Lafargue e Marquis

LES APPLICATIONS DE ROCK-EVAL 6 DANSL'EXPLORATION ET LA PRODUCTION DESHYDROCARBURES, ET DANS LES �TUDES DECONTAMINATION DES SOLS

Le succ�s d'une exploration p�troli�re repose sur l'analysed�taill�e du syst�me p�trolier dans une zone donn�e.L'identification des roches m�res potentielles, la d�termination deleur maturit�, de leurs param�tres cin�tiques et de leur r�partitionsont r�alis�es au mieux � partir d'examens rapides d'�chantillonsde roches (carottes ou d�blais) au moyen de la pyrolyse Rock-Eval. Cette technique a �t� utilis�e en routine pendant unequinzaine d'ann�es et elle est devenue un outil standard pourl'exploration des hydrocarbures.

Cet article d�crit comment les nouvelles fonctionnalit�s de laderni�re version de l'appareil Rock-Eval (Rock-Eval 6) ont permisune expansion des applications de la m�thode en g�osciencesp�troli�res. Des exemples d'applications nouvelles sont illustr�sdans les domaines de la caract�risation des roches m�res, de lag�ochimie de r�servoir et des �tudes environnementales incluantla quantification et la description des hydrocarbures dans des solscontamin�s.

ROCK-EVAL 6 APPLICATIONS IN HYDROCARBONEXPLORATION, PRODUCTION, AND SOILCONTAMINATION STUDIES

Successful petroleum exploration relies on detailed analysis of thepetroleum system in a given area. Identification of potential sourcerocks, their maturity and kinetic parameters, and their regionaldistribution are best accomplished by rapid screening of rocksamples (cores and/or cuttings) using the Rock-Eval apparatus.The technique has been routinely used for about fifteen years andhas become a standard tool for hydrocarbon exploration.

This paper describes how the new functions of the latest version ofRock-Eval (Rock-Eval 6) have expanded applications of themethod in petroleum geoscience. Examples of new applicationsare illustrated for source rock characterization, reservoirgeochemistry, and environmental studies, including quantificationand typing of hydrocarbons in contaminated soils.

REVUE DE L’INSTITUT FRANÇAIS DU PÉTROLEVOL. 53, N° 4, JUILLET-AOÛT 1998

421

ROCK-EVAL 6 APPLICATIONS INHYDROCARBON EXPLORATION,PRODUCTION, AND SOILCONTAMINATION STUDIES

E. LAFARGUE, F. MARQUIS and D. PILLOTInstitut français du pétrole1

(1) 1 et 4, avenue de Bois-Pr�au,92852 Rueil-Malmaison Cedex - France

bihorel

copyright C 1998, Institut Français du Pétrole

http://www.ifp.fr/

http://ogst.ifp.fr/

LAS APLICACIONES DE ROCK-EVAL 6 EN LAEXPLORACIîN Y LA PRODUCCIîN DE LOSHIDROCARBUROS, Y EN LOS ESTUDIOS DECONTAMINACIîN DE LOS SUELOS

El �xito de una exploraci�n petrolera se funda en el an�lisisdetallado del sistema de hidrocarburos en un �rea determinada.La identificaci�n de las rocas madre potenciales, la determinaci�nde su grado de madurez, de sus par�metros cin�ticos y de sudistribuci�n, se llevan a cabo de mejor modo tomando como puntode partida ex�menes r�pidos de muestras de rocas (testigos oresiduos de perforaci�n) por medio de la pir�lisis Rock-Eval. Seha aplicado esta t�cnica en rutina durante unos quince a�os y hallegado a ser una herramienta est�ndar para la exploraci�n de loshidrocarburos.

Se describe en este art�culo c�mo las nuevas funcionalidades dela �ltima versi�n del aparato Rock-Eval (Rock-Eval 6) hanpermitido una expansi�n de las aplicaciones en geocienciaspetroleras. Se ilustran diversos ejemplos de nuevas aplicacionesen los sectores de la caracterizaci�n de las rocas madre, de lageoqu�mica de yacimiento y de los estudios medioambientales enque se incluyen la cuantificaci�n y la descripci�n de loshidrocarburos en los suelos contaminados.

INTRODUCTION

Characterizing the organic matter from sedimentaryrocks is one of the main objectives of organicgeochemistry and is now widely recognized as a criticalstep in the evaluation of the hydrocarbon potential of aprospect. During the last two decades, various authors(e.g., Barker, 1974; Claypool and Reed, 1976; Espitaliéet al., 1977 and 1984; Clementz et al., 1979; Larter andDouglas, 1980; Horsfield, 1985; Peters and Simoneit,1982; Peters, 1986) have used pyrolysis methods toprovide data on the potential, maturity and type of thesource rocks in different sedimentary basins.

Among these techniques, Rock-Eval pyrolysis hasbeen widely used in the industry as a standard methodin petroleum exploration. This technique usestemperature programmed heating of a small amount ofrock (100 mg) in an inert atmosphere (helium ornitrogen) so as to determine (Fig. 1): the quantity offree hydrocarbons present in the sample (S1 peak) andthe amount of hydrocarbons and compounds containingoxygen (CO2) that are produced during the thermalcracking of the insoluble organic matter (kerogen) inthe rock (S2 and S3 peaks respectively).

Furthermore, the Total Organic Carbon (TOC)content of the rock is determined by oxidation underair, in a second oven, of the residual organic carbonafter pyrolysis (S4 peak). The method has not evolvedmuch over the years. However, the new Rock-Eval 6apparatus incorporates major changes in the method-ology. As a consequence, new scientific applications ofthe method are proposed.

As we will see in the following, some of theseapplications are new and therefore expand the fields ofuse of Rock-Eval methodology, while others aremainly improvements of the existing method. In thisrespect, we will first present the Rock-Eval 6 functionsand we will then detail their different applications (useof mineral carbon determination in carbonatesequences, application of new oxygen indices,application of high temperature Tmax (temperaturemeasured at the top of the S2 peak), and use of theimproved total organic carbon (TOC) determination).Finally, we will illustrate more specific applications ofRock-Eval 6 data in reservoir geochemistry and in soilcontamination studies.

ROCK-EVAL 6 APPLICATIONSIN HYDROCARBON EXPLORATION, PRODUCTION, AND SOIL CONTAMINATION STUDIES


422

1 METHODOLOGY: ROCK-EVAL 6 NEWFUNCTIONS

A number of technical changes have been introducedin Rock-Eval 6. They lead to the new functionalitiesdescribed in the next paragraphs.

1.1 New Temperature Range for thePyrolysis and Oxidation Ovens

In Rock-Eval 6, programmed heating of both thepyrolysis and the oxidation ovens is conducted from100°C (instead of 180°C in the previous versions) up to850°C (instead of 600°C in the previous versions).

Pyrolysis at temperatures up to 850°C is necessaryfor complete thermal degradation of terrestrial type IIIorganic matter. Previous studies have shown thatthermal cracking of terrestrial organic matter is notalways complete at 600°C and therefore thecorresponding S2 peak can be underestimated. Theconsequence is a possible underestimation of thepetroleum potential for type III source rocks. Byincreasing the maximum pyrolysis temperature inRock-Eval 6, the measure of the hydrogen index(HI = S2/TOC) is more accurate and the range ofvalidity of Tmax is extended to higher values.

Another important outcome of this new functionalityis a better determination of kinetic parameters for

type III organic matter. For such samples, wefrequently observed pyrolysis curves that did not returnto the baseline when using the low heating rates. Thisexperimental problem was a source of uncertainty inthe determination of the kinetic parameters for type IIIsource rocks.

The oxidation temperature of the sample has alsobeen increased (up to 850°C) in order to avoidincomplete combustion of refractory material, such asheavy oils, tar-mats, coke or pyrobitumen in sourcerocks or reservoir rocks, or polyaromatic hydrocarbons(PAH) in soils from polluted sites. This newfunctionality has resulted in a better TOC determination.As we will see later, high temperature oxidation alsoallows mineral carbon determination, which is one ofthe main improvements provided by Rock-Eval 6.

The initial pyrolysis temperature has been reduced to100°C instead of 180°C to allow detailed study of thefree hydrocarbons in rocks. This is particularly useful forthe analysis of cuttings on site (oil shows during drilling),for reservoir studies (better evaluation of API gravityfrom Rock-Eval data) and especially for soilcontamination studies where contaminants are often verylight hydrocarbon compounds (gasoline, diesel oil, jetfuel). In conjunction with this lower temperature, it isnow possible to select the temperature program in orderto optimize the separation of these light compounds, thusapproaching a separation according to bubble points.

ROCK-EVAL 6 APPLICATIONS IN HYDROCARBON EXPLORATION, PRODUCTION, AND SOIL CONTAMINATION STUDIES


423

TotalOrganicMatter

Free HC

Kerogen

Volatilization

Pyrolysis

OxidationResidualFraction

PyrolyzedFraction

Oil

Gas

Organic matter fractions analyzed RecordsParameters

S1

S4

S2

S3

Tmax

S1 S2 S3 S4

Gas+

OilHC

CompoundsCO2

Pyrol.CO2Oxid.

Tmax

Figure 1

General diagram showing the different fractions of the total organic matter of analyzed rocks, the corresponding parameters and their recordings.

Furthermore, the location of the thermocouple thatdrives the ovens has been changed. In Rock-Eval 6, thethermocouple is now located in the crucibleimmediately below the sample (Fig. 2) instead ofagainst the oven wall as it was in Rock-Eval 2.

Therefore, the measured temperature is very close tothe real sample temperature. As a result, the temperaturemeasurement is more accurate. As illustrated inFigure 2, the shape of the crucible has also been

Figure 2

Schematic diagram indicating the location of the thermocouplefor temperature regulation in the pyrolysis oven.

changed in order to accommodate the thermocouple(TC1). The second thermocouple (TC2) is used to ensurecontinuity of temperature control when the crucible isremoved from the pyrolysis oven at the end of theanalysis. The same is true for the oxidation oven.

1.2 Continuous On-Line Detection of COand CO2 with Infra-Red Detectors

For earlier versions of Rock-Eval, the CO2 releasedduring pyrolysis was trapped and quantified using acatharometer. Although the S3 peak quantifies the totalCO2 yield, it does not reflect its temperature-programmed release during kerogen cracking. Thislimits interpretation of the S3 response. Similarly duringoxidation, the CO2 released was trapped at 600°C andthen quantified by the catharometer. For practicalreasons, CO was not analyzed during pyrolysis oroxidation.

These limitations have led us to add sensitive infrareddetectors to Rock-Eval 6, that are capable of continuouson-line recording of the amount of both CO and CO2released during pyrolysis and oxidation of samples. As aresult, five characteristic curves are obtained for eachsample analyzed using Rock-Eval 6 (Fig. 3): (1) theamount of hydrocarbons (flame ionization detector, FIDtrace of S1 and S2 peaks), (2–3) CO and CO2 producedduring pyrolysis (CO and CO2 pyrolysis curves), (4–5)CO and CO2 produced during oxidation (CO and CO2oxidation curves). The sixth curve in Figure 3 is thesuperposition of CO and CO2 curves and is very usefulfor a rapid visualization of the presence or absence ofcarbonate in the sample.

As shown in following results, continuous recordingof both CO and CO2 has revealed a number of usefulnew parameters. For instance, the character of CO andCO2 pyrograms produced can be used to distinguish thetypes of kerogen in rock samples. The different peakson the pyrograms represent different classes offunctional groups containing oxygen (e.g. ethers,carboxyls, carbonyls) which vary from one organicmatter type to another. This approach has already beensuccessfully used for CO pyrograms from immature toearly mature source rocks (Daly and Peters; 1982). Thisnew functionality allows measurement of the mineralcarbon in the rock in addition to the classical organiccarbon determination. Furthermore, mineral typedetermination (e.g. calcite, dolomite, siderite) ispossible.

PROGRAMMER

FID

Crucible

Sample

Furnace

N2

Piston

Furnace open: temperature controlled by TC2

Furnace closed: temperature controlled by TC1

TC2

TC1



424

1.3 Processing of Multi-Heating RatesCycles

Because of new applications of the Rock-Evalmethodology to reservoir geochemistry and soilcontamination studies, it was necessary to allowenough flexibility in the apparatus to account for thevariety and the specificity of the hydrocarbons to bedetected and quantified. For example, in soilcontamination studies, the residence time of the S1peak and the starting heating rates are critical for agood determination of pollutants. For Rock-Eval 6,there is now a true “temperature versus time”function, meaning that initial temperatures, heatingrates and residence times can be adjusted by the user for any specific application. In addition, anumber of pre-programmed cycles are available thathave already been successfully tested for soil

contamination studies and for reservoir geochemistry(e.g., detection of tar-mats).

2 RESULTS

2.1 Mineral Carbon Determination

The amount of mineral carbon is calculated byaddition of the CO2 released during pyrolysis above400°C and the CO2 from carbonate decompositionduring the oxidation phase from 650°C to 850°C(Fig. 4). Carbonate minerals such as magnesite andsiderite begin to decompose when the pyrolysistemperature approaches 400°C whereas dolomite andcalcite (the most important minerals in carbonatesequences) decompose during oxidation (Fig. 5). In thisfigure, we can observe that dolomite oxidative



425

Figure 3

Illustration of Rock-Eval 6 standard recordings for hydrocarbons, CO and CO2 during the pyrolysis and oxidation phases.

decomposition gives two characteristic peaks corres-ponding to the following reactions:

CaMg(CO3)2 ® Ca CO3 + MgO + CO2(1)

and at higher temperature

CaCO3 ® CaO + CO2 (2)

Calcite decomposition occurs at about 800°C and isindicated by a single peak.

Because of this sequential decomposition of thecarbonate minerals during the analytical cycle, it ispossible to quantify the amount of mineral carbon andestimate which species is dominant in the sample.

Although it is less abundant than calcite anddolomite, siderite is quite common in source rocks,particularly in fermentation zones where thepreservation of organic matter is enhanced (Curtis,1977; Berner, 1981). Its range of decompositiontemperature is large, from 400°C to 650°C, andtherefore it can interfere, during pyrolysis, with the CO2



426

Min C (%) = ( S3b+ S5) x 12/440

Programmed Temperature (T peak¡C)

� �

S2

S1

HC

OXIDATIONPYROLYSIS

CO

CO

CO2

CO2S3b

S5

S'3a

400 400500 500600 600

650

700 700 800

400

300200

Figure 4

Principle of determining mineral carbon content (MINC; %) using CO2 pyrolysis and oxidation curves.

released by the oxygenated functions in the organicmatter. In Rock-Eval 6, thanks to the continuous on-linerecording of CO and CO2 released during pyrolysis, itseems that the combination of S3CO, S3CO2 values andT peak CO2 (temperature at the maximum of CO2pyrolysis curve; “Tmax CO2”) on bulk rocks can pointout very rapidly when and where siderite is occurring.

Samples with siderite present S3CO values greater than5 mg CO/g rock, S3CO2 greater than 10 mg CO2/g rockand T peak CO2 in the 485–520°C temperature range.As a result, we have a very good indicator of thepresence of siderite in source rock samples which inturn may be used as a redox indicator of the wholeseries (siderite occurs only in specific environments).



427

Dolomite

Calcite

Siderite

Bicarbonate

300¡C 400¡C 500¡C 600¡C 700¡C 800¡C 900¡C

Temperature

OXIDATION

PYROLYSIS

Figure 5

Rock-Eval 6 response of different types of carbonate minerals illustrating their progressive decomposition with increasing temperature.

Furthermore, the comparison of a series of bulk rocksand acid-treated rocks indicated that, when a great dealof siderite is present, we can observe the production ofCO during pyrolysis according to the following set ofchemical reactions:

FeCO3 ® FeO + CO2(1)

3FeO + CO2 ® Fe3O4 + CO (2)

Once siderite is removed by acid treatment, the COproduction stops, resulting in small values of S3CO(organic CO alone).

Mineral carbon content based on Rock-Eval 6analysis correlates well with that obtained by routinechemical attack techniques throughout the rangeobserved in most sedimentary rocks (0 –12%; Fig. 6).

This rapid estimate of both the organic and mineralcarbon is very useful in studies of the relationshipbetween organic matter and mineral carbon incarbonate petroleum systems. Such work has beensuccessfully conducted recently by van Buchem et al.(1996) on the Upper Devonian mixed carbonate-siliciclastic system of western Canada. These authorsshow a distinct pattern in plots of the carbonate versusorganic carbon content within each sequence, each

being characteristic of the equilibrium betweencarbonate precipitation, organic matter dilution, andoccurrence of dysaerobic conditions.

2.2 Organic Carbon Determination

Like previous Rock-Eval systems, Rock-Eval 6determines the amount of organic carbon by addingpyrolyzed carbon (PC) and residual carbon (RC).Nevertheless, the continuous CO and CO2 detection hasimproved PC and RC quantification (Fig. 7).

Pyrolyzed carbon is computed from:– the hydrocarbon compounds released in peaks S1 and

S2, assuming that they contain about 83% of theorganic carbon;

– the CO released during pyrolysis up to 500°C (S'3apeak);

– the CO2 released during pyrolysis up to 400°C (S3apeak). To avoid interference from the release ofpyrolytic CO2 from carbonate minerals such assiderite, only the first part of the CO2 pyrolysis curveis taken into account. Likewise, in order to avoidpossible interference caused by Boudouard'sreaction, where the CO2 released early duringcarbonate decomposition can react with the residualcarbon to produce CO (CO2 + C ® 2 CO), we limit



428

Conventional mineral C (% weight)

Roc

k-E

val 6

min

eral

C (

%w

eigh

t)

00

2

2

4

4

6

6

8

8

10

10

12

12

Figure 6

Comparison of mineral carbon determinationusing Rock-Eval 6 and acid attack.

the calculation for CO to a temperature of 500°C.The residual carbon is obtained during the oxidationphase by summing the organic carbon oxidized intoCO (S4' peak) and CO2 up to 650°C (S4 peak). Athigher temperatures, there is no more CO productionand the remaining CO2 comes generally from thedecomposition of carbonates. The progress made in organic carbon determination

has brought higher performance as a whole (Fig. 8;

comparison between TOC measurements using Rock-Eval 6 and LECO), but this is particularly true whendealing with coal samples where Rock-Eval 2 responsewas often poor. The organic carbon now measured formature coal samples (Vitrinite Reflectance (Ro) > 2%) isdrastically improved since high temperature oxidationallows complete combustion of the organic matter, thusproviding the true residual carbon in the sample (Fig. 9).



429

TOC = PC + RC


S2

S1

HC

OXIDATIONPYROLYSIS

CO

CO

CO2

CO2

S3aS4

S4'

S'3a

400 400500 500600 600

650

700 700 800

400

300200

REMAINING C (RC)

RC (%) = (S4 x 12/440) + (S'4 x 12/280)PC (%) = (S1+ S2) x 0.083

+ (S3a x 12/440)

+ (S'3a x 12/280)

PYROLYSED C (PC)

Figure 7

Principle of determining total organic carbon content (TOC; %) using FID, CO2 pyrolysis and oxidation curves to calculate the amounts ofpyrolyzed carbon (PC) and residual carbon (RC).

2.3 Oxygen Index Determination

In the previous Rock-Eval apparatus, the oxygenindex (OI in mgCO2/g TOC) is calculated from theamount of CO2 released and trapped over the 300°C to390°C pyrolysis range. For Rock-Eval 6, both CO andCO2 are continuously monitored during theprogrammed pyrolysis of a rock sample. From thesecurves, oxygen indices specific to CO2 (OICO2

) and toCO (OICO) can be defined (Fig. 10). The combinationof these two values gives the true oxygen index of thesample (OIRE6).

This improvement in the calculation of the oxygenindex allows the use of the OICO2/ OICO ratio and theOIRE6, to estimate changes during source rockdeposition. For instance, on a series of source rocksamples from the Bucomazi formation (Fig. 11), wewere able to differentiate between “basin fill” versus“sheet drape” geological assemblages where theorgano-facies variations depend upon paleoen-vironment of deposition (Lafargue and Burwood,1997). These differences were also illustrated by the

CO and CO2 pyrolysis curves that vary from onedepositional environment to the other thus indicatingvariations in the chemical composition of the organicmatter (Fig. 12).

This modification of the apparatus improves itsapplication to studies of organic facies variations inrecent sediments, where the organic matter is rich inoxygenated functions.

2.4 Residual Petroleum Potential (S2)and High Temperature Tmax

For Rock-Eval 6, the final pyrolysis temperature is850°C instead of 600°C. As a consequence, thepetroleum potential of many type III rock samples isincreased and the resulting hydrogen index is morerepresentative. Another improvement of this hightemperature pyrolysis is the ability to measure highTmax values for type III samples with vitrinitereflectance values (Ro) greater than 2%. Figure 13shows differences between the S2 peaks obtained byRock-Eval 2 and Rock-Eval 6 for coal samples with a



430

20 40 60 80 100

0

25

50

75

100

0

TOC measured with LECO

TOC

mea

sure

d w

ith R

ock-

Eva

l

Rock-Eval 2Rock-Eval 6

Coal Samples

(R0 = 2.50)

(R0 = 3.25)

(R0 = 4.00)

0 1 2 3 4 50

1

2

3

4

5

Figure 8

Comparison of organic carbon determination using Rock-Eval 6 and LECO.

large range of maturities. The figure shows that thevalue of S2 changes significantly and that Tmax valuesover 600°C are significant and are not an artifact of themeasurement caused by very small S2 peaks. Thecorrelation between Tmax and Ro (Fig. 14) is thusextended to the very mature samples, which was notpossible with the previous apparatus.

In type III source rocks, small residual potential S2makes it difficult to know whether there is anyremaining gas potential. When such source rocks arelocated in thrusted zones, it is very important todetermine their relative maturities in different parts ofthe thrust in order to reconstruct the deformationkinematics of the basin. This reconstruction requiresmaturity measurements that were difficult to obtainwith previous versions of Rock-Eval when the sampleswere highly mature. In addition, determination of theresidual potential for gas and the role of coal maturity

on its retention properties is also of interest for theevaluation of coal bed methane (Scott et al., 1994).

3 ROCK-EVAL 6 ENLARGED APPLICATIONS

This section describes how the new functions of Rock-Eval 6 expand applications of the method in reservoirgeochemistry and in soil contamination studies.

3.1 Reservoir Geochemistry

Geochemistry of reservoirs is an area of growinginterest with remarkable economic importance becauseit can be used to evaluate reservoir continuity duringfield appraisal, to identify non-productive reservoirzones, and to analyze commingled oils for productionallocation calculations (e.g. Kaufman, 1990; England



431

Temperature (¡C)

IR S

igna

l (pp

m)

10

800700600500

15

10

5

400300

CO2

CO2

CO2

CO

CO

CO

TOCRE6 = 53%

TOCRE2 = 56%

R0 = 2.2TOCRE6 = 92%TOCRE2 = 72%

R0 = 4.0

R0 = 0.5

TOCRE6 = 96%

TOCRE2 = 19%

15

10

5

15

5

Figure 9

Examples of oxidation curves for a series of coal samples at increasing maturity. The resulting TOC calculation shows the differences betweenRE6 and RE2 measurements.

and Cubitt, 1995). The Rock-Eval method has alreadybeen successfully applied in reservoir geochemistry,especially for the detection of tar-mats and for theprediction of oil API gravities (Trabelsi et al., 1994).For reservoir geochemistry, the main advantage ofRock-Eval 6 is its capability to perform both pyrolysisand oxidation of the sample up to 850°C at variousrates.

Figure 15 is an example of Rock-Eval 6 results forfour different reservoir rocks: three sandstones and onecarbonate. The first sample represents a conventional

oil accumulation that produces a large S1 peak and asmaller S2 peak. Almost no CO and CO2 are generatedduring oxidation.

The second sample shows a small S1 peak, a bimodalS2 peak (small S2a and large S2b) and significantamounts of CO and CO2 released during oxidation.This sample is from a conventional tar-mat, i.e. areservoir rock accumulated with crude oil enriched inresins and asphaltenes. For the last sandstone reservoir,we still observe the S1, S2a and S2b peaks but we alsoobserve an important CO2 peak produced during



432

OI (CO2)� = S 3a x 100/TOC

OI (CO)� = S' 3a x 100/TOC

OI RE6� = OI (CO2) x 32/44 + OI (CO) x 16/28


S2

S1

HC

OXIDATIONPYROLYSIS

CO

CO

CO2CO2

S3a

S4

S'3a

400 400500 500600 600700 700 800

400

300200

Figure 10

Principle of determining the oxygen indices using CO2 and CO pyrolysis curves.

oxidation at high temperature (near 800°C). Since wealso observed this peak in the rock sample afterdecarbonatation, it cannot be caused by carbonatedecomposition of minerals in the sandstone matrix.This peak corresponds to the combustion of refractorymaterial associated with pyrobitumen in the sample.Therefore it seems possible, from the comparison of theoxidation curves of two tar-mat levels, to distinguish aconventional tar-mat deposited in the reservoir from apyrobitumen produced by in-place secondary crackingof an oil accumulation. This distinction is veryimportant since it can guide exploration and productionstrategies in oil fields with tar-mats.

The carbonate reservoir sample shows the samepattern in the FID trace as that observed for theconventional tar-mat in the sandstone reservoir. This istypical of the large amounts of resins and asphaltenespresent in the rock. During the oxidation phase of thesample, the strong CO2 production at about 500°C andthe CO production correspond to the residual organiccarbon, whereas the very large amount of CO2produced at higher temperature corresponds to thedecomposition of the carbonate matrix.

3.2 Soil Contamination Studies

Due to its economic, environmental and industrialimportance, the characterization of soils contaminatedby hydrocarbons is another area where research is veryactive. Rock-Eval 6 expands the application ofpyrolysis methodology to oil-contaminated sites bymaking it possible to start the analysis at lowtemperature (100°C). Furthermore, heating rates can beadjusted so as to release the different petroleum cuts(e.g., gasoline, diesel oil, heavy oils, lubricant oils, gasplant distillation residues). In this application (calledPollut-Eval), the vaporized hydrocarbons are identifiedby the FID and the signal is integrated for fullquantification. A complete carbon mass balance is thencarried out through oxidation of the residue andcontinuous quantification of CO2 by the infrareddetectors.

For these types of studies, the apparatus is equippedwith a cooled autosampler that reduces the loss of lightcompounds. The equipment thus provides theparameters needed to characterize a contaminated site:what pollutant, how much and where? Due to the shortduration of the analysis (30 min), the time needed to



433

9000

9500

10000

10500

0 500 0 20

8000

8500

9000

9500

0 500 0 20

Dep

th (f

eet)

7000

7500

8000

8500

9000

0 500 0 20

OIRE6mg O2/g TOC

OICO2/OICO

OIRE6mg O2/g TOC

OICO2/OICO

OIRE6mg O2/g TOC

OICO2/OICO

Sheet Drape Assemblage

Basin Fill Assemblage

Figure 11

Correlation of Rock-Eval 6 oxygen indices across different wells of the Lower Congo Basin illustrating the transition from basin fill to sheetdrape assemblages.

evaluate the extent of a contaminated site is drasticallydecreased compared to routine techniques that involvethe extraction of the pollutant prior to its analysis bychromatography, infrared or chromatography-massspectrometry. Furthermore, when applied on site, themeasurements can be used to optimize the drillingprogram (Ducreux et al., 1997).

Rock-Eval 6 data can be correlated to standardenvironmental data such as infrared response. They arealso complementary to infrared or gas chromatographicanalyses because they allow rapid screening of a largenumber of samples, thus helping to identify the samplesthat are worthy of additional study. An example of the

application of Rock-Eval 6 data for two industrial sitesis presented in Figure 16. The upper part of the figureshows contamination by diesel oil and polyaromatichydrocarbons in a soil near an old gas plant. The FIDtrace indicates two main peaks corresponding to themixture of these hydrocarbons and the CO and CO2traces during oxidation are characteristic of thecombustion of the heavy residue accompanying theseproducts in the pollution. The second example is takenfrom contaminated soil near a service station. Lightgasoline-range compounds are released early duringpyrolysis and no significant amounts of CO and CO2are recorded during oxidation.



434

2.5 5.0 7.5 10.0 12.5 17.5 20.0 min

2.5

5.0

7.5

10.0

300 350 400 450 500 550 600 650 ¡C

2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 min 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 min

0.075

0.100

0.025

0.050

0.125

0.150

300 350 400 450 500 550 600 650 ¡C

0.075

0.100

0.025

0.050

0.125

0.150

300 350 400 450 500 550 600 650 ¡C

2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 min

0.25

0.50

0.75

1.00

1.25

1.50

300 350 400 450 500 550 600 650 ¡C

CO

2C

O

15.0

Sheet DrapeBasin Fill

Figure 12

CO2 and CO pyrolysis curves illustrating the differences in the composition of the organic matter in the basin fill and sheet drape organo-facies.



435


650

Tmax = 420¡C S2 = 115 mg/g

Tmax = 421¡C S2 = 116 mg/g

Tmax = 563¡C S2 = 9 mg/g

Tmax = 596 ¡C S2 = 1,3 mg/g

Tmax = 578¡C

S2 = 15 mg/g

Tmax = 653¡C

S2 = 6.5 mg/g

800 800700600400 500 400 600 700500

Rock-Eval 6Rock-Eval 2

FID

SIG

NA

L (m

V) 10

50

3

0

0

0

Figure 13

Illustration of the effect of high final pyrolysis temperature with Rock-Eval 6 on the calculation of the residual potential S2 and on themeasurement of Tmax.

Vitrinite reflectance (%)

T max

(¡C

)

0.6 0.8 1 1.6 1.8 2 2.2 2.4 2.6 2.8 31.41.2 3.2 3.4 3.6 3.8 4 4.2 4.4 4.6 4.8

700

680

580

420

440

460

480

500

520

540

560

600

620

640

660

Tmax RE2

Tmax RE6

Figure 14

Correlation between Tmax and Ro for Rock-Eval 6and Rock-Eval 2 on a series of coal samples.



436

OXIDATIONPYROLYSIS

5

10

15

5

10

15

5

10

15

5

10

15

200 300 400 500 600 700 800 400 500 600 700 800

FID

Sig

nal (

mV

)

IR S

igna

l (pp

m)

SA

ND

ST

ON

EC

AR

BO

NA

TE

CO CO2

CO2

CO

CO

CO2

CO2

CO

Conventionaloil

Tar Level

Tar Level

Pyrobitumen

Figure 15

Examples of applying Rock-Eval 6 to the study of reservoir rocks.


CO2

CO

CO2

CO

200 300 400 500 600 700 400

40

20

40

20

600500 700 800100

PYROLYSE OXIDATION

Sig

nal F

ID (

mV

)

Sig

nal I

R (

ppm

)

10

10

20

20

Diesel+ PAH

Gasoline

Figure 16

Examples of applying Rock-Eval 6 to the studyof contaminated soils.

CONCLUSIONS

The new Rock-Eval 6 pyroanalyser marks animportant step in the development of programmedpyrolysis systems. This apparatus provides newfunctions and parameters that expand applications ofthe technique in petroleum geoscience. Problems withthe older Rock-Eval systems have been reduced. Themajor improvements and their scientific impact can besummarized as follows:– Mineral carbon determination:

· improved characterization of marly/carbonatesource rocks;

· detection of carbonate types (e.g., siderite, calcite,dolomite);

· enhanced characterization of hydrocarbons incarbonate reservoirs;

· possible correction of matrix effects.

– Oxygen indices:· impact on source rock facies analysis;· impact on the knowledge of source rock

preservation conditions.

– Improved measurements of TOC and Tmax:· better analysis of type III source rocks;·. better analysis of heavy bitumen in reservoirs (tar-

mat studies);· better characterization of coals.

ACKNOWLEDGMENTS

The support granted by the European Community(Thermie Project OG/0146/93) for this project is greatlyappreciated. We also wish to thank A.Y. Huc, A. Mascle and K. Peters for their very helpful review ofa preliminary version of the manuscript.

REFERENCES

Barker C. (1974) Pyrolysis techniques for source rock evaluation.Amer. Assoc. Pet. Geol. Bull., 58, 2349-2361.

Berner R.A. (1981) A new geochemical classification ofsedimentary environments. J. Sed. Petrol. 51, 359-365.

Claypool G.E. and Reed P.R. (1976) Thermal analysis techniquefor source rock evaluation: Quantitative estimate of organicrichness and effects of lithologic variation. Am. Ass. Pet. Geol.Bull., 60, 608-626.

Clementz D.M., Demaison G.J. and Daly A.R. (1979) Well sitegeochemistry by programmed pyrolysis. Proceedings of the 11thAnnual Offshore Technology Conference, Houston, OTC 3410, 1,465-470.

Curtis C.D. (1977) Sedimentary geochemistry: environments andprocesses dominated by involvement of an aqueous phase. Phil.Trans. R. Soc. London. A., 286, 353-372.

Daly A.R. and Peters K.E. (1982) Continuous detection ofpyrolytic carbon monoxide: A rapid method for determiningsedimentary organic facies. Amer. Assoc. Pet. Geol. Bull., 66,2672-268.

Ducreux J., Lafargue E., Marquis F., Pillot D. and Bocard C.(1997) Use of Rock-Eval method for the evaluation of soilscontaminated by hydrocarbons. Analusis, 25, 9-10, 40-44.

England W. and Cubitt J. (1995) The geochemistry of reservoirs.Geol. Soc. Sp. Publ., 86.

Espitalie J., Laporte J. L., Madec M., Marquis F., Leplat P. andPaulet J. (1977) Méthode rapide de caractérisation des rochesmères, de leur potentiel pétrolier et de leur degré d’évolution.Rev. Inst. fr. petr., 32, 23-45.

Espitalie J., Marquis F. and Barsony I. (1984) Geochemicallogging: In Analytical Pyrolysis-Techniques and Applications, K.J. Voorhees ed., Boston, Butterworth, 276-304.

Horsfield B. (1985) Pyrolysis studies in petroleum exploration.In: Advances in Petroleum Geochemistry (eds. J. Brooks andD. Welte), 1, 247-298, Academic Press, New York.

Kaufman R., Ahmed A. and Elsinger R.J. (1990) Gaschromatography as a development and production tool forfingerprints in oils from individual reservoirs: Application in theGulf of Mexico. In: GC-SSEPM Foundation Ninth AnnualResearch Conference Proceedings, October 1, 1990, 263-282.

Lafargue E. and Burwood R.(1997) Improved source rockcharacterisation utilising Rock-Eval 6 derived Oxygen Indices.In: 18th Int. Meeting on Org. Geoch. Sept. 22-26, Maastricht, pp.727-728, Book of abstacts.

Larter S.R. and Douglas A.G. (1982) Pyrolysis methods inorganic geochemistry: An overview. J. Anal. Appl. Pyrol., 4,1–19.

Peters K.E. (1986) Guidelines for evaluating petroleum sourcerocks using programmed pyrolysis. AAPG Bull., 70, 329.

Peters K.E and Simoneit B.R.T (1982) Rock-Eval pyrolysis ofQuaternary sediments from Leg 64, sites 479 and 480, Gulf ofCalifornia. Initial Report of the Deep Sea Drilling Project., 64,925-93.

Scott A.R., Kaiser W.R. and Ayers W.B. Jr. (1984) Thermalmaturity of Fruitland coal and composition of FruitlandFormation and Pictured Cliffs sandstone. In: Coal Bed Methanein the Upper Cretaceous Fruitland Formation, San Juan Basin,New Mexico and Colorado, New Mexico and Colorado, Bull.146, 165-185.

Trabelsi K., Espitalie J. and Huc A.Y. (1994) Characterisation ofextra heavy oils and tar deposits by modified pyrolysis methods.In: European Symposium on Heavy Oil Technologies in a WiderEurope, Proceedings, June 7&8, 1994, 30-40.

Van Buchem F.S.P., Eberli G.P., Whalen M.T., Mountjoy E.W.and Homewood P. (1996) The basinal geochemical signature andplatform margin geometries in the Upper Devonian mixedcarbonate-siliciclastic system of western Canada. Bull. Soc. Geol.France, 167, 685-699.

Final manuscript received in April 1998



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