Pyrolytic and spectroscopic study of a sulphur-rich ... oil shale reserves of Russia and ... The Gorodische outcrop is located near the city of Ulyanovsk along the Volga ... this paper

Pyrolytic and spectroscopic study of a sulphur-richkerogen 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

aLaboratoiredeChimieBioorganique etOrganiquePhysique,CNRSUMR7573,ENSCP,11 ruePierre etMarieCurie, 75231Paris cedex05,

FrancebLaboratoire de Stratigraphie, CNRS ESA 7073, UPMC, 4 place Jussieu, 75252 Paris cedex 05, France

cChemistry Department, University of Western Ontario, N6A 5B7, London, Ontario, CanadadElf 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 studiedusing 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, nanoscopi-

cally amorphous organic matter, exhibits a relatively high aliphatic character; organic sulphur is mainly present asdi(poly)sulphides and alkylsulphides. The f top kerogen was chie¯y formed via intermolecular incorporation of sulphurin algal or cyanobacterial lipids and carbohydrates. However, its formation also involved oxidative condensation viaether 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. Allrights 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 accumu-lation of sedimentary organic matter (OM) and impor-tant gas and oil source-rocks are Upper Jurassic in age

(Ulmishek and Klemme, 1990), especially in the north-ern hemisphere, including the North Sea and Siberianoil ®elds. Episodes of extensive OM deposition also

took place on the Russian Platform, located betweenthese two basins, but the relative stability of this plat-form since the Triassic did not allow su�cient burial

of OM for hydrocarbon production. Although suchepisodes are relatively less numerous on the Russian

platform than in the neighbouring basins, large accu-mulation occurred during the Middle Volgian (LateTithonian, 140 Ma) and several basins of the Russian

platform display organic-rich levels of this age (Shmuret al., 1983). In the Ulyanovsk region, these organic-richsediments crop out along the Volga river and are known

as the ``Kashpir oil shales''. At Gorodische (Fig. 1), thisorganic-rich deposit represents a 6 m thick layer of greyto 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 shalesrepresent, after the Baltic oil shales, one of the mostimportant oil shale reserves of Russia and they have

been mined for oil production since the 1850s (Shmur etal., 1983; Russell, 1990). However, production fromthese oil shales has almost ceased today because they are

0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.

PI I : S0146-6380(00 )00088-7

Organic Geochemistry 31 (2000) 1641±1661

www.elsevier.nl/locate/orggeochem

* Corresponding author. Tel.: + 33-1-4427-6716; fax: + 33-

1-4325-7975.

E-mail address: [email protected] (S. Derenne).1 Current address: LGIT-Groupe de GeÂ ochimie de l'Envir-

onnement, UniversiteÂ Joseph Fourier, BP 53, 38041 Grenoble

cedex, France.

characterised by high sulphur contents, and particularlySorg, leading to high atmospheric pollution uponreforming (Popov et al., 1986). Nevertheless these shalesare 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 ofreduced mineral sulphur (H2S, S

0 or SX2ÿ) in functiona-

lised lipids or carbohydrates during early diagenesis (e.g.

Francois, 1987; review by Sinninghe DamsteÂ and deLeeuw, 1989; van Kaam-Peters et al., 1998a). S-rich OMis mostly found in sediments from evaporitic and anoxiccarbonate platform environments (Sinninghe DamsteÂ et

al., 1989; Schae�er et al., 1995; Mongenot et al., 1997;Baudin et al., 1999). Sulphur incorporation, from theH2S produced by sulphate-reducing bacteria under

anoxic conditions, can occur during very early diagen-esis in the upper layers of the sediment (Hartgers et al.,1997).

Several paleoecological indices in the Gorodischeblack shales, like the presence of bioturbations andbrachiopods in several levels, testify for oxygenation of

the upper centimeters of the sediments, at least duringpart of the deposition of these black shales. The resultspresented here, concerning the most OM-rich level,termed ``f top'', are part of a broader study. This study

aims at determining the paleoenvironmental conditionsthat led to the deposition of these black shales andexplaining their strong variations in OM quality and

quantity. The kerogen of f top was examined by a com-bination of various methods in order to characterise itsstructure, to specify the nature of the sulphur functionsand to determine the source organisms of the OM and

its preservation pathway(s), in this especially rich level.

2. Geological setting and sampling

The Gorodische outcrop is located near the city of

Ulyanovsk along the Volga river (Fig. 1). It is mainlycomposed 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 tobrown and despite a laminated feature, bioturbationsare visible in the lower and upper parts of the unit. It isoverlain by sandstone of Middle to Upper Volgian age.

A more detailed description of the section is given inHantzpergue et al. (1998). TOC and HI values for thisorganic-rich formation are presented in Fig. 2. The f top

level which is studied in this paper, is a 7 cm thick bedlocated 2.70 m above the base of the OM-rich depositand presents the highest TOC and HI values (Fig. 2). In

contrast to most part of the formation, no bioturbationnor benthic organisms are observed in this level.

3. Experimental

Shale samples were collected in 1995. Subsamples

were ground for Rock-Eval analysis, the remainder wasstored at room temperature away from light. Prior tofurther analysis, the surface of the sample was carefully

removed in order to eliminate oxidised or pollutedmaterial.Rock-Eval pyrolysis, OSA device, was performed on

10 mg samples of powdered bulk shale. The sample washeated at 300�C for 3 min followed by a programmedpyrolysis at 25�C/min up to 600�C under a He ¯ow andthen oxidised at 600�C for 7 min under an oxygen ¯ow.

After grinding, ca. 50 g of shale were extracted withCHCl3/MeOH, 2:1, v/v (stirring for 12 h at room tem-perature) before isolation of the kerogen via the classical

HCl/HF treatment (Durand and Nicaise, 1980). Thekerogen concentrate was then extracted as describedabove and dried under vacuum.

An aliquot of the kerogen was ®xed with 2% OsO4

for electron microscopy. For transmission electronmicroscopy (TEM), the ®xed kerogen was embedded in

Araldite, cut in ultra-thin sections and stained withuranyl acetate and lead citrate. Observations were car-ried out with a Philips 300 microscope. For scanningelectron microscopy (SEM), the ®xed kerogen was

dehydrated using the CO2 critical point technique andcoated with gold prior to observation with a Jeol 840microscope.

Fig. 1. Location map of the Gorodische outcrop.

1642 A. Riboulleau et al. / Organic Geochemistry 31 (2000) 1641±1661

FTIR spectra of the kerogen and its pyrolysis residues

were recorded on a Bruker IFS 48 spectrometer as 5 mmKBr pellets. Solid state 13C NMR spectroscopy wasperformed on the kerogen and its pyrolysis residues with

a Bruker MSL 400 spectrometer using high powerdecoupling, cross polarisation and magic angle spin-ning. Spectra were recorded at 3 and 4 kHz in order todiscriminate spinning side bands (SSB).

X-ray absorption near edge structure (XANES) spec-troscopy was performed at the Canadian SynchrotronRadiation Facility situated on the 1 GeV electron sto-

rage ring, Alladin, University of Wisconsin. Experi-mental details have been described previously (Kasrai etal., 1994). All the S K- and L-edge spectra presented in

this paper were recorded using total yield (TEY) detec-tion mode. They were background subtracted using alinear 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 edgejump for the K-edge. S K-edge spectra were simulatedby a linear combination of reference spectra using a

least-squares ®tting program. Analytical procedure aswell as interpretation and comparison with standardspectra are described in Sarret et al. (1999).

Elemental analyses of the kerogen and of its pyrolysisresidues were performed at the Service Central d'Ana-lyse of the CNRS. The O content of the unheated kero-

gen was determined by coulometry (LaboratoiresWol�).Bulk isotopic measurement of d13C was performed on

the kerogen with a Carlo-Erba CHN coupled to a VG-

SIRA 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 releasedproducts are trapped in cold chloroform. After eachtreatment, the residue is extracted with CHCl3/MeOH

as previously described. The ®rst thermal treatment at300�C was aimed at eliminating thermolabile compo-nents. It has been recently demonstrated that such athermal treatment, when applied to S-rich kerogens also

promotes some aromatisation and lowers the globale�ciency of the subsequent cracking at 400�C. How-ever, it was also shown that more detailed information

on the chemical structure of such kerogens can ®nally beobtained by this two step treatment at 300 and 400�Cthan via direct pyrolysis at 400�C (Mongenot, 1998;

Sarret et al., unpublished results).The 400�C pyrolysate was separated by column chro-

matography (Al2O3, Act II) into three fractions ofincreasing polarity eluted with heptane, toluene and

methanol, respectively. An additional elution withCHCl3 was performed, and yielded a small amount ofproducts which were combined with the MeOH-eluted

fraction. Carboxylic acids were separated from theMeOH±CHCl3 fraction using a double extraction withether under base and acid conditions and analysed by

GC and GC/MS as their methyl esters. Unsaturatedmethyl esters were further analysed after derivatisationwith DMDS as described by Scribe et al. (1988).

A part of the total 400�C pyrolysate and of its hep-tane- and toluene-eluted fractions was desulphurisedwith Raney Ni using the conditions previously describedby Sinninghe DamsteÂ 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

Fig. 2. Stratigraphic section of Gorodische outcrop (after Hantzpergue et al., 1998) and variations of TOC and HI along the organic-

rich formation.

A. Riboulleau et al. / Organic Geochemistry 31 (2000) 1641±1661 1643

column, ®lm thickness of 0.4 mm, heating program 100to 300�C at 4�C minÿ1, injector and FID at 320�C,helium carrier gas), coupled with a HP 5989 mass spec-trometer 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 ferromag-netic wire with a Curie point of 610�C. The wire placedin a glass tube was introduced in the Curie point pyr-

olyser (Fisher 0316M) coupled to a Hewlett-PackardHP 5890 gas chromatograph with FID (30 m fused silicacapillary column, ®lm thickness 0.4 mm, heating pro-

gram 35�C for 10 min and from 35 to 300�C at 4�Cminÿ1, He carrier gas). The chromatograph was coupledwith a HP 5989 mass spectrometer with a mass rangem/z= 40±600, operated at 70 eV.

4. Results

4.1. Bulk features

Rock-Eval pyrolysis of the unextracted shale shows anexceptionally high TOC of 45% for f top and a highhydrogen index (HI) of 700 mg HC/g TOC (Fig. 2). The

Tmax value is very low, 396�C, re¯ecting the immaturityof the sample. Bulk isotopic measurement indicate ad13CTOC of ÿ20.5% (PDB) for f top kerogen. Elementalanalysis of the isolated kerogen (Table 1) indicate a

relatively high H/C ratio of 1.39 consistent with the highHI, an O/C ratio of 0.20, a low N/C ratio of 0.014, a highcontent of total S of 11.7 wt.% and a weak Fe content of

0.67 wt.%. Sorg content was then calculated in the clas-sical way: it was considered that Fe is only present aspyrite after the acid treatment and that non-pyritic sul-

phur is organic, giving a high Sorg/C ratio of 0.068. F topthus belongs to Type II-S kerogens, according to theclassi®cation of Orr (1986). Sulphur-rich kerogens areknown to exhibit a relatively low thermal stability

(Baskin and Peters, 1992; Tomic et al., 1995) thusexplaining the very low Tmax value observed for f top.

4.2. Microscopic study

Under the light microscope, f top kerogen appears

chie¯y constituted of a single type of amorphous

organic matter (AOM) associated with rare pyrite. ThisAOM is yellow to orange and appears as gel-like parti-cles of varying size (from 5 to 200 mm). When observedby SEM and TEM, the AOM appears homogeneous

and amorphous, even at high magni®cation (nanometricscale). Similar morphological features have been pre-viously observed in S-rich kerogens from the Kimmer-

idge Clay Formation (Boussa®r et al., 1995) and thepaleolagoon 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 asmall shoulder at 15 ppm, corresponding to CH2 inalkyl 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. Arelatively intense broad signal centred at 75 ppm corre-

sponding to aliphatic C linked to N or O is also noticed.According to the weak N/C ratio revealed by elementalanalysis the latter signal must be chie¯y due to C±O

bonds. The 30 ppm peak exhibits a broad shoulderbetween 40 and 60 ppm, which can be assigned to car-bons in C±S bonds and/or to C b to the functions that

gave the 75 ppm signal. Two weak signals at 130 and175 ppm are observed corresponding to unsaturatedcarbons 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 bythe comparison of the spectra recorded at two di�erentspinning rates.

Table 1

Atomic ratios of f top kerogen and insoluble pyrolysis residues

H/C Sorg/C N/C

f top 1.39 0.068 0.014

f top res 300 1.19 0.047 0.016

f top res 400 0.57 0.030 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).


4.3.2. FTIR spectroscopyThe 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 centredat 3400 cmÿ1 corresponding to O±H groups, (ii) a bandat 1700 cmÿ1 (C�O), 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 ppmobserved in the 13C NMR spectrum. A band at 1630cmÿ1 is also observed indicating that the unsaturated Cdetected at 130 ppm via 13C NMR are mostly ole®nic. A

narrow band with a rather high intensity is noted at 750cmÿ1; it is due to CaF2 neoformed during the HCl/HFtreatment since the latter mineral was identi®ed by

XRD (A. Person, pers. comm.).

4.3.3. XANES spectroscopy

Sulphur K-edge spectrum for f top kerogen is pre-sented in Fig. 5a. The spectrum was simulated by a lin-ear combination of spectra of model compounds in

order to quantify the di�erent sulphur species which canbe di�erentiated by this mode, i.e. Sÿ2, disulphides,alkyl and/or heterocyclic sulphides, sulphoxides, sul-phones, sulphonates and sulphates (Sarret et al., 1999).

Results of the simulation are presented in Table 2. Themajor sulphur species in f top kerogen are alkyl and/orheterocyclic sulphides (76%) and di(poly)sulphides

occur in lower proportion (11%). A small contributionof sulphoxides, sulphonates and sulphates is alsonoticed.

S L-edge spectrum for f top kerogen is compared withspectra of various reference compounds in Fig. 6. FromS K-edge results, one could foresee spectral similarities

with the alkylsulphide and/or heterocyclic sulphide(thiophene) references. These two types of compounds,which are di�cult to distinguish on K-edge spectra,exhibit clearly di�erent peak positions on L-edge spectra,

Fig. 4. FTIR spectra of (a) f top kerogen and of its (b) 300 and

(c) 400�C pyrolysis residues.

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.


as this latter method is more sensitive to the reducedforms of sulphur (Kasrai et al., 1996a). F top spectrumis 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 sul-phides 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 shoulderat 162.8 eV. This spectrum is well reproduced by a linearcombination of 30% di(poly)sulphides+70% alkyl sul-

phides (Fig. 6a, dotted line). Contrary to the K-edgemode, S L-edge XANES spectroscopy does not allow aprecise 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 simu-lated (Sarret et al., 1999). Accordingly, S L-edge resultsare not directly comparable with K-edge results, e.g.

concerning the di(poly)sulphide content (30% comparedto 11%, respectively). However, S L-edge analysisallows to conclude that the major reduced sulphur spe-

cies correspond to alkyl sulphides, and to a lesser extentdi(poly)sulphides. The f top spectrum also contains asmall 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 o�-line pyrolysis residues

After the thermal treatment at 300�C, the weight lossamounts to ca. 18% of the initial organic matter, mainlycorresponding to volatile compounds (17%) while trap-

ped products only represent 1% of the initial OM. Afterpyrolysis 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 and28% 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 takesplace upon pyrolysis at 400�C. The N/C ratio shows aslight 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 kerogenresidues after pyrolysis. In contrast, the Sorg/C ratioregularly decreases.

The 13C NMR spectrum of the 300�C residue (Fig.3b) is still dominated by the peak centred at 30 ppm butthe relative intensity of the 40±60 ppm shoulder is lower

compared to the unheated kerogen. The 75 ppm peakexhibits a lower intensity and becomes thinner; in thesame time, the peak at 130 ppm increases and the car-

boxyl peak at 175 ppm remains approximately constantwith respect to the 30 ppm band. The important peak at165 ppm corresponds to a spinning side band of the 130

ppm peak. As expected, the 13C NMR spectrum of the400�C residue (not shown) is dominated by an aromaticsignal at 130 ppm and only shows a weak signal at 30ppm. 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 C�C signal towards 1600 cmÿ1 indi-cates some aromatisation at this temperature as pre-viously observed for the Orbagnoux kerogen

(Mongenot, 1998). The C±O absorption decreases shar-ply at 300�C, in agreement with the decrease of the 75ppm signal in the 13C NMR spectrum. In the 400�Cresidue (Fig. 4c) the absorptions corresponding to CH2

and CH3 are of much lower intensity in agreement withthe large decrease of the H/C ratio, the C�C band isbroader and now centred at 1600 cmÿ1, correspondingto aromatic carbons and the broad band around 1100cmÿ1, due to C±O bonds, is no longer detected.The S K- and L-edge XANES spectra for the inso-

luble pyrolysis residues are presented in Figs. 5b and cand 6b and c, respectively. The edge of the K-edgespectrum for the 300�C residue is slightly shifted to

higher energy compared to the unheated kerogen. Thisshift corresponds to the removal of di(poly)sulphidesupon heating. Indeed, the simulation shows that thedi(poly)sulphides are no longer present in the 300�Cresidue (Table 2). The edge of the spectrum for the400�C residue is even more shifted to the right. As thedi(poly)sulphides are already eliminated in the 300�C

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

FMa S(-II) Disulphides S aliph./heterocycl.b Sulphoxides Sulphones Sulphonates Sulphates

f top 0.08 0 11 76 5 0 4 4

f top res 300 0.05 0 0 94 5 0 0 1

f top res 400 0.18 4 0 84 9 0 0 3

a FM: ®gure of merit of the ®t, FM= S(f kerogen-f ®t)2

b 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.


residue, this shift can only be attributed to a change inthe alkyl/heterocyclic sulphides distribution. S L-edgeanalysis will provide more information about thischange. Concerning the oxidised sulphur species, the

oscillation situated at about 2481 eV is weaker for the300�C residue than for f top, so their contribution issmaller. Indeed, Table 2 shows that their content is of 6

and 13%, respectively. The 400�C residue containsabout 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. Thisoxidation state does not exist for organic sulphides,

whose oxidation number is 0. Therefore, the 400�Cresidue contains a small amount of inorganic sulphidesthat 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�Cresidue 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 particularlyhigh, 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 replacesimple thiophene (i.e. not included in a polyaromatic

structure) by dibenzothiophene (DBT), the position ofthe 164.7 eV peak does not match so well. Thus, the300�C residue contains thiophenes as major species, andto a lesser extent alkyl sulphides. No di(poly)sulphides

are detected in this sample, which is in agreement withK-edge results. For the 400�C residue, the L-edge spec-trum shows peak positions similar to that of the 300�Cresidue, but with di�erent relative intensities. Alkylsul-phides are no longer observed and the linear combina-tion of 70% thiophene+30% sulphoxide a�ords a good

®t. Again, if we replace simple thiophene by DBT, thepeak at 164.6 eV does not match. As explained pre-viously, the percentages determined on S L-edge spectrado not a�ord precise quantitative information on sul-

phur distribution; however they re¯ect the changes indisulphide, alkyl sulphide and thiophene contents uponthermal stress. During heating, the disulphides (300�C)and the sulphides (400�C) present in f top disappearwhile 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 ppmsignal corresponding to aromatic carbons in the 13CNMR spectrum of the 300 and 400�C residues, com-

pared to that of the kerogen. Such changes are alsoresponsible for the shifts observed on the K-edge spec-trum. It can be noted that the sulphoxide contributionfor the 400�C residue, calculated by S L-edge spectro-

scopy, is particularly high compared to its actual con-tent determined via K-edge analysis (9%). Thisdisagreement can be due to a higher oxidation of the

Fig. 6. S L-edge XANES spectra for some reference com-

pounds: (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 pre-

sented 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).


surface of the samples, as the sampling depths of S K-and L-edge XANES spectroscopy are di�erent. Indeed,

using TEY detection mode, it is about 70 nm for the K-edge 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 complexand shows an important hump, due to the coelutions ofnumerous products as commonly observed for pyr-

olysates of S-rich kerogens (Sinninghe DamsteÂ 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 andmethanol, respectively.

4.4.2.1. Heptane-eluted fraction. This fraction represents17% of the pyrolysate. Its GC trace (Fig. 7a) is stillcomplex and exhibits an important hump due tonumerous coelutions. Nevertheless, selective ion detec-

tion (SID) of characteristic fragments allowed the iden-ti®cation of a number of compounds and homologousseries (Table 3). However, due to coelutions, their rela-

tive abundance could not be determined and only arough estimation of their relative intensities can begiven. Hydrocarbons mainly consist of n-alkane 1/n-alk-1-ene 2 doublets up to C33 (maximum C17) without any

odd- or even- carbon-number predominance. n-Alkyl-cyclohexanes 9 and cyclopentanes 10 are observed insmall amount. The former are frequently reported in

kerogen pyrolysis products (Ho�mann 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-alkylmethylben-zene isomers 6, the o-isomer being predominant. It canbe 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 lowamount. Isoprenoid compounds correspond to prist-1-ene 3 and regular saturated C16 and C18 hydrocarbons

4, but their abundance relative to n-alkanes could not bedetermined. Neither pristane, nor phytane was detected,possibly because of a too low abundance. The other

isoprenoid compounds identi®ed are hopanes andhopenes 28 from C27 to C31. These compounds are gen-erally 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 thio-phenes 11-14, thiolanes 15, thianes 16 and benzothio-phenes 19 are identi®ed. These compounds are majorcomponents of most pyrolysates of S-rich kerogens

(Sinninghe DamsteÂ et al, 1988b; Payzant et al., 1989).Most of the identi®ed series of OSC have a linear skele-ton. However, C10 to C16 2,3-dimethyl-5-n-alkylthio-

phenes 13 [identi®ed by their mass spectra and elutiontime from Sinninghe DamsteÂ et al. (1989)], are alsoobserved along with branched alkylthiophenes 14 from

C15 to C17. Di�erent series of polyaromatic OSC pre-viously observed by van Kaam-Peters and SinningheDamsteÂ (1997) and van Kaam-Peters et al. (1998b) inthe pyrolysate of S-rich kerogen from the paleolagoon

of Orbagnoux, are also detected: C9 to C18 alkylatedbithiophenes 17±18 (Appendix I, characteristic frag-ments at m/z= 179, 193 and 207) and C10 to C13 n-

alkylphenylthiophenes 20 (II, characteristic fragments atm/z= 173 and 187). Other series of compounds pre-viously detected in the pyrolysate of Orbagnoux kero-

gen are also observed in f top pyrolysate: series 26 and27 characterised by intense fragments at m/z=229±230and 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 intensefragments at m/z=203 and 217, respectively, (Mon-genot et al., 1999). As these compounds are also present

in the toluene-eluted fraction (Table 4), the assignmentof these di�erent series is discussed later on. Two series22, not reported so far, characterised by intense frag-

Fig. 7. TIC of the heptane-eluted fraction of the 400�C pyr-

olysate of f top kerogen (a) before and (b) after desulphurisa-

tion by Raney Ni and hydrogenation (*, n-alkanes 1; *,

pollutants).


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 ofthese two series (e.g. Fig. 8a) points to the presence of a

n-alkyl side chain and their disappearance after desul-phurisation indicate that they correspond to OSC.These compounds are tentatively identi®ed as n-alkyl-

methyl- and n-alkyldimethylthienothiophenes (III) onthe basis of their mass spectra (e.g. Fig. 8b). Thie-nothiophenes have already been observed in crude oils

(Orr and Sinninghe DamsteÂ , 1989) but, as far as we areaware, alkylated homologues of such compounds havenot been reported so far.Following Raney Ni desulphurisation and hydro-

genation, the heptane fraction is dominated by a series ofn-alkanes (Fig. 7b), thus con®rming that most of theOSC have a linear skeleton. Nevertheless, branched and

isoprenoid compounds are also present in small

amounts. In particular, pristane and phytane areobserved, which were not detected in the non-desul-phurised fraction. Pristane must be directly derived from

hydrogenation of prist-1-ene. Phytane can originatefrom hydrogenation of phytenes and/or desulphurisa-tion of C20 isoprenoid OSCs. The latter have been com-

monly observed in extracts and kerogen pyrolysates(Sinninghe DamsteÂ and de Leeuw, 1987, and referencestherein). Based on previously published mass spectra

and elution times (Sinninghe DamsteÂ et al., 1986; Sin-ninghe DamsteÂ and de Leeuw, 1987), C20 isoprenoidOSCs were searched for in the untreated fraction butnone could be detected. Therefore, it is possible that

several C20 isoprenoid OSCs are present but that theyare undetectable amongst the coelution hump due totoo low individual abundances and/or that phytane

Table 3

Compounds detected in the heptane-eluted fraction of the 400�C pyrolysate of f top kerogen

Series Range Maximum

1 n-Alkanes C13±C35 C17

2 n-Alk-1-enesa C13±C33 C16

3 Prist-1-ene C19 C19

4 Regular isoprenoid alkanes C16, C18 ±

5 n-Alkylbenzenes C12±C32 C15

6 n-Alkylmethylbenzenesb C12±C33 C14

7 o n-Alkenylbenzenes C13±C29 C15±C16

8 Substituted n-alkylbenzenesb C12±C26 C14

9 n-Alkylcyclohexanes C13±C26 C17

10 n-Alkylcyclopentanes C13±C27 C16

11 2 n-Alkylthiophenes C10±C28 C12

12 2,5-Di-n-alkylthiophenesc C10±C28 C13

13 2,3-Dimethyl-5-n-alkylthiophenes C10±C16 C13

14 Branched alkylthiophenesb C15±C17 ±

15 2-n-Alkylthiolanesd C9±C25 C12

16 2-n-Alkylthianes C10±C21 C13

17 2-n-Alkyl-5,50 bithiophenes C9±C18 C10

18 2,20-Di-n-alkyl-5,50-bithiophenes C10±C18 C11±C12

19 n-Alkylbenzo[b]thiophenese C9±C22 C10

20 n-Alkylphenylthiophenesf C10±C13 C11

21 n-Alkyldibenzo- or naphtothiophenes C12±C15 C12±C13

22 n-Alkylmethylthienothiophenesg C8±C15 C9

23 Compound in 190 C10 C10

24, 25 Series in 203±217b C11±C19 C11±C12

26, 27 Series in 229±243b C13±C18 C13±C14

28 Hopanes and hopenes C27±C31 ±

a A series of n-alk-2-enes was also identi®ed with the same range and a maximum at C18.b 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-alkylmethyl-

benzo[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.


skeletons are involved through S-linkage in non-polar,

non-GC-amenable, high molecular weight pyrolysisproducts, so that they are not observed in the heptane-eluted fraction but released after desulphurisation.

Three series of branched alkanes (iso-, anteiso- and 4-

methylalkanes) occur in trace amount. Branchedhydrocarbon skeletons are generally considered tore¯ect a bacterial input (Albro, 1976; Shiea et al., 1990).

4.4.2.2. Toluene-eluted fraction. This fraction, whichrepresents 19% of the total pyrolysate, also appearshighly complex and its GC trace shows an important

hump (Fig. 9). Nevertheless, as in the case of the hep-tane-eluted fraction, numerous series of compoundswere identi®ed via selective detection of characteristic

ions. Identi®ed compounds, listed in Table 4, can besubdivided into two major groups: ketones and OSC.Some polyaromatic compounds 34±37 are also detectedin low amounts.

Ketones and especially n-alkan-2-ones often occur inkerogen pyrolysates where they are supposed to bederived from the thermal cleavage of ether bonds (van

de Meent et al., 1980; Largeau et al., 1986). Several ser-ies of n-alkanones 29 were identi®ed in the toluenefraction of f top pyrolysate as shown by the ion chro-

matogram at m/z=58 (Fig. 10). They comprise series ofmid-chain n-alkanones with di�erent locations of theketo group, from C(3) to C(12) (Fig. 11). The occur-

rence of so many series of mid-chain ketones has beenrarely reported. As suggested by Gillaizeau et al. (1996)in a study concerned with the kerogen of the GoÈ ynuÈ k oilshale, such feature should re¯ect the presence of ether

linkages at various locations of the alkyl chain. Thewide range of location of the ether links in f top kerogenis similar to recent results of Jenisch-Anton et al. (1999)

Table 4

Compounds detected in the toluene-eluted fraction of the 400�C pyrolysate of f top kerogen

Series Range Maximum

29 n-Alkan-2-onesa C10±C29 C13

30 n-Alkylcyclopentanones C11±C24 C13

31 n-Alkylcyclohexanones C11±C18 C14

32 1-Phenyl-n-alkan-1-ones C10±C16 C12

33 1-Phenyl-n-alkan-2-onesb C9±C29 C12

34 Alkyl¯uorenesc C13±C16 C14

35 Alkylanthracenes or phenanthrenesc C14±C17 ±

36 Alkylpyrenes C14±C15 ±

37 Alkylbenzo¯uorenes C12±C13 ±

17 2-n-Alkyl-5,50-bithiophenesc C9±C11 C10

20 n-Alkylphenylthiophenesc C11±C15 C12

19 n-Alkylbenzo[b]thiophenes C9±C12 C12

21 n-Alkyldibenzo- or naphtothiophenesc C12±C15 ±

23 Compound in 190 C10 C10

24, 25 Series in 203±217 C11±C14 ±

26, 27 Series in 229±243 C13±C15 ±

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.

Fig. 8. (a) Ion chromatogram at m/z=167 of the heptane-

eluted fraction of the 400�C pyrolysate of f top kerogen; ^,

series 22. (b) Mass spectrum of the C10 isomer.


who reported data for two oils (Marvejols and RozelPoint) and a kerogen (Gibellina), all S-rich, showing thepresence of ether links at di�erent locations along alkyl

chains. Cyclic ketones (cyclopentanones 30 and cyclo-hexanones 31) are minor constituents, while two seriesof phenyl ketones (1-phenyl-n-alkan-1-ones 32 and 1-

phenyl-n-alkan-2-ones 33, structures IV and V, respec-tively) are present in signi®cant amount. Their origin,however, is unknown.

Most of the OSC present in this fraction have alreadybeen observed in the heptane fraction and they mainlyconsist of short-chain polycyclic compounds. Amongthem, bithiophenes 17, benzo[b]thiophenes 19, phe-

nylthiophenes 20, and dibenzothiophenes or naph-tothiophenes 21. The four series characterised by theions 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 desul-

phurised fraction is consistent with the thie-nylbenzothiophene (VI) structure proposed by vanKaam-Peters and Sinninghe DamsteÂ (1997) and vanKaam-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 withsimilar mass fragmentation in the pyrolysate of the S-

rich kerogen from the Kimmeridgian paleolagoon ofOrbagnoux. On the basis of mass spectra, desulphurisa-tion products and presence of thiochromans in the

extracts of Orbagnoux (van Kaam-Peters and SinningheDamsteÂ , 1997; van Kaam-Peters et al., 1998b), Mon-genot et al. (1999) tentatively identi®ed these com-

pounds as tetramethylthiochromenes (VII). So far,thiochromenes have not been ®rmly identi®ed in rockextracts, crude oils or kerogen pyrolysates. However,their saturated counterparts, thiochromans, have been

observed in Oligocene crude oils and rock extracts (Sin-ninghe DamsteÂ et al., 1987; Adam, 1991; Schae�er,1993; van Kaam-Peters and Sinninghe DamsteÂ , 1997;

Fig. 10. Ion chromatogram at m/z=58 of the toluene-eluted

fraction of the 400�C pyrolysate of f top kerogen (!, n-alkan-

2-ones 29; �, mid-chain n-alkanones).

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-2-

to -4-ones are characterised by an even fragment (58, 72 and 86,

respectively) due to a McLa�erty rearrangement, while n-

alkan-5- to -8-ones are characterised by an odd fragment due to

a cleavage (85, 99, 113 and 127, respectively) (�, mid-chain n-

alkanones).

Fig. 9. TIC of the toluene-eluted fraction of the 400�C pyr-

olysate of f top kerogen.


van Kaam-Peters et al., 1998b) and are formed by sub-stitution of the O atom in the chroman structure by asulphur (Adam, 1991). A compound characterised by amajor peak at 203 was previously observed by Adam

(1991) in extracts of Oligocene samples and crude oiland 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 pyr-olysate (Fig. 12a and c) points to the occurrence of an n-

alkyl side chain and not of an isoprenoid one in thepresent case. Consequently, this pattern is consistent,neither with the structure proposed by Adam (1991),

nor with a thiochromene structure. In contrast, as dis-cussed below, these structures should correspond tobenzodithiophenes (IX) or thienobenzothiophenes (X).Indeed, a compound characterised by an intense

mass peak at 190 (23) is observed in the heptane andtoluene fractions of f top pyrolysate which retentiontime indicates that it can correspond to the lowest

homologue of the series 24 and 25 (Fig. 12a±d). Com-parison with mass spectra of reference compoundsshowed that 23 can correspond to benzodithiophene

(IX) or thienobenzothiophene (X). The addition of a n-alkyl side chain to one of these structures would give aseries of compounds characterised in MS by a fragment

at 203, and the addition of two n-alkyl side chainswould give a series of compounds characterised in MSby a fragment at 217. The presence of di-n-alkylben-zenes in the desuphurisation products of the heptane-

and toluene-eluted fractions is consistent with thishypothesis.After desulphurisation, the toluene-eluted fraction

still shows an important hump, with only a few resolvedpeaks corresponding to n-alkanes. The di�erent series ofketones observed before desulphurisation are still

detected but the mass fragmentogram at m/z=58 indi-cates that mid-chain linear ketones are relatively moreabundant with respect to n-alkan-2-ones than in thenon-desulphurised fraction. It therefore appears that

some ketones are also linked via S-bonds in the pyr-olysate of f top kerogen. Such pyrolysis products shouldcorrespond to moieties which were linked both by sul-

phur and ether bonds in the macromolecular structureof the kerogen. Similar interpretations were previouslyobtained by Richnow et al. (1992) concerning the mac-

romolecular structure of an oil and a kerogen from theMonterey Formation.

4.4.2.3. Methanol/chloroform-eluted fraction. This frac-tion which represents 41% of the pyrolysate was sepa-rated into an acid and a non-acid subfraction and theformer 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-numberedcompounds (CPI=0.18) (Fig. 13). The main compo-nents are palmitic acid, n-C16 and stearic acid, n-C18,

Fig. 12. (a) Ion chromatogram atm/z=190+203 of the heptane-

eluted fraction of the 400�C pyrolysate of f top kerogen; &,

compound 23; *, series 24; (b) mass spectrum of the com-

pound 23; (c) ion chromatogram at m/z=217; *, series 25; (d)

mass spectrum of a C13 isomer of series 25.


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®cantamount, they represent 17% of the saturated fatty acids.

These C20+ acids are generally considered of terrestrialorigin (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 abacterial origin has also recently been considered (Gongand Hollander, 1997). Unsaturated fatty acids 39±40,

identi®ed after DMDS derivatisation (Fig. 13, inset), aredominated by oleic acid (C18:1o9) and a C16 mono-unsaturated acid (C16:1o10). Oleic acid is ubiquitous but

would chie¯y be of phytoplanktonic origin, whereas theC16:1o10 acid is considered as a bacterial marker (Bar-ouxis et al., 1988). Other unsaturated acids are alsoobserved in lower amount, such as C18:2 40, which is

also common in green microalgae (Weete, 1976). Thepresence of these unsaturated compounds, known to behighly 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 lowamounts, 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 previouslyobserved by Mongenot et al. (1999) in the same sub-

fraction of Orbagnoux pyrolysate. However, contrary tothe latter study, no unsaturated alcohol is observed in ftop 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 ligninand/or melanoidins (Saiz-Jimenez and de Leeuw, 1986;

Zegouagh et al., 1999) are not detected in the presentcase. Long chain n-alkylphenols have previously beenobserved in the pyrolysates of various marine kerogens:

two samples from the Kimmeridge Clay Formation(Gelin et al., 1995), a Cenomanian black shale fromCentral Italy (Salmon et al., 1997) and OrdovicianKukersite from Estonia (Derenne et al., 1990). Kuker-

site is known to derive from the selective preservation ofthe cell walls from a colonial microorganisms, termedGloeocapsomorpha prisca which were shown to be the

phenol source (Derenne et al., 1990, 1992a). In contrast,no precise source could be attributed for the long-chainn-alkylphenols in the Cenomanian black shale (Salmon

et al., 1997), in the Kimmeridge Clay samples (Gelin etal., 1995) and in the present case as well. Some alkyl-thiophenes from C11 to C15 and even-carbon-numberedbranched alkanes from C16 to C30 (3-methyl- and 2,2-

dimethylalkanes) are also present in low amount in themethanol-eluted, non-acid, subfraction. Due to theirlow polarity, such compounds should not appear in this

subfraction and are considered to re¯ect thermal degra-dation, during GC/MS injection, of high molecularweight OSCs present in the subfraction. A similar fea-

ture 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 Orbagnouxkerogen, desulphurisation of the total pyrolysate yieldshydrocarbons, the distribution of which is very di�erent

to that expected on the basis of analytical data fromcolumn-eluted fractions and desulphurized counter-parts. Such a discrepancy is mainly due to the high

amount of polar and/or high molecular weight com-pounds retained on the alumina column for the Orbag-noux pyrolysate (ca. 55%). Although the amount of

retained compounds is less here (23%), the total pyr-olysate of f top was desulphurised in order to determineif these polar and/or high molecular weight compoundshave a speci®c signature.

The GC of the desulphurised pyrolysate is dominatedby a series of n-alkanes from C12 to C31, similar to the n-alkanes observed in the heptane-eluted and desulphurised

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 C12±C30 C16 1

39 Monounsaturated fatty acids C14±C18 C18 0.3

40 Diunsaturated fatty acids C18 C18 tr

41 Branched fatty acidsa C15±C17 ± 0.02

a Anteiso C15 and C17, iso C16.

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.


toluene-eluted fractions. The GC trace also contains asmall hump of numerous coeluting products, which allare observed in the column-eluted fractions with similardistributions. It therefore appears that in f top pyr-

olysate, column-eluted and column-retained compoundshave probably the same type of structure, and hence thesame precursors, and the latter mostly di�er by higher

molecular weights.

4.4.3. Flash pyrolysis

Recent isotopic and pyrolytic studies on the Kim-meridge Clay Formation (van Kaam-Peters et al.,1998a,b; Sinninghe DamsteÂ et al., 1998) indicated that

short chain alkylthiophenes liberated upon ¯ash pyr-olysis of S-rich kerogens can originate from moietiescorresponding to sulphurised carbohydrates in thesekerogens. Flash pyrolysis was therefore performed on f

top kerogen in order to determine the distribution ofshort chain alkylthiophenes and assess the contributionof sulphurised carbohydrates.

The ¯ash pyrogram of f top (not shown) is dominatedby short chain alkylthiophenes, the distribution ofwhich is shown in Fig. 14. This distribution is domi-

nated by compounds with a linear skeleton, and is closeto the distribution obtained by pyrolysis of sulphurisedcarbohydrate-containing kerogens and sulphurised

algae (Sinninghe DamsteÂ et al., 1998). In particular, it isvery similar to that observed from the organic-richBlackstone Band from the Kimmeridge Clay Formation(Sinninghe DamsteÂ et al., 1998; van Kaam-Peters et al.,

1998a,b).

5. Discussion

5.1. Chemical structure and source organisms of f top

kerogen

Elemental analysis and spectroscopic (FTIR and solidstate 13C NMR) features pointed to a pronounced

aliphatic character for the S-rich kerogen isolated froman especially organic-rich level, f top, from the Gor-odische outcrop of the Kashpir oil shales. Although a

rather low amount of n-alkanes and n-alk-1-enes isreleased upon pyrolysis of this kerogen, the occurrenceof series of n-alkanones and, above all, of OSC with a

linear skeleton con®rmed this aliphatic character.XANES spectroscopy revealed that, in f top, sulphurmainly occurs as alkylsulphides and, in lesser amount,

as disulphides. It thus appears that the chemical struc-ture of f top comprises long n-alkyl chains linked byether and sulphide bridges. A similar structure wasrecently suggested, by selective degradation studies for

kerogens from the Monterey Formation (Richnow etal., 1992) and Gessoso-Sol®fera (Schae�er-Reiss et al.,1998). Along with these ether and sulphide bridges,

some ester functions re¯ecting the rapid incorporation

of weakly altered fatty acids within the macromolecularnetwork, also occur in f top.The sharp predominance of n-alkyl skeletons over

branched ones observed in the o�-line pyrolysate isconsistent with a major algal or cyanobacterial origin oflipids in f top kerogen. Indeed, these lipids are known tobe predominantly unbranched (Weete, 1976). A low

bacterial contribution is shown by the occurrence of afew branched hydrocarbons and of the hopanoidsobserved either directly in the pyrolysate or after desul-

phurisation. The lack of lignin-derived pyrolysis pro-ducts rules out a large terrestrial input, however theoccurrence of C20+ acids may re¯ect a weak contribu-

tion of higher plant lipids.In addition, a substantial contribution of sulphurised

carbohydrates is also inferred in f top kerogen based onthe high amount of short chain alkylthiophenes

obtained by ¯ash pyrolysis. This is consistent with thevery high value of d13Corg of f top kerogen compared toother samples from the Gorodische section (unpub-

lished results).

5.2. Mechanism of OM accumulation

The presence of alkane/alkene doublets in kerogenpyrolysates is often associated with the selective pre-

servation of highly aliphatic macromolecules such asalgaenans and cutans. However, when this type ofpathway is implicated, some morphological features ofthe source organisms are, at least partly, retained in the

resulting kerogen (Derenne et al., 1991, 1992b). Thenanoscopically amorphous nature of f top kerogen evi-denced by TEM observations therefore rules out a sig-

Fig. 14. Partial ion chromatogram at m/z=97+98+

111+112+125+126+139+140+153+154 revealing the dis-

tribution of C1±C5 alkylthiophenes in the ¯ash pyrolysate of f

top kerogen. These compounds are the most abundant in f top

pyrolysate. Black: alkylthiophenes with a linear skeleton.


ni®cant role for the selective preservation pathway in itsformation. In contrast, such morphological features, inconjunction with the high Sorg content and the abun-dance of OSC in the pyrolysate indicate that natural

sulphurisation played a major role in OM preservationin this organic-rich level. Based on XANES results, itappears that intermolecular sulphur incorporation was

dominant over intramolecular processes.Some alkyl chains in f top kerogen are linked by ether

bridges, sometimes in addition to S linkages. The pre-

sence of ether bridges in S-rich kerogens and macro-molecular fractions of bitumens and oils has beenpreviously observed via selective degradation and/or

pyrolytic studies (Richnow et al., 1992, 1993; Koop-mans et al., 1996; HoÈ ld et al., 1998; Putschew et al.,1998; Schae�er-Reiss et al., 1998; Jenisch-Anton et al.,1999). In addition, S- and O-containing compounds

were recently described in bitumens of the PosidoniaShales (Wilkes and Hors®eld, 1999). The similar dis-tributions of the compounds released from sulphide and

ether cleavage indicate that S- and O-bound moieties areprobably derived from the same precursors in the Mon-terey samples studied by Richnow et al. (1992, 1993).

Likewise, relatively close distributions of the n-alkanes,alkylthiophenes and alkanones released by pyrolysis areobserved for f top kerogen. The presence of ether

bridges in S-rich kerogens is not fully explained. Rich-now et al. (1992, 1993) and Jenisch-Anton et al. (1999)proposed that part of the oxygenated functions presentin S-rich kerogens are directly derived from the biologi-

cal precursors. However, Jenisch-Anton et al. (1999)also noticed that such an origin could not account forthe variety of oxygenated molecules they observed in S-

rich oils and kerogen from Marvejols, Rozel Point andGibellina, and proposed that they may be partly derivedfrom diagenetic incorporation of oxygen. Indeed, oxi-

dative incorporation in macromolecular structures waspreviously shown to be responsible for the preservationof some lipids under highly oxic depositional conditions(i.e. Gatellier et al., 1993). Jenisch-Anton et al. (1999)

considered that the oxic/anoxic interface should be theseat of a competition between S- and O-incorporationinto OM, depending on the availability of each species.

The absence of bioturbations and of benthic organismsin f top level reveals that the sediment/water interfacewas severely dysoxic. However, the presence of shells

from nectonic organisms such as ammonites or belem-nites indicate that anoxia did not invade the wholewater column. Accordingly the OM was partly oxidized

and began to form macromolecular units in the watercolumn. Moreover, as considered in previous studies(Schouten et al., 1994; Carmo et al., 1997), this partialoxidation would have favoured subsequent sulphurisa-

tion of the OM in the sediment by increasing its reac-tivity towards inorganic sulphur. Added to a highplanktonic productivity, a high level of OM preserva-

tion, linked to the incorporation of both sulphur andoxygen, would thus account for the particularly highamount of OM preserved in f top level.

5.3. Comparison with other S-rich kerogens

This study of f top chemical structure, source organ-

isms and formation pathway involved a combination ofa number of methods: electron microscopy, FTIR, solidstate 13C NMR, XANES spectroscopy and GC/MS

analysis of pyrolysis products before and after desul-phurisation. Such a multidisciplinary approach has onlybeen applied so far to a limited number of immature S-

rich kerogens including samples of the Monterey For-mation (Richnow et al., 1992; Nelson et al., 1995; Stan-kiewicz et al., 1996; HoÈ ld et al., 1998) and from thebituminous laminites of Orbagnoux (Mongenot et al.,

1997, 1999; van Kaam-Peters and Sinninghe DamsteÂ ,1997; Mongenot, 1998; van Kaam-Peters et al., 1998b).We have thus compared f top with these two types of

kerogens so as to examine their similarities and di�er-ences, especially concerning sulphur linkage and beha-viour upon pyrolysis. Interestingly the three deposits

correspond to di�erent sedimentary contexts: a calmsilty clayey platform for the Kashpir oil shales (Hantz-pergue et al., 1998), a carbonated lagoon for Orbagnoux

(Mongenot et al., 1997) and a restricted basin withupwellings and siliceous deposit for the Monterey For-mation (Ulmishek and Klemme, 1990).The bulk characteristics of whole rocks and kerogens

from f top, Orbagnoux (TM9sa) and di�erent samplesfrom the Monterey Formation (Naples Beach) are pre-sented in Table 6. It can be seen that these samples are

rather di�erent, in terms of TOC content but also inaliphaticity and sulphur content. Indeed, a regulardecrease both in aliphaticity and sulphur content is

noticed from Orbagnoux to f top and to the Montereykerogens. Relatively high O/C ratios are noted for bothf top and Monterey kerogens. The O/C ratio is notavailable for the Orbagnoux kerogen, but a low OI of 15

mg CO2/g TOC (Mongenot, 1998) added to spectro-scopic features and to the low proportion of oxygen-containing pyrolysis products (Mongenot et al., 1999)

indicate that this ratio must be low.Light and TEM observations of Orbagnoux kerogen

(Mongenot et al., 1997) revealed similar features to

those observed for f top, i.e. a predominance of orangegel-like amorphous particles, shown to be nanoscopi-cally amorphous by TEM. Similar features were also

reported for S-rich kerogens of the Kimmerige ClayFormation (Boussa®r et al., 1995) and were consideredto be characteristic of sulphurised OM. The microscopiccharacterisation of Monterey kerogens has been per-

formed by Pytte (1989) and Stankiewicz et al. (1996).These kerogens are dominated by red ¯uorescing amor-phous organic matter (AOM) termed `àmorphinite II''


(Pytte, 1989). This AOM has not been observedunder TEM but it has been shown to be enriched insulphur, after isolation by density gradient fractionation

(Stankiewicz et al., 1996). It is therefore likely that`àmorphinite II'' corresponds to the gel-like orangeparticles. This comparison reveals that, despite their

di�erent chemical compositions, these S-rich kerogens,when observed via microscopy present similar featureswhich are highly characteristic and can be related to

their high sulphur content.The thermal behaviour of a kerogen can be char-

acterised by its pyrolysis products and residues. A direct

comparison of the pyrolysis products of f top andOrbagnoux kerogens can be performed since the sameanalytical procedure was used, i.e. indirect `ò�-line''pyrolysis with a preliminary 300�C thermal treatment.

Upon 400�C pyrolysis, the two kerogens exhibit similarweight losses (58% of initial OM for f top and 62% forTM9sa). However 85% of this loss represents trapped

products in the case of TM9sa while hardly 50% istrapped in the case of f top. This indicates that pyrolysisproducts of f top are more volatile and therefore of

lower mean molecular weight than those of TM9sa. Thisdi�erence is also re¯ected upon the fractionation of thepyrolysate on alumina column: ca. 20% are retained onthe column for f top while ca. 55% are retained in the

case of TM9sa, once again meaning that the pyrolysisproducts of Orbagnoux kerogen are more polar and/orof higher molecular weight than those of f top. Several

heating experiments on Monterey kerogens have beenpreviously reported but the experimental conditions weresharply di�erent from those used in our study so that

precise comparison is di�cult. Baskin and Peters (1992)and Nelson et al. (1995) used hydrous pyrolysis which isusually considered as a�ording higher yields than anhy-

drous pyrolysis and as promoting the release of lowermolecular weights products (Lewan, 1997). Idiz et al.(1990) and Tomic et al. (1995) carried out pyrolysis insealed vessels, i.e. under conditions which also favour the

formation of volatile products. Taken together, theseexperiments indicated that both the Monterey kerogensand TM9sa tend to generate large amounts of high mole-

cular weight products upon pyrolysis. For example, afterheating in sealed tubes at 300�C for 100 h, the volatilecompounds formed only accounted for 25 wt.% of the

initial kerogen for theMonterey samples (Idiz et al., 1990).XANES spectroscopy has been so far performed on a

limited number of kerogen samples: from Monterey,

Orbagnoux and Recent sediments from the Peru upwel-ling area (Eglinton et al., 1994; Nelson et al., 1995; Sar-ret et al., unpublished data). This spectroscopic method

has been shown to be a powerful technique to char-acterise sulphur forms, in various coals and asphaltenes(George and Gorbaty, 1989; Kasrai et al., 1996a) and is

especially appropriate to follow the behaviour of sul-phur functions upon pyrolysis. The present study indi-cated that the major sulphur forms in f top kerogen arealkylsulphides and in lesser amount di(poly)sulphides. A

similar distribution of sulphur species in the Montereykerogens was determined by Nelson et al. (1995) andEglinton et al. (1994). In contrast, the kerogen of

Orbagnoux was shown to contain thiophenes as themajor sulphur form and in lesser amount di(poly)sul-phides and sulphides. These features imply that sulphur

incorporation was di�erent for f top and Montereykerogens on the one hand and TM9sa on the otherhand: essentially intermolecular for f top and Montereykerogens (polysulphides and sulphides) and mostly

intramolecular for TM9sa (thiophenes). However, thethree kerogens present similar behaviour upon thermalstress, i.e. a progressive decrease in di(poly)sulphides

and sulphides while the relative abundance of thio-phenes increases (Nelson et al., 1995; Sarret et al.,unpublished data). This disappearance of polysulphides

and sulphides, and relative increase in the contributionof thiophenes with increasing temperature, is consistentwith the accepted idea of preferential cleavage of weak

polysulphide and sulphide links and aromatisation ofthe residue during pyrolysis (Baskin and Peters, 1992;Tomic et al., 1995).A conspicuous feature of TM9sa when compared to f

top and Monterey kerogens is the evolution of the Sorg/C ratio in the pyrolysis residues. As in the case of f topresidues, a progressive decrease of the Sorg/C ratio was

Table 6

Bulk features of f top, TM9sa (Orbagnoux) and di�erent Naples Beach samples (Monterey Formation)

Whole rock Kerogen

TOC(%) HI(mg HC/g TOC) H/C O/C Sorg/C

f top 45 700 1.39 0.20 0.07

TM9saa 7.2 909 1.42 ± 0.09

Naples Beach 4.7b±20.6c 490d 1.23c±1.29b 0.15d 0.05b,d

a Data were collected from Mongenot et al. (1997).b Data from Stankiewicz et al. (1996).c Data from Nelson et al. (1995).d Data from HoÈ ld et al. (1998).


observed on the Monterey residues by Idiz et al. (1990)and Nelson et al. (1995). This decrease was consideredby these authors to be consistent with the preferentialcleavage of sulphide bonds and also with the dis-

appearance of sulphides in the pyrolysis residuesrevealed by XANES (Nelson et al., 1995). In contrast,for TM9sa kerogen, the Sorg/C ratio of the residues

increases with temperature (Mongenot et al., 1999). Thelatter authors proposed that this increase indicated theformation of sulphur-rich, polyaromatic, refractory

material in the pyrolysis residues. A similar increase ofthe Sorg/C ratio in the pyrolysis residues was observedfor a S-poor Type I kerogen from GoÈ ynuÈ k oil shales

(Gillaizeau et al., 1997). This behaviour could be relatedto the high proportion of thiophenes in the TM9sakerogen, as thiophenes are known to be thermally morestable than sulphides.

Taken together, elemental composition, spectroscopicfeatures (especially XANES) and evolution of the Sorg/Cratio upon a thermal stress point to a closer similarity

between f top and the Monterey kerogens than with theOrbagnoux kerogen in terms of sulphur content and thenature of sulphur linkages. It thus appears that the less

S-rich kerogens, i.e. f top and Monterey, are mostlybased on intermolecular incorporation of sulphur whilethe Orbagnoux kerogen which is extremely S-rich, is

based on inter- and intramolecular incorporation, thelatter being predominant. In addition, the oxygen con-tent might also be an important factor since the role ofoxygen in the thermal behaviour of S-rich kerogens of

the Monterey Formation was recently discussed (Rey-nolds et al., 1995). Indeed, as noticed above, f top andMonterey kerogens are characterised by relatively high

O/C ratios while Orbagnoux kerogen contains a lowamount of oxygen. The medium S-richness of f top andMonterey kerogens might be linked to this high oxygen

content: incorporation of both O and S would haveoccurred during diagenesis, contrary to Orbagnoux,where only S- would have been incorporated.

6. Conclusions

An organic-rich sample, termed f top (45% TOC,Sorg/C=0.068), from the Gorodische section (Upper

Jurassic, Russian platform) was studied using a combi-nation of microscopic, spectroscopic and pyrolyticmethods. Microscopic observations indicated that thekerogen is mainly composed of orange gel-like, nanos-

copically amorphous organic matter. The kerogen exhi-bits a relatively high aliphatic character as shown byFTIR and solid state 13C NMR while XANES spectra

revealed that organic sulphur is mainly present asdi(poly)sulphides and sulphides. Pyrolysis products aremainly linear compounds: n-alkanes, n-alkanones and a

large amount of OSC with a linear skeleton, as shownby desulphurisation. High amounts of short chain-alkylthiophenes are also generated upon ¯ash pyrolysis.

Taken together, these results indicate that the kerogenof f top is mostly derived from algal or cyanobacteriallipids and from carbohydrates, rapidly incorporatedinto a macromolecular network by natural sulphurisa-

tion, consistent with its high d13C value. The relativelyhigh abundance of oxygenated products, consistent withthe high O/C ratio of this kerogen, indicates that oxi-

dative incorporation via ether linkages was alsoinvolved. Comparison of f top sample with other S-richkerogens pointed to a closer similarity with the Mon-

terey kerogens rather than with a kerogen from thebituminous laminites of Orbagnoux when both sulphurincorporation (intermolecular vs. intramolecular) and

the extent of oxygen incorporation during kerogen for-mation are considered.

Acknowledgements

This is a contribution to Peri-Tethys project 95-96/28.We thank CIME Jussieu for SEM observations, Mr. B.Rousseau (ENS) for preparation of ultrathin sections

observed by TEM, Mrs. M. Grably (Laboratoire deBiogeÂ ochimie Isotopique, UPMC) for isotopic mea-surement, Mrs. J. Maquet (Laboratoire de Chimie de laMatieÁ re CondenseÂ e, UPMC) for NMR spectroscopy

and SocieÂ teÂ de Secours des Amis des Sciences for®nancial support to A.R. Dr. P. Farrimond, Dr. A.Bishop and an anonymous reviewer are acknowledged

for helpful comments on an earlier version of thispaper.

Appendix on next page


Appendix

When several isomers are possible for the above structures, only one is presented.

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