a function of Temperature. All the mixtures are 50:50.

Through the observation of Figure 37, it is concluded that the end of the mass loss depends on the

hydrocarbon with the highest molecular weight in a given mixture. Thus, the mixtures Decane +

Hexadecane and dodecane + hexadecane end up at almost the same temperature. Remind the

difference of mg of HC in rock/mg rock is related to the sample analysed contains more or fewer

hydrocarbons.

4.3.2. Mathematical model for a mixture of two hydrocarbons

Figure 38 compares the mass loss for the mixture of Decane + Dodecane with the corresponding

mass losses for the pure components. It shows a very important aspect for mixtures of two

hydrocarbons: the mass loss of the mixture occurs at temperatures that lie between the those for the

mass loss of its pure constituents.

Figure 38 - Thermogravimetric analysis for Decane, Dodecane and mixture of Decane + Dodecane.

0

0,05

0,1

0,15

0,2

0,25

0,3

0 50 100 150 200 250 300 350 400

mg

of

HC

in r

ock

/mg

rock

Temperature (°C)

Decane + Dodecane

Dodecane + Hexadecane

Decane + Hexadecane

0

0,05

0,1

0,15

0,2

0,25

0,3

0 50 100 150 200 250 300 350

mg

of

HC

in r

ock

/mg

rock

Temperature (°C)

Decane

Dodecane

Decane + Dodecane

37

The explanation for the behaviour of the mass loss curves of Figure 37 is because as it is a mixture,

and each hydrocarbon has it own partial pressure, depending on the temperature, there will be

evaporation of the two components simultaneously, but with different rates, depending on the

composition of the liquid in the rock and the volatility of the pure components. To describe this effect,

the rate law for evaporation has not only to take into account the amount of the component under

consideration but also the composition of the mixture at any given time. To account for this fact the

change of vapour pressure as a function of the composition, as described Raoult’s law was added to

the mathematical model. Therefore, its necessary to add one term to the equation 11, whose term is the

introduction of Raoult´s law, thus the equation of the derivative for mass loss of a hydrocarbon can be

written as:

−dwHCa

(t)

dt= kTref

e− Ea

R(

1T

−1

Tref)

∗ wHCa(t) ×

wHCa(t)

wHC(t) (Eq. 13)

Where dw𝐻𝐶𝑎(𝑡)

dt Derived of mass loss of the hydrocarbon a at time t

kTref Kinetic constant for the reference temperature

Ea Activation energy

R Universal gas constant

T Temperature of samples in absolute temperature

Tref Reference temperature in absolute temperature

w𝐻𝐶𝑎(𝑡) Weight of hydrocarbon a at time t

𝑤𝐻𝐶(𝑡) Total weight of hydrocarbons at time t

Equation 13 was used, in a way similar to the one that was used for pure component, to estimate

the activation energy and kinetic constant for each hydrocarbon now in the mixtures under study. Again

the Euler method was used to integrate the relevant differential equations corresponding to the material

balances of the different compounds present and the result was fitted to the experimental data varying

the kinetic parameters and the estimated composition. In Figure 39 it is possible to see that the mass

loss computed by the model for the first assay of a mixture of Decane + Dodecane describes very well

the experimental mass loss.

38

Figure 39 - Experimental mass loss and model mass loss to assay 1 of a mixture of Decane + Dodecane.

Table 6 shows the values of the activation energies and kinetic constants estimated for the mixtures

used.

Table 6 - Kinetic constant and the activation energy for each hydrocarbon that forming mixtures studied.

Mixture Assay 1 Assay 2

kTref(s-1) Ea (J/mol) kTref

(s-1) Ea (J/mol)

Decane 1.1E-02 55466.2 7.3E-03 57047.9

Dodecane 9.3E-07 116450.0 4.8E-07 120803.8

Decane 7.7E-03 51500.4 3.6E-03 48300.0

Hexadecane 4.6E-06 81701.3 2.4E-06 85322.1

Decane 5.5E-03 35787.7 3.6E-03 40356.4

Eicosane 5.2E-13 162441.2 1.9E-13 163484.0

Decane 5.9E-03 40428.1 3.6E-03 44298.8

Squalane 7.6E-15 167904.9 8.7E-15 166532.0

Dodecane 9.2E-04 58051.7 7.1E-04 61145.8

Hexadecane 2.6E-08 119499.0 3.6E-09 130619.3

Hexadecane 3.0E-06 77100.0 3.0E-06 78903.8

Squalane 8.1E-15 167624.9 9.0E-15 166818.0

Hexadecane 3.1E-06 88957.4 1.1E-06 94848.1

Eicosane 8.9E-14 171833.0 7.3E-14 170622.0

Figure 40 represents the activation energies obtained for all the compounds that were teste in the

mixtures vs. the corresponding values of ktref. As it can be seen there is good correlation between the

two parameters. Note that in Figure 40 the values obtained for the pure components were also plotted.

0

0,05

0,1

0,15

0,2

0,25

0,3

0 5 10 15 20 25 30 35

mg

of

HC

in r

ock

/mg

rock

Time (min)


Model mass loss

39

Figure 40 - Energy activation as a function logarithmic of a kinetic constant to show the correlation between these two parameters for the different components under study.

This relationship will be the basis for the mathematical model that will be used for the estimation the

composition of the oil in the rock for a more complex mixture.

4.4. Estimation of percentages for a mixture of three Hydrocarbon

The objective in this subchapter is to estimate the fractions of each hydrocarbon in a mixture of three

hydrocarbons that was impregnated into the rock. After the mixture is prepared, an analysis by gas

chromatography was carried-out to check the actual composition of the sample so as to allow the

comparison of the know composition with the results obtained with the estimation using the

mathematical model. The obtained by the gas chromatography analysis were 32.19wt.% for Decane,

34.87wt.% for Dodecane and 32.94wt.% for Hexadecane. For the estimation of the composition by the

thermal analysis results, using mass loss model described, values for the kinetic parameters of each of

the components were obtained using the data in Table 6. The base values that were chosen were:

kTref=1.12E-2s-1 and Ea=55466J/mol for Decane (assay 1 of mixture Decane + Dodecane), kTref

=9.33E-

7s-1 and Ea=116450J/mol for Dodecane (assay 1 of mixture Decane + Dodecane) finally for Hexadecane

was chosen kTref=2.59E-9s-1 and Ea=119499J/mol (assay 1 of mixture Dodecane + Hexadecane). In

Figures 41 and 42 are presented the curves of experimental and model mass loss, for the model mass

loss was used the percentages calculated by gas chromatography analysis.

0

20000

40000

60000

80000

100000

120000

140000

160000

180000

200000

-35 -30 -25 -20 -15 -10 -5 0

E a(J

/mo

l)

ln (kTref)

Hydrocarbon + rock

Assay 1 of mixtures

Assay 2 of mixtures

40

Figure 41 - Experimental and model mass loss and to assay 1 of a mixture of Decane + Dodecane + Hexadecane.

Figure 42 - Experimental and model mass loss and to assay 2 of a mixture of Decane + Dodecane + Hexadecane.

In Figures 41 and 42 is possible observed that the curve of model mass loss describes perfectly to

the curve of experimental mass loss, that is to say, that the model can explain what happens

experimentally. Lastly, the objective is to calculate with the mathematical model the proportions of each

hydrocarbon in the mixture. The percentages calculated are in Table 7 as well as the relative error for

each hydrocarbon in each assay.

Through equation 16 the relative error is calculated.

Relative error =Real% − Model%

Real% (Eq. 14)

Where 𝑅𝑒𝑎𝑙% Value obtained by the analysis of gas chromatography

𝑀𝑜𝑑𝑒𝑙% Value obtained by the mathematical model

0

0,05

0,1

0,15

0,2

0 5 10 15 20 25 30 35

mg

of

HC

in r

ock

/mg

rock

Time (min)


Model mass loss

0

0,05

0,1

0,15

0,2

0,25

0,3

0 5 10 15 20 25 30 35

mg

of

HC

in r

ock

/mg

rock

Time (min)


Model mass loss

41

Table 7 – Real composition, the estimated composition of the model and relative error of each hydrocarbon for each assay.

Decane Dodecane Hexadecane

Assay 1

Real composition 0.322 0.349 0.329

Estimated composition 0.334 0.350 0.316

Relative error 3.70% 0.48% 4.13%

Assay 2

Real composition 0.322 0.349 0.329

Estimated composition 0.318 0.370 0.312

Relative error 1.30% 6.18% 5.27%

The average relative error of each assay is 2.77% for assay 1 and 4.25% for assay 2. Analysing the

calculated values for the composition determined by the mathematical model and for the relative error

concluded that the model determines the percentages of each hydrocarbon with a low relative error, so

this model gives consistent results. Therefore, this mathematical model to determine this type of values

for a mixture of three hydrocarbons is good enough.

4.5. Estimation of percentages for a mixture of four Hydrocarbons

The objective in this subchapter is to predict the fractions of each hydrocarbon in a mixture of four

hydrocarbons. The procedure was the same as in subchapter 4.4. and in this case, the gas

chromatography results were 20,29wt.% for Decane, 25.50wt.% for Dodecane, 31,94wt.% for

Hexadecane and 22,27wt.% for Eicosane. Appendix C contains the chromatogram and the gas

chromatography analysis report for this mixture. For the forecast (curve of model mass loss) the values

used were kTref=7.69E-3s-1 and Ea=51500.42J/mol for Decane, kTref

=7.07E-4s-1 and Ea=61145.83J/mol

for Dodecane, kTref=2.59E-8s-1 and Ea=119498.95J/mol for Hexadecane finally for Hexadecane was

chosen kTref=5.24E-13s-1 and Ea=162441.2J/mol. In Figures 43 and 44 are presented the curves of

experimental and model mass loss, for the model mass loss was used the percentages calculated by

gas chromatography analysis.

Figure 43 - Experimental and model mass loss and to assay 1 of a mixture of Decane + Dodecane + Hexadecane + Eicosane.

0

0,05

0,1

0,15

0,2

0,25

0,3

0 5 10 15 20 25 30 35

mg

of

HC

in r

ock

/mg

rock

Time (min)


Model mass loss

42

Figure 44 - Experimental and model mass loss and to assay 2 of a mixture of Decane + Dodecane + Hexadecane + Eicosane.

Observing the Figures 43 and 44 conclude that the curve of model mass loss describes perfectly to

the curve of experimental mass loss for assay 1, while that for assay 2 the curve of model mass loss

don´t fit perfectly in certain regions, more properly in the domain of Eicosane mass loss, but anyway is

a good representation of mass loss in experience. Lastly, it was calculated the proportions of each

hydrocarbon present in the mixture, analogously as in sub-chapter 4.4., and relative error in relation to

of proportions calculated by analysis of gas chromatography, the values are in Table 8.

Table 8 - Real composition, the estimated composition of the model and relative error of each hydrocarbon for each assay.

Decane Dodecane Hexadecane Eicosane

Assay 1

Real composition 0.2029 0.2550 0.3194 0.2227

Estimated composition 0.2235 0.2210 0.3288 0.2266

Relative error 10.17% 13.33% 2.960% 1.760%

Assay 2

Real composition 0.2029 0.2550 0.3194 0.2227

Estimated composition 0.2489 0.2110 0.3390 0.2010

Relative error 22.72% 17.25% 6.140% 9.740%

The average relative error of each assay is 7.05% for assay 1 and 13.96% for assay 2. the

percentages calculated by the mathematical model are not very different from the percentages obtained

by gas chromatography analysis although in some cases the relative error is large, for example, 22.72%

for Decane in assay 2. Although there are high relative errors, the relative errors of the tests are

satisfactory for the hydrocarbons in question, the mathematical model continues to provide quite

satisfactory results given that each analysis has an associated error.

0

0,05

0,1

0,15

0,2

0,25

0 5 10 15 20 25 30 35

mg

of

HC

in r

ock

/mg

rock

Time (min)


Model mass loss

43

4.6. Estimation of percentages for a mixture of five Hydrocarbon

The objective in this subchapter is to predict the fractions of each hydrocarbon in a mixture of five

hydrocarbons. The procedure was the same as in subchapter 4.4. and in this case, the gas

chromatography results were 21.27wt.% for Decane, 21.44wt.% for Dodecane, 29.06wt.% for

Hexadecane, 16.32wt.% for Eicosane and 11.91wt.% for Squalane. For this mixture, the values utilized

to make the curve of model mass loss were kTref=7.69E-3s-1 and Ea=51500.42J/mol for Decane,

kTref=9.15E-4s-1 and Ea=58051.66J/mol for Dodecane, kTref

=2.59E-9s-1 and Ea=119498.99J/mol for

Hexadecane, kTref=8.85E-14s-1 and Ea=171833J/mol for Eicosane and kTref

=7.58E-15s-1 and

Ea=197904.93J/mol for Squalane. In Figures 45 and 46 are presented the curves of experimental and

model mass loss, for the model mass loss.

Figure 45 - Experimental and model mass loss and to assay 1 of a mixture of Decane + Dodecane + Hexadecane + Eicosane + Squalane.

Figure 46 - Experimental and model mass loss and to assay 2 of a mixture of Decane + Dodecane + Hexadecane + Eicosane + Squalane.

0

0,05

0,1

0,15

0,2

0,25

0 5 10 15 20 25 30 35 40

mg

of

HC

in r

ock

/mg

rock

Time (min)


Model mass loss

0

0,05

0,1

0,15

0,2

0,25

0 5 10 15 20 25 30 35 40

mg

of

HC

in r

ock

/mg

rock

Time (min)


Model mass loss

44

Observing the Figures 45 and 46 it was concluded that the mathematical model cannot fully describe

the loss of mass for this mixture. The model describes well the loss of mass corresponding in most of

the mass loss of Decane, Dodecane, Hexadecane and almost all of Eicosane, but when it cannot

accurately describe the mass loss of Squalane.

45

5. Conclusion

The rocky matrix used in this work was a carbonate and through thermogravimetric analysis the

mass losses suggest, taking into account the temperature interval at which they occur, that it

corresponds to Siderite and Calcite.

In the experiments carried-out with one single hydrocarbon impregnated in the rock, it was observed

that the higher the molecular weight the temperature for the mass loss to occur; this is due to the fact

that the prevailing process is evaporation and that the higher the molecular weight the higher the boiling

point due to the increase in molecular forces, in this case, London forces. Kinetic parameters to describe

the evaporation of each of the components studied were obtain (the kinetic rate constant at a reference

temperature of 298K - kTref - and the activation energy) and, as expected, it was observed that the

higher molecular weight the lower the value of kTref and the higher that of the activation energy. The

decrease in the value of kTrefis due the fact that higher molecular weights led to lower evaporation rates

at lower temperature. The process associated with mass loss is the thermo-vaporization and activation

energy is defined as the amount of energy required for process to occur. The difference in the values

for the activation energy and kTref is also related to molecular weight: the higher is the molecular weight

the higher is the value of activation energy and the lower is the value of kTref.

In the experiments carried-out with mixtures of two hydrocarbons impregnated in the rock, Raoult's

law was introduced in the mathematical model to allow for the changes in vapour pressure due to

dilution, and these tests were used to verify the values calculated of kTref and for the activation energy,

as well as if the process could be adequately described by a first-order Arrhenius law. The results

obtained indicated that the model used was able to describe the evaporation of the binary mixtures.

In the experiments carried-out with mixtures of three different hydrocarbons impregnated in the rock

the objective was to determine if the composition of the mixture in the rock, containing Decane,

Dodecane and Hexadecane could be estimated using the values of kTref and the activation energy

calculated previously for the mixtures of two hydrocarbon. Again the results were positive and the model

was able to adequately describe the evaporation process.

The same procedure was applied to mixtures with four different hydrocarbons impregnated in the

rock, and with the same objective. Again it was concluded that the mathematical model was able to

obtain very good estimates. Thus it was concluded that the mathematical model provides a good method

to the composition of the mixtures with four hydrocarbons.

In the lastly set of tests five hydrocarbons were simultaneously impregnated in the rock, the last

hydrocarbon being squalene. The results obtained were not as good probably due to the fact that

squalene is a very heavy component that is evaporated right at the end of the heating ramp for the

desorption of the hydrocarbons. This is consistent with the observation that when mixtures of two

hydrocarbons including Squalane and another hydrocarbon, the end of Squalane mass loss is very close

46

to 300°C and at this temperature, not only occurs thermo-vaporization also occurring thermal

degradation, due to the boiling point equal to 500.3ºC.

In this work it is demonstrated that the concept of deconvolution applied to the data obtained in the

evaporation of hydrocarbons from a rock can be a useful tool to estimate the composition of the oil

present in the rock. This deconvolution requires a numerical model to be used and adequate calibration.

Furthermore, this methodology can be added in an almost seamless way to the Rock-Eval technique,

adding value to the analysis.

47

6. Future Work

With the work done in this thesis, it was concluded that the mathematical model can estimate the

composition of oil mixtures that are evaporated from the rocks during the Rock-Eval process The

concept was well demonstrated for unsaturated straight chain hydrocarbons although the quality of the

description was not was good for the only ramified hydrocarbon that was used (squalene) a problem

that might be associated with the high boiling point of the latter. Future work to be considered could

consist of the inclusion of a larger range of hydrocarbons, with carbon chains between 10 and 30 carbon

atoms and consequently, more complex mixtures, incorporating also hydrocarbons of different nature,

like aromatics, in order to make the model more robust.

Additionally the model can be applied to already existing data from real source or reservoir rocks for

which Rock-Eval and composition data is available to allow the correct calibration of the methodology

under real-world conditions. Other future work can be the change of rocky matrix for example for the

sandstone because as it was written sandstone represent half of the reservoir rocks in the world.

48


49

7. References

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[2] A.-Y. Huc, Geochemistry of fossil fuels - From Conventional to Unconventional

Hydrocarbon Systems, Paris: Editions TECHNIP, 2013.

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[Accessed 27 04 2018].

[4] A. G. Collins, Geochemistry of oilfield waters - Developments in Petroleum Science 1, New

York: Elsevier Scientific Publishing Company, 1975.

[5] Geolog Surface Logging, “Geosource - Real-time Total Organic Carbon (TOC) and

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[7] H. H. Schobert, Chemistry of Fossil Fuels and Biofuels, New York: Cambridge University

Press, 2013.

[8] Perkin Elmer, “The beginners Guide of thermogravimetric analysis (TGA),” Waltham.

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source rock samples”.

[11] B. Tissot and D.H.welte, Petroleum Formation and Occurrence, New York: Springer-Verlag

Berlin Heidelberg GmbH, 1984.

[12] J. G. M. Filho, “Geoquímica do Petróleo,” Universidade Federal do Rio de Janeiro, Rio de

Janeiro.

[13] G. J. Herndl and T. Reinthaler, “Microbial control of the dark end of the biological pump,”

Nature of geoscience, 29 August 2013.

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[17] I. Breger, Kerogen McGraw Hill encyclopedia of science and technology, McGraw: New

York, 1961.

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2007.

50

[19] D. v. Klevelen, “Graphical-Statistical method for the study of structure and reaction process

of coal,” Fuel, 1950.

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sc.ft.ugm.ac.id/w/source-rock-and-kerogen/. [Accessed 8 May 2018].

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Lenigrad: Gostoptekhizdat, 1957.

[22] P. K. Singh, “Peat swamps at Giral lignite field of Barmer basin, Rajasthan, Western India:

understanding the evolution through petrological modelling,” 22 July 2016.

[23] B. R. T. Simoneit, “Biomarkers (molecular fossils) as geochemical indicators of life,”

Elsevier, 23 April 2003.

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of diagenetically transformed carotenoids,” Philosophical transactions of Royal Society, 2010.

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science and technology, Rueil-Malmaison, Éditions Technip, 2003.

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et de leur degré d’évolution,” Oil & Gas Science of Technology, 1977.

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and Developments,” in Oil & Gas Science and Technology , Rueil-Malmaison , Éditions

Technip, 2001.

[28] E. Lafargue, F. Marquis and D. Pillot, “Rock-Eval 6 Applications in Hydrocabon exploration,

Production and soil contamination studies,” in Revue de l´Institut Français du Pétrole, Rueil-

Malmaison, 1998.

[29] “Oilfield Glossary,” Schlumberger, [Online]. Available:

http://www.glossary.oilfield.slb.com/Terms/t/toc.aspx. [Accessed 12 May 2018].

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2018].

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shales of the Dakhla and Duwi formations in Abu Tartur area, Western Desert, Egypt:

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evaluation of soils,” Analysis, 1997.

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51

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52


a

Appendix A

Formulas of calculated parameters of Rock-Eval 6

Table 9 – Formulas of calculated parameters of Rock-Eval 6

Calculated parameters Formula

Tmax TpS2 − ΔTmax

PI 𝑆1

𝑆1 + 𝑆2

PC [(S1 + S2) × 0.83] + [S3 ×

1244

] + [(S3CO +S3`CO

2) ∗

1228

]

10

RC CO S4CO ×

1228

10

RC CO2 S4CO2 ×

1244

10

RC RC CO + RC CO2

TOC PC + RC

S1/TOC S1 × 100

TOC

HI S2 × 100

TOC

OI S3 × 100

TOC

OI CO S3CO × 100

TOC

PyroMinC [S3` ×

1244

] + [(S3`CO

2) ×

1228

]

10

OxiMinC S5 ×

1244

10

MinC PyroMinC + OxiMinC

b

Appendix B

Specifications of hydrocarbons

Table 10 - specifications of the Hydrocarbons

Compound name

Molecular form

boiling point (°C)

Molecular weight

Density (g/mL)

% Purity Brand

n-Decane C10H22 174.1 142.28 0.73 ≥94 Merck KGaA

Dodecane C16H26 216.2 170.34 0.75 ≥99 VWR Chemicals

Hexadecane C12H34 286.8 226.44 0.77 ≥99 CARLO ERBA

Icosane C20H42 342.7 282.55 0.79 ≥99 Sigma Aldrich

Squalane C30H62 500.3 422.81 0.81 ≥99 Merck KGaA

c

Appendix C

Chromatogram of mixture Decane. Dodecane. Hexadecane and Eicosane

Figure 47 - Chromatogram of mixture Decane. Dodecane. Hexadecane and Eicosane

d

Report of the chromatographic analysis of the mixture Decane,

Dodecane, Hexadecane and Eicosane.

Table 11 – Peak of Chromatographic analysis of the mixture Decane, Dodecane, Hexadecane and Eicosane.

Peak Time (min) Area (μV.s) Height (μV) Area (%) Norm. Area (%) BL Area/Height (s)

1 7.047 1468848 431695.49 20.29 20.3 BE 3.4025

2 10.215 1846114 486729.37 25.50 25.5 BB 3.7929

3 15.476 2311812 443183.89 31.94 31.9 VV 5.2164

4 24.045 1612436 132627.78 22.27 20.3 BB 12.1576

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