-
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: [email protected] (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