lirehe

k M

enttry,

ised

atography (L.C), and gas chromatography-mass spectrometry (GCMS)

polycyclic biomarkers to help the characterization of organicmatter type, depositional conditions and to assess the maturity 2. Geological setting

The Ypresian Metlaoui Fm successions crop out 40 km to the

Fuel Processing Technology 8level etc. Such source and physico-chemical relationship1. Introduction

The organic geochemical characterization of sedimentarysource rocks is typically based on various analytical techniques.Tow marked geochemical methods are Rock-Eval pyrolysis[1,2], and compound class given by liquid chromatography [35] are appropriate techniques for a good assessment ofgeochemical nature and characterization of source rockssedimentary organic matter.

In addition, GC/MS analysis has been popular in the past fewdecades, mainly because of its usefulness in detecting complex

features have made biomarkers invaluable to oil correlations.[610].

The main objective of this work is to attempt to understandthe organic matter type, its depositional environment andthermal maturity by confrontation of RE pyrolysis, L.C., andGCMS parameters at the Ypresian organic-rich carbonates ofJebel Chaker, which outcrop in western of Kairouan city, incentral-northern Tunisia (Figs. 1 and 2). These facies belongingto the Metlaoui Group, which shows a wide range of facies fromNorth to South [11], and have been intensively studied becauseof the commercial interest in phosphates and hydrocarbons.data from a geochemical study of Jebel Chaker organic-rich facies, in central-northern Tunisia, in order to obtain independent parameters onorganic matter source, composition, and thermal maturity.

This study shows a clear evidence of planktonic organic matter as indicated by the hydrogen index, n-alkanes distribution, predominance ofsaturated, and the high concentration of cholestane. The thermal maturity of Ypresian organic matter was estimated by Tmax, abundance of hetero(N.S.O) compounds, and sterane geochemical parameters such as C29 20S/ (20S+20R) and C29 (+) maturity ratios, to be of lowthermal maturity (end of diagenesisbeginning of catagenesis). These data reveal a close concordance between RE, L.C., and GC/MS data, andshow that these methods remain valuable and practical for geochemical characterization of sedimentary organic matter. 2007 Elsevier B.V. All rights reserved.

Keywords: Ypresian; Organic matter; Rock-Eval pyrolysis; GCMS; TunisiaThe present paper compares Rock-Eval pyrolysis (RE), liquid chromAbstractUsefulness of Rock-Eval pyrolysis,in the characterization of Yp

central-nort

Adel Arfaoui , Mabrou

University of Sfax, Sciences Faculty, DepartmLaboratory of Organic Geochemis

Received 11 December 2006; received in rev Corresponding author. Fax: +216 74 274 437.E-mail address: arfmon@yahoo.fr (A. Arfaoui).

0378-3820/$ - see front matter 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.fuproc.2007.05.004quid chromatography, and GC/MSsian Chaker organic matter,rn Tunisia

ontacer, Dorra Mehdi

of Earth Sciences, GEOGLOB: 03/UR/10-02,Po. Box, 802, 3038 Sfax, Tunisia

form 17 April 2007; accepted 9 May 2007

8 (2007) 959966www.elsevier.com/locate/fuprocWest of Kairouan (central-northern Tunisia; Fig. 1), in the centraland eastern parts of the Ypresian basin [11]. These successionsshow a wide facies variation from their northern to southern

ribu

960 A. Arfaoui et al. / Fuel Processing Technology 88 (2007) 959966Fig. 1. Paleogeographic map of Tunisia showing the distoutcrops and coeval with the main phosphatic facies (ChouabineFormation) that crop out extensively in western Tunisia [12].

In the eastern part of the Ypresian basin, outcrops of the JebelChaker limestone [13] are bounded by the Pelagian Platform to

Fig. 2. Location and morphostructural setting of Jetion of Ypresian facies (modified from Zar et al. [29]).the East, by the Jebel Ousselat to the West, and the Jebel Es-Sfea to the South (Fig. 2).

West of Kairouan, it exhibits a carbonate megasequence,which is bounded by Palaeocene clays (El Haria Formation) at

bel Chaker (modified from Rigane et al. [14]).

the base and Lutetian-Priabonian marls (Souar Formation) at thetop [1416].

3. Materials and methods

3.1. Materials of study

The organic-rich limestones have been sampled at the Jebel Chaker outcrops inan average amount of 5001000 g. The samples (28) are collected on account oftheir organicmatter richness (dark grey limestones). The geographic location of thecollected samples is shown in Fig. 2. Immediately after collection, all samples havebeen dried at 40 C. The sample was finely powdered prior to analyses.

3.2. Techniques of study

3.2.1. Rock-Eval pyrolysisRock-Eval analyses were performed on 08 selected samples. These analyses

were undertaken mainly by the Entreprise Tunisienne dActivits Ptrolire(E.T.A.P.) using Rock-Eval II instrument, according to Espitali et al. [1]procedure.

3.2.2. Bitumen extractionBitumen, from powdered sample (3040 g), was extracted with dichlor-

omethane (300400 cm3) for 1 h at 40 C. After filtration the solvent wasevaporated (rotary evaporator with water aspirator, evaporation temperature40 C). Then, the organic extracts (free oils or bitumen) were concentrated by

Table 1Results of rock-Eval analyses of Chaker samples

Samplinglocation

Sample TOC(wt.%)

Mg hydrocarbonper g of rock

HI OI PI Tmax(C)

S1 S2

J. Chaker JC-01 1.04 0.18 4.81 463 16 0.04 433JC-04 1.82 0.42 8.77 482 34 0.05 433JC-08 1.15 0.14 4.97 432 42 0.03 436JC-12 1.23 0.72 5.51 448 49 0.12 434JC-16 1.57 0.31 5.47 348 52 0.05 435JC-21 1.42 0.20 5.51 388 49 0.04 436JC-24 1.48 0.28 5.41 366 65 0.05 438JC-28 1.68 0.27 6.28 385 56 0.04 436Average 1.42 0.31 5.34 414 45.4 0.05 435.1

TOC: Total organic carbon (%); HI: hydrogen index (S2/TOC100); OI: oxygen index (mg CO2 sample/TOC100); PI: production index (S1/S1+S2); Tmax:maximum pyrolysis temperature.

961A. Arfaoui et al. / Fuel Processing Technology 88 (2007) 959966Fig. 3. Stratigraphic log of the Ypresian sequence in Jebel Chaker with location of thorganic matter rich limestones. Upper part II, grey limestone package.e samples of organic matter rich rocks. Lower part I: Predominant gray to dark

ngallowing the oilsolvent solution to stand at room temperature until the CH2Cl2was removed.

Fig. 4. HI versus OI crossplot referred to as pseudo-van Krevelen diagram withindications for diagenetic evolutionary pathways for Type -I, -II and -III organicmatter.962 A. Arfaoui et al. / Fuel Processi3.2.3. Liquid chromatographyOrganic extracts (C0) were separated via liquid-column chromatography on

alumina over silica gel. Aliphatic (F1), aromatic (F2) and polar (F3) fractions,were obtained. Only two fractions from bitumen, were eluted by hexane (F1),and a mixture of hexane/dichloromethane (65:35 Vol/Vol) (F2). F3 is deducedfollowing this equation: F3=C0 (F1+F2).

3.2.4. Gc/msAliphatic hydrocarbons were analysed by gas chromatography/mass spec-

trometryHP6890-HP 5973MSDcombination (Agilent Technologies,WilmingtonDE,USA). TheGCwas usedwith a 30m fused-silica column (0.25mm i.d.) coatedwith 5%phenylmethyl siloxane. Heliumwas used as the carrier gas at a flow rate of1.4mlmin1. The following temperature programmewas employed: 100290 Cwith ramping at 4 Cmin1. The samples were injected in the splitless mode withan injector temperature of 280 C. Samples were run in the electron impact mode at70 eV with a 2.9 s scan time over a 50550 a.m.u. range resolution. The GC/MSanalysis was based on fragmentograms atm/z 217 (steranes). The relative contentsof particular compounds were calculated from peak areas.

4. Results and discussion

4.1. Origin and depositional environment of organic matter

4.1.1. Rock-Eval pyrolysis dataIn the present study the thermovaporized free hydrocarbo-

naceous compounds (S1) present in the rock, are extremely lowranging from 0.14 to 0.72 mg hydrocarbon per gram of rock(Table 1). These low values suggest that: (i) few thehydrocarbons have been expelled from the source rocks dueto their low thermal maturity, (ii) the low hydrocarbonsrepresented by S1 may have migrated from kerogen.The (S2) are much higher ranging from 4.81 to 8.77 mghydrocarbon per gram of rock (Table 1), combining a significantpetroleum potential of Ypresian facies.

The hydrocarbon index (HI) values reached 482 mg HC/gTOC (Table 1). Such result suggests that the Ypresian organicmatter is of low thermal maturity and of Type-II [1,17].

Furthermore, it seems that no external contribution ofmigrated hydrocarbons was added to the Ypresian organic-rich facies as indicated by the low S1 values, and the productionindex (IP=S1 /S1+S2) (Fig. 3; Table 1).

In a standard HI/OI-discrimination diagram [1,2,4], a trendof decreasing HI and complementary increasing OI values(Fig. 4) is noted. Such distribution is, probably, reflecting theType-II organic matter with high plants contribution.

4.1.2. Abundance and composition of extracted hydrocarbonsThe results of the hydrocarbon extractions and their

chromatographic separations are shown in Fig. 5. The saturatedaliphatic hydrocarbons (i.e. alkanes) values fluctuate between13 and 91% of the three fractions. The high concentration ofsaturate aliphatics is commonly attributed to marine sources [4].In contrast the aromatics, which reflect a terrigenous plantscontribution, are very low (0123%). Such results areconsistent for Type-II organic matter [4,18,19].

4.1.3. Steranes (m/z 217) distribution

4.1.3.1. Regular steranes. The resulting m/z 217 masschromatograms for representative samples are shown inFig. 6. Labelled peaks are summarized in Table 2.

In this work, following compounds have been identified:C27-cholestane, C28-ergostane, C29-stigmastane and propylcho-lestanes with their 20S and 20R epimers (Philp, 1985).Cholestane (C27) was consistently the most abundant pseudo-homologue comprising 43 to 46% of the total C27 to C29-regular-steranes), followed by ergostane (C28) comprising 29 to32%, and stigmastane (C29) comprising 25%. (Table 3).

As known, the sterane distributions can be used as effectivesource facies discriminators to group oils in a region on thebasis of genetic relationships. A predominance of C29 steranesis related to a strong terrestrial contribution, whereas apredominance of C27 and C28 indicate a predominance ofmarine phytoplankton and lacustrine algae, respectively [20]. Inthis respect, the relative high abundance of C27 steranes (4346%) reflects significant marine input.

On the other hand, if the predominant primary producers ofC29-sterols are photosynthetic organisms, including land plants,the relatively high stigmastane proportions suggest a probablecontribution of terrestrial organic matter as indicated by the C27/C29 sterane ratios which rise from 1.72 to 1.84 (Table 3) [21,22].

The homologous distributions of steranes, C27C28C29, areoften expressed in ternary plots [10], to show similarity ordissimilarity in source facies among the oils of interest (Fig. 7).In this present study, the ternary plot shows that the outcrop

Technology 88 (2007) 959966samples are indistinguishable by sterane distributions indicatingthat these oils were derived from similar origin (marine sourcewith contribution of higher plants).

ingA. Arfaoui et al. / Fuel Process4.1.3.2. Diasteranes. Table 3 shows that the diasteranesindex, of all outcrop sample extracts, rise from 0.82 to 1.03. Theoccurrence of diasteranes in relative low abundance in theYpresian organic matter implies generation of these oils fromorganic-rich limestones. Either, low diasterane/sterane ratios areassociated with anoxic, clay-poor, carbonate source rocks,whilst high ratios are generally found in oils derived fromclastic sediments, where clay minerals may act as catalysts intheir formation from other steranes [20,23].

4.2. Thermal evolution of organic matter

4.2.1. Abundance of organic extractsThe total organic extracts are low, ranging from 0.28 to

3.63mg/g dry weight for the analyzed samples (Table 4). The lowconcentrations probably result from the low thermal maturity oforganic matter as indicated by the predominance of the hetero(NSO) compounds (0364% of the three fractions) [4,18,19], andeventually to a possible removal of aliphatic compounds by post-generation processes such as weathering and biodegradation [24].

Fig. 5. Geochemical logs of Je963Technology 88 (2007) 9599664.2.2. Maximum pyrolysis temperature: TmaxIn the present work, a general assessment of thermal maturity

using Tmax values shows no major difference between thestudied samples. However, Tmax values rise from 433438 C(Table 1). Generally such values suggest that the organic matterhas reached a relative low thermal maturity in west of Kairouanwhich corresponds to the end of diagenesis and beginning ofcatagenesis (Fig. 8) [25,1].

4.2.3. Sterane maturity parameters

4.2.3.1. 20S / (20S+20R)- and (+)-C29 steranesratios. 20S / (20S+20R)- and (+)-C29 steranesare two ratios that could be measured with confidence(Table 3).

In the present study, 20S/(20S+20R)-and (+)-C29steranes ratios of in the Ypresian outcrop samples (Table 3), risefrom 0.38 to 0.41 and from 0.35 to 0.43, respectively. Theserelative low values suggesting that these oils were generateddominantly prior to the peak stage of the conventional oil window

bel Chaker cross-section.

ng Technology 88 (2007) 959966964 A. Arfaoui et al. / Fuel Processi[23,26] consistent with Rock-Eval data of Tmax values and Tmax/HI diagram (Fig. 8).

4.2.3.2. Pregnane index. The pregnanes were identified bycomparison with the retention time and mass spectral datareported by Winger and Pomerantz [27] and Jiang et al. (1990)(Fig. 6; Table 3).

Low molecular weight steranes, i.e. pregnane (C21-sterane)and methylpregnane (C22-sterane), typically appear in high-ly mature condensates [28]. As indicated by the PregnaneIndex (PI) (Table 3), the occurrence of pregnanes with lowconcentrations in the studied samples confirms again therelative low thermal maturity of organic matter at ChakerJebel.

5. Conclusions

The main conclusions of this work are summarized below:

(i) Generally, the organic matter may be autochthonous tothe environment where it is deposited with predominanceof marine source.

(ii) The thermal maturity level has been estimated to bewithin end of diagenesis/beginning of catagenesis andcorrespond to theoretical vitrinite values (Ro) in the range0.530.63%.

(iii) The Ypresian is a favourable episode for the accumulationand preservation of respectable quantities of organic

Fig. 6. Mass chromatograms (m/z 217) of steranes fomatter. Owing to their geochemical characteristics, thesefacies represent new source rocks in a promising areasuch as the central-northern Tunisia.

Acknowledgements

Financial support for this study was provided by GEOGLOB(code: 03/UR/10-02, Tunisia).We are grateful to Dr. D.Mehdi forhis helpful comments and suggestions that improved this paper.We would like to thank anonymous reviewer for helpful reviews.

r some representative samples from Chaker area.

Fig. 7. Ternary plot showing the relative distribution of C27C28C29 steranes inthe studied samples.

Table 2Steranes (m/z 217) identification

Peaknumber

Formula Molecularweight

Compound

1 C21H36 288 Diapregnane2 C21H36 288 5(H),14(H),17(H)-pregnane3 C22H38 302 Diahomopregnane4 C22H38 302 5(H),14(H),17(H)-methylpregnane5 C27H48 372 13(H),17(H)-diacholestane (20S)6 C27H48 372 13(H),17(H)-diacholestane (20R)7 C27H48 372 13(H),17(H)-diacholestane (20S)8 C27H48 372 13(H),17(H)-diacholestane (20R)9 C28H50 386 13(H),17(H)-24-methyldiacholestane

(20S) 24R/S isomers (?)10 C28H50 386 13(H),17(H)-24-methyldiacholestane

(20S) 24R/S isomers (?)11,12 C28H50 386 13(H),17(H)-24-methyldiacholestane

(20S) 24R/S isomers (?)13 C28H50 386 13(H),17(H)-24-methyldiacholestane (20S)14 C27H48 372 5(H),14(H),17(H)-cholestane (20S)15 C27H48 372 5(H),14(H),17(H)-cholestane (20R)+

C29H52 400 13(H),17(H)-24-ethyldiacholestane (20S)16 C27H48 372 5(H),14(H),17(H)-cholestane (20S)17 C28H50 386 13(H),17(H)-24-methyldiacholestane (20R)18 C27H48 372 5(H),14(H),17(H)-cholestane (20R)19 C29H52 400 13(H),17(H)-24-ethyldiacholestane (20R)20 C29H52 400 13(H),17(H)-24-ethyldiacholestane (20S)21 C28H50 386 5(H),14(H),17(H)-24-methyldiacholestane

(20S)22 C29H52 400 13(H),17(H)-24-ethyldiacholestane (20R)23 C28H50 386 5(H),14(H),17(H)-24-methylcholestane

(20R)24 C28H50 386 5(H),14(H),17(H)-24-methylcholestane

(20S)25 C28H50 386 5(H),14(H),17(H)-24-methylcholestane

(20R)26 C29H52 400 5(H),14(H),17(H)-24-ethylcholestane

(20S)27 C29H52 400 5(H),14(H),17(H)-24-ethylcholestane

(20R)28 C29H52 400 5(H),14(H),17(H)-24-ethylcholestane

(20S)29 C29H52 400 5(H),14(H),17(H)-24-ethylcholestane

(20R)30 C30H54 414 5(H),14(H),17(H)-24-propylcholestane

(20S)31 C30H54 414 5(H),14(H),17(H)-24-propylcholestane

(20R)32 C30H54 414 5(H),14(H),17(H)-24-propylcholestane

(20S)33 C30H54 414 5(H),14(H),17(H)-24-propylcholestane

(20R)

Table 3Sterane parameters (m/z 217) of Chaker representative samples

Sample Sterane (%) a (C27/C29)

bDiasteranesindex c

20S/(20S+20R) d

/(+) e

PI f

C27 C28 C29

JC-01 44 29 25 1.76 0.85 0.41 0.43 26.00JC-16 46 29 25 1.84 0.49 0.40 0.35 12.00JC-28 43 32 25 1.72 0.59 0.38 0.37 13.00Average 44 30 25 1.77 0.64 0.40 0.39 17.00a 5(H),14(H),17(H)-20R-steranes.b C27/C29 (5(H),14(H),17(H)-20R-steranes).c 24-ethyl-13(H)-17(H)-diacholestanes (20R+20S) / [24-ethyl-14(H)-

17(H)-cholestanes (20R+20S)+24-ethyl-14(H)-17(H)-cholestanes (20R+20S)].d 20S / (20S+20R) for C29-5(H),14(H),17(H)-steranes.e 5(H),14(H),17(H) / [5(H),14(H),17(H)+5(H),14(H),17(H)]

for C29-steranes.f Pregnane index, sum of concentrations of C21 and C22 steranes (pregnanes)

over total concentration of steranes100.

Table 4Extract yields and relative percentages of saturated hydrocarbons, aromatichydrocarbons and asphaltic (NSO) compounds of the representative samplesfrom Chaker area

Sample Total bitumenextract

Aliphatichydrocarbons

Aromatichydrocarbons

Asphaltic(NSO)compounds

F1/F2

Total yield(mg/g dryweight)

F1(% of threefractions)

F2(% of threefractions)

F3(% of threefractions)

JC-01 1.82 57 1 42 57JC-02 1.46 41 9 50 4.5JC-03 0.91 47 9 44 5.2JC-04 1.54 91 6 03 15JC-05 1.88 47 6 47 7.8JC-06 0.96 41 11 48 3.7JC-07 0.77 24 9 67 2.7JC-08 1.36 48 4 48 12JC-09 1.87 39 35 26 1.1JC-10 0.92 36 16 48 2.2JC-11 1.65 36 11 53 3.2JC-12 2.97 82 8 10 10JC-13 0.96 74 13 13 5.7JC-14 0.88 34 14 52 2.4JC-15 0.28 13 23 64 0.5JC-16 1.78 39 1 60 39JC-17 0.97 38 9 53 4.2JC-18 0.67 40 2 58 20JC-19 1.21 32 15 53 2.1JC-20 3.48 35 4 61 8.7JC-21 3.00 45 9 46 5JC-22 0.86 37 14 49 2.6JC-23 0.40 48 19 33 2.5JC-24 0.74 25 23 52 1JC-25 0.59 40 16 44 2.5JC-26 1.54 69 6 25 11JC-27 0.74 37 14 49 2.6JC-28 3.63 53 16 31 3.3JC-29 2.70 66 6 28 11Average 1.47 45.31 11.35 43.35 8.6

965A. Arfaoui et al. / Fuel Processing Technology 88 (2007) 959966

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Usefulness of Rock-Eval pyrolysis, liquid chromatography, and GC/MS in the characterization of .....IntroductionGeological settingMaterials and methodsMaterials of studyTechniques of studyRock-Eval pyrolysisBitumen extractionLiquid chromatographyGc/ms

Results and discussionOrigin and depositional environment of organic matterRock-Eval pyrolysis dataAbundance and composition of extracted hydrocarbonsSteranes (m/z 217) distributionRegular steranesDiasteranes

Thermal evolution of organic matterAbundance of organic extractsMaximum pyrolysis temperature: TmaxSterane maturity parameters20S/(20S+20R)- and (+)-C29 steranes ratiosPregnane index

ConclusionsAcknowledgementsReferences