Directory UMM :Data Elmu:jurnal:O:Organic Geochemistry:Vol31.Issue12.Dec2000:
Organic Geochemistry 31 (2000) 1641±1661
www.elsevier.nl/locate/orggeochem
Pyrolytic and spectroscopic study of a sulphur-rich
kerogen from the ``Kashpir oil shales''
(Upper Jurassic, Russian platform)
A. Riboulleau a,b, S. Derenne a,*, G. Sarret c,1, C. Largeau a, F. Baudin b,
J. Connan d
a
Laboratoire de Chimie Bioorganique et Organique Physique, CNRS UMR 7573, ENSCP, 11 rue Pierre et Marie Curie, 75231 Paris cedex 05,
France
b
Laboratoire de Stratigraphie, CNRS ESA 7073, UPMC, 4 place Jussieu, 75252 Paris cedex 05, France
c
Chemistry Department, University of Western Ontario, N6A 5B7, London, Ontario, Canada
d
Elf Aquitaine CSTJF, Avenue Larribau, 64018 Pau cedex, France
Abstract
The kerogen of an organic-rich sample, termed f top, from the Gorodische section (Russian platform) was studied
using a combination of microscopic, spectroscopic and pyrolytic methods so as to examine its chemical structure,
source organisms and formation pathway(s). This kerogen, which is mainly composed of orange gel-like, nanoscopically amorphous organic matter, exhibits a relatively high aliphatic character; organic sulphur is mainly present as
di(poly)sulphides and alkylsulphides. The f top kerogen was chie¯y formed via intermolecular incorporation of sulphur
in algal or cyanobacterial lipids and carbohydrates. However, its formation also involved oxidative condensation via
ether linkages. Comparison of f top sample with other S-rich kerogens points to a closer similarity with Monterey
kerogens rather than with a kerogen from the bituminous laminites of Orbagnoux. # 2000 Elsevier Science Ltd. All
rights reserved.
Keywords: Kashpir oil shales; Pyrolysis; FTIR; Solid state 13C NMR; XANES spectroscopy; Type II-S kerogen
1. Introduction
The Upper Jurassic was a period of intense accumulation of sedimentary organic matter (OM) and important gas and oil source-rocks are Upper Jurassic in age
(Ulmishek and Klemme, 1990), especially in the northern hemisphere, including the North Sea and Siberian
oil ®elds. Episodes of extensive OM deposition also
took place on the Russian Platform, located between
these two basins, but the relative stability of this platform since the Triassic did not allow sucient burial
* Corresponding author. Tel.: + 33-1-4427-6716; fax: + 331-4325-7975.
E-mail address: sderenne@ext.jussieu.fr (S. Derenne).
1
Current address: LGIT-Groupe de GeÂochimie de l'Environnement, Universite Joseph Fourier, BP 53, 38041 Grenoble
cedex, France.
of OM for hydrocarbon production. Although such
episodes are relatively less numerous on the Russian
platform than in the neighbouring basins, large accumulation occurred during the Middle Volgian (Late
Tithonian, 140 Ma) and several basins of the Russian
platform display organic-rich levels of this age (Shmur
et al., 1983). In the Ulyanovsk region, these organic-rich
sediments crop out along the Volga river and are known
as the ``Kashpir oil shales''. At Gorodische (Fig. 1), this
organic-rich deposit represents a 6 m thick layer of grey
to dark-brown shales whose TOC contents vary between
0.5 and 45% and HI between 50 and 700 mg HC/g TOC
(Hantzpergue et al., 1998). The Kashpir oil shales
represent, after the Baltic oil shales, one of the most
important oil shale reserves of Russia and they have
been mined for oil production since the 1850s (Shmur et
al., 1983; Russell, 1990). However, production from
these oil shales has almost ceased today because they are
0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(00)00088-7
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A. Riboulleau et al. / Organic Geochemistry 31 (2000) 1641±1661
quantity. The kerogen of f top was examined by a combination of various methods in order to characterise its
structure, to specify the nature of the sulphur functions
and to determine the source organisms of the OM and
its preservation pathway(s), in this especially rich level.
2. Geological setting and sampling
Fig. 1. Location map of the Gorodische outcrop.
characterised by high sulphur contents, and particularly
Sorg, leading to high atmospheric pollution upon
reforming (Popov et al., 1986). Nevertheless these shales
are still mined near Kashpir village for pharmaceutical
and industrial purposes.
S-rich kerogens have been intensively studied during
the last decade and it is now well established that this
type of fossil OM originates from incorporation of
reduced mineral sulphur (H2S, S0 or S2ÿ
X ) in functionalised lipids or carbohydrates during early diagenesis (e.g.
Francois, 1987; review by Sinninghe Damste and de
Leeuw, 1989; van Kaam-Peters et al., 1998a). S-rich OM
is mostly found in sediments from evaporitic and anoxic
carbonate platform environments (Sinninghe Damste et
al., 1989; Schaeer et al., 1995; Mongenot et al., 1997;
Baudin et al., 1999). Sulphur incorporation, from the
H2S produced by sulphate-reducing bacteria under
anoxic conditions, can occur during very early diagenesis in the upper layers of the sediment (Hartgers et al.,
1997).
Several paleoecological indices in the Gorodische
black shales, like the presence of bioturbations and
brachiopods in several levels, testify for oxygenation of
the upper centimeters of the sediments, at least during
part of the deposition of these black shales. The results
presented here, concerning the most OM-rich level,
termed ``f top'', are part of a broader study. This study
aims at determining the paleoenvironmental conditions
that led to the deposition of these black shales and
explaining their strong variations in OM quality and
The Gorodische outcrop is located near the city of
Ulyanovsk along the Volga river (Fig. 1). It is mainly
composed of grey clays of Middle Volgian age (Fig. 2).
At the top of the middle Volgian, appears the 6 m thick
black shale unit. Its colour varies from dark grey to
brown and despite a laminated feature, bioturbations
are visible in the lower and upper parts of the unit. It is
overlain by sandstone of Middle to Upper Volgian age.
A more detailed description of the section is given in
Hantzpergue et al. (1998). TOC and HI values for this
organic-rich formation are presented in Fig. 2. The f top
level which is studied in this paper, is a 7 cm thick bed
located 2.70 m above the base of the OM-rich deposit
and presents the highest TOC and HI values (Fig. 2). In
contrast to most part of the formation, no bioturbation
nor benthic organisms are observed in this level.
3. Experimental
Shale samples were collected in 1995. Subsamples
were ground for Rock-Eval analysis, the remainder was
stored at room temperature away from light. Prior to
further analysis, the surface of the sample was carefully
removed in order to eliminate oxidised or polluted
material.
Rock-Eval pyrolysis, OSA device, was performed on
10 mg samples of powdered bulk shale. The sample was
heated at 300 C for 3 min followed by a programmed
pyrolysis at 25 C/min up to 600 C under a He ¯ow and
then oxidised at 600 C for 7 min under an oxygen ¯ow.
After grinding, ca. 50 g of shale were extracted with
CHCl3/MeOH, 2:1, v/v (stirring for 12 h at room temperature) before isolation of the kerogen via the classical
HCl/HF treatment (Durand and Nicaise, 1980). The
kerogen concentrate was then extracted as described
above and dried under vacuum.
An aliquot of the kerogen was ®xed with 2% OsO4
for electron microscopy. For transmission electron
microscopy (TEM), the ®xed kerogen was embedded in
Araldite, cut in ultra-thin sections and stained with
uranyl acetate and lead citrate. Observations were carried out with a Philips 300 microscope. For scanning
electron microscopy (SEM), the ®xed kerogen was
dehydrated using the CO2 critical point technique and
coated with gold prior to observation with a Jeol 840
microscope.
A. Riboulleau et al. / Organic Geochemistry 31 (2000) 1641±1661
1643
Fig. 2. Stratigraphic section of Gorodische outcrop (after Hantzpergue et al., 1998) and variations of TOC and HI along the organicrich formation.
FTIR spectra of the kerogen and its pyrolysis residues
were recorded on a Bruker IFS 48 spectrometer as 5 mm
KBr pellets. Solid state 13C NMR spectroscopy was
performed on the kerogen and its pyrolysis residues with
a Bruker MSL 400 spectrometer using high power
decoupling, cross polarisation and magic angle spinning. Spectra were recorded at 3 and 4 kHz in order to
discriminate spinning side bands (SSB).
X-ray absorption near edge structure (XANES) spectroscopy was performed at the Canadian Synchrotron
Radiation Facility situated on the 1 GeV electron storage ring, Alladin, University of Wisconsin. Experimental details have been described previously (Kasrai et
al., 1994). All the S K- and L-edge spectra presented in
this paper were recorded using total yield (TEY) detection mode. They were background subtracted using a
linear function extrapolated from the pre-edge region,
and then normalised to the height of the maximum of
peak B for the L-edge, and to the height of the edge
jump for the K-edge. S K-edge spectra were simulated
by a linear combination of reference spectra using a
least-squares ®tting program. Analytical procedure as
well as interpretation and comparison with standard
spectra are described in Sarret et al. (1999).
Elemental analyses of the kerogen and of its pyrolysis
residues were performed at the Service Central d'Analyse of the CNRS. The O content of the unheated kerogen was determined by coulometry (Laboratoires
Wol).
Bulk isotopic measurement of d13C was performed on
the kerogen with a Carlo-Erba CHN coupled to a VGSIRA 10 spectrometer.
O-line pyrolysis was performed on the kerogen as
previously described by Largeau et al. (1986). Brie¯y,
the sample is successively heated at 300 C for 20 min
and 400 C for 1 hour under an He ¯ow. The released
products are trapped in cold chloroform. After each
treatment, the residue is extracted with CHCl3/MeOH
as previously described. The ®rst thermal treatment at
300 C was aimed at eliminating thermolabile components. It has been recently demonstrated that such a
thermal treatment, when applied to S-rich kerogens also
promotes some aromatisation and lowers the global
eciency of the subsequent cracking at 400 C. However, it was also shown that more detailed information
on the chemical structure of such kerogens can ®nally be
obtained by this two step treatment at 300 and 400 C
than via direct pyrolysis at 400 C (Mongenot, 1998;
Sarret et al., unpublished results).
The 400 C pyrolysate was separated by column chromatography (Al2O3, Act II) into three fractions of
increasing polarity eluted with heptane, toluene and
methanol, respectively. An additional elution with
CHCl3 was performed, and yielded a small amount of
products which were combined with the MeOH-eluted
fraction. Carboxylic acids were separated from the
MeOH±CHCl3 fraction using a double extraction with
ether under base and acid conditions and analysed by
GC and GC/MS as their methyl esters. Unsaturated
methyl esters were further analysed after derivatisation
with DMDS as described by Scribe et al. (1988).
A part of the total 400 C pyrolysate and of its heptane- and toluene-eluted fractions was desulphurised
with Raney Ni using the conditions previously described
by Sinninghe Damste et al. (1988a) and hydrogenated
before GC and GC/MS analyse.
All the fractions were analysed by GC and GC/MS
using a HP 5890 gas chromatograph (60 m capillary
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A. Riboulleau et al. / Organic Geochemistry 31 (2000) 1641±1661
column, ®lm thickness of 0.4 mm, heating program 100
to 300 C at 4 C minÿ1, injector and FID at 320 C,
helium carrier gas), coupled with a HP 5989 mass spectrometer with a mass range m/z= 40±600, operated at
70 eV.
Curie point pyrolysis-gas chromatography±mass
spectrometry (CuPy-GC±MS) was performed on ca. 2
mg of sample. The sample was loaded on a ferromagnetic wire with a Curie point of 610 C. The wire placed
in a glass tube was introduced in the Curie point pyrolyser (Fisher 0316M) coupled to a Hewlett-Packard
HP 5890 gas chromatograph with FID (30 m fused silica
capillary column, ®lm thickness 0.4 mm, heating program 35 C for 10 min and from 35 to 300 C at 4 C
minÿ1, He carrier gas). The chromatograph was coupled
with a HP 5989 mass spectrometer with a mass range
m/z= 40±600, operated at 70 eV.
4. Results
4.1. Bulk features
Rock-Eval pyrolysis of the unextracted shale shows an
exceptionally high TOC of 45% for f top and a high
hydrogen index (HI) of 700 mg HC/g TOC (Fig. 2). The
Tmax value is very low, 396 C, re¯ecting the immaturity
of the sample. Bulk isotopic measurement indicate a
d13CTOC of ÿ20.5 % (PDB) for f top kerogen. Elemental
analysis of the isolated kerogen (Table 1) indicate a
relatively high H/C ratio of 1.39 consistent with the high
HI, an O/C ratio of 0.20, a low N/C ratio of 0.014, a high
content of total S of 11.7 wt.% and a weak Fe content of
0.67 wt.%. Sorg content was then calculated in the classical way: it was considered that Fe is only present as
pyrite after the acid treatment and that non-pyritic sulphur is organic, giving a high Sorg/C ratio of 0.068. F top
thus belongs to Type II-S kerogens, according to the
classi®cation of Orr (1986). Sulphur-rich kerogens are
known to exhibit a relatively low thermal stability
(Baskin and Peters, 1992; Tomic et al., 1995) thus
explaining the very low Tmax value observed for f top.
organic matter (AOM) associated with rare pyrite. This
AOM is yellow to orange and appears as gel-like particles of varying size (from 5 to 200 mm). When observed
by SEM and TEM, the AOM appears homogeneous
and amorphous, even at high magni®cation (nanometric
scale). Similar morphological features have been previously observed in S-rich kerogens from the Kimmeridge Clay Formation (Boussa®r et al., 1995) and the
paleolagoon of Orbagnoux (Mongenot et al., 1999).
4.3. Spectroscopic study
4.3.1. Solid state 13C NMR spectroscopy
The solid state 13C NMR spectrum of f top kerogen
(Fig. 3a) is dominated by a broad peak at 30 ppm with a
small shoulder at 15 ppm, corresponding to CH2 in
alkyl chains and to CH3 groups, respectively. Such a
dominance re¯ects the aliphatic character of the sample,
in agreement with both high H/C ratio and HI value. A
relatively intense broad signal centred at 75 ppm corresponding to aliphatic C linked to N or O is also noticed.
According to the weak N/C ratio revealed by elemental
analysis the latter signal must be chie¯y due to C±O
bonds. The 30 ppm peak exhibits a broad shoulder
between 40 and 60 ppm, which can be assigned to carbons in C±S bonds and/or to C b to the functions that
gave the 75 ppm signal. Two weak signals at 130 and
175 ppm are observed corresponding to unsaturated
carbons and to C in carboxylic groups, respectively.
Signals observed at 140 and 215 ppm correspond to
spinning side bands of the carboxylic peak as shown by
the comparison of the spectra recorded at two dierent
spinning rates.
4.2. Microscopic study
Under the light microscope, f top kerogen appears
chie¯y constituted of a single type of amorphous
Table 1
Atomic ratios of f top kerogen and insoluble pyrolysis residues
f top
f top res 300
f top res 400
H/C
Sorg/C
N/C
1.39
1.19
0.57
0.068
0.047
0.030
0.014
0.016
0.021
Fig. 3. Solid state CP/MAS 13C NMR spectra of (a) f top
kerogen and (b) of its 300 C pyrolysis residues. (X, O and/or
N; SSB, spinning side bands).
A. Riboulleau et al. / Organic Geochemistry 31 (2000) 1641±1661
1645
4.3.2. FTIR spectroscopy
The FTIR spectrum of f top kerogen (Fig. 4a) shows
intense absorptions at 2920, 2850, 1445 and 1375 cmÿ1
due to CH2 and CH3 groups. Oxygenated functions are
detected as (i) a broad band of medium intensity centred
at 3400 cmÿ1 corresponding to O±H groups, (ii) a band
at 1700 cmÿ1 (CO), and (iii) a broad and intense band
centred at 1050 cmÿ1 corresponding to C±O bonds, in
agreement with the relatively strong signal at 75 ppm
observed in the 13C NMR spectrum. A band at 1630
cmÿ1 is also observed indicating that the unsaturated C
detected at 130 ppm via 13C NMR are mostly ole®nic. A
narrow band with a rather high intensity is noted at 750
cmÿ1; it is due to CaF2 neoformed during the HCl/HF
treatment since the latter mineral was identi®ed by
XRD (A. Person, pers. comm.).
Fig. 4. FTIR spectra of (a) f top kerogen and of its (b) 300 and
(c) 400 C pyrolysis residues.
4.3.3. XANES spectroscopy
Sulphur K-edge spectrum for f top kerogen is presented in Fig. 5a. The spectrum was simulated by a linear combination of spectra of model compounds in
order to quantify the dierent sulphur species which can
be dierentiated by this mode, i.e. Sÿ2, disulphides,
alkyl and/or heterocyclic sulphides, sulphoxides, sulphones, sulphonates and sulphates (Sarret et al., 1999).
Results of the simulation are presented in Table 2. The
major sulphur species in f top kerogen are alkyl and/or
heterocyclic sulphides (76%) and di(poly)sulphides
occur in lower proportion (11%). A small contribution
of sulphoxides, sulphonates and sulphates is also
noticed.
S L-edge spectrum for f top kerogen is compared with
spectra of various reference compounds in Fig. 6. From
S K-edge results, one could foresee spectral similarities
with the alkylsulphide and/or heterocyclic sulphide
(thiophene) references. These two types of compounds,
which are dicult to distinguish on K-edge spectra,
exhibit clearly dierent peak positions on L-edge spectra,
Fig. 5. S K-edge XANES spectra (solid lines) and simulations (dotted lines) calculated as indicated in Table 2 for (a) f top kerogen
and its (b) 300 and (c) 400 C pyrolysis residues.
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A. Riboulleau et al. / Organic Geochemistry 31 (2000) 1641±1661
Table 2
Distribution of the sulphur species in f top kerogen and insoluble pyrolysis residues determined by simulation of the S K-edge XANES
spectra
f top
f top res 300
f top res 400
FMa
S(-II)
Disulphides
S aliph./heterocycl.b
Sulphoxides
Sulphones
Sulphonates
Sulphates
0.08
0.05
0.18
0
0
4
11
0
0
76
94
84
5
5
9
0
0
0
4
0
0
4
1
3
a
FM: ®gure of merit of the ®t, FM= S(f kerogen-f ®t)2
As indicated by S L-edge spectroscopy, these reduced forms correspond to (i) alkylsulphides for the unheated kerogen, (ii) alkyl
and heterocylic sulphides for the 300 C residue, (iii) heterocyclic sulphides for the 400 C residue.
b
as this latter method is more sensitive to the reduced
forms of sulphur (Kasrai et al., 1996a). F top spectrum
is clearly closer to the alkyl sulphide reference (Fig. 6-2)
than to the thiophene (Fig. 6-5) and dibenzothiophene
(Fig. 6-4) ones; it is therefore concluded that alkyl sulphides are the major sulphur species in f top kerogen.
However, f top spectrum is slightly shifted to the left
compared to the reference and presents a small shoulder
at 162.8 eV. This spectrum is well reproduced by a linear
combination of 30% di(poly)sulphides+70% alkyl sulphides (Fig. 6a, dotted line). Contrary to the K-edge
mode, S L-edge XANES spectroscopy does not allow a
precise quanti®cation of sulphur species due to (i) some
uncertainties on the normalisation of the spectra, and
(ii) the fact that only one part of the spectrum is simulated (Sarret et al., 1999). Accordingly, S L-edge results
are not directly comparable with K-edge results, e.g.
concerning the di(poly)sulphide content (30% compared
to 11%, respectively). However, S L-edge analysis
allows to conclude that the major reduced sulphur species correspond to alkyl sulphides, and to a lesser extent
di(poly)sulphides. The f top spectrum also contains a
small peak at about 167.6 eV, which corresponds to the
position of the main peak for the sulphoxide reference
(Fig. 6-6), which is consistent with K-edge results.
4.4. Pyrolytic study
4.4.1. Mass balance and spectroscopic features of oline pyrolysis residues
After the thermal treatment at 300 C, the weight loss
amounts to ca. 18% of the initial organic matter, mainly
corresponding to volatile compounds (17%) while trapped products only represent 1% of the initial OM. After
pyrolysis at 400 C, the total weight loss represents 58%
of the initial OM. This loss corresponds in similar
amount to volatile and trapped products (ca. 30 and
28% of the initial OM, respectively).
Elemental analyses of the pyrolysis insoluble residues
(Table 1) show, as expected, a small decrease in the H/C
ratio upon 300 C heating whereas a large decrease takes
place upon pyrolysis at 400 C. The N/C ratio shows a
slight increase upon pyrolysis which is consistent with
the results of Barth et al. (1996) and of Gillaizeau et al.
(1997) attesting for the concentration of N in kerogen
residues after pyrolysis. In contrast, the Sorg/C ratio
regularly decreases.
The 13C NMR spectrum of the 300 C residue (Fig.
3b) is still dominated by the peak centred at 30 ppm but
the relative intensity of the 40±60 ppm shoulder is lower
compared to the unheated kerogen. The 75 ppm peak
exhibits a lower intensity and becomes thinner; in the
same time, the peak at 130 ppm increases and the carboxyl peak at 175 ppm remains approximately constant
with respect to the 30 ppm band. The important peak at
165 ppm corresponds to a spinning side band of the 130
ppm peak. As expected, the 13C NMR spectrum of the
400 C residue (not shown) is dominated by an aromatic
signal at 130 ppm and only shows a weak signal at 30
ppm. This indicates the intense aromatisation of the
material.
The FTIR spectrum of the 300 C residue (Fig. 4b)
shows the same absorptions as the unheated kerogen.
The shift of the CC signal towards 1600 cmÿ1 indicates some aromatisation at this temperature as previously observed for the Orbagnoux kerogen
(Mongenot, 1998). The C±O absorption decreases sharply at 300 C, in agreement with the decrease of the 75
ppm signal in the 13C NMR spectrum. In the 400 C
residue (Fig. 4c) the absorptions corresponding to CH2
and CH3 are of much lower intensity in agreement with
the large decrease of the H/C ratio, the CC band is
broader and now centred at 1600 cmÿ1, corresponding
to aromatic carbons and the broad band around 1100
cmÿ1, due to C±O bonds, is no longer detected.
The S K- and L-edge XANES spectra for the insoluble pyrolysis residues are presented in Figs. 5b and c
and 6b and c, respectively. The edge of the K-edge
spectrum for the 300 C residue is slightly shifted to
higher energy compared to the unheated kerogen. This
shift corresponds to the removal of di(poly)sulphides
upon heating. Indeed, the simulation shows that the
di(poly)sulphides are no longer present in the 300 C
residue (Table 2). The edge of the spectrum for the
400 C residue is even more shifted to the right. As the
di(poly)sulphides are already eliminated in the 300 C
A. Riboulleau et al. / Organic Geochemistry 31 (2000) 1641±1661
Fig. 6. S L-edge XANES spectra for some reference compounds: (1) dl-cystine, (2) dl-methionine, (3) poly(phenylene
sulphide), (4) dibenzothiophene, (5) 3-(2-thienyl)-dl-alanine,
(6) dl-methionine sulphoxide. S L-edge XANES spectra (solid
lines) and simulations (dotted lines) for (a) f top kerogen and its
(b) 300 and (c) 400 C pyrolysis residues. Simulations consist of
a linear combination of the reference compounds spectra presented above. These combinations are the following: f top: 30%
(1)+70% (2), 300 C residue: 40% (2)+60% (5), 400 C residue:
70% (5)+30% (6).
1647
residue, this shift can only be attributed to a change in
the alkyl/heterocyclic sulphides distribution. S L-edge
analysis will provide more information about this
change. Concerning the oxidised sulphur species, the
oscillation situated at about 2481 eV is weaker for the
300 C residue than for f top, so their contribution is
smaller. Indeed, Table 2 shows that their content is of 6
and 13%, respectively. The 400 C residue contains
about the same total amount of oxidised species as f top
(12%), but more sulphoxides. This latter sample also
contains 4% of sulphur in the oxidation state (-II),
which gives rise to a small shoulder at 2469 eV. This
oxidation state does not exist for organic sulphides,
whose oxidation number is 0. Therefore, the 400 C
residue contains a small amount of inorganic sulphides
that has formed upon heating.
The S L-edge spectra for the residues are compared to
f top spectrum in Fig. 6. The spectrum of the 300 C
residue is shifted to higher energy compared to f top,
and its three peaks roughly match with those of the
reference corresponding to simple thiophene. However,
the amplitude of the peak at 164.7 eV is particularly
high, which indicates the presence of alkyl sulphides. A
good ®t is obtained with a linear combination of 60%
simple thiophene+40% alkyl sulphide. If we replace
simple thiophene (i.e. not included in a polyaromatic
structure) by dibenzothiophene (DBT), the position of
the 164.7 eV peak does not match so well. Thus, the
300 C residue contains thiophenes as major species, and
to a lesser extent alkyl sulphides. No di(poly)sulphides
are detected in this sample, which is in agreement with
K-edge results. For the 400 C residue, the L-edge spectrum shows peak positions similar to that of the 300 C
residue, but with dierent relative intensities. Alkylsulphides are no longer observed and the linear combination of 70% thiophene+30% sulphoxide aords a good
®t. Again, if we replace simple thiophene by DBT, the
peak at 164.6 eV does not match. As explained previously, the percentages determined on S L-edge spectra
do not aord precise quantitative information on sulphur distribution; however they re¯ect the changes in
disulphide, alkyl sulphide and thiophene contents upon
thermal stress. During heating, the disulphides (300 C)
and the sulphides (400 C) present in f top disappear
while the relative abundance of thiophenes increases.
These trends are consistent with the decrease of the 40±
60 ppm shoulder and relative increase of the 130 ppm
signal corresponding to aromatic carbons in the 13C
NMR spectrum of the 300 and 400 C residues, compared to that of the kerogen. Such changes are also
responsible for the shifts observed on the K-edge spectrum. It can be noted that the sulphoxide contribution
for the 400 C residue, calculated by S L-edge spectroscopy, is particularly high compared to its actual content determined via K-edge analysis (9%). This
disagreement can be due to a higher oxidation of the
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A. Riboulleau et al. / Organic Geochemistry 31 (2000) 1641±1661
Fig. 7. TIC of the heptane-eluted fraction of the 400 C pyrolysate of f top kerogen (a) before and (b) after desulphurisation by Raney Ni and hydrogenation (*, n-alkanes 1; *,
pollutants).
surface of the samples, as the sampling depths of S Kand L-edge XANES spectroscopy are dierent. Indeed,
using TEY detection mode, it is about 70 nm for the Kedge compared to only 5 nm for S L- edge (Kasrai et al.,
1996b). Similar features have been previously observed
on asphaltene and kerogen samples (Sarret et al., 1999;
Sarret et al., unpublished results).
4.4.2. O-line pyrolysis products
The GC trace of the 400 C pyrolysate is very complex
and shows an important hump, due to the coelutions of
numerous products as commonly observed for pyrolysates of S-rich kerogens (Sinninghe Damste et al.,
1990). In order to make easier GC/MS identi®cations,
the pyrolysate was fractionated into three fractions of
increasing polarity, eluted with heptane, toluene and
methanol, respectively.
4.4.2.1. Heptane-eluted fraction. This fraction represents
17% of the pyrolysate. Its GC trace (Fig. 7a) is still
complex and exhibits an important hump due to
numerous coelutions. Nevertheless, selective ion detection (SID) of characteristic fragments allowed the identi®cation of a number of compounds and homologous
series (Table 3). However, due to coelutions, their rela-
tive abundance could not be determined and only a
rough estimation of their relative intensities can be
given. Hydrocarbons mainly consist of n-alkane 1/n-alk1-ene 2 doublets up to C33 (maximum C17) without any
odd- or even- carbon-number predominance. n-Alkylcyclohexanes 9 and cyclopentanes 10 are observed in
small amount. The former are frequently reported in
kerogen pyrolysis products (Homann et al., 1987),
however, their origin is still a matter of debate.
n-Alkylbenzene 5/n-alkenylbenzene 7 doublets are also
observed, associated with the three n-alkylmethylbenzene isomers 6, the o-isomer being predominant. It can
be noticed that the latter isomer is the only one which
can be formed by cyclisation of linear compounds.
Polysubstituted alkylbenzenes 8 are also observed in low
amount. Isoprenoid compounds correspond to prist-1ene 3 and regular saturated C16 and C18 hydrocarbons
4, but their abundance relative to n-alkanes could not be
determined. Neither pristane, nor phytane was detected,
possibly because of a too low abundance. The other
isoprenoid compounds identi®ed are hopanes and
hopenes 28 from C27 to C31. These compounds are generally considered to be of bacterial origin (Rohmer et
al., 1984).
Numerous series of organic sulphur compounds
(OSC) are identi®ed and their coelution accounts for the
bulk of the hump. Several isomers of alkylated thiophenes 11-14, thiolanes 15, thianes 16 and benzothiophenes 19 are identi®ed. These compounds are major
components of most pyrolysates of S-rich kerogens
(Sinninghe Damste et al, 1988b; Payzant et al., 1989).
Most of the identi®ed series of OSC have a linear skeleton. However, C10 to C16 2,3-dimethyl-5-n-alkylthiophenes 13 [identi®ed by their mass spectra and elution
time from Sinninghe Damste et al. (1989)], are also
observed along with branched alkylthiophenes 14 from
C15 to C17. Dierent series of polyaromatic OSC previously observed by van Kaam-Peters and Sinninghe
Damste (1997) and van Kaam-Peters et al. (1998b) in
the pyrolysate of S-rich kerogen from the paleolagoon
of Orbagnoux, are also detected: C9 to C18 alkylated
bithiophenes 17±18 (Appendix I, characteristic fragments at m/z= 179, 193 and 207) and C10 to C13 nalkylphenylthiophenes 20 (II, characteristic fragments at
m/z= 173 and 187). Other series of compounds previously detected in the pyrolysate of Orbagnoux kerogen are also observed in f top pyrolysate: series 26 and
27 characterised by intense fragments at m/z=229±230
and 243±244 (van Kaam-Peters and Sinninghe DamsteÂ,
1997; van Kaam-Peters et al., 1998b; Mongenot et al.,
1999), and series 24 and 25 characterised by intense
fragments at m/z=203 and 217, respectively, (Mongenot et al., 1999). As these compounds are also present
in the toluene-eluted fraction (Table 4), the assignment
of these dierent series is discussed later on. Two series
22, not reported so far, characterised by intense frag-
A. Riboulleau et al. / Organic Geochemistry 31 (2000) 1641±1661
1649
Table 3
Compounds detected in the heptane-eluted fraction of the 400 C pyrolysate of f top kerogen
Series
Range
Maximum
1 n-Alkanes
2 n-Alk-1-enesa
3 Prist-1-ene
4 Regular isoprenoid alkanes
5 n-Alkylbenzenes
6 n-Alkylmethylbenzenesb
7 o n-Alkenylbenzenes
8 Substituted n-alkylbenzenesb
9 n-Alkylcyclohexanes
10 n-Alkylcyclopentanes
11 2 n-Alkylthiophenes
12 2,5-Di-n-alkylthiophenesc
13 2,3-Dimethyl-5-n-alkylthiophenes
14 Branched alkylthiophenesb
15 2-n-Alkylthiolanesd
16 2-n-Alkylthianes
17 2-n-Alkyl-5,50 bithiophenes
18 2,20 -Di-n-alkyl-5,50 -bithiophenes
19 n-Alkylbenzo[b]thiophenese
20 n-Alkylphenylthiophenesf
21 n-Alkyldibenzo- or naphtothiophenes
22 n-Alkylmethylthienothiophenesg
23 Compound in 190
24, 25 Series in 203±217b
26, 27 Series in 229±243b
28 Hopanes and hopenes
C13±C35
C13±C33
C19
C16, C18
C12±C32
C12±C33
C13±C29
C12±C26
C13±C26
C13±C27
C10±C28
C10±C28
C10±C16
C15±C17
C9±C25
C10±C21
C9±C18
C10±C18
C9±C22
C10±C13
C12±C15
C8±C15
C10
C11±C19
C13±C18
C27±C31
C17
C16
C19
±
C15
C14
C15±C16
C14
C17
C16
C12
C13
C13
±
C12
C13
C10
C11±C12
C10
C11
C12±C13
C9
C10
C11±C12
C13±C14
±
a
A series of n-alk-2-enes was also identi®ed with the same range and a maximum at C18.
Several isomers were detected in the series.
c
Three series were detected: C10±C30 (max C13) 2-n-alkyl-5-methylthiophenes, C10±C29 (max C12) 2-n-alkyl-5-ethylthiophenes and
C11±C25 (max C12) 2-n-alkyl-5-propylthiophenes.
d
2-n-Alkyl-5-methylthiolanes (C10±C21, max C13) were also observed.
e
Two series (2-n-alkyl- and 4-n-alkylbenzo[b]thiophenes) were observed with the same distribution. Two series of n-alkylmethylbenzo[b]thiophenes (C10±C20, max C11) were also observed.
f
A series of C12±C15 (max C13) n-alkylmethylphenylthiophenes was also observed.
g
A series of C9±C16 (max C10) n-alkyldimethylthienothiophenes was also observed.
b
ments at m/z= 167 and 181 and a molecular ion at
168+14n and 182+14n (n from 1 to 10), respectively,
are also observed. The regular distribution pattern of
these two series (e.g. Fig. 8a) points to the presence of a
n-alkyl side chain and their disappearance after desulphurisation indicate that they correspond to OSC.
These compounds are tentatively identi®ed as n-alkylmethyl- and n-alkyldimethylthienothiophenes (III) on
the basis of their mass spectra (e.g. Fig. 8b). Thienothiophenes have already been observed in crude oils
(Orr and Sinninghe DamsteÂ, 1989) but, as far as we are
aware, alkylated homologues of such compounds have
not been reported so far.
Following Raney Ni desulphurisation and hydrogenation, the heptane fraction is dominated by a series of
n-alkanes (Fig. 7b), thus con®rming that most of the
OSC have a linear skeleton. Nevertheless, branched and
isoprenoid compounds are also present in small
amounts. In particular, pristane and phytane are
observed, which were not detected in the non-desulphurised fraction. Pristane must be directly derived from
hydrogenation of prist-1-ene. Phytane can originate
from hydrogenation of phytenes and/or desulphurisation of C20 isoprenoid OSCs. The latter have been commonly observed in extracts and kerogen pyrolysates
(Sinninghe Damste and de Leeuw, 1987, and references
therein). Based on previously published mass spectra
and elution times (Sinninghe Damste et al., 1986; Sinninghe Damste and de Leeuw, 1987), C20 isoprenoid
OSCs were searched for in the untreated fraction but
none could be detected. Therefore, it is possible that
several C20 isoprenoid OSCs are present but that they
are undetectable amongst the coelution hump due to
too low individual abundances and/or that phytane
1650
A. Riboulleau et al. / Organic Geochemistry 31 (2000) 1641±1661
Table 4
Compounds detected in the toluene-eluted fraction of the 400 C pyrolysate of f top kerogen
Series
a
29 n-Alkan-2-ones
30 n-Alkylcyclopentanones
31 n-Alkylcyclohexanones
32 1-Phenyl-n-alkan-1-ones
33 1-Phenyl-n-alkan-2-onesb
34 Alkyl¯uorenesc
35 Alkylanthracenes or phenanthrenesc
36 Alkylpyrenes
37 Alkylbenzo¯uorenes
17 2-n-Alkyl-5,50 -bithiophenesc
20 n-Alkylphenylthiophenesc
19 n-Alkylbenzo[b]thiophenes
21 n-Alkyldibenzo- or naphtothiophenesc
23 Compound in 190
24, 25 Series in 203±217
26, 27 Series in 229±243
Range
Maximum
C10±C29
C11±C24
C11±C18
C10±C16
C9±C29
C13±C16
C14±C17
C14±C15
C12±C13
C9±C11
C11±C15
C9±C12
C12±C15
C10
C11±C14
C13±C15
C13
C13
C14
C12
C12
C14
±
±
±
C10
C12
C12
±
C10
±
±
a
Other series of ketones were also observed: n-alkan-3-ones, distr.: C11±C26 (C14); n-alkan-4-ones, C11±C25 (C13); n-alkan-5-ones,
C11±C27 (C16, C19); n-alkan-6-ones, C12±C21 (C13); n-alkan-7-ones, C13±C24 (C16); n-alkan-8-ones, C15±C26 (C17); n-alkan-9-ones,
C17±C26 (C18); n-alkan-10-ones, C19±C24 (C19); n-alkan-11-ones, C21±C26 (C21); n-alkan-12-ones, C23±C26(C23).
b
Three series were observed with the same distribution: 1-phenyl-n-alkan-2-ones and two 1-(methylphenyl)-n-alkan-1-ones.
c
Numerous isomers were detected in the series.
Three series of branched alkanes (iso-, anteiso- and 4methylalkanes) occur in trace amount. Branched
hydrocarbon skeletons are generally considered to
re¯ect a bacterial input (Albro, 1976; Shiea et al., 1990).
Fig. 8. (a) Ion chromatogram at m/z=167 of the heptaneeluted fraction of the 400 C pyrolysate of f top kerogen; ^,
series 22. (b) Mass spectrum of the C10 isomer.
skeletons are involved through S-linkage in non-polar,
non-GC-amenable, high molecular weight pyrolysis
products, so that they are not observed in the heptaneeluted fraction but released after desulphurisation.
4.4.2.2. Toluene-eluted fraction. This fraction, which
represents 19% of the total pyrolysate, also appears
highly complex and its GC trace shows an important
hump (Fig. 9). Nevertheless, as in the case of the heptane-eluted fraction, numerous series of compounds
were identi®ed via selective detection of characteristic
ions. Identi®ed compounds, listed in Table 4, can be
subdivided into two major groups: ketones and OSC.
Some polyaromatic compounds 34±37 are also detected
in low amounts.
Ketones and especially n-alkan-2-ones often occur in
kerogen pyrolysates where they are supposed to be
derived from the thermal cleavage of ether bonds (van
de Meent et al., 1980; Largeau et al., 1986). Several series of n-alkanones 29 were identi®ed in the toluene
fraction of f top pyrolysate as shown by the ion chromatogram at m/z=58 (Fig. 10). They comprise series of
mid-chain n-alkanones with dierent locations of the
keto group, from C(3) to C(12) (Fig. 11). The occurrence of so many series of mid-chain ketones has been
rarely reported. As suggested by Gillaizeau et al. (1996)
in a study concerned with the kerogen of the GoÈynuÈk oil
shale, such feature should re¯ect the presence of ether
linkages at various locations of the alkyl chain. The
wide range of location of the ether links in f top kerogen
is similar to recent results of Jenisch-Anton et al. (1999)
A. Riboulleau et al. / Organic Geochemistry 31 (2000) 1641±1661
1651
Fig. 9. TIC of the toluene-eluted fraction of the 400 C pyrolysate of f top kerogen.
Fig. 11. Partial mass fragmentograms at m/z= 58, 72, 86, 85,
99, 113 and 127 revealing the presence of C16 n-alkan-2- (!),-3,-4-,-5-,-6-,-7- and-8-ones (*), respectively. Note that n-alkan-2to -4-ones are characterised by an even fragment (58, 72 and 86,
respectively) due to a McLaerty rearrangement, while nalkan-5- to -8-ones are characterised by an odd fragment due to
a cleavage (85, 99, 113 and 127, respectively) (, mid-chain nalkanones).
Fig. 10. Ion chromatogram at m/z=58 of the toluene-eluted
fraction of the 400 C pyrolysate of f top kerogen (!, n-alkan2-ones 29; , mid-chain n-alkanones).
who reported data for two oils (Marvejols and Rozel
Point) and a kerogen (Gibellina), all S-rich, showing the
presence of ether links at dierent locations along alkyl
chains. Cyclic ketones (cyclopentanones 30 and cyclohexanones 31) are minor constituents, while two series
of phenyl ketones (1-phenyl-n-alkan-1-ones 32 and 1phenyl-n-alkan-2-ones 33, structures IV and V, respectively) are present in signi®cant amount. Their origin,
however, is unknown.
Most of the OSC present in this fraction have already
been observed in the heptane fraction and they mainly
consist of short-chain polycyclic compounds. Among
them, bithiophenes 17, benzo[b]thiophenes 19, phenylthiophenes 20, and dibenzothiophenes or naphtothiophenes 21. The four series characterised by the
ions 203 (24), 217 (25), 229 (26) and 243 (27) previously
observed in the heptane-eluted fraction are also present.
The occurrence of 1,2-di-n-alkylbenzenes in the desulphurised fraction is consistent with the thienylbenzothiophene (VI) structure proposed by van
Kaam-Peters and Sinninghe Damste (1997) and van
Kaam-Peters et al. (1998b) for series 26 and 27. Such a
structure, however, cannot be considered for series 24
and 25. Mongenot et al. (1999) observed series with
similar mass fragmentation in the pyrolysate of the Srich kerogen from the Kimmeridgian paleolagoon of
Orbagnoux. On the basis of mass spectra, desulphurisation products and presence of thiochromans in the
extracts of Orbagnoux (van Kaam-Peters and Sinninghe
DamsteÂ, 1997; van Kaam-Peters et al., 1998b), Mongenot et al. (1999) tentatively identi®ed these compounds as tetramethylthiochromenes (VII). So far,
thiochromenes have not been ®rmly identi®ed in rock
extracts, crude oils or kerogen pyrolysates. However,
their saturated counterparts, thiochromans, have been
observed in Oligocene crude oils and rock extracts (Sinninghe Damste et al., 1987; Adam, 1991; Schaeer,
1993; van Kaam-Peters and Sinninghe DamsteÂ, 1997;
1652
A. Riboulleau et al. / Organic Geochemistry 31 (2000) 1641±1661
van Kaam-Peters et al., 1998b) and are formed by substitution of the O atom in the chroman structure by a
sulphur (Adam, 1991). A compound characterised by a
major peak at 203 was previously observed by Adam
(1991) in extracts of Oligocene samples and crude oil
and an isoprenoid trimethylbenzo[b]thiophene structure
(VIII) was considered for this compound. Nevertheless,
the regular pattern of the 203 and 217 series, 24 and 25,
observed in the heptane-eluted fraction of f top pyrolysate (Fig. 12a and c) points to the occurrence of an nalkyl side chain and not of an isoprenoid one in the
present case. Consequently, this pattern is consistent,
neither with the structure proposed by Adam (1991),
nor with a thiochromene structure. In contrast, as discussed below, these structures should correspond to
benzodithiophenes (IX) or thienobenzothiophenes (X).
Indeed, a compound characterised by an intense
mass peak at 190 (23) is observed in the heptane and
toluene fractions of f top pyrolysate which retention
time indicates that it can correspond to the lowest
homologue of the series 24 and 25 (Fig. 12a±d). Comparison with mass spectra of reference compounds
showed that 23 can correspond to benzodithiophene
(IX) or thienobenzothiophene (X). The addition of a nalkyl side chain to one of these structures would give a
series of compounds characterised in MS by a fragment
at 203, and the addition of two n-alkyl side chains
would give a series of compounds characterised in MS
by a fragment at 217. The presence of di-n-alkylbenzenes in the desuphurisation products of the heptaneand toluene-eluted fractions is consistent with this
hypothesis.
After desulphurisation, the toluene-eluted fraction
still shows an important hump, with only a few resolved
peaks corresponding to n-alkanes. The dierent series of
ketones observed before desulphurisation are still
detected but the mass fragmentogram at m/z=58 indicates that mid-chain linear ketones are relatively more
abundant with respect to n-alkan-2-ones than in the
non-desulphurised fraction. It therefore appears that
some ketones are also linked via S-bonds in the pyrolysate of f top kerogen. Such pyrolysis products should
correspond to moieties which were linked both by sulphur and ether bonds in the macromolecular structure
of the kerogen. Similar interpretations were previously
obtained by Richnow et al. (1992) concerning the macromolecular structure of an oil and a kerogen from the
Monterey Formation.
4.4.2.3. Methanol/chloroform-eluted fraction. This fraction which represents 41% of the pyrolysate was separated into an acid and a non-acid subfraction and the
former was esteri®ed by MeOH/MeCOCl prior to GC/
MS analysis (Table 5).
The esteri®ed acid subfraction is dominated by
methylesters of saturated fatty acids 38 from C12 to C30
with a strong predominance of even-carbon-numbered
compounds (CPI=0.18) (Fig. 13). The main components are palmitic acid, n-C16 and stearic acid, n-C18,
Fig. 12. (a) Ion chromatogram at m/z=190+203 of the heptaneeluted fraction of the 400 C pyrolysate of f top kerogen; &,
compound 23; *, series 24; (b) mass spectrum of the compound 23; (c) ion chromatogram at m/z=217; *, series 25; (d)
mass spectrum of a C13 isomer of series 25.
A. Riboulleau et al. / Organic Geochemistry 31 (2000) 1641±1661
1653
Table 5
Compounds detected in the methanol-eluted acid subfraction
of the 400 C pyrolysate of f top kerogen, tr: trace amount
Series
Range
Max.
Rel.
intensity
38 Saturated fatty acids
39 Monounsaturated fatty acids
40 Diunsaturated fatty acids
41 Branched fatty acidsa
C12±C30
C14±C18
C18
C15±C17
C16
C18
C18
±
1
0.3
tr
0.02
a
Anteiso C15 and C17, iso C16.
which is a very common feature in kerogen pyrolysates
(e.g. Kawamura et al., 1986; Largeau et al., 1986).
Long-chain, C20+, fatty acids occur in signi®cant
amount, they represent 17% of the saturated fatty acids.
These C20+ acids are generally considered of terrestrial
origin (Volkman et al., 1980; Barouxis et al., 1988).
However, long chain fatty acids have been observed in
certain algae (e.g. diatoms; Volkman et al., 1980) and a
bacterial origin has also recently been considered (Gong
and Hollander, 1997). Unsaturated fatty acids 39±40,
identi®ed after DMDS derivatisation (Fig. 13, inset), are
dominated by oleic acid (C18:1o9) and a C16 monounsaturated acid (C16:1o10). Oleic acid is ubiquitous but
would chie¯y be of phytoplanktonic origin, whereas the
C16:1o10 acid is considered as a bacterial marker (Barouxis et al., 1988). Other unsaturated acids are also
observed in lower amount, such as C18:2 40, which is
also common in green microalgae (Weete, 1976). The
presence of these unsaturated compounds, known to be
highly sensitive to degradation, attests for a rapid and
early incorporation of lipidic moieties in the kerogen.
Iso- and anteiso- branched acids 41 are observed in low
amounts, re¯ecting bacterial input (Perry et al., 1979;
Goossens et al., 1986).
The non-acid subfraction is dominated by two linear,
saturated, C16 and C18 primary alcohols as previously
observed by Mongenot et al. (1999) in the same subfraction of Orbagnoux pyrolysate. However, contrary to
the latter study, no unsaturated alcohol is observed in f
top pyrolysate. Series of n-alkylphenols from C9 to C16
and n-alkoxyphenols from C10 to C15 are also observed.
Short chain alkyl phenols (C1±C3), derived from lignin
and/or melanoidins (Saiz-Jimenez and de Leeuw, 1986;
Zegouagh et al., 1999) are not detected in the present
case. Long chain n-alkylphenols have previously been
observed in the pyrolysates of various marine kerogens:
two samples from the Kimmeridge Clay Formation
(Gelin et al., 1995), a Cenomanian black shale from
Central Italy (Salmon et al., 1997) and Ordovician
Kukersite from Estonia (Derenne et al., 1990). Kukersite is known to derive from the selective preservation of
the cell walls from a colonial microorganisms, termed
Gloeocapsomorpha prisca which were shown to be the
Fig. 13. TIC of the esteri®ed methanol-eluted acid subfraction
of the 400 C pyrolysate of f top kerogen: ^, methyl esters of
saturated fatty acids; ^, methyl esters of monounsaturated
fatty acids. Inset: ion chromatogram at m/z=61 showing the
unsaturated esters after DMDS derivatisation.
phenol source (Derenne et al., 1990, 1992a). In contrast,
no precise source could be attributed for the long-chain
n-alkylphenols in the Cenomanian black shale (Salmon
et al., 1997), in the Kimmeridge Clay samples (Gelin et
al., 1995) and in the present case as well. Some alkylthiophenes from C11 to C15 and even-carbon-numbered
branched alkanes from C16 to C30 (3-methyl- and 2,2dimethylalkanes) are also present in low amount in the
methanol-eluted, non-acid, subfraction. Due to their
low polarity, such compounds should not appear in this
subfraction and are considered to re¯ect thermal degradation, during GC/MS injection, of high molecular
weight OSCs present in the subfraction. A similar feature was previously observed by Mongenot et al. (1999)
in the case of Orbagnoux kerogen.
4.4.2.4. Desulphurized pyrolysate. Mongenot et al.
(1999) recently observed that, in the case of Orbagnoux
kerogen, desulphurisation of the total pyrolysate yields
hydrocarbons, the distribution of which is very dierent
to that expected on the basis of analytical data from
column-eluted fractions and desulphurized counterparts. Such a discrepancy is mainly due to the high
amount of polar and/or high molecular weight compounds retained on the alumina column for the Orbagnoux pyrolysate (ca. 55%). Although the amount of
retained compounds is less here (23%), the total pyrolysate of f top was desulphurised in order to determine
if these polar and/or high molecular weight compounds
have a speci®c signature.
The GC of the desulphurised pyrolysate is dominated
by a series of n-alkanes from C12 to C31, similar to the nalkanes observed in the heptane-eluted and desulphurised
1654
A. Riboulleau et al. / Organic Geochemistry 31 (2000) 1641±1661
toluene-eluted fractions. The GC trace also contains a
small hump of numerous coeluting products, which all
are observed in the column-eluted fractions with similar
distributions. It therefore appears that in f top pyrolysate, column-eluted and column-retained compounds
have probably the same type of structure, and hence the
same precursors, and the latter mostly dier by higher
molecular weights.
4.4.3. Flash pyrolysis
Recent isotopic and pyrolytic studies on the Kimmeridge Clay Formation (van Kaam-Peters et al.,
1998a,b; Sinninghe Damste et al., 1998) indicated that
short chain alkylthiophenes liberated upon ¯ash pyrolysis of S-rich kerogens can originate from moieties
corresponding to sulphurised carbohydrates in these
kerogens. Flash pyrolysis was therefore performed on f
top kerogen in order to determine the distribution of
short chain alkylthiophenes and assess the contribution
of sulphurised carbohydrates.
The ¯ash pyrogram of f top (not shown) is dominated
by short chain alkylthiophenes, the distribution of
which is shown in Fig. 14. This distribution is dominated by compounds with a linear skeleton, and is close
to the distribution obtained by pyrolysis of sulphurised
carbohydrate-containing kerogens and sulphurised
algae (Sinninghe Damste
www.elsevier.nl/locate/orggeochem
Pyrolytic and spectroscopic study of a sulphur-rich
kerogen from the ``Kashpir oil shales''
(Upper Jurassic, Russian platform)
A. Riboulleau a,b, S. Derenne a,*, G. Sarret c,1, C. Largeau a, F. Baudin b,
J. Connan d
a
Laboratoire de Chimie Bioorganique et Organique Physique, CNRS UMR 7573, ENSCP, 11 rue Pierre et Marie Curie, 75231 Paris cedex 05,
France
b
Laboratoire de Stratigraphie, CNRS ESA 7073, UPMC, 4 place Jussieu, 75252 Paris cedex 05, France
c
Chemistry Department, University of Western Ontario, N6A 5B7, London, Ontario, Canada
d
Elf Aquitaine CSTJF, Avenue Larribau, 64018 Pau cedex, France
Abstract
The kerogen of an organic-rich sample, termed f top, from the Gorodische section (Russian platform) was studied
using a combination of microscopic, spectroscopic and pyrolytic methods so as to examine its chemical structure,
source organisms and formation pathway(s). This kerogen, which is mainly composed of orange gel-like, nanoscopically amorphous organic matter, exhibits a relatively high aliphatic character; organic sulphur is mainly present as
di(poly)sulphides and alkylsulphides. The f top kerogen was chie¯y formed via intermolecular incorporation of sulphur
in algal or cyanobacterial lipids and carbohydrates. However, its formation also involved oxidative condensation via
ether linkages. Comparison of f top sample with other S-rich kerogens points to a closer similarity with Monterey
kerogens rather than with a kerogen from the bituminous laminites of Orbagnoux. # 2000 Elsevier Science Ltd. All
rights reserved.
Keywords: Kashpir oil shales; Pyrolysis; FTIR; Solid state 13C NMR; XANES spectroscopy; Type II-S kerogen
1. Introduction
The Upper Jurassic was a period of intense accumulation of sedimentary organic matter (OM) and important gas and oil source-rocks are Upper Jurassic in age
(Ulmishek and Klemme, 1990), especially in the northern hemisphere, including the North Sea and Siberian
oil ®elds. Episodes of extensive OM deposition also
took place on the Russian Platform, located between
these two basins, but the relative stability of this platform since the Triassic did not allow sucient burial
* Corresponding author. Tel.: + 33-1-4427-6716; fax: + 331-4325-7975.
E-mail address: sderenne@ext.jussieu.fr (S. Derenne).
1
Current address: LGIT-Groupe de GeÂochimie de l'Environnement, Universite Joseph Fourier, BP 53, 38041 Grenoble
cedex, France.
of OM for hydrocarbon production. Although such
episodes are relatively less numerous on the Russian
platform than in the neighbouring basins, large accumulation occurred during the Middle Volgian (Late
Tithonian, 140 Ma) and several basins of the Russian
platform display organic-rich levels of this age (Shmur
et al., 1983). In the Ulyanovsk region, these organic-rich
sediments crop out along the Volga river and are known
as the ``Kashpir oil shales''. At Gorodische (Fig. 1), this
organic-rich deposit represents a 6 m thick layer of grey
to dark-brown shales whose TOC contents vary between
0.5 and 45% and HI between 50 and 700 mg HC/g TOC
(Hantzpergue et al., 1998). The Kashpir oil shales
represent, after the Baltic oil shales, one of the most
important oil shale reserves of Russia and they have
been mined for oil production since the 1850s (Shmur et
al., 1983; Russell, 1990). However, production from
these oil shales has almost ceased today because they are
0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(00)00088-7
1642
A. Riboulleau et al. / Organic Geochemistry 31 (2000) 1641±1661
quantity. The kerogen of f top was examined by a combination of various methods in order to characterise its
structure, to specify the nature of the sulphur functions
and to determine the source organisms of the OM and
its preservation pathway(s), in this especially rich level.
2. Geological setting and sampling
Fig. 1. Location map of the Gorodische outcrop.
characterised by high sulphur contents, and particularly
Sorg, leading to high atmospheric pollution upon
reforming (Popov et al., 1986). Nevertheless these shales
are still mined near Kashpir village for pharmaceutical
and industrial purposes.
S-rich kerogens have been intensively studied during
the last decade and it is now well established that this
type of fossil OM originates from incorporation of
reduced mineral sulphur (H2S, S0 or S2ÿ
X ) in functionalised lipids or carbohydrates during early diagenesis (e.g.
Francois, 1987; review by Sinninghe Damste and de
Leeuw, 1989; van Kaam-Peters et al., 1998a). S-rich OM
is mostly found in sediments from evaporitic and anoxic
carbonate platform environments (Sinninghe Damste et
al., 1989; Schaeer et al., 1995; Mongenot et al., 1997;
Baudin et al., 1999). Sulphur incorporation, from the
H2S produced by sulphate-reducing bacteria under
anoxic conditions, can occur during very early diagenesis in the upper layers of the sediment (Hartgers et al.,
1997).
Several paleoecological indices in the Gorodische
black shales, like the presence of bioturbations and
brachiopods in several levels, testify for oxygenation of
the upper centimeters of the sediments, at least during
part of the deposition of these black shales. The results
presented here, concerning the most OM-rich level,
termed ``f top'', are part of a broader study. This study
aims at determining the paleoenvironmental conditions
that led to the deposition of these black shales and
explaining their strong variations in OM quality and
The Gorodische outcrop is located near the city of
Ulyanovsk along the Volga river (Fig. 1). It is mainly
composed of grey clays of Middle Volgian age (Fig. 2).
At the top of the middle Volgian, appears the 6 m thick
black shale unit. Its colour varies from dark grey to
brown and despite a laminated feature, bioturbations
are visible in the lower and upper parts of the unit. It is
overlain by sandstone of Middle to Upper Volgian age.
A more detailed description of the section is given in
Hantzpergue et al. (1998). TOC and HI values for this
organic-rich formation are presented in Fig. 2. The f top
level which is studied in this paper, is a 7 cm thick bed
located 2.70 m above the base of the OM-rich deposit
and presents the highest TOC and HI values (Fig. 2). In
contrast to most part of the formation, no bioturbation
nor benthic organisms are observed in this level.
3. Experimental
Shale samples were collected in 1995. Subsamples
were ground for Rock-Eval analysis, the remainder was
stored at room temperature away from light. Prior to
further analysis, the surface of the sample was carefully
removed in order to eliminate oxidised or polluted
material.
Rock-Eval pyrolysis, OSA device, was performed on
10 mg samples of powdered bulk shale. The sample was
heated at 300 C for 3 min followed by a programmed
pyrolysis at 25 C/min up to 600 C under a He ¯ow and
then oxidised at 600 C for 7 min under an oxygen ¯ow.
After grinding, ca. 50 g of shale were extracted with
CHCl3/MeOH, 2:1, v/v (stirring for 12 h at room temperature) before isolation of the kerogen via the classical
HCl/HF treatment (Durand and Nicaise, 1980). The
kerogen concentrate was then extracted as described
above and dried under vacuum.
An aliquot of the kerogen was ®xed with 2% OsO4
for electron microscopy. For transmission electron
microscopy (TEM), the ®xed kerogen was embedded in
Araldite, cut in ultra-thin sections and stained with
uranyl acetate and lead citrate. Observations were carried out with a Philips 300 microscope. For scanning
electron microscopy (SEM), the ®xed kerogen was
dehydrated using the CO2 critical point technique and
coated with gold prior to observation with a Jeol 840
microscope.
A. Riboulleau et al. / Organic Geochemistry 31 (2000) 1641±1661
1643
Fig. 2. Stratigraphic section of Gorodische outcrop (after Hantzpergue et al., 1998) and variations of TOC and HI along the organicrich formation.
FTIR spectra of the kerogen and its pyrolysis residues
were recorded on a Bruker IFS 48 spectrometer as 5 mm
KBr pellets. Solid state 13C NMR spectroscopy was
performed on the kerogen and its pyrolysis residues with
a Bruker MSL 400 spectrometer using high power
decoupling, cross polarisation and magic angle spinning. Spectra were recorded at 3 and 4 kHz in order to
discriminate spinning side bands (SSB).
X-ray absorption near edge structure (XANES) spectroscopy was performed at the Canadian Synchrotron
Radiation Facility situated on the 1 GeV electron storage ring, Alladin, University of Wisconsin. Experimental details have been described previously (Kasrai et
al., 1994). All the S K- and L-edge spectra presented in
this paper were recorded using total yield (TEY) detection mode. They were background subtracted using a
linear function extrapolated from the pre-edge region,
and then normalised to the height of the maximum of
peak B for the L-edge, and to the height of the edge
jump for the K-edge. S K-edge spectra were simulated
by a linear combination of reference spectra using a
least-squares ®tting program. Analytical procedure as
well as interpretation and comparison with standard
spectra are described in Sarret et al. (1999).
Elemental analyses of the kerogen and of its pyrolysis
residues were performed at the Service Central d'Analyse of the CNRS. The O content of the unheated kerogen was determined by coulometry (Laboratoires
Wol).
Bulk isotopic measurement of d13C was performed on
the kerogen with a Carlo-Erba CHN coupled to a VGSIRA 10 spectrometer.
O-line pyrolysis was performed on the kerogen as
previously described by Largeau et al. (1986). Brie¯y,
the sample is successively heated at 300 C for 20 min
and 400 C for 1 hour under an He ¯ow. The released
products are trapped in cold chloroform. After each
treatment, the residue is extracted with CHCl3/MeOH
as previously described. The ®rst thermal treatment at
300 C was aimed at eliminating thermolabile components. It has been recently demonstrated that such a
thermal treatment, when applied to S-rich kerogens also
promotes some aromatisation and lowers the global
eciency of the subsequent cracking at 400 C. However, it was also shown that more detailed information
on the chemical structure of such kerogens can ®nally be
obtained by this two step treatment at 300 and 400 C
than via direct pyrolysis at 400 C (Mongenot, 1998;
Sarret et al., unpublished results).
The 400 C pyrolysate was separated by column chromatography (Al2O3, Act II) into three fractions of
increasing polarity eluted with heptane, toluene and
methanol, respectively. An additional elution with
CHCl3 was performed, and yielded a small amount of
products which were combined with the MeOH-eluted
fraction. Carboxylic acids were separated from the
MeOH±CHCl3 fraction using a double extraction with
ether under base and acid conditions and analysed by
GC and GC/MS as their methyl esters. Unsaturated
methyl esters were further analysed after derivatisation
with DMDS as described by Scribe et al. (1988).
A part of the total 400 C pyrolysate and of its heptane- and toluene-eluted fractions was desulphurised
with Raney Ni using the conditions previously described
by Sinninghe Damste et al. (1988a) and hydrogenated
before GC and GC/MS analyse.
All the fractions were analysed by GC and GC/MS
using a HP 5890 gas chromatograph (60 m capillary
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A. Riboulleau et al. / Organic Geochemistry 31 (2000) 1641±1661
column, ®lm thickness of 0.4 mm, heating program 100
to 300 C at 4 C minÿ1, injector and FID at 320 C,
helium carrier gas), coupled with a HP 5989 mass spectrometer with a mass range m/z= 40±600, operated at
70 eV.
Curie point pyrolysis-gas chromatography±mass
spectrometry (CuPy-GC±MS) was performed on ca. 2
mg of sample. The sample was loaded on a ferromagnetic wire with a Curie point of 610 C. The wire placed
in a glass tube was introduced in the Curie point pyrolyser (Fisher 0316M) coupled to a Hewlett-Packard
HP 5890 gas chromatograph with FID (30 m fused silica
capillary column, ®lm thickness 0.4 mm, heating program 35 C for 10 min and from 35 to 300 C at 4 C
minÿ1, He carrier gas). The chromatograph was coupled
with a HP 5989 mass spectrometer with a mass range
m/z= 40±600, operated at 70 eV.
4. Results
4.1. Bulk features
Rock-Eval pyrolysis of the unextracted shale shows an
exceptionally high TOC of 45% for f top and a high
hydrogen index (HI) of 700 mg HC/g TOC (Fig. 2). The
Tmax value is very low, 396 C, re¯ecting the immaturity
of the sample. Bulk isotopic measurement indicate a
d13CTOC of ÿ20.5 % (PDB) for f top kerogen. Elemental
analysis of the isolated kerogen (Table 1) indicate a
relatively high H/C ratio of 1.39 consistent with the high
HI, an O/C ratio of 0.20, a low N/C ratio of 0.014, a high
content of total S of 11.7 wt.% and a weak Fe content of
0.67 wt.%. Sorg content was then calculated in the classical way: it was considered that Fe is only present as
pyrite after the acid treatment and that non-pyritic sulphur is organic, giving a high Sorg/C ratio of 0.068. F top
thus belongs to Type II-S kerogens, according to the
classi®cation of Orr (1986). Sulphur-rich kerogens are
known to exhibit a relatively low thermal stability
(Baskin and Peters, 1992; Tomic et al., 1995) thus
explaining the very low Tmax value observed for f top.
organic matter (AOM) associated with rare pyrite. This
AOM is yellow to orange and appears as gel-like particles of varying size (from 5 to 200 mm). When observed
by SEM and TEM, the AOM appears homogeneous
and amorphous, even at high magni®cation (nanometric
scale). Similar morphological features have been previously observed in S-rich kerogens from the Kimmeridge Clay Formation (Boussa®r et al., 1995) and the
paleolagoon of Orbagnoux (Mongenot et al., 1999).
4.3. Spectroscopic study
4.3.1. Solid state 13C NMR spectroscopy
The solid state 13C NMR spectrum of f top kerogen
(Fig. 3a) is dominated by a broad peak at 30 ppm with a
small shoulder at 15 ppm, corresponding to CH2 in
alkyl chains and to CH3 groups, respectively. Such a
dominance re¯ects the aliphatic character of the sample,
in agreement with both high H/C ratio and HI value. A
relatively intense broad signal centred at 75 ppm corresponding to aliphatic C linked to N or O is also noticed.
According to the weak N/C ratio revealed by elemental
analysis the latter signal must be chie¯y due to C±O
bonds. The 30 ppm peak exhibits a broad shoulder
between 40 and 60 ppm, which can be assigned to carbons in C±S bonds and/or to C b to the functions that
gave the 75 ppm signal. Two weak signals at 130 and
175 ppm are observed corresponding to unsaturated
carbons and to C in carboxylic groups, respectively.
Signals observed at 140 and 215 ppm correspond to
spinning side bands of the carboxylic peak as shown by
the comparison of the spectra recorded at two dierent
spinning rates.
4.2. Microscopic study
Under the light microscope, f top kerogen appears
chie¯y constituted of a single type of amorphous
Table 1
Atomic ratios of f top kerogen and insoluble pyrolysis residues
f top
f top res 300
f top res 400
H/C
Sorg/C
N/C
1.39
1.19
0.57
0.068
0.047
0.030
0.014
0.016
0.021
Fig. 3. Solid state CP/MAS 13C NMR spectra of (a) f top
kerogen and (b) of its 300 C pyrolysis residues. (X, O and/or
N; SSB, spinning side bands).
A. Riboulleau et al. / Organic Geochemistry 31 (2000) 1641±1661
1645
4.3.2. FTIR spectroscopy
The FTIR spectrum of f top kerogen (Fig. 4a) shows
intense absorptions at 2920, 2850, 1445 and 1375 cmÿ1
due to CH2 and CH3 groups. Oxygenated functions are
detected as (i) a broad band of medium intensity centred
at 3400 cmÿ1 corresponding to O±H groups, (ii) a band
at 1700 cmÿ1 (CO), and (iii) a broad and intense band
centred at 1050 cmÿ1 corresponding to C±O bonds, in
agreement with the relatively strong signal at 75 ppm
observed in the 13C NMR spectrum. A band at 1630
cmÿ1 is also observed indicating that the unsaturated C
detected at 130 ppm via 13C NMR are mostly ole®nic. A
narrow band with a rather high intensity is noted at 750
cmÿ1; it is due to CaF2 neoformed during the HCl/HF
treatment since the latter mineral was identi®ed by
XRD (A. Person, pers. comm.).
Fig. 4. FTIR spectra of (a) f top kerogen and of its (b) 300 and
(c) 400 C pyrolysis residues.
4.3.3. XANES spectroscopy
Sulphur K-edge spectrum for f top kerogen is presented in Fig. 5a. The spectrum was simulated by a linear combination of spectra of model compounds in
order to quantify the dierent sulphur species which can
be dierentiated by this mode, i.e. Sÿ2, disulphides,
alkyl and/or heterocyclic sulphides, sulphoxides, sulphones, sulphonates and sulphates (Sarret et al., 1999).
Results of the simulation are presented in Table 2. The
major sulphur species in f top kerogen are alkyl and/or
heterocyclic sulphides (76%) and di(poly)sulphides
occur in lower proportion (11%). A small contribution
of sulphoxides, sulphonates and sulphates is also
noticed.
S L-edge spectrum for f top kerogen is compared with
spectra of various reference compounds in Fig. 6. From
S K-edge results, one could foresee spectral similarities
with the alkylsulphide and/or heterocyclic sulphide
(thiophene) references. These two types of compounds,
which are dicult to distinguish on K-edge spectra,
exhibit clearly dierent peak positions on L-edge spectra,
Fig. 5. S K-edge XANES spectra (solid lines) and simulations (dotted lines) calculated as indicated in Table 2 for (a) f top kerogen
and its (b) 300 and (c) 400 C pyrolysis residues.
1646
A. Riboulleau et al. / Organic Geochemistry 31 (2000) 1641±1661
Table 2
Distribution of the sulphur species in f top kerogen and insoluble pyrolysis residues determined by simulation of the S K-edge XANES
spectra
f top
f top res 300
f top res 400
FMa
S(-II)
Disulphides
S aliph./heterocycl.b
Sulphoxides
Sulphones
Sulphonates
Sulphates
0.08
0.05
0.18
0
0
4
11
0
0
76
94
84
5
5
9
0
0
0
4
0
0
4
1
3
a
FM: ®gure of merit of the ®t, FM= S(f kerogen-f ®t)2
As indicated by S L-edge spectroscopy, these reduced forms correspond to (i) alkylsulphides for the unheated kerogen, (ii) alkyl
and heterocylic sulphides for the 300 C residue, (iii) heterocyclic sulphides for the 400 C residue.
b
as this latter method is more sensitive to the reduced
forms of sulphur (Kasrai et al., 1996a). F top spectrum
is clearly closer to the alkyl sulphide reference (Fig. 6-2)
than to the thiophene (Fig. 6-5) and dibenzothiophene
(Fig. 6-4) ones; it is therefore concluded that alkyl sulphides are the major sulphur species in f top kerogen.
However, f top spectrum is slightly shifted to the left
compared to the reference and presents a small shoulder
at 162.8 eV. This spectrum is well reproduced by a linear
combination of 30% di(poly)sulphides+70% alkyl sulphides (Fig. 6a, dotted line). Contrary to the K-edge
mode, S L-edge XANES spectroscopy does not allow a
precise quanti®cation of sulphur species due to (i) some
uncertainties on the normalisation of the spectra, and
(ii) the fact that only one part of the spectrum is simulated (Sarret et al., 1999). Accordingly, S L-edge results
are not directly comparable with K-edge results, e.g.
concerning the di(poly)sulphide content (30% compared
to 11%, respectively). However, S L-edge analysis
allows to conclude that the major reduced sulphur species correspond to alkyl sulphides, and to a lesser extent
di(poly)sulphides. The f top spectrum also contains a
small peak at about 167.6 eV, which corresponds to the
position of the main peak for the sulphoxide reference
(Fig. 6-6), which is consistent with K-edge results.
4.4. Pyrolytic study
4.4.1. Mass balance and spectroscopic features of oline pyrolysis residues
After the thermal treatment at 300 C, the weight loss
amounts to ca. 18% of the initial organic matter, mainly
corresponding to volatile compounds (17%) while trapped products only represent 1% of the initial OM. After
pyrolysis at 400 C, the total weight loss represents 58%
of the initial OM. This loss corresponds in similar
amount to volatile and trapped products (ca. 30 and
28% of the initial OM, respectively).
Elemental analyses of the pyrolysis insoluble residues
(Table 1) show, as expected, a small decrease in the H/C
ratio upon 300 C heating whereas a large decrease takes
place upon pyrolysis at 400 C. The N/C ratio shows a
slight increase upon pyrolysis which is consistent with
the results of Barth et al. (1996) and of Gillaizeau et al.
(1997) attesting for the concentration of N in kerogen
residues after pyrolysis. In contrast, the Sorg/C ratio
regularly decreases.
The 13C NMR spectrum of the 300 C residue (Fig.
3b) is still dominated by the peak centred at 30 ppm but
the relative intensity of the 40±60 ppm shoulder is lower
compared to the unheated kerogen. The 75 ppm peak
exhibits a lower intensity and becomes thinner; in the
same time, the peak at 130 ppm increases and the carboxyl peak at 175 ppm remains approximately constant
with respect to the 30 ppm band. The important peak at
165 ppm corresponds to a spinning side band of the 130
ppm peak. As expected, the 13C NMR spectrum of the
400 C residue (not shown) is dominated by an aromatic
signal at 130 ppm and only shows a weak signal at 30
ppm. This indicates the intense aromatisation of the
material.
The FTIR spectrum of the 300 C residue (Fig. 4b)
shows the same absorptions as the unheated kerogen.
The shift of the CC signal towards 1600 cmÿ1 indicates some aromatisation at this temperature as previously observed for the Orbagnoux kerogen
(Mongenot, 1998). The C±O absorption decreases sharply at 300 C, in agreement with the decrease of the 75
ppm signal in the 13C NMR spectrum. In the 400 C
residue (Fig. 4c) the absorptions corresponding to CH2
and CH3 are of much lower intensity in agreement with
the large decrease of the H/C ratio, the CC band is
broader and now centred at 1600 cmÿ1, corresponding
to aromatic carbons and the broad band around 1100
cmÿ1, due to C±O bonds, is no longer detected.
The S K- and L-edge XANES spectra for the insoluble pyrolysis residues are presented in Figs. 5b and c
and 6b and c, respectively. The edge of the K-edge
spectrum for the 300 C residue is slightly shifted to
higher energy compared to the unheated kerogen. This
shift corresponds to the removal of di(poly)sulphides
upon heating. Indeed, the simulation shows that the
di(poly)sulphides are no longer present in the 300 C
residue (Table 2). The edge of the spectrum for the
400 C residue is even more shifted to the right. As the
di(poly)sulphides are already eliminated in the 300 C
A. Riboulleau et al. / Organic Geochemistry 31 (2000) 1641±1661
Fig. 6. S L-edge XANES spectra for some reference compounds: (1) dl-cystine, (2) dl-methionine, (3) poly(phenylene
sulphide), (4) dibenzothiophene, (5) 3-(2-thienyl)-dl-alanine,
(6) dl-methionine sulphoxide. S L-edge XANES spectra (solid
lines) and simulations (dotted lines) for (a) f top kerogen and its
(b) 300 and (c) 400 C pyrolysis residues. Simulations consist of
a linear combination of the reference compounds spectra presented above. These combinations are the following: f top: 30%
(1)+70% (2), 300 C residue: 40% (2)+60% (5), 400 C residue:
70% (5)+30% (6).
1647
residue, this shift can only be attributed to a change in
the alkyl/heterocyclic sulphides distribution. S L-edge
analysis will provide more information about this
change. Concerning the oxidised sulphur species, the
oscillation situated at about 2481 eV is weaker for the
300 C residue than for f top, so their contribution is
smaller. Indeed, Table 2 shows that their content is of 6
and 13%, respectively. The 400 C residue contains
about the same total amount of oxidised species as f top
(12%), but more sulphoxides. This latter sample also
contains 4% of sulphur in the oxidation state (-II),
which gives rise to a small shoulder at 2469 eV. This
oxidation state does not exist for organic sulphides,
whose oxidation number is 0. Therefore, the 400 C
residue contains a small amount of inorganic sulphides
that has formed upon heating.
The S L-edge spectra for the residues are compared to
f top spectrum in Fig. 6. The spectrum of the 300 C
residue is shifted to higher energy compared to f top,
and its three peaks roughly match with those of the
reference corresponding to simple thiophene. However,
the amplitude of the peak at 164.7 eV is particularly
high, which indicates the presence of alkyl sulphides. A
good ®t is obtained with a linear combination of 60%
simple thiophene+40% alkyl sulphide. If we replace
simple thiophene (i.e. not included in a polyaromatic
structure) by dibenzothiophene (DBT), the position of
the 164.7 eV peak does not match so well. Thus, the
300 C residue contains thiophenes as major species, and
to a lesser extent alkyl sulphides. No di(poly)sulphides
are detected in this sample, which is in agreement with
K-edge results. For the 400 C residue, the L-edge spectrum shows peak positions similar to that of the 300 C
residue, but with dierent relative intensities. Alkylsulphides are no longer observed and the linear combination of 70% thiophene+30% sulphoxide aords a good
®t. Again, if we replace simple thiophene by DBT, the
peak at 164.6 eV does not match. As explained previously, the percentages determined on S L-edge spectra
do not aord precise quantitative information on sulphur distribution; however they re¯ect the changes in
disulphide, alkyl sulphide and thiophene contents upon
thermal stress. During heating, the disulphides (300 C)
and the sulphides (400 C) present in f top disappear
while the relative abundance of thiophenes increases.
These trends are consistent with the decrease of the 40±
60 ppm shoulder and relative increase of the 130 ppm
signal corresponding to aromatic carbons in the 13C
NMR spectrum of the 300 and 400 C residues, compared to that of the kerogen. Such changes are also
responsible for the shifts observed on the K-edge spectrum. It can be noted that the sulphoxide contribution
for the 400 C residue, calculated by S L-edge spectroscopy, is particularly high compared to its actual content determined via K-edge analysis (9%). This
disagreement can be due to a higher oxidation of the
1648
A. Riboulleau et al. / Organic Geochemistry 31 (2000) 1641±1661
Fig. 7. TIC of the heptane-eluted fraction of the 400 C pyrolysate of f top kerogen (a) before and (b) after desulphurisation by Raney Ni and hydrogenation (*, n-alkanes 1; *,
pollutants).
surface of the samples, as the sampling depths of S Kand L-edge XANES spectroscopy are dierent. Indeed,
using TEY detection mode, it is about 70 nm for the Kedge compared to only 5 nm for S L- edge (Kasrai et al.,
1996b). Similar features have been previously observed
on asphaltene and kerogen samples (Sarret et al., 1999;
Sarret et al., unpublished results).
4.4.2. O-line pyrolysis products
The GC trace of the 400 C pyrolysate is very complex
and shows an important hump, due to the coelutions of
numerous products as commonly observed for pyrolysates of S-rich kerogens (Sinninghe Damste et al.,
1990). In order to make easier GC/MS identi®cations,
the pyrolysate was fractionated into three fractions of
increasing polarity, eluted with heptane, toluene and
methanol, respectively.
4.4.2.1. Heptane-eluted fraction. This fraction represents
17% of the pyrolysate. Its GC trace (Fig. 7a) is still
complex and exhibits an important hump due to
numerous coelutions. Nevertheless, selective ion detection (SID) of characteristic fragments allowed the identi®cation of a number of compounds and homologous
series (Table 3). However, due to coelutions, their rela-
tive abundance could not be determined and only a
rough estimation of their relative intensities can be
given. Hydrocarbons mainly consist of n-alkane 1/n-alk1-ene 2 doublets up to C33 (maximum C17) without any
odd- or even- carbon-number predominance. n-Alkylcyclohexanes 9 and cyclopentanes 10 are observed in
small amount. The former are frequently reported in
kerogen pyrolysis products (Homann et al., 1987),
however, their origin is still a matter of debate.
n-Alkylbenzene 5/n-alkenylbenzene 7 doublets are also
observed, associated with the three n-alkylmethylbenzene isomers 6, the o-isomer being predominant. It can
be noticed that the latter isomer is the only one which
can be formed by cyclisation of linear compounds.
Polysubstituted alkylbenzenes 8 are also observed in low
amount. Isoprenoid compounds correspond to prist-1ene 3 and regular saturated C16 and C18 hydrocarbons
4, but their abundance relative to n-alkanes could not be
determined. Neither pristane, nor phytane was detected,
possibly because of a too low abundance. The other
isoprenoid compounds identi®ed are hopanes and
hopenes 28 from C27 to C31. These compounds are generally considered to be of bacterial origin (Rohmer et
al., 1984).
Numerous series of organic sulphur compounds
(OSC) are identi®ed and their coelution accounts for the
bulk of the hump. Several isomers of alkylated thiophenes 11-14, thiolanes 15, thianes 16 and benzothiophenes 19 are identi®ed. These compounds are major
components of most pyrolysates of S-rich kerogens
(Sinninghe Damste et al, 1988b; Payzant et al., 1989).
Most of the identi®ed series of OSC have a linear skeleton. However, C10 to C16 2,3-dimethyl-5-n-alkylthiophenes 13 [identi®ed by their mass spectra and elution
time from Sinninghe Damste et al. (1989)], are also
observed along with branched alkylthiophenes 14 from
C15 to C17. Dierent series of polyaromatic OSC previously observed by van Kaam-Peters and Sinninghe
Damste (1997) and van Kaam-Peters et al. (1998b) in
the pyrolysate of S-rich kerogen from the paleolagoon
of Orbagnoux, are also detected: C9 to C18 alkylated
bithiophenes 17±18 (Appendix I, characteristic fragments at m/z= 179, 193 and 207) and C10 to C13 nalkylphenylthiophenes 20 (II, characteristic fragments at
m/z= 173 and 187). Other series of compounds previously detected in the pyrolysate of Orbagnoux kerogen are also observed in f top pyrolysate: series 26 and
27 characterised by intense fragments at m/z=229±230
and 243±244 (van Kaam-Peters and Sinninghe DamsteÂ,
1997; van Kaam-Peters et al., 1998b; Mongenot et al.,
1999), and series 24 and 25 characterised by intense
fragments at m/z=203 and 217, respectively, (Mongenot et al., 1999). As these compounds are also present
in the toluene-eluted fraction (Table 4), the assignment
of these dierent series is discussed later on. Two series
22, not reported so far, characterised by intense frag-
A. Riboulleau et al. / Organic Geochemistry 31 (2000) 1641±1661
1649
Table 3
Compounds detected in the heptane-eluted fraction of the 400 C pyrolysate of f top kerogen
Series
Range
Maximum
1 n-Alkanes
2 n-Alk-1-enesa
3 Prist-1-ene
4 Regular isoprenoid alkanes
5 n-Alkylbenzenes
6 n-Alkylmethylbenzenesb
7 o n-Alkenylbenzenes
8 Substituted n-alkylbenzenesb
9 n-Alkylcyclohexanes
10 n-Alkylcyclopentanes
11 2 n-Alkylthiophenes
12 2,5-Di-n-alkylthiophenesc
13 2,3-Dimethyl-5-n-alkylthiophenes
14 Branched alkylthiophenesb
15 2-n-Alkylthiolanesd
16 2-n-Alkylthianes
17 2-n-Alkyl-5,50 bithiophenes
18 2,20 -Di-n-alkyl-5,50 -bithiophenes
19 n-Alkylbenzo[b]thiophenese
20 n-Alkylphenylthiophenesf
21 n-Alkyldibenzo- or naphtothiophenes
22 n-Alkylmethylthienothiophenesg
23 Compound in 190
24, 25 Series in 203±217b
26, 27 Series in 229±243b
28 Hopanes and hopenes
C13±C35
C13±C33
C19
C16, C18
C12±C32
C12±C33
C13±C29
C12±C26
C13±C26
C13±C27
C10±C28
C10±C28
C10±C16
C15±C17
C9±C25
C10±C21
C9±C18
C10±C18
C9±C22
C10±C13
C12±C15
C8±C15
C10
C11±C19
C13±C18
C27±C31
C17
C16
C19
±
C15
C14
C15±C16
C14
C17
C16
C12
C13
C13
±
C12
C13
C10
C11±C12
C10
C11
C12±C13
C9
C10
C11±C12
C13±C14
±
a
A series of n-alk-2-enes was also identi®ed with the same range and a maximum at C18.
Several isomers were detected in the series.
c
Three series were detected: C10±C30 (max C13) 2-n-alkyl-5-methylthiophenes, C10±C29 (max C12) 2-n-alkyl-5-ethylthiophenes and
C11±C25 (max C12) 2-n-alkyl-5-propylthiophenes.
d
2-n-Alkyl-5-methylthiolanes (C10±C21, max C13) were also observed.
e
Two series (2-n-alkyl- and 4-n-alkylbenzo[b]thiophenes) were observed with the same distribution. Two series of n-alkylmethylbenzo[b]thiophenes (C10±C20, max C11) were also observed.
f
A series of C12±C15 (max C13) n-alkylmethylphenylthiophenes was also observed.
g
A series of C9±C16 (max C10) n-alkyldimethylthienothiophenes was also observed.
b
ments at m/z= 167 and 181 and a molecular ion at
168+14n and 182+14n (n from 1 to 10), respectively,
are also observed. The regular distribution pattern of
these two series (e.g. Fig. 8a) points to the presence of a
n-alkyl side chain and their disappearance after desulphurisation indicate that they correspond to OSC.
These compounds are tentatively identi®ed as n-alkylmethyl- and n-alkyldimethylthienothiophenes (III) on
the basis of their mass spectra (e.g. Fig. 8b). Thienothiophenes have already been observed in crude oils
(Orr and Sinninghe DamsteÂ, 1989) but, as far as we are
aware, alkylated homologues of such compounds have
not been reported so far.
Following Raney Ni desulphurisation and hydrogenation, the heptane fraction is dominated by a series of
n-alkanes (Fig. 7b), thus con®rming that most of the
OSC have a linear skeleton. Nevertheless, branched and
isoprenoid compounds are also present in small
amounts. In particular, pristane and phytane are
observed, which were not detected in the non-desulphurised fraction. Pristane must be directly derived from
hydrogenation of prist-1-ene. Phytane can originate
from hydrogenation of phytenes and/or desulphurisation of C20 isoprenoid OSCs. The latter have been commonly observed in extracts and kerogen pyrolysates
(Sinninghe Damste and de Leeuw, 1987, and references
therein). Based on previously published mass spectra
and elution times (Sinninghe Damste et al., 1986; Sinninghe Damste and de Leeuw, 1987), C20 isoprenoid
OSCs were searched for in the untreated fraction but
none could be detected. Therefore, it is possible that
several C20 isoprenoid OSCs are present but that they
are undetectable amongst the coelution hump due to
too low individual abundances and/or that phytane
1650
A. Riboulleau et al. / Organic Geochemistry 31 (2000) 1641±1661
Table 4
Compounds detected in the toluene-eluted fraction of the 400 C pyrolysate of f top kerogen
Series
a
29 n-Alkan-2-ones
30 n-Alkylcyclopentanones
31 n-Alkylcyclohexanones
32 1-Phenyl-n-alkan-1-ones
33 1-Phenyl-n-alkan-2-onesb
34 Alkyl¯uorenesc
35 Alkylanthracenes or phenanthrenesc
36 Alkylpyrenes
37 Alkylbenzo¯uorenes
17 2-n-Alkyl-5,50 -bithiophenesc
20 n-Alkylphenylthiophenesc
19 n-Alkylbenzo[b]thiophenes
21 n-Alkyldibenzo- or naphtothiophenesc
23 Compound in 190
24, 25 Series in 203±217
26, 27 Series in 229±243
Range
Maximum
C10±C29
C11±C24
C11±C18
C10±C16
C9±C29
C13±C16
C14±C17
C14±C15
C12±C13
C9±C11
C11±C15
C9±C12
C12±C15
C10
C11±C14
C13±C15
C13
C13
C14
C12
C12
C14
±
±
±
C10
C12
C12
±
C10
±
±
a
Other series of ketones were also observed: n-alkan-3-ones, distr.: C11±C26 (C14); n-alkan-4-ones, C11±C25 (C13); n-alkan-5-ones,
C11±C27 (C16, C19); n-alkan-6-ones, C12±C21 (C13); n-alkan-7-ones, C13±C24 (C16); n-alkan-8-ones, C15±C26 (C17); n-alkan-9-ones,
C17±C26 (C18); n-alkan-10-ones, C19±C24 (C19); n-alkan-11-ones, C21±C26 (C21); n-alkan-12-ones, C23±C26(C23).
b
Three series were observed with the same distribution: 1-phenyl-n-alkan-2-ones and two 1-(methylphenyl)-n-alkan-1-ones.
c
Numerous isomers were detected in the series.
Three series of branched alkanes (iso-, anteiso- and 4methylalkanes) occur in trace amount. Branched
hydrocarbon skeletons are generally considered to
re¯ect a bacterial input (Albro, 1976; Shiea et al., 1990).
Fig. 8. (a) Ion chromatogram at m/z=167 of the heptaneeluted fraction of the 400 C pyrolysate of f top kerogen; ^,
series 22. (b) Mass spectrum of the C10 isomer.
skeletons are involved through S-linkage in non-polar,
non-GC-amenable, high molecular weight pyrolysis
products, so that they are not observed in the heptaneeluted fraction but released after desulphurisation.
4.4.2.2. Toluene-eluted fraction. This fraction, which
represents 19% of the total pyrolysate, also appears
highly complex and its GC trace shows an important
hump (Fig. 9). Nevertheless, as in the case of the heptane-eluted fraction, numerous series of compounds
were identi®ed via selective detection of characteristic
ions. Identi®ed compounds, listed in Table 4, can be
subdivided into two major groups: ketones and OSC.
Some polyaromatic compounds 34±37 are also detected
in low amounts.
Ketones and especially n-alkan-2-ones often occur in
kerogen pyrolysates where they are supposed to be
derived from the thermal cleavage of ether bonds (van
de Meent et al., 1980; Largeau et al., 1986). Several series of n-alkanones 29 were identi®ed in the toluene
fraction of f top pyrolysate as shown by the ion chromatogram at m/z=58 (Fig. 10). They comprise series of
mid-chain n-alkanones with dierent locations of the
keto group, from C(3) to C(12) (Fig. 11). The occurrence of so many series of mid-chain ketones has been
rarely reported. As suggested by Gillaizeau et al. (1996)
in a study concerned with the kerogen of the GoÈynuÈk oil
shale, such feature should re¯ect the presence of ether
linkages at various locations of the alkyl chain. The
wide range of location of the ether links in f top kerogen
is similar to recent results of Jenisch-Anton et al. (1999)
A. Riboulleau et al. / Organic Geochemistry 31 (2000) 1641±1661
1651
Fig. 9. TIC of the toluene-eluted fraction of the 400 C pyrolysate of f top kerogen.
Fig. 11. Partial mass fragmentograms at m/z= 58, 72, 86, 85,
99, 113 and 127 revealing the presence of C16 n-alkan-2- (!),-3,-4-,-5-,-6-,-7- and-8-ones (*), respectively. Note that n-alkan-2to -4-ones are characterised by an even fragment (58, 72 and 86,
respectively) due to a McLaerty rearrangement, while nalkan-5- to -8-ones are characterised by an odd fragment due to
a cleavage (85, 99, 113 and 127, respectively) (, mid-chain nalkanones).
Fig. 10. Ion chromatogram at m/z=58 of the toluene-eluted
fraction of the 400 C pyrolysate of f top kerogen (!, n-alkan2-ones 29; , mid-chain n-alkanones).
who reported data for two oils (Marvejols and Rozel
Point) and a kerogen (Gibellina), all S-rich, showing the
presence of ether links at dierent locations along alkyl
chains. Cyclic ketones (cyclopentanones 30 and cyclohexanones 31) are minor constituents, while two series
of phenyl ketones (1-phenyl-n-alkan-1-ones 32 and 1phenyl-n-alkan-2-ones 33, structures IV and V, respectively) are present in signi®cant amount. Their origin,
however, is unknown.
Most of the OSC present in this fraction have already
been observed in the heptane fraction and they mainly
consist of short-chain polycyclic compounds. Among
them, bithiophenes 17, benzo[b]thiophenes 19, phenylthiophenes 20, and dibenzothiophenes or naphtothiophenes 21. The four series characterised by the
ions 203 (24), 217 (25), 229 (26) and 243 (27) previously
observed in the heptane-eluted fraction are also present.
The occurrence of 1,2-di-n-alkylbenzenes in the desulphurised fraction is consistent with the thienylbenzothiophene (VI) structure proposed by van
Kaam-Peters and Sinninghe Damste (1997) and van
Kaam-Peters et al. (1998b) for series 26 and 27. Such a
structure, however, cannot be considered for series 24
and 25. Mongenot et al. (1999) observed series with
similar mass fragmentation in the pyrolysate of the Srich kerogen from the Kimmeridgian paleolagoon of
Orbagnoux. On the basis of mass spectra, desulphurisation products and presence of thiochromans in the
extracts of Orbagnoux (van Kaam-Peters and Sinninghe
DamsteÂ, 1997; van Kaam-Peters et al., 1998b), Mongenot et al. (1999) tentatively identi®ed these compounds as tetramethylthiochromenes (VII). So far,
thiochromenes have not been ®rmly identi®ed in rock
extracts, crude oils or kerogen pyrolysates. However,
their saturated counterparts, thiochromans, have been
observed in Oligocene crude oils and rock extracts (Sinninghe Damste et al., 1987; Adam, 1991; Schaeer,
1993; van Kaam-Peters and Sinninghe DamsteÂ, 1997;
1652
A. Riboulleau et al. / Organic Geochemistry 31 (2000) 1641±1661
van Kaam-Peters et al., 1998b) and are formed by substitution of the O atom in the chroman structure by a
sulphur (Adam, 1991). A compound characterised by a
major peak at 203 was previously observed by Adam
(1991) in extracts of Oligocene samples and crude oil
and an isoprenoid trimethylbenzo[b]thiophene structure
(VIII) was considered for this compound. Nevertheless,
the regular pattern of the 203 and 217 series, 24 and 25,
observed in the heptane-eluted fraction of f top pyrolysate (Fig. 12a and c) points to the occurrence of an nalkyl side chain and not of an isoprenoid one in the
present case. Consequently, this pattern is consistent,
neither with the structure proposed by Adam (1991),
nor with a thiochromene structure. In contrast, as discussed below, these structures should correspond to
benzodithiophenes (IX) or thienobenzothiophenes (X).
Indeed, a compound characterised by an intense
mass peak at 190 (23) is observed in the heptane and
toluene fractions of f top pyrolysate which retention
time indicates that it can correspond to the lowest
homologue of the series 24 and 25 (Fig. 12a±d). Comparison with mass spectra of reference compounds
showed that 23 can correspond to benzodithiophene
(IX) or thienobenzothiophene (X). The addition of a nalkyl side chain to one of these structures would give a
series of compounds characterised in MS by a fragment
at 203, and the addition of two n-alkyl side chains
would give a series of compounds characterised in MS
by a fragment at 217. The presence of di-n-alkylbenzenes in the desuphurisation products of the heptaneand toluene-eluted fractions is consistent with this
hypothesis.
After desulphurisation, the toluene-eluted fraction
still shows an important hump, with only a few resolved
peaks corresponding to n-alkanes. The dierent series of
ketones observed before desulphurisation are still
detected but the mass fragmentogram at m/z=58 indicates that mid-chain linear ketones are relatively more
abundant with respect to n-alkan-2-ones than in the
non-desulphurised fraction. It therefore appears that
some ketones are also linked via S-bonds in the pyrolysate of f top kerogen. Such pyrolysis products should
correspond to moieties which were linked both by sulphur and ether bonds in the macromolecular structure
of the kerogen. Similar interpretations were previously
obtained by Richnow et al. (1992) concerning the macromolecular structure of an oil and a kerogen from the
Monterey Formation.
4.4.2.3. Methanol/chloroform-eluted fraction. This fraction which represents 41% of the pyrolysate was separated into an acid and a non-acid subfraction and the
former was esteri®ed by MeOH/MeCOCl prior to GC/
MS analysis (Table 5).
The esteri®ed acid subfraction is dominated by
methylesters of saturated fatty acids 38 from C12 to C30
with a strong predominance of even-carbon-numbered
compounds (CPI=0.18) (Fig. 13). The main components are palmitic acid, n-C16 and stearic acid, n-C18,
Fig. 12. (a) Ion chromatogram at m/z=190+203 of the heptaneeluted fraction of the 400 C pyrolysate of f top kerogen; &,
compound 23; *, series 24; (b) mass spectrum of the compound 23; (c) ion chromatogram at m/z=217; *, series 25; (d)
mass spectrum of a C13 isomer of series 25.
A. Riboulleau et al. / Organic Geochemistry 31 (2000) 1641±1661
1653
Table 5
Compounds detected in the methanol-eluted acid subfraction
of the 400 C pyrolysate of f top kerogen, tr: trace amount
Series
Range
Max.
Rel.
intensity
38 Saturated fatty acids
39 Monounsaturated fatty acids
40 Diunsaturated fatty acids
41 Branched fatty acidsa
C12±C30
C14±C18
C18
C15±C17
C16
C18
C18
±
1
0.3
tr
0.02
a
Anteiso C15 and C17, iso C16.
which is a very common feature in kerogen pyrolysates
(e.g. Kawamura et al., 1986; Largeau et al., 1986).
Long-chain, C20+, fatty acids occur in signi®cant
amount, they represent 17% of the saturated fatty acids.
These C20+ acids are generally considered of terrestrial
origin (Volkman et al., 1980; Barouxis et al., 1988).
However, long chain fatty acids have been observed in
certain algae (e.g. diatoms; Volkman et al., 1980) and a
bacterial origin has also recently been considered (Gong
and Hollander, 1997). Unsaturated fatty acids 39±40,
identi®ed after DMDS derivatisation (Fig. 13, inset), are
dominated by oleic acid (C18:1o9) and a C16 monounsaturated acid (C16:1o10). Oleic acid is ubiquitous but
would chie¯y be of phytoplanktonic origin, whereas the
C16:1o10 acid is considered as a bacterial marker (Barouxis et al., 1988). Other unsaturated acids are also
observed in lower amount, such as C18:2 40, which is
also common in green microalgae (Weete, 1976). The
presence of these unsaturated compounds, known to be
highly sensitive to degradation, attests for a rapid and
early incorporation of lipidic moieties in the kerogen.
Iso- and anteiso- branched acids 41 are observed in low
amounts, re¯ecting bacterial input (Perry et al., 1979;
Goossens et al., 1986).
The non-acid subfraction is dominated by two linear,
saturated, C16 and C18 primary alcohols as previously
observed by Mongenot et al. (1999) in the same subfraction of Orbagnoux pyrolysate. However, contrary to
the latter study, no unsaturated alcohol is observed in f
top pyrolysate. Series of n-alkylphenols from C9 to C16
and n-alkoxyphenols from C10 to C15 are also observed.
Short chain alkyl phenols (C1±C3), derived from lignin
and/or melanoidins (Saiz-Jimenez and de Leeuw, 1986;
Zegouagh et al., 1999) are not detected in the present
case. Long chain n-alkylphenols have previously been
observed in the pyrolysates of various marine kerogens:
two samples from the Kimmeridge Clay Formation
(Gelin et al., 1995), a Cenomanian black shale from
Central Italy (Salmon et al., 1997) and Ordovician
Kukersite from Estonia (Derenne et al., 1990). Kukersite is known to derive from the selective preservation of
the cell walls from a colonial microorganisms, termed
Gloeocapsomorpha prisca which were shown to be the
Fig. 13. TIC of the esteri®ed methanol-eluted acid subfraction
of the 400 C pyrolysate of f top kerogen: ^, methyl esters of
saturated fatty acids; ^, methyl esters of monounsaturated
fatty acids. Inset: ion chromatogram at m/z=61 showing the
unsaturated esters after DMDS derivatisation.
phenol source (Derenne et al., 1990, 1992a). In contrast,
no precise source could be attributed for the long-chain
n-alkylphenols in the Cenomanian black shale (Salmon
et al., 1997), in the Kimmeridge Clay samples (Gelin et
al., 1995) and in the present case as well. Some alkylthiophenes from C11 to C15 and even-carbon-numbered
branched alkanes from C16 to C30 (3-methyl- and 2,2dimethylalkanes) are also present in low amount in the
methanol-eluted, non-acid, subfraction. Due to their
low polarity, such compounds should not appear in this
subfraction and are considered to re¯ect thermal degradation, during GC/MS injection, of high molecular
weight OSCs present in the subfraction. A similar feature was previously observed by Mongenot et al. (1999)
in the case of Orbagnoux kerogen.
4.4.2.4. Desulphurized pyrolysate. Mongenot et al.
(1999) recently observed that, in the case of Orbagnoux
kerogen, desulphurisation of the total pyrolysate yields
hydrocarbons, the distribution of which is very dierent
to that expected on the basis of analytical data from
column-eluted fractions and desulphurized counterparts. Such a discrepancy is mainly due to the high
amount of polar and/or high molecular weight compounds retained on the alumina column for the Orbagnoux pyrolysate (ca. 55%). Although the amount of
retained compounds is less here (23%), the total pyrolysate of f top was desulphurised in order to determine
if these polar and/or high molecular weight compounds
have a speci®c signature.
The GC of the desulphurised pyrolysate is dominated
by a series of n-alkanes from C12 to C31, similar to the nalkanes observed in the heptane-eluted and desulphurised
1654
A. Riboulleau et al. / Organic Geochemistry 31 (2000) 1641±1661
toluene-eluted fractions. The GC trace also contains a
small hump of numerous coeluting products, which all
are observed in the column-eluted fractions with similar
distributions. It therefore appears that in f top pyrolysate, column-eluted and column-retained compounds
have probably the same type of structure, and hence the
same precursors, and the latter mostly dier by higher
molecular weights.
4.4.3. Flash pyrolysis
Recent isotopic and pyrolytic studies on the Kimmeridge Clay Formation (van Kaam-Peters et al.,
1998a,b; Sinninghe Damste et al., 1998) indicated that
short chain alkylthiophenes liberated upon ¯ash pyrolysis of S-rich kerogens can originate from moieties
corresponding to sulphurised carbohydrates in these
kerogens. Flash pyrolysis was therefore performed on f
top kerogen in order to determine the distribution of
short chain alkylthiophenes and assess the contribution
of sulphurised carbohydrates.
The ¯ash pyrogram of f top (not shown) is dominated
by short chain alkylthiophenes, the distribution of
which is shown in Fig. 14. This distribution is dominated by compounds with a linear skeleton, and is close
to the distribution obtained by pyrolysis of sulphurised
carbohydrate-containing kerogens and sulphurised
algae (Sinninghe Damste