Directory UMM :Data Elmu:jurnal:O:Organic Geochemistry:Vol31.Issue9.2000:
Organic Geochemistry 31 (2000) 787±798
www.elsevier.nl/locate/orggeochem
Origin of variations in organic matter abundance and
composition in a lithologically homogeneous maar-type oil
shale deposit (GeÂrce, Pliocene, Hungary)
Sylvie Derenne a,*, Claude Largeau a, Alice Brukner-Wein b,
Magdolna Hetenyi c, GeÂrard Bardoux d, Andre Mariotti d
a
Laboratoire de Chimie Bioorganique et Organique Physique, UMR CNRS 7573, ENSCP, 11 rue P. et M. Curie,
75231 Paris cedex 05, France
b
Geological Institute of Hungary, StefaÂnia uÂt 14, H-1143 Budapest, Hungary
c
Institute of Mineralogy, Geochemistry and Petrography, JoÂzsef Attila University, PO Box 651, H-6701 Szeged, Hungary
d
Laboratoire de BiogeÂochimie Isotopique, INRA-CNRS-UPMC, 7 place Jussieu, 75252 Paris cedex 05, France
Received 26 January 2000; accepted 20 June 2000
(Returned to author for revision 3 May 2000)
Abstract
Despite having an homogeneous lithology, the largest Hungarian maar-type deposit (GeÂrce oil shale, Pliocene) has
previously been shown to exhibit substantial variations in organic matter quantity and quality with depth. This heterogeneity is also re¯ected, in the present study, by large variations in bitumen abundance and composition, for 23
samples from GeÂrce well-6 core. Based on the above bitumen data, four samples were selected that were representative
of the whole set which exhibit contrasting features. Scanning and transmission electron microscopy showed the
occurrence of extensively altered Botryococcus colonies in this deposit. GC/MS and GC-C-ir-MS of the saturated
hydrocarbon fractions of the bitumen of these samples reveal a predominant algal contribution along with a variable
bacterial input. The relative abundance of these two contributions in the four selected samples is also re¯ected by differences in FTIR and solid-state 13C NMR spectra of the isolated kerogens. Curie point pyrolysis/GC/MS of these
kerogens revealed a relatively high terrestrial contribution in one sample and con®rmed the variable input of algae and
bacteria. The above dierences in relative contributions account for the variations in organic matter quantity and
quality observed along the core. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Maar-type deposit; Botryococcus; Bitumen; Kerogen; Hydrocarbons; GC-C-ir-MS; FTIR; Solid-state 13C NMR; Pyrolysis/
GC/MS; Electron microscopy
1. Introduction
Four maar-type oil shales have been discovered in
Hungary in the last 25 years: Pula, GeÂrce, VaÂrkeszoÈ and
EgyhaÂzaskeszoÈ (Ravasz and Solti, 1987) (Fig. 1). These
organic-rich deposits are the result of intense volcanic
eruptions which took place 4 to 4.3 million years ago
* Corresponding author. Tel.: +33-1-4427-6716; fax: +331-4325-7975.
E-mail address: [email protected] (S. Derenne).
and, after volcanic activity ceased, the subsequent invasions of water from the Pannonian lake into the
resulting tu rings. The lakes thus formed were currentfree and warm (more than 29 C, as shown by a high
aragonite content) due to periodic heating by post-volcanic geysers. Favourable conditions for planktonic life
developed in these lakes thanks to the important nutrient supply provided by the weathering of the crater
walls. Based on palynological observations, these four
oil shales contain an abundant contribution of fossil
colonies of Botryococcus microalgae, especially in the
case of the Pula deposit (Nagy, 1978). A sample from
0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(00)00093-0
788
S. Derenne et al. / Organic Geochemistry 31 (2000) 787±798
involved gas chromatographic/mass spectrometric (GC/
MS) analysis of the saturate fraction of bitumens, stable
carbon isotope analysis of individual alkanes in this
fraction by GC-C-irMS, examination of isolated kerogens via Fourier transform infra-red (FTIR) and solid
state 13C nuclear magnetic resonance (NMR) spectroscopies, Curie point pyrolysis coupled with GC/MS
(CuPy-GC/MS), scanning and transmission electron
microscopy (SEM and TEM).
2. Experimental
Fig. 1. Map of Hungary showing the location of the GeÂrce and
Pula deposits.
the massive alginite section (Brukner-Wein et al., 1991)
of the Pula oil shale was previously studied in detail
(Derenne et al., 1997). This study con®rmed that
Botryococcus braunii provided a major input to this
massive alginite, both via selective preservation of algaenan (resistant biomacromolecule building up the thick
outer walls of Botryococcus) and via incorporation of
some high molecular weight lipids of Botryococcus into
macromolecular structures. In addition, this study speci®ed the nature of the B. braunii races that contributed
to the Pula deposit: i.e. the n-alkadiene- and lycopadiene-producing ones, termed A and L, respectively.
The present study is focused on another deposit,
GeÂrce (Fig. 1), which is the largest Hungarian maar-type
oil shale deposit. Indeed, it covers 2.1 km2 with a
maximum alginite thickness of ca. 70 m. In contrast
with Pula where alternations of massive and laminated
alginite occur, the lithology of the GeÂrce deposit has
been shown to be homogeneous and to exclusively consist of laminated alginite, with lamination thickness
ranging from 0.1 to 0.5 mm (JaÂmbor and Solti, 1975).
Nevertheless, a previous study (Brukner-Wein and
Hetenyi, 1993) performed on 23 samples cored at various depths (between 16.3 and 65 m) in GeÂrce well-6,
revealed substantial dierences in Rock-Eval parameters between these samples. The aim of the present
work was therefore to (i) understand the origin of the
above dierences in spite of a similar lithology exhibited
by all these samples and (ii) to derive information on the
factors that control organic matter quality and quantity
in maar-type deposits.
To this end, bitumen abundance and group composition (saturates, aromatics, resins and asphaltenes) was
determined for the same set of 23 samples as previously
studied. From previous Rock-Eval data and the present
results on bitumen, four samples representative of the
whole set were selected for further studies. The latter
2.1. Bitumen analysis
Chloroform extraction of the ground oil shale samples was carried out in a Soxhlet apparatus and bitumen
fractionation was performed as previously described by
Brukner-Wein (1995). The less polar fraction was analysed by GC and GC/MS using an HP 5890 gas chromatograph with a CP Sil 5CB capillary column (length
25 m, i.d. 0.32 mm, ®lm thickness 0.4 mm). The oven was
heated from 100 to 300 C at 4 C minÿ1. For GC/MS
analyses, the chromatograph was coupled with a HP
5989A mass spectrometer operated at 70 eV. Isotopic
analyses of individual alkanes using the GC-C-irMS
technique were performed using a HP 5890 gas chromatograph (50 m BPX 5 capillary column, i.d. 0.32 mm,
®lm thickness 0.25 mm; heating program 100 to 350 C at
3 C minÿ1, splitless injector at 320 C) coupled to a
combustion (CuO) furnace (850 C), a cryogenic water
trap, and a VG Optima isotope ratio mass spectrometer.
Carbon isotopic compositions are expressed in per mil
relative to the Pee Dee Belemnite standard.
2.2. Kerogen analysis
Kerogens were isolated from the bitumen-free samples by the classical HF/HCl treatment (Durand and
Nicaise, 1980) and further extracted by stirring at room
temperature overnight with CH2Cl2/MeOH, 2/1, v/v.
Elemental analyses were performed at the Service Central d'Analyse du CNRS, Vernaison, France.
Kerogen FTIR spectra were recorded as KBr pellets.
Solid state 13C NMR spectra were obtained at 100.62
MHz on a Bruker MSL400 spectrometer using high
power decoupling, cross polarization (contact time 1
ms) and magic angle spinning (spinning rate 4 kHz) in a
double bearing probe. The spectra were the results of ca.
5000 scans.
Curie point pyrolyses were performed under an
helium ¯ow with ferromagnetic wires with a Curie temperature of 610 C in a Fisher 0316M pyrolyser. A pyrolysis time of 10 s was used and the reactor was
maintained at 250 C to prevent condensation of pyrolysis products. The pyrolyser was directly connected to
S. Derenne et al. / Organic Geochemistry 31 (2000) 787±798
the GC injector (heated at 280 C) of the same GC/MS
system as above. The GC oven was heated from 50 to
300 C at 3 C minÿ1.
The kerogens were ®xed by 1% glutaraldehyde in
cacodylate buer (pH 7.4) and post-®xed in 1% OsO4.
SEM observations were carried out after kerogen dehydration in ethanol, complete removal of water using the
CO2 critical point technique and coating with gold prior
to observations on a Jeol 840. The material was embedded in Araldite and sections were stained with uranyl
acetate and lead citrate prior to TEM observations on a
Philips 300 microscope.
3. Results and discussion
3.1. Bitumen
3.1.1. Abundance and group composition
The total amount of bitumen and the relative abundances of the dierent fractions (saturates, aromatics,
resins and asphaltenes) are shown for the 23 samples of
the core in Table 1. As previously observed for RockEval parameters, substantial dierences can be noted
down the core. However, no simple relationship could
be established between bitumen abundance and composition on the one hand and organic matter quantity and/
or quality (as shown by TOC/HI) on the other hand.
Additional studies were therefore carried out to elucidate the origin of these variations. To this end, both the
bitumen and the isolated kerogen from four selected
samples (located at 28, 37.3, 46 and 54.5 m depth) were
further studied. The selection of these four samples was
based on their contrasting features, as listed below: (i)
28 m, relatively low TOC and HI values, substantial
percentage of asphaltenes, (ii) 37.3 m, high TOC and HI
values, (iii) 46 m, substantial amount of bitumen, relatively low abundance of asphaltenes whereas aromatic
and resin contributions are relatively high and (iv) 54.5
m, relatively low amount of bitumen that has an extremely low abundance of saturated hydrocarbons.
3.1.2. GC/MS analysis
The saturated hydrocarbon fraction from the four
samples was analysed by GC and GC/MS. As shown in
Fig. 2, all these fractions are dominated by a homologous series of n-alkanes, ranging from C21 to C33 with
a strong odd-over-even carbon number predominance
[carbon preference index, CPI from 3.7 to 11.7 calculated according to Bray and Evans (1961) in the C22±C32
range]. Such a distribution is generally considered as
re¯ecting a strong terrestrial input with the long-chain
n-alkanes derived from epicuticular waxes of higher
plants. However, a similar distribution was previously
observed in the case of Pula oil shale, where a signi®cant
terrestrial input was not supported by palynological
789
observations. Indeed, stable carbon isotope ratios of
these individual n-alkanes do not match with those of
either C3 or C4 higher plants (Lichtfouse et al., 1994).
Such values thus show that the above long chain nalkanes, in the Pula deposit, in fact result from the
diagenetic reduction of B. braunii alkadienes. However,
in the case of the GeÂrce deposit, both algal and terrestrial origins could be a priori considered for the nalkanes since a substantial higher plant input was indicated by palynological observations (Nagy, 1978). d13C
measurements were therefore performed on the nalkanes present in the extracts of the 28, 37.3 and 54.5 m
samples (Table 2). The values thus obtained for the
long, C28 to C33, n-alkanes are in the same range as
those previously reported for Pula (Lichtfouse et al.,
1994). Given this similarity, it is likely that terrestrial
input was minor for these long chain hydrocarbons and
that Botryococcus contribution predominated.
In addition to n-alkanes, several relatively minor series also occur in the saturated hydrocarbon fraction of
the bitumen of the 28 m sample (Fig. 2A). Most of them
correspond to branched alkanes: C22 to C30 iso alkanes
with a strong even over odd predominance, C24 to C30
even-carbon-numbered anteiso alkanes and C24 to C30
even-carbon-numbered alkanes characterized by intense
fragments at m/z 57, 85 and (Mÿ57)+ and thus assigned
to 5-methylalkanes. All these series likely re¯ect a bacterial contribution since bacterial lipids are generally
considered as characterized by the occurrence of branched components (Kolattukudy, 1976). This bacterial
contribution is also evidenced by the presence of some
hopanoid compounds eluting between the C30 and C35
n-alkanes. These polycyclic compounds were identi®ed
on the basis of their mass spectra as C27, C29 and C30
hopanes along with a C30 hopene. Two series of branched alkanes eluting just before the 5-methylalkanes
were also detected. Their mass spectra are characterized
by a peak at m/z 168 or 196 but no precise structure
could be established. All these branched series are characterized by a substantially higher d13C values (ranging
from ÿ22 to ÿ25%) thus suggesting that they are not of
algal origin.
When comparing the four TIC traces (Fig. 2A±D), it
appears that bacterial contribution is lower in the 46 m
and 54.5 m samples and negligible in the 37.3 m one
since the GC trace of the latter only shows the n-alkane
peaks (Fig. 2B). Moreover, this sample is characterized
by the highest HI value and the lowest OI one in the
whole set and hence could be classi®ed as a type I
kerogen (Tables 1 and 3). In addition, when compared
to the other three samples, the n-alkane distribution in
the 37.3 m sample is more markedly dominated by oddcarbon-numbered compounds (CPI of 11.7 against 3.7,
5.5 and 5.3 for the 28, 46 and 54.5 m samples, respectively) with a clear maximum at C27 and C29 and much
lower levels of the shortest, C21 to C25, n-alkanes. In the
790
S. Derenne et al. / Organic Geochemistry 31 (2000) 787±798
Table 1
Bulk geochemical parameters (TOC, HI) from Brukner-Wein and Hetenyi (1993), bitumen abundance and composition in the 23
samples from GeÂrce well-6a
Depth
(m)
TOC
(%)
HI
(mg HC/g TOC)
Bitumen
(g/g TOC)
Asphaltenes
(%)
Saturates
(%)
Aromatics
(%)
Resin
(%)
16.3
18.8
19.0
19.8
28.0
31.2
32.0
34.8
35.0
36.5
37.3
38.8
41.5
42.0
44.0
46.0
52.5
54.5
56.7
57.3
61.3
62.3
65.0
3.86
6.45
7.22
6.58
5.76
4.72
6.63
4.69
5.10
6.28
9.36
7.48
5.94
5.87
4.97
7.08
6.39
9.17
2.54
5.76
3.62
5.11
2.39
475
571
637
620
441
473
526
534
577
746
748
502
608
596
501
570
583
637
487
486
489
470
372
0.16
0.27
0.26
0.26
0.37
0.38
0.29
0.33
0.31
0.32
0.32
0.51
0.24
0.21
0.35
0.45
0.23
0.28
0.23
0.24
0.13
0.26
0.21
5.0
27.8
44.4
35.4
35.0
34.4
45.8
32.1
36.9
37.0
27.7
34.0
23.6
18.7
14.3
19.1
23.9
24.6
26.0
17.5
17.6
32.6
21.8
2.8
1.3
1.3
1.2
1.4
1.5
1.3
1.3
1.3
1.5
1.1
1.2
1.9
1.4
1.1
0.7
1.7
0.1
1.4
0.7
1.0
0.7
1.8
3.8
2.1
1.7
2.7
1.3
2.1
1.5
1.0
1.1
1.4
1.6
1.0
2.2
2.2
2.5
6.3
2.8
2.0
2.5
1.1
3.5
1.6
3.7
76.0
58.5
44.4
50.0
58.3
54.9
42.7
58.6
51.1
47.6
60.4
63.7
58.6
62.5
73.3
72.6
60.1
55.1
61.2
68.0
63.9
46.4
62.9
a
Bold-faced data correspond to the four samples selected for further studies.
other samples, the latter alkanes are likely due to an
additional source. This is con®rmed by the d13C values
of the C22±C24 n-alkanes which are higher, for a given
compound, in the 28 and 54.5 m samples than in the
37.3 m one (Table 2). Indeed, for example, the C23
alkane exhibits d13C values of ÿ25.5 and ÿ27% in the 28
and 54.5 m samples, respectively, whereas it is ÿ30.6% in
the 37.3 m one. Based on this shift in isotopic composition, this additional contribution is likely to be of similar origin as the branched alkanes, i.e. bacterial.
3.2. Kerogen
3.2.1. Elemental analysis
Elemental composition was determined for the four
kerogens (Table 3). H/C atomic ratios are relatively high
(1.44±1.66) in agreement with their low maturity and
high oil potential (Brukner-Wein and Hetenyi, 1993). In
addition, a good correlation can be noted between H/C
ratios and HI values. S content is very low in the four
samples (< 2%), thus indicating that natural sulphur-
Table 2
Carbon isotope composition (d13C versus PDB, 0.3%) of the C21 to C33 n-alkanes from the extracts of the 28.0, 37.3 and 54.5 m
samples from GeÂrce well-6 compared with those from Pula (Lichtfouse et al., 1994)
Depth
(m)
Pula
28.0
37.3
54.5a
a
C21
C22
C23
C24
C25
C26
C27
ÿ23.9
ÿ26.5
ÿ29.4
ÿ27.9
ÿ27.7
ÿ25.5
ÿ30.6
ÿ27.0
ÿ27.0
ÿ27.5
ÿ30.1
ÿ28.6
ÿ28.9
ÿ27.0
ÿ27.9
ÿ28.1
ÿ28.4
ÿ28.2
ÿ29.4
ÿ29.2
ÿ29.5
ÿ28.2
ÿ29.1
ÿ29.0
ÿ29.2
ÿ26.2
ÿ25.9
Analysis of the 54.5 m sample was performed in duplicate.
C28
C29
C31
C33
ÿ28.7
ÿ29.0
ÿ30.9
ÿ30.7
ÿ30.0
ÿ31.1
ÿ29.2
ÿ29.6
ÿ30.9
ÿ35.4
ÿ32.6
ÿ31.4
ÿ33.0
ÿ30.0
ÿ30.9
ÿ30.1
ÿ33.2
ÿ33.5
S. Derenne et al. / Organic Geochemistry 31 (2000) 787±798
791
Fig. 2. Total ion current chromatogram showing the composition of the saturated hydrocarbon fraction of the bitumen from GeÂrce
well-6 oil shales [28 m (A), 37.3 m (B), 46 m (C) and 54.5 m (D)]. n-alkanes; * iso alkanes; x anteiso alkanes; ! 5- methylalkanes; 0
other branched alkanes; ~ hopanoids.
792
S. Derenne et al. / Organic Geochemistry 31 (2000) 787±798
ization was not an important process for OM preservation in these oil shales. TOC, HI and H/C values tend to
increase when the bacterial contribution (assessed from
the abundance of branched and hopanoid saturated
hydrocarbons in the bitumen) decreases.
3.2.2. FTIR
All the FTIR spectra (Fig. 3) are characterized by
intense bands at 2920, 2850, 1460 and 1375 cmÿ1 indicating an abundant contribution of alkyl chains. This is
con®rmed by the occurrence of a sharp band at 720
cmÿ1 due to (CH2)n with n 5 4. This high aliphaticity is
in agreement with the H/C ratios derived from elemental analysis and the Rock-Eval HI values. Moreover, the 37.3 m sample which was shown to exhibit the
highest H/C and HI values is characterized by the most
intense band at 720 cmÿ1. In addition, the relative
intensities of the 1460 (CH2+CH3) and 1375 cmÿ1
(CH3) bands can be used to assess the average chain
length or the branching level in the alkyl chains. As a
result, a higher contribution of methyl groups and/or
the occurrence of shorter chains is noted in the 28 and
46 m samples when compared to the other two. The
relative abundances of the OH (3400 cmÿ1), CO (1710
cmÿ1) and ole®nic CC bands (1617 cmÿ1) with respect
to the aliphatic ones (2920 and 2850 cmÿ1) are also
higher in the 28 and 46 m samples.
Fig. 3. FTIR spectra of GeÂrce well-6 kerogens isolated from
the 28 m (A), 37.3 m (B), 46 m (C) and 54.5 m (D) samples.
3.2.3. Solid state 13C NMR
The spectra of the four kerogens (Fig. 4) exhibit peaks
at the same chemical shifts. They are dominated by an
intense peak due to aliphatic carbons. This peak maximizes at 30 ppm (carbons from polymethylenic chains)
and shows shoulders at 15 and 35 ppm due to methyl
groups and substituted carbons, respectively. These
shoulders are much more signi®cant in the 28 and 46 m
samples thus resulting in a broader peak (width at half
height of ca. 14 ppm against ca. 6 ppm in the other two
samples). These observations are consistent with the
relative abundance of the CH3 and CH2 groups derived
from FTIR spectra. Signals at 130 ppm corresponding
to unsaturated carbons and at 175 ppm (esters and/or
amides) are observed with very low intensities in the
37.3 and 54.5 m kerogens whereas their intensities are
substantially higher in the other two samples, in agreement with FTIR data.
3.2.4. CuPy-GC/MS
Curie point pyrolysis was performed on the four
kerogens so as to obtain (i) more precise information on
the nature and length of the alkyl chains and (ii) better
insight into the building blocks of these geomacromolecules. The pyrochromatograms (Fig. 5) all show an
abundant homologous series of doublets corresponding
to n-alkanes and n-alk-1-enes up to C27. These doublets
result from the homolytic cleavage of long alkyl chains.
Fig. 4. Solid state 13C NMR spectra of GeÂrce well-6 kerogens
isolated from the 28 m (A), 37.3 m (B), 46 m (C) and 54.5 m
(D) samples (*spinning side bands).
793
S. Derenne et al. / Organic Geochemistry 31 (2000) 787±798
Table 3
Elemental composition and H/C atomic ratio of GeÂrce well-6 kerogens and Rock-Eval data of the corresponding crude oil shales
Depth
(m)
C
(%)
H
(%)
N
(%)
S
(%)
Ash
(%)
H/C
HI
(mgHC/gTOC)
OI
(mgCO2/gTOC)
Tmax
( C)
28.0
37.3
46.0
54.5
60.2
63.9
52.3
61.8
7.2
8.8
6.3
8.1
2.61
1.58
1.91
1.75
0.95
0.87
1.75
0.80
9
11
21
32
1.44
1.66
1.44
1.57
441
748
570
637
72
44
54
56
402
431
408
430
They are relatively more intense with respect to the
other pyrolysis products (such as prist-1-ene) in the 37.3
and 54.5 m samples thus con®rming their higher aliphaticity as shown by Rock-Eval parameters, H/C
ratios, FTIR and NMR data. Several minor series of
products containing n-alkyl chains are also identi®ed in
these pyrolysates; they are C7 to C29 n-alkan-2-ones and
n-alken-2-ones, C7 to C29 n-alkylbenzenes. The relative
abundance of these series with respect to the n-alkanes is
similar in all the samples. n-Alkanones were previously
shown to be derived from the thermal cleavage of ether
linkages between alkyl chains and are commonly
observed in kerogen pyrolysates (e.g. van de Meent et
al., 1980). n-Alkylbenzenes likely result from cyclization
and aromatization upon pyrolysis since no aromatic
moieties could be detected in the FTIR spectra (Mulik
and Erdman, 1963).
Long alkyl chains are known to build up the macromolecular network of the resistant biomacromolecules,
termed algaenans, occurring in the cell walls of various
species of microalgae and, as a result, alkane/alkene
doublets, alkanones and alkylbenzenes are detected in
algaenan pyrolysates (Largeau et al., 1984, 1986; Kadouri
et al., 1988; Derenne et al., 1991, 1992a). These algaenans
were shown to provide an important input to kerogens in
a number of organic-rich deposits via the selective preservation pathway (Largeau et al., 1986; Derenne et al.,
1991). In this mechanism, while the labile compounds
and classical biomacromolecules (like proteins and
polysaccharides) are rapidly degraded during the ®rst
steps of biomass fossilization, such resistant biomacromolecules are selectively preserved and hence selectively
enriched in kerogen (Tegelaar et al., 1989; Largeau and
Derenne, 1993). The ®rst evidence of the involvement of
this pathway in kerogen formation was obtained from a
comparative study of the algaenan isolated from the
extant microalga B. braunii and an immature, Botryococcus-derived oil shale (Torbanite) (Largeau et al.,
1984). Since Botryococcus, from palynological studies, is
known to contribute to the GeÂrce deposit, the above
alkyl-containing pyrolysis products are likely to be
derived from Botryococcus algaenan. Moreover, three
dierent chemical races, termed A, B and L, were distinguished in extant B. braunii on the basis of the nature
of the hydrocarbons they produce: alkadienes, terpenic
CnH2n-10 botryococcenes with n ranging from 30 to 37
and a lycopadiene, respectively. The chemical structure
of the algaenans from the A and B races is based on
long polymethylenic chains whereas that of the L race
also comprises C40 isoprenoid chains with a lycopane
skeleton thus yielding a number of isoprenoid compounds upon pyrolysis (Derenne et al., 1990a). The only
isoprenoid hydrocarbon which signi®cantly contributes
to the pyrolysates of the four GeÂrce samples is prist-1ene. However, pristenes are ubiquitous pyrolysis products of kerogens and several assumptions have been
put forward to account for their origin, including the
phytyl chain of chlorophyll (Didyk et al., 1978). The
lack of other isoprenoid compounds rules out a signi®cant contribution of the L race of B. braunii in the
GeÂrce deposit in contrast with what was observed in the
case of Pula deposit (Derenne et al., 1997). Moreover, as
stressed above, the long chain n-alkanes of the bitumen
indicate a contribution from the A race of B. braunii. As
a result, the n-alkane/n-alk-1-ene doublets in GeÂrce
pyrolysates likely originate from the algaenan of the A
race of B. braunii.
Phenol and higher substituted homologues with total
carbon number up to C23 are present in the pyrolysates
of the 28, 37.3 and 46 m GeÂrce kerogens. Phenols and
methoxyphenols substituted by short alkyl chains (4
C3) in kerogen pyrolysates are usually considered to be
related to lignin-derived compounds (Saiz-Jimenez and
de Leeuw, 1986) and therefore to re¯ect a terrestrial
input. Such low molecular weight phenols are especially
abundant in the 28 m sample thus indicating a relatively
high terrestrial input and they signi®cantly contribute to
the pyrolysates of the 46 and 37.3 m samples, as shown
by C7 phenol/C15 n-alkane ratios of 0.8, 0.4 and 0.2,
respectively. In sharp contrast, no C10-alkylphenols
were detected in the case of the 54.5 m sample. The
higher terrestrial contribution in the 28 m sample is in
agreement with its low HI and high OI values and with
the relatively high OH, CO and CC levels indicated
by FTIR and 13C NMR. Long chain (C10+) alkyl phenols are relatively important constituents of the pyrolysate of the 37.3 m sample whereas they are only
detected in trace amounts in the other two samples.
Such alkyl phenols were previously reported in the pyrolysate of Kukersite, a marine Ordovician deposit
794
S. Derenne et al. / Organic Geochemistry 31 (2000) 787±798
Fig. 5. Curie point Py-GC/MS (610 C) of GeÂrce well-6 kerogens isolated from the 28 m (A), 37.3 m (B), 46 m (C) and 54.5 m (D)
samples. * n-alkane/n-alk-1-ene doublets; phenols; p, prist-1-ene; H, hopanoid compounds.
S. Derenne et al. / Organic Geochemistry 31 (2000) 787±798
795
Fig. 6. Scanning electron microscopy (scale bar 10 mm) of a typical, poorly preserved, Botryococcus colony (A) and of a ligneous
debris (B) in kerogen from GeÂrce well-6 (37.3 m sample); well preserved Botryococcus colony typical of the Pula deposit (C). Transmission electron microscopy of Botryococcus in kerogen from GeÂrce well-6 (37.3 m sample): part of a colony (D), 16,000; coalesced
walls (E), 40,000 (V, cell voids; OW, algaenan-composed outer walls).
796
S. Derenne et al. / Organic Geochemistry 31 (2000) 787±798
chie¯y composed of fossil remains of Gloeocapsomorpha
prisca (Klesment, 1974; Klesment and Nappa, 1980;
Derenne et al., 1990b). These fossil microorganisms
were suggested to be related to the microalga B. braunii
which can adapt to large salinity variations although
this relationship is still a matter of debate (Stasiuk and
Osadetz, 1990). Under saline conditions, (i) the morphology of the colony is markedly modi®ed, with thick,
multilayered, outer walls entirely surrounding the cells
whereas the apical part is only covered by a thin trilaminar layer in a freshwater environment, and (ii) a substantial content of long chain alkylphenols is noted both
in the biosynthesized lipids and in the pyrolysis products
of the algaenan (Derenne et al., 1992b). Examination of
the organic matter from GeÂrce deposit by SEM revealed
the occurrence of lignous debris (Fig. 6B) and showed
the predominance of Botryococcus colonies. However,
the latter underwent extensive morphological diagenetic
alterations thus leading to hardly recognizable colonies
(Fig. 6A) especially when compared to the well-preserved ones occurring in Pula deposit (Fig. 6C). As a
result, it is not possible from these observations to
derive information on the morphological type of B.
braunii and hence on the salinity of the crater lake.
However, based on the geological features of this lake
(closed, warm with an intense weathering), a relatively
high salinity can be expected. Moreover, the important
morphological alteration of Botryococcus colonies may
re¯ect evaporitic events. Indeed, when the ultrastructure
of Coorongite (a Recent rubbery material formed from
Botryococcus biomass on the shores of some lakes or in
dried up basins) is examined, coalescence of the outer
walls and partial fusion of the colonies is noted
(Dubreuil et al., 1989). TEM observations of the 37.3 m
sample con®rmed the high level of alteration of the
morphology of Botryococcus colonies and revealed the
accumulation of outer walls around cell voids (Fig. 6D).
Coalescence of these walls was clearly observed via
TEM at high magni®cation (Fig. 6E).
Hopanes ranging from C27 to C31 are present in the
pyrolysates along with C27 and C29 hopenes. Such
polycyclic compounds were tightly bound to the macromolecular structure since they were not released upon
solvent extraction. Bound hopanoids have been previously described in the pyrolysates of a number of
kerogens (Tannenbaum et al., 1986; van Graas, 1986;
Eglinton and Douglas, 1988; Boreham et al., 1994;
Innes et al, 1997; Salmon et al., 1997). They originate
from bacterial lipid incorporation during diagenesis and
thus re¯ect bacterial input. As shown on the pyrochromatograms (Fig. 5), the relative amount of the
hopanoids in the four samples exhibits strong variations. It is rather high in the 46 and 28 m samples (relative abundance of the C27 hopene with respect to the C15
n-alkane of 0.8 and 0.6, respectively) and, to a lesser
extent, in the 54.5 m one (C27 hopene/C15 alkane ratio
of 0.3) whereas hopanoid abundance is very low in the
37.3 m sample (C27 hopene/C15 alkane of 0.1). However,
the same distribution is observed in the four samples.
Variations in bacterial contribution, derived from the
above observations on hopanoid abundance, are in
agreement with the branching level deduced from FTIR
and NMR spectra. Moreover, bitumen analysis suggested a lower bacterial contribution for the 37.3 m
sample which is fully con®rmed by hopane abundance
in the pyrolysates.
4. Conclusion
The largest Hungarian maar-type deposit, GeÂrce oil
shale, is known to exhibit substantial variations in
organic matter quantity and quality with depth
although its lithology is homogeneous (laminated alginite). The heterogeneity revealed by bulk geochemical
parameters was con®rmed by bitumen analysis performed on 23 core samples. Based on the above features, four samples were selected for detailed study,
using a large array of techniques, on both the soluble
and the insoluble fraction of the organic matter. The
nature and isotopic composition of the saturated
hydrocarbons of the bitumens, along with the spectroscopic features of the isolated kerogens and identi®cation of the products released upon kerogen pyrolysis
show that the observed dierences can be chie¯y attributed to variations in the relative contribution of the
various source organisms (Botryococcus microalgae,
higher plants and bacteria) and not to selective degradation during the ®rst stages of fossilization. Organic
matter quality in the GeÂrce oil shale, as re¯ected by HI
values, therefore appears closely correlated to the algal
contribution. In contrast, the relative increase in bacteria and higher plant contributions, especially pronounced in the case of the 28 m sample, is associated
with HI lowering. The above features are consistent
with the extremely highly aliphatic nature of Botryococcus algaenan which accounts, owing to selective preservation, for the bulk of the algal-derived material.
Moreover, they indicate that accumulated organic matter of bacteria and higher plant origin was characterized
by a rather low oil potential and hence was not dominated by waxy components. In addition, scanning and
transmission electron microscopy revealed an extensive
alteration of the morphology of the colonies, possibly
related to arid periods and associated increases in the
salinity of the crater lake.
Acknowledgements
This study was partly supported by the Action InteÂgreÂe Franco-Hongroise (Balaton Programme). J. Maquet
S. Derenne et al. / Organic Geochemistry 31 (2000) 787±798
(UPMC, Paris) is acknowledged for technical assistance
in solid state NMR and B. Rousseau (ENS, Paris) for
ultrathin section preparation. Scanning electron microscopy was performed at the CIME Jussieu, Paris.
Associate Editor Ð M.G. Fowler
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www.elsevier.nl/locate/orggeochem
Origin of variations in organic matter abundance and
composition in a lithologically homogeneous maar-type oil
shale deposit (GeÂrce, Pliocene, Hungary)
Sylvie Derenne a,*, Claude Largeau a, Alice Brukner-Wein b,
Magdolna Hetenyi c, GeÂrard Bardoux d, Andre Mariotti d
a
Laboratoire de Chimie Bioorganique et Organique Physique, UMR CNRS 7573, ENSCP, 11 rue P. et M. Curie,
75231 Paris cedex 05, France
b
Geological Institute of Hungary, StefaÂnia uÂt 14, H-1143 Budapest, Hungary
c
Institute of Mineralogy, Geochemistry and Petrography, JoÂzsef Attila University, PO Box 651, H-6701 Szeged, Hungary
d
Laboratoire de BiogeÂochimie Isotopique, INRA-CNRS-UPMC, 7 place Jussieu, 75252 Paris cedex 05, France
Received 26 January 2000; accepted 20 June 2000
(Returned to author for revision 3 May 2000)
Abstract
Despite having an homogeneous lithology, the largest Hungarian maar-type deposit (GeÂrce oil shale, Pliocene) has
previously been shown to exhibit substantial variations in organic matter quantity and quality with depth. This heterogeneity is also re¯ected, in the present study, by large variations in bitumen abundance and composition, for 23
samples from GeÂrce well-6 core. Based on the above bitumen data, four samples were selected that were representative
of the whole set which exhibit contrasting features. Scanning and transmission electron microscopy showed the
occurrence of extensively altered Botryococcus colonies in this deposit. GC/MS and GC-C-ir-MS of the saturated
hydrocarbon fractions of the bitumen of these samples reveal a predominant algal contribution along with a variable
bacterial input. The relative abundance of these two contributions in the four selected samples is also re¯ected by differences in FTIR and solid-state 13C NMR spectra of the isolated kerogens. Curie point pyrolysis/GC/MS of these
kerogens revealed a relatively high terrestrial contribution in one sample and con®rmed the variable input of algae and
bacteria. The above dierences in relative contributions account for the variations in organic matter quantity and
quality observed along the core. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Maar-type deposit; Botryococcus; Bitumen; Kerogen; Hydrocarbons; GC-C-ir-MS; FTIR; Solid-state 13C NMR; Pyrolysis/
GC/MS; Electron microscopy
1. Introduction
Four maar-type oil shales have been discovered in
Hungary in the last 25 years: Pula, GeÂrce, VaÂrkeszoÈ and
EgyhaÂzaskeszoÈ (Ravasz and Solti, 1987) (Fig. 1). These
organic-rich deposits are the result of intense volcanic
eruptions which took place 4 to 4.3 million years ago
* Corresponding author. Tel.: +33-1-4427-6716; fax: +331-4325-7975.
E-mail address: [email protected] (S. Derenne).
and, after volcanic activity ceased, the subsequent invasions of water from the Pannonian lake into the
resulting tu rings. The lakes thus formed were currentfree and warm (more than 29 C, as shown by a high
aragonite content) due to periodic heating by post-volcanic geysers. Favourable conditions for planktonic life
developed in these lakes thanks to the important nutrient supply provided by the weathering of the crater
walls. Based on palynological observations, these four
oil shales contain an abundant contribution of fossil
colonies of Botryococcus microalgae, especially in the
case of the Pula deposit (Nagy, 1978). A sample from
0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(00)00093-0
788
S. Derenne et al. / Organic Geochemistry 31 (2000) 787±798
involved gas chromatographic/mass spectrometric (GC/
MS) analysis of the saturate fraction of bitumens, stable
carbon isotope analysis of individual alkanes in this
fraction by GC-C-irMS, examination of isolated kerogens via Fourier transform infra-red (FTIR) and solid
state 13C nuclear magnetic resonance (NMR) spectroscopies, Curie point pyrolysis coupled with GC/MS
(CuPy-GC/MS), scanning and transmission electron
microscopy (SEM and TEM).
2. Experimental
Fig. 1. Map of Hungary showing the location of the GeÂrce and
Pula deposits.
the massive alginite section (Brukner-Wein et al., 1991)
of the Pula oil shale was previously studied in detail
(Derenne et al., 1997). This study con®rmed that
Botryococcus braunii provided a major input to this
massive alginite, both via selective preservation of algaenan (resistant biomacromolecule building up the thick
outer walls of Botryococcus) and via incorporation of
some high molecular weight lipids of Botryococcus into
macromolecular structures. In addition, this study speci®ed the nature of the B. braunii races that contributed
to the Pula deposit: i.e. the n-alkadiene- and lycopadiene-producing ones, termed A and L, respectively.
The present study is focused on another deposit,
GeÂrce (Fig. 1), which is the largest Hungarian maar-type
oil shale deposit. Indeed, it covers 2.1 km2 with a
maximum alginite thickness of ca. 70 m. In contrast
with Pula where alternations of massive and laminated
alginite occur, the lithology of the GeÂrce deposit has
been shown to be homogeneous and to exclusively consist of laminated alginite, with lamination thickness
ranging from 0.1 to 0.5 mm (JaÂmbor and Solti, 1975).
Nevertheless, a previous study (Brukner-Wein and
Hetenyi, 1993) performed on 23 samples cored at various depths (between 16.3 and 65 m) in GeÂrce well-6,
revealed substantial dierences in Rock-Eval parameters between these samples. The aim of the present
work was therefore to (i) understand the origin of the
above dierences in spite of a similar lithology exhibited
by all these samples and (ii) to derive information on the
factors that control organic matter quality and quantity
in maar-type deposits.
To this end, bitumen abundance and group composition (saturates, aromatics, resins and asphaltenes) was
determined for the same set of 23 samples as previously
studied. From previous Rock-Eval data and the present
results on bitumen, four samples representative of the
whole set were selected for further studies. The latter
2.1. Bitumen analysis
Chloroform extraction of the ground oil shale samples was carried out in a Soxhlet apparatus and bitumen
fractionation was performed as previously described by
Brukner-Wein (1995). The less polar fraction was analysed by GC and GC/MS using an HP 5890 gas chromatograph with a CP Sil 5CB capillary column (length
25 m, i.d. 0.32 mm, ®lm thickness 0.4 mm). The oven was
heated from 100 to 300 C at 4 C minÿ1. For GC/MS
analyses, the chromatograph was coupled with a HP
5989A mass spectrometer operated at 70 eV. Isotopic
analyses of individual alkanes using the GC-C-irMS
technique were performed using a HP 5890 gas chromatograph (50 m BPX 5 capillary column, i.d. 0.32 mm,
®lm thickness 0.25 mm; heating program 100 to 350 C at
3 C minÿ1, splitless injector at 320 C) coupled to a
combustion (CuO) furnace (850 C), a cryogenic water
trap, and a VG Optima isotope ratio mass spectrometer.
Carbon isotopic compositions are expressed in per mil
relative to the Pee Dee Belemnite standard.
2.2. Kerogen analysis
Kerogens were isolated from the bitumen-free samples by the classical HF/HCl treatment (Durand and
Nicaise, 1980) and further extracted by stirring at room
temperature overnight with CH2Cl2/MeOH, 2/1, v/v.
Elemental analyses were performed at the Service Central d'Analyse du CNRS, Vernaison, France.
Kerogen FTIR spectra were recorded as KBr pellets.
Solid state 13C NMR spectra were obtained at 100.62
MHz on a Bruker MSL400 spectrometer using high
power decoupling, cross polarization (contact time 1
ms) and magic angle spinning (spinning rate 4 kHz) in a
double bearing probe. The spectra were the results of ca.
5000 scans.
Curie point pyrolyses were performed under an
helium ¯ow with ferromagnetic wires with a Curie temperature of 610 C in a Fisher 0316M pyrolyser. A pyrolysis time of 10 s was used and the reactor was
maintained at 250 C to prevent condensation of pyrolysis products. The pyrolyser was directly connected to
S. Derenne et al. / Organic Geochemistry 31 (2000) 787±798
the GC injector (heated at 280 C) of the same GC/MS
system as above. The GC oven was heated from 50 to
300 C at 3 C minÿ1.
The kerogens were ®xed by 1% glutaraldehyde in
cacodylate buer (pH 7.4) and post-®xed in 1% OsO4.
SEM observations were carried out after kerogen dehydration in ethanol, complete removal of water using the
CO2 critical point technique and coating with gold prior
to observations on a Jeol 840. The material was embedded in Araldite and sections were stained with uranyl
acetate and lead citrate prior to TEM observations on a
Philips 300 microscope.
3. Results and discussion
3.1. Bitumen
3.1.1. Abundance and group composition
The total amount of bitumen and the relative abundances of the dierent fractions (saturates, aromatics,
resins and asphaltenes) are shown for the 23 samples of
the core in Table 1. As previously observed for RockEval parameters, substantial dierences can be noted
down the core. However, no simple relationship could
be established between bitumen abundance and composition on the one hand and organic matter quantity and/
or quality (as shown by TOC/HI) on the other hand.
Additional studies were therefore carried out to elucidate the origin of these variations. To this end, both the
bitumen and the isolated kerogen from four selected
samples (located at 28, 37.3, 46 and 54.5 m depth) were
further studied. The selection of these four samples was
based on their contrasting features, as listed below: (i)
28 m, relatively low TOC and HI values, substantial
percentage of asphaltenes, (ii) 37.3 m, high TOC and HI
values, (iii) 46 m, substantial amount of bitumen, relatively low abundance of asphaltenes whereas aromatic
and resin contributions are relatively high and (iv) 54.5
m, relatively low amount of bitumen that has an extremely low abundance of saturated hydrocarbons.
3.1.2. GC/MS analysis
The saturated hydrocarbon fraction from the four
samples was analysed by GC and GC/MS. As shown in
Fig. 2, all these fractions are dominated by a homologous series of n-alkanes, ranging from C21 to C33 with
a strong odd-over-even carbon number predominance
[carbon preference index, CPI from 3.7 to 11.7 calculated according to Bray and Evans (1961) in the C22±C32
range]. Such a distribution is generally considered as
re¯ecting a strong terrestrial input with the long-chain
n-alkanes derived from epicuticular waxes of higher
plants. However, a similar distribution was previously
observed in the case of Pula oil shale, where a signi®cant
terrestrial input was not supported by palynological
789
observations. Indeed, stable carbon isotope ratios of
these individual n-alkanes do not match with those of
either C3 or C4 higher plants (Lichtfouse et al., 1994).
Such values thus show that the above long chain nalkanes, in the Pula deposit, in fact result from the
diagenetic reduction of B. braunii alkadienes. However,
in the case of the GeÂrce deposit, both algal and terrestrial origins could be a priori considered for the nalkanes since a substantial higher plant input was indicated by palynological observations (Nagy, 1978). d13C
measurements were therefore performed on the nalkanes present in the extracts of the 28, 37.3 and 54.5 m
samples (Table 2). The values thus obtained for the
long, C28 to C33, n-alkanes are in the same range as
those previously reported for Pula (Lichtfouse et al.,
1994). Given this similarity, it is likely that terrestrial
input was minor for these long chain hydrocarbons and
that Botryococcus contribution predominated.
In addition to n-alkanes, several relatively minor series also occur in the saturated hydrocarbon fraction of
the bitumen of the 28 m sample (Fig. 2A). Most of them
correspond to branched alkanes: C22 to C30 iso alkanes
with a strong even over odd predominance, C24 to C30
even-carbon-numbered anteiso alkanes and C24 to C30
even-carbon-numbered alkanes characterized by intense
fragments at m/z 57, 85 and (Mÿ57)+ and thus assigned
to 5-methylalkanes. All these series likely re¯ect a bacterial contribution since bacterial lipids are generally
considered as characterized by the occurrence of branched components (Kolattukudy, 1976). This bacterial
contribution is also evidenced by the presence of some
hopanoid compounds eluting between the C30 and C35
n-alkanes. These polycyclic compounds were identi®ed
on the basis of their mass spectra as C27, C29 and C30
hopanes along with a C30 hopene. Two series of branched alkanes eluting just before the 5-methylalkanes
were also detected. Their mass spectra are characterized
by a peak at m/z 168 or 196 but no precise structure
could be established. All these branched series are characterized by a substantially higher d13C values (ranging
from ÿ22 to ÿ25%) thus suggesting that they are not of
algal origin.
When comparing the four TIC traces (Fig. 2A±D), it
appears that bacterial contribution is lower in the 46 m
and 54.5 m samples and negligible in the 37.3 m one
since the GC trace of the latter only shows the n-alkane
peaks (Fig. 2B). Moreover, this sample is characterized
by the highest HI value and the lowest OI one in the
whole set and hence could be classi®ed as a type I
kerogen (Tables 1 and 3). In addition, when compared
to the other three samples, the n-alkane distribution in
the 37.3 m sample is more markedly dominated by oddcarbon-numbered compounds (CPI of 11.7 against 3.7,
5.5 and 5.3 for the 28, 46 and 54.5 m samples, respectively) with a clear maximum at C27 and C29 and much
lower levels of the shortest, C21 to C25, n-alkanes. In the
790
S. Derenne et al. / Organic Geochemistry 31 (2000) 787±798
Table 1
Bulk geochemical parameters (TOC, HI) from Brukner-Wein and Hetenyi (1993), bitumen abundance and composition in the 23
samples from GeÂrce well-6a
Depth
(m)
TOC
(%)
HI
(mg HC/g TOC)
Bitumen
(g/g TOC)
Asphaltenes
(%)
Saturates
(%)
Aromatics
(%)
Resin
(%)
16.3
18.8
19.0
19.8
28.0
31.2
32.0
34.8
35.0
36.5
37.3
38.8
41.5
42.0
44.0
46.0
52.5
54.5
56.7
57.3
61.3
62.3
65.0
3.86
6.45
7.22
6.58
5.76
4.72
6.63
4.69
5.10
6.28
9.36
7.48
5.94
5.87
4.97
7.08
6.39
9.17
2.54
5.76
3.62
5.11
2.39
475
571
637
620
441
473
526
534
577
746
748
502
608
596
501
570
583
637
487
486
489
470
372
0.16
0.27
0.26
0.26
0.37
0.38
0.29
0.33
0.31
0.32
0.32
0.51
0.24
0.21
0.35
0.45
0.23
0.28
0.23
0.24
0.13
0.26
0.21
5.0
27.8
44.4
35.4
35.0
34.4
45.8
32.1
36.9
37.0
27.7
34.0
23.6
18.7
14.3
19.1
23.9
24.6
26.0
17.5
17.6
32.6
21.8
2.8
1.3
1.3
1.2
1.4
1.5
1.3
1.3
1.3
1.5
1.1
1.2
1.9
1.4
1.1
0.7
1.7
0.1
1.4
0.7
1.0
0.7
1.8
3.8
2.1
1.7
2.7
1.3
2.1
1.5
1.0
1.1
1.4
1.6
1.0
2.2
2.2
2.5
6.3
2.8
2.0
2.5
1.1
3.5
1.6
3.7
76.0
58.5
44.4
50.0
58.3
54.9
42.7
58.6
51.1
47.6
60.4
63.7
58.6
62.5
73.3
72.6
60.1
55.1
61.2
68.0
63.9
46.4
62.9
a
Bold-faced data correspond to the four samples selected for further studies.
other samples, the latter alkanes are likely due to an
additional source. This is con®rmed by the d13C values
of the C22±C24 n-alkanes which are higher, for a given
compound, in the 28 and 54.5 m samples than in the
37.3 m one (Table 2). Indeed, for example, the C23
alkane exhibits d13C values of ÿ25.5 and ÿ27% in the 28
and 54.5 m samples, respectively, whereas it is ÿ30.6% in
the 37.3 m one. Based on this shift in isotopic composition, this additional contribution is likely to be of similar origin as the branched alkanes, i.e. bacterial.
3.2. Kerogen
3.2.1. Elemental analysis
Elemental composition was determined for the four
kerogens (Table 3). H/C atomic ratios are relatively high
(1.44±1.66) in agreement with their low maturity and
high oil potential (Brukner-Wein and Hetenyi, 1993). In
addition, a good correlation can be noted between H/C
ratios and HI values. S content is very low in the four
samples (< 2%), thus indicating that natural sulphur-
Table 2
Carbon isotope composition (d13C versus PDB, 0.3%) of the C21 to C33 n-alkanes from the extracts of the 28.0, 37.3 and 54.5 m
samples from GeÂrce well-6 compared with those from Pula (Lichtfouse et al., 1994)
Depth
(m)
Pula
28.0
37.3
54.5a
a
C21
C22
C23
C24
C25
C26
C27
ÿ23.9
ÿ26.5
ÿ29.4
ÿ27.9
ÿ27.7
ÿ25.5
ÿ30.6
ÿ27.0
ÿ27.0
ÿ27.5
ÿ30.1
ÿ28.6
ÿ28.9
ÿ27.0
ÿ27.9
ÿ28.1
ÿ28.4
ÿ28.2
ÿ29.4
ÿ29.2
ÿ29.5
ÿ28.2
ÿ29.1
ÿ29.0
ÿ29.2
ÿ26.2
ÿ25.9
Analysis of the 54.5 m sample was performed in duplicate.
C28
C29
C31
C33
ÿ28.7
ÿ29.0
ÿ30.9
ÿ30.7
ÿ30.0
ÿ31.1
ÿ29.2
ÿ29.6
ÿ30.9
ÿ35.4
ÿ32.6
ÿ31.4
ÿ33.0
ÿ30.0
ÿ30.9
ÿ30.1
ÿ33.2
ÿ33.5
S. Derenne et al. / Organic Geochemistry 31 (2000) 787±798
791
Fig. 2. Total ion current chromatogram showing the composition of the saturated hydrocarbon fraction of the bitumen from GeÂrce
well-6 oil shales [28 m (A), 37.3 m (B), 46 m (C) and 54.5 m (D)]. n-alkanes; * iso alkanes; x anteiso alkanes; ! 5- methylalkanes; 0
other branched alkanes; ~ hopanoids.
792
S. Derenne et al. / Organic Geochemistry 31 (2000) 787±798
ization was not an important process for OM preservation in these oil shales. TOC, HI and H/C values tend to
increase when the bacterial contribution (assessed from
the abundance of branched and hopanoid saturated
hydrocarbons in the bitumen) decreases.
3.2.2. FTIR
All the FTIR spectra (Fig. 3) are characterized by
intense bands at 2920, 2850, 1460 and 1375 cmÿ1 indicating an abundant contribution of alkyl chains. This is
con®rmed by the occurrence of a sharp band at 720
cmÿ1 due to (CH2)n with n 5 4. This high aliphaticity is
in agreement with the H/C ratios derived from elemental analysis and the Rock-Eval HI values. Moreover, the 37.3 m sample which was shown to exhibit the
highest H/C and HI values is characterized by the most
intense band at 720 cmÿ1. In addition, the relative
intensities of the 1460 (CH2+CH3) and 1375 cmÿ1
(CH3) bands can be used to assess the average chain
length or the branching level in the alkyl chains. As a
result, a higher contribution of methyl groups and/or
the occurrence of shorter chains is noted in the 28 and
46 m samples when compared to the other two. The
relative abundances of the OH (3400 cmÿ1), CO (1710
cmÿ1) and ole®nic CC bands (1617 cmÿ1) with respect
to the aliphatic ones (2920 and 2850 cmÿ1) are also
higher in the 28 and 46 m samples.
Fig. 3. FTIR spectra of GeÂrce well-6 kerogens isolated from
the 28 m (A), 37.3 m (B), 46 m (C) and 54.5 m (D) samples.
3.2.3. Solid state 13C NMR
The spectra of the four kerogens (Fig. 4) exhibit peaks
at the same chemical shifts. They are dominated by an
intense peak due to aliphatic carbons. This peak maximizes at 30 ppm (carbons from polymethylenic chains)
and shows shoulders at 15 and 35 ppm due to methyl
groups and substituted carbons, respectively. These
shoulders are much more signi®cant in the 28 and 46 m
samples thus resulting in a broader peak (width at half
height of ca. 14 ppm against ca. 6 ppm in the other two
samples). These observations are consistent with the
relative abundance of the CH3 and CH2 groups derived
from FTIR spectra. Signals at 130 ppm corresponding
to unsaturated carbons and at 175 ppm (esters and/or
amides) are observed with very low intensities in the
37.3 and 54.5 m kerogens whereas their intensities are
substantially higher in the other two samples, in agreement with FTIR data.
3.2.4. CuPy-GC/MS
Curie point pyrolysis was performed on the four
kerogens so as to obtain (i) more precise information on
the nature and length of the alkyl chains and (ii) better
insight into the building blocks of these geomacromolecules. The pyrochromatograms (Fig. 5) all show an
abundant homologous series of doublets corresponding
to n-alkanes and n-alk-1-enes up to C27. These doublets
result from the homolytic cleavage of long alkyl chains.
Fig. 4. Solid state 13C NMR spectra of GeÂrce well-6 kerogens
isolated from the 28 m (A), 37.3 m (B), 46 m (C) and 54.5 m
(D) samples (*spinning side bands).
793
S. Derenne et al. / Organic Geochemistry 31 (2000) 787±798
Table 3
Elemental composition and H/C atomic ratio of GeÂrce well-6 kerogens and Rock-Eval data of the corresponding crude oil shales
Depth
(m)
C
(%)
H
(%)
N
(%)
S
(%)
Ash
(%)
H/C
HI
(mgHC/gTOC)
OI
(mgCO2/gTOC)
Tmax
( C)
28.0
37.3
46.0
54.5
60.2
63.9
52.3
61.8
7.2
8.8
6.3
8.1
2.61
1.58
1.91
1.75
0.95
0.87
1.75
0.80
9
11
21
32
1.44
1.66
1.44
1.57
441
748
570
637
72
44
54
56
402
431
408
430
They are relatively more intense with respect to the
other pyrolysis products (such as prist-1-ene) in the 37.3
and 54.5 m samples thus con®rming their higher aliphaticity as shown by Rock-Eval parameters, H/C
ratios, FTIR and NMR data. Several minor series of
products containing n-alkyl chains are also identi®ed in
these pyrolysates; they are C7 to C29 n-alkan-2-ones and
n-alken-2-ones, C7 to C29 n-alkylbenzenes. The relative
abundance of these series with respect to the n-alkanes is
similar in all the samples. n-Alkanones were previously
shown to be derived from the thermal cleavage of ether
linkages between alkyl chains and are commonly
observed in kerogen pyrolysates (e.g. van de Meent et
al., 1980). n-Alkylbenzenes likely result from cyclization
and aromatization upon pyrolysis since no aromatic
moieties could be detected in the FTIR spectra (Mulik
and Erdman, 1963).
Long alkyl chains are known to build up the macromolecular network of the resistant biomacromolecules,
termed algaenans, occurring in the cell walls of various
species of microalgae and, as a result, alkane/alkene
doublets, alkanones and alkylbenzenes are detected in
algaenan pyrolysates (Largeau et al., 1984, 1986; Kadouri
et al., 1988; Derenne et al., 1991, 1992a). These algaenans
were shown to provide an important input to kerogens in
a number of organic-rich deposits via the selective preservation pathway (Largeau et al., 1986; Derenne et al.,
1991). In this mechanism, while the labile compounds
and classical biomacromolecules (like proteins and
polysaccharides) are rapidly degraded during the ®rst
steps of biomass fossilization, such resistant biomacromolecules are selectively preserved and hence selectively
enriched in kerogen (Tegelaar et al., 1989; Largeau and
Derenne, 1993). The ®rst evidence of the involvement of
this pathway in kerogen formation was obtained from a
comparative study of the algaenan isolated from the
extant microalga B. braunii and an immature, Botryococcus-derived oil shale (Torbanite) (Largeau et al.,
1984). Since Botryococcus, from palynological studies, is
known to contribute to the GeÂrce deposit, the above
alkyl-containing pyrolysis products are likely to be
derived from Botryococcus algaenan. Moreover, three
dierent chemical races, termed A, B and L, were distinguished in extant B. braunii on the basis of the nature
of the hydrocarbons they produce: alkadienes, terpenic
CnH2n-10 botryococcenes with n ranging from 30 to 37
and a lycopadiene, respectively. The chemical structure
of the algaenans from the A and B races is based on
long polymethylenic chains whereas that of the L race
also comprises C40 isoprenoid chains with a lycopane
skeleton thus yielding a number of isoprenoid compounds upon pyrolysis (Derenne et al., 1990a). The only
isoprenoid hydrocarbon which signi®cantly contributes
to the pyrolysates of the four GeÂrce samples is prist-1ene. However, pristenes are ubiquitous pyrolysis products of kerogens and several assumptions have been
put forward to account for their origin, including the
phytyl chain of chlorophyll (Didyk et al., 1978). The
lack of other isoprenoid compounds rules out a signi®cant contribution of the L race of B. braunii in the
GeÂrce deposit in contrast with what was observed in the
case of Pula deposit (Derenne et al., 1997). Moreover, as
stressed above, the long chain n-alkanes of the bitumen
indicate a contribution from the A race of B. braunii. As
a result, the n-alkane/n-alk-1-ene doublets in GeÂrce
pyrolysates likely originate from the algaenan of the A
race of B. braunii.
Phenol and higher substituted homologues with total
carbon number up to C23 are present in the pyrolysates
of the 28, 37.3 and 46 m GeÂrce kerogens. Phenols and
methoxyphenols substituted by short alkyl chains (4
C3) in kerogen pyrolysates are usually considered to be
related to lignin-derived compounds (Saiz-Jimenez and
de Leeuw, 1986) and therefore to re¯ect a terrestrial
input. Such low molecular weight phenols are especially
abundant in the 28 m sample thus indicating a relatively
high terrestrial input and they signi®cantly contribute to
the pyrolysates of the 46 and 37.3 m samples, as shown
by C7 phenol/C15 n-alkane ratios of 0.8, 0.4 and 0.2,
respectively. In sharp contrast, no C10-alkylphenols
were detected in the case of the 54.5 m sample. The
higher terrestrial contribution in the 28 m sample is in
agreement with its low HI and high OI values and with
the relatively high OH, CO and CC levels indicated
by FTIR and 13C NMR. Long chain (C10+) alkyl phenols are relatively important constituents of the pyrolysate of the 37.3 m sample whereas they are only
detected in trace amounts in the other two samples.
Such alkyl phenols were previously reported in the pyrolysate of Kukersite, a marine Ordovician deposit
794
S. Derenne et al. / Organic Geochemistry 31 (2000) 787±798
Fig. 5. Curie point Py-GC/MS (610 C) of GeÂrce well-6 kerogens isolated from the 28 m (A), 37.3 m (B), 46 m (C) and 54.5 m (D)
samples. * n-alkane/n-alk-1-ene doublets; phenols; p, prist-1-ene; H, hopanoid compounds.
S. Derenne et al. / Organic Geochemistry 31 (2000) 787±798
795
Fig. 6. Scanning electron microscopy (scale bar 10 mm) of a typical, poorly preserved, Botryococcus colony (A) and of a ligneous
debris (B) in kerogen from GeÂrce well-6 (37.3 m sample); well preserved Botryococcus colony typical of the Pula deposit (C). Transmission electron microscopy of Botryococcus in kerogen from GeÂrce well-6 (37.3 m sample): part of a colony (D), 16,000; coalesced
walls (E), 40,000 (V, cell voids; OW, algaenan-composed outer walls).
796
S. Derenne et al. / Organic Geochemistry 31 (2000) 787±798
chie¯y composed of fossil remains of Gloeocapsomorpha
prisca (Klesment, 1974; Klesment and Nappa, 1980;
Derenne et al., 1990b). These fossil microorganisms
were suggested to be related to the microalga B. braunii
which can adapt to large salinity variations although
this relationship is still a matter of debate (Stasiuk and
Osadetz, 1990). Under saline conditions, (i) the morphology of the colony is markedly modi®ed, with thick,
multilayered, outer walls entirely surrounding the cells
whereas the apical part is only covered by a thin trilaminar layer in a freshwater environment, and (ii) a substantial content of long chain alkylphenols is noted both
in the biosynthesized lipids and in the pyrolysis products
of the algaenan (Derenne et al., 1992b). Examination of
the organic matter from GeÂrce deposit by SEM revealed
the occurrence of lignous debris (Fig. 6B) and showed
the predominance of Botryococcus colonies. However,
the latter underwent extensive morphological diagenetic
alterations thus leading to hardly recognizable colonies
(Fig. 6A) especially when compared to the well-preserved ones occurring in Pula deposit (Fig. 6C). As a
result, it is not possible from these observations to
derive information on the morphological type of B.
braunii and hence on the salinity of the crater lake.
However, based on the geological features of this lake
(closed, warm with an intense weathering), a relatively
high salinity can be expected. Moreover, the important
morphological alteration of Botryococcus colonies may
re¯ect evaporitic events. Indeed, when the ultrastructure
of Coorongite (a Recent rubbery material formed from
Botryococcus biomass on the shores of some lakes or in
dried up basins) is examined, coalescence of the outer
walls and partial fusion of the colonies is noted
(Dubreuil et al., 1989). TEM observations of the 37.3 m
sample con®rmed the high level of alteration of the
morphology of Botryococcus colonies and revealed the
accumulation of outer walls around cell voids (Fig. 6D).
Coalescence of these walls was clearly observed via
TEM at high magni®cation (Fig. 6E).
Hopanes ranging from C27 to C31 are present in the
pyrolysates along with C27 and C29 hopenes. Such
polycyclic compounds were tightly bound to the macromolecular structure since they were not released upon
solvent extraction. Bound hopanoids have been previously described in the pyrolysates of a number of
kerogens (Tannenbaum et al., 1986; van Graas, 1986;
Eglinton and Douglas, 1988; Boreham et al., 1994;
Innes et al, 1997; Salmon et al., 1997). They originate
from bacterial lipid incorporation during diagenesis and
thus re¯ect bacterial input. As shown on the pyrochromatograms (Fig. 5), the relative amount of the
hopanoids in the four samples exhibits strong variations. It is rather high in the 46 and 28 m samples (relative abundance of the C27 hopene with respect to the C15
n-alkane of 0.8 and 0.6, respectively) and, to a lesser
extent, in the 54.5 m one (C27 hopene/C15 alkane ratio
of 0.3) whereas hopanoid abundance is very low in the
37.3 m sample (C27 hopene/C15 alkane of 0.1). However,
the same distribution is observed in the four samples.
Variations in bacterial contribution, derived from the
above observations on hopanoid abundance, are in
agreement with the branching level deduced from FTIR
and NMR spectra. Moreover, bitumen analysis suggested a lower bacterial contribution for the 37.3 m
sample which is fully con®rmed by hopane abundance
in the pyrolysates.
4. Conclusion
The largest Hungarian maar-type deposit, GeÂrce oil
shale, is known to exhibit substantial variations in
organic matter quantity and quality with depth
although its lithology is homogeneous (laminated alginite). The heterogeneity revealed by bulk geochemical
parameters was con®rmed by bitumen analysis performed on 23 core samples. Based on the above features, four samples were selected for detailed study,
using a large array of techniques, on both the soluble
and the insoluble fraction of the organic matter. The
nature and isotopic composition of the saturated
hydrocarbons of the bitumens, along with the spectroscopic features of the isolated kerogens and identi®cation of the products released upon kerogen pyrolysis
show that the observed dierences can be chie¯y attributed to variations in the relative contribution of the
various source organisms (Botryococcus microalgae,
higher plants and bacteria) and not to selective degradation during the ®rst stages of fossilization. Organic
matter quality in the GeÂrce oil shale, as re¯ected by HI
values, therefore appears closely correlated to the algal
contribution. In contrast, the relative increase in bacteria and higher plant contributions, especially pronounced in the case of the 28 m sample, is associated
with HI lowering. The above features are consistent
with the extremely highly aliphatic nature of Botryococcus algaenan which accounts, owing to selective preservation, for the bulk of the algal-derived material.
Moreover, they indicate that accumulated organic matter of bacteria and higher plant origin was characterized
by a rather low oil potential and hence was not dominated by waxy components. In addition, scanning and
transmission electron microscopy revealed an extensive
alteration of the morphology of the colonies, possibly
related to arid periods and associated increases in the
salinity of the crater lake.
Acknowledgements
This study was partly supported by the Action InteÂgreÂe Franco-Hongroise (Balaton Programme). J. Maquet
S. Derenne et al. / Organic Geochemistry 31 (2000) 787±798
(UPMC, Paris) is acknowledged for technical assistance
in solid state NMR and B. Rousseau (ENS, Paris) for
ultrathin section preparation. Scanning electron microscopy was performed at the CIME Jussieu, Paris.
Associate Editor Ð M.G. Fowler
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