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 aLaboratoire de Chimie Bioorganique et Organique Physique, UMR CNRS 7573, ENSCP, 11 rue P. et M. Curie,75231 Paris cedex 05, France
bGeological Institute of Hungary, StefaÂnia uÂt 14, H-1143 Budapest, Hungary
cInstitute of Mineralogy, Geochemistry and Petrography, JoÂzsef Attila University, PO Box 651, H-6701 Szeged, Hungary dLaboratoire 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 het-erogeneity 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 dif-ferences in FTIR and solid-state13C 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-state13C 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
and, after volcanic activity ceased, the subsequent inva-sions of water from the Pannonian lake into the resulting tu rings. The lakes thus formed were current-free and warm (more than 29C, as shown by a high
aragonite content) due to periodic heating by post-vol-canic geysers. Favourable conditions for planktonic life developed in these lakes thanks to the important nutri-ent 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
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* Corresponding author. Tel.: 1-4427-6716; fax: +33-1-4325-7975.
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 algae-nan (resistant biomacromolecule building up the thick outer walls of Botryococcus) and via incorporation of some high molecular weight lipids ofBotryococcusinto macromolecular structures. In addition, this study spe-ci®ed the nature of theB. brauniiraces that contributed to the Pula deposit: i.e. the n-alkadiene- and lycopa-diene-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 con-sist 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 var-ious depths (between 16.3 and 65 m) in GeÂrce well-6, revealed substantial dierences in Rock-Eval para-meters 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 composi-tion (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
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 kero-gens via Fourier transform infra-red (FTIR) and solid state 13C nuclear magnetic resonance (NMR) spectro-scopies, Curie point pyrolysis coupled with GC/MS (CuPy-GC/MS), scanning and transmission electron microscopy (SEM and TEM).
2. Experimental
2.1. Bitumen analysis
Chloroform extraction of the ground oil shale sam-ples was carried out in a Soxhlet apparatus and bitumen fractionation was performed as previously described by Brukner-Wein (1995). The less polar fraction was ana-lysed by GC and GC/MS using an HP 5890 gas chro-matograph with a CP Sil 5CB capillary column (length 25 m, i.d. 0.32 mm, ®lm thickness 0.4mm). The oven was heated from 100 to 300C at 4C 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 chro-matograph (50 m BPX 5 capillary column, i.d. 0.32 mm, ®lm thickness 0.25mm; heating program 100 to 350C at
3C minÿ1, splitless injector at 320C) coupled to a
combustion (CuO) furnace (850C), 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 sam-ples 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 Cen-tral 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 tem-perature of 610C in a Fisher 0316M pyrolyser. A
pyr-olysis time of 10 s was used and the reactor was maintained at 250C to prevent condensation of
pyr-olysis products. The pyrolyser was directly connected to
the GC injector (heated at 280C) of the same GC/MS
system as above. The GC oven was heated from 50 to 300C at 3C 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 dehy-dration in ethanol, complete removal of water using the CO2critical point technique and coating with gold prior to observations on a Jeol 840. The material was embed-ded 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 abun-dances 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 Rock-Eval parameters, substantial dierences can be noted down the core. However, no simple relationship could be established between bitumen abundance and compo-sition 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 eluci-date 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, rela-tively 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 extre-mely 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 homo-logous series ofn-alkanes, ranging from C21to C33with a strong odd-over-even carbon number predominance [carbon preference index, CPI from 3.7 to 11.7 calcu-lated 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
observations. Indeed, stable carbon isotope ratios of these individual n-alkanes do not match with those of either C3 or C4higher plants (Lichtfouse et al., 1994). Such values thus show that the above long chain n -alkanes, in the Pula deposit, in fact result from the diagenetic reduction ofB. brauniialkadienes. However, in the case of the GeÂrce deposit, both algal and terres-trial origins could be a priori considered for the n -alkanes since a substantial higher plant input was indi-cated by palynological observations (Nagy, 1978).d13C measurements were therefore performed on the n -alkanes 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 thatBotryococcuscontribution predominated.
In addition ton-alkanes, several relatively minor ser-ies also occur in the saturated hydrocarbon fraction of the bitumen of the 28 m sample (Fig. 2A). Most of them correspond to branched alkanes: C22to C30iso alkanes with a strong even over odd predominance, C24to C30 even-carbon-numbered anteiso alkanes and C24to C30 even-carbon-numbered alkanes characterized by intense fragments atm/z57, 85 and (Mÿ57)+and thus assigned to 5-methylalkanes. All these series likely re¯ect a bac-terial contribution since bacbac-terial lipids are generally considered as characterized by the occurrence of bran-ched components (Kolattukudy, 1976). This bacterial contribution is also evidenced by the presence of some hopanoid compounds eluting between the C30and C35
n-alkanes. These polycyclic compounds were identi®ed on the basis of their mass spectra as C27, C29and C30 hopanes along with a C30hopene. Two series of bran-ched 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 char-acterized by a substantially higherd13C values (ranging fromÿ22 toÿ25%) thus suggesting that they are not of algal origin.
other samples, the latter alkanes are likely due to an additional source. This is con®rmed by thed13C 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 exhibitsd13C values of
ÿ25.5 andÿ27%in the 28 and 54.5 m samples, respectively, whereas it isÿ30.6%in the 37.3 m one. Based on this shift in isotopic composi-tion, this additional contribution is likely to be of simi-lar 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 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 TOC HI Bitumen Asphaltenes Saturates Aromatics Resin
(m) (%) (mg HC/g TOC) (g/g TOC) (%) (%) (%) (%)
16.3 3.86 475 0.16 5.0 2.8 3.8 76.0
18.8 6.45 571 0.27 27.8 1.3 2.1 58.5
19.0 7.22 637 0.26 44.4 1.3 1.7 44.4
19.8 6.58 620 0.26 35.4 1.2 2.7 50.0
28.0 5.76 441 0.37 35.0 1.4 1.3 58.3
31.2 4.72 473 0.38 34.4 1.5 2.1 54.9
32.0 6.63 526 0.29 45.8 1.3 1.5 42.7
34.8 4.69 534 0.33 32.1 1.3 1.0 58.6
35.0 5.10 577 0.31 36.9 1.3 1.1 51.1
36.5 6.28 746 0.32 37.0 1.5 1.4 47.6
37.3 9.36 748 0.32 27.7 1.1 1.6 60.4
38.8 7.48 502 0.51 34.0 1.2 1.0 63.7
41.5 5.94 608 0.24 23.6 1.9 2.2 58.6
42.0 5.87 596 0.21 18.7 1.4 2.2 62.5
44.0 4.97 501 0.35 14.3 1.1 2.5 73.3
46.0 7.08 570 0.45 19.1 0.7 6.3 72.6
52.5 6.39 583 0.23 23.9 1.7 2.8 60.1
54.5 9.17 637 0.28 24.6 0.1 2.0 55.1
56.7 2.54 487 0.23 26.0 1.4 2.5 61.2
57.3 5.76 486 0.24 17.5 0.7 1.1 68.0
61.3 3.62 489 0.13 17.6 1.0 3.5 63.9
62.3 5.11 470 0.26 32.6 0.7 1.6 46.4
65.0 2.39 372 0.21 21.8 1.8 3.7 62.9
a Bold-faced data correspond to the four samples selected for further studies.
Table 2
Carbon isotope composition (d13C versus PDB,
0.3%) of the C21to C33n-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 C21 C22 C23 C24 C25 C26 C27 C28 C29 C31 C33
(m)
Pula ÿ28.7 ÿ30.7 ÿ30.9 ÿ30.0
28.0 ÿ23.9 ÿ26.5 ÿ25.5 ÿ27.5 ÿ27.0 ÿ28.2 ÿ28.2 ÿ29.0 ÿ30.0 ÿ35.4 ÿ30.9
37.3 ÿ29.4 ÿ30.6 ÿ30.1 ÿ27.9 ÿ29.4 ÿ29.1 ÿ30.9 ÿ31.1 ÿ32.6
54.5a
ÿ26.2 ÿ27.9 ÿ27.0 ÿ28.6 ÿ28.1 ÿ29.2 ÿ29.0 ÿ29.2 ÿ31.4 ÿ33.2
ÿ25.9 ÿ27.7 ÿ27.0 ÿ28.9 ÿ28.4 ÿ29.5 ÿ29.2 ÿ30.1 ÿ29.6 ÿ33.0 ÿ33.5
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;xanteiso alkanes;!5- methylalkanes;0
ization was not an important process for OM preserva-tion 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 indi-cating an abundant contribution of alkyl chains. This is con®rmed by the occurrence of a sharp band at 720 cmÿ1due to (CH
2)nwithn54. This high aliphaticity is in agreement with the H/C ratios derived from ele-mental analysis and the Rock-Eval HI values. More-over, 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), C
O (1710 cmÿ1) and ole®nic C
C bands (1617 cmÿ1) with respect to the aliphatic ones (2920 and 2850 cmÿ1) are also higher in the 28 and 46 m samples.
3.2.3. Solid state13C 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 max-imizes 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 CH3and CH2groups 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 agree-ment 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 geomacromole-cules. The pyrochromatograms (Fig. 5) all show an abundant homologous series of doublets corresponding ton-alkanes andn-alk-1-enes up to C27. These doublets result from the homolytic cleavage of long alkyl chains.
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.
Fig. 4. Solid state13C NMR spectra of GeÂrce well-6 kerogens
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 ali-phaticity as shown by Rock-Eval parameters, H/C ratios, FTIR and NMR data. Several minor series of products containingn-alkyl chains are also identi®ed in these pyrolysates; they are C7to C29n-alkan-2-ones and
n-alken-2-ones, C7to C29n-alkylbenzenes. The relative abundance of these series with respect to then-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 macro-molecular 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 depositsviathe selective pre-servation 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 biomacro-molecules 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, Botryo-coccus-derived oil shale (Torbanite) (Largeau et al., 1984). SinceBotryococcus,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 dis-tinguished in extantB. brauniion the basis of the nature of the hydrocarbons they produce: alkadienes, terpenic
CnH2n-10 botryococcenes withnranging 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 com-pounds upon pyrolysis (Derenne et al., 1990a). The only isoprenoid hydrocarbon which signi®cantly contributes to the pyrolysates of the four GeÂrce samples is prist-1-ene. However, pristenes are ubiquitous pyrolysis pro-ducts 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 sig-ni®cant contribution of the L race ofB. 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 chainn-alkanes of the bitumen indicate a contribution from the A race ofB. braunii. As a result, the n-alkane/n-alk-1-ene doublets in GeÂrce pyrolysates likely originate from the algaenan of the A race ofB. braunii.
Phenol and higher substituted homologues with total carbon number up to C23are 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 and13C NMR. Long chain (C
10+) alkyl phe-nols are relatively important constituents of the pyr-olysate 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 pyr-olysate of Kukersite, a marine Ordovician deposit
Table 3
Elemental composition and H/C atomic ratio of GeÂrce well-6 kerogens and Rock-Eval data of the corresponding crude oil shales
Fig. 5. Curie point Py-GC/MS (610C) of GeÂrce well-6 kerogens isolated from the 28 m (A), 37.3 m (B), 46 m (C) and 54.5 m (D)
chie¯y composed of fossil remains ofGloeocapsomorpha prisca (Klesment, 1974; Klesment and Nappa, 1980; Derenne et al., 1990b). These fossil microorganisms were suggested to be related to the microalgaB. 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 mor-phology 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 trila-minar layer in a freshwater environment, and (ii) a sub-stantial 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-pre-served 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 Botryococcuscolonies may re¯ect evaporitic events. Indeed, when the ultrastructure of Coorongite (a Recent rubbery material formed from
Botryococcusbiomass 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 ofBotryococcus 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 C27to C31are present in the pyrolysates along with C27 and C29 hopenes. Such polycyclic compounds were tightly bound to the mac-romolecular structure since they were not released upon solvent extraction. Bound hopanoids have been pre-viously 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 pyr-ochromatograms (Fig. 5), the relative amount of the hopanoids in the four samples exhibits strong varia-tions. It is rather high in the 46 and 28 m samples (rela-tive abundance of the C27hopene 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 (C27hopene/C15alkane ratio
of 0.3) whereas hopanoid abundance is very low in the 37.3 m sample (C27hopene/C15alkane 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 sug-gested 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 algi-nite). The heterogeneity revealed by bulk geochemical parameters was con®rmed by bitumen analysis per-formed on 23 core samples. Based on the above fea-tures, 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 spectro-scopic features of the isolated kerogens and identi®ca-tion of the products released upon kerogen pyrolysis show that the observed dierences can be chie¯y attrib-uted to variations in the relative contribution of the various source organisms (Botryococcus microalgae, higher plants and bacteria) and not to selective degra-dation 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 bac-teria and higher plant contributions, especially pro-nounced 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 Botryo-coccusalgaenan which accounts, owing to selective pre-servation, for the bulk of the algal-derived material. Moreover, they indicate that accumulated organic mat-ter of bacmat-teria and higher plant origin was characmat-terized by a rather low oil potential and hence was not domi-nated 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
(UPMC, Paris) is acknowledged for technical assistance in solid state NMR and B. Rousseau (ENS, Paris) for ultrathin section preparation. Scanning electron micro-scopy was performed at the CIME Jussieu, Paris.
Associate EditorÐM.G. Fowler
References
Boreham, C.J., Summons, R.E., Roksandic, Z., Dowling, L.M., Hutton, A.C., 1994. Chemical, molecular and isotopic dierentiation of organic facies in the Tertiary lacustrine Duaringa oil shale deposit, Queensland, Australia. Organic Geochemistry 21, 685±712.
Bray, E.E., Evans, E.D., 1961. Distribution ofn-parans as a clue to recognition of source beds. Geochimica et Cosmo-chimica Acta 22, 2±15.
Brukner-Wein, A., 1995. Infrared spectrometric and gas chro-matographic determination of the soluble organis matter from rock samples (oil shales). Analyst 120, 1687±1691. Brukner-Wein, A., HeteÂnyi, M., 1993. Relationship of the
organic geochemical features of two maar-type Hungarian oil shales. Acta Geologica Hungarica 36, 223±239.
Brukner-Wein, A., HeteÂnyi, M., Solti, G., 1991. Organic geo-chemistry of alginite deposited in a volcanic crater lake. In: Manning, D. (Ed.), Organic Geochemistry, Manchester University Press, (pp. 402±404).
Derenne, S., Largeau, C., Casadevall, E., Sellier, N., 1990a. Direct relationship between the resistant biopolymer and the tetraterpenic hydrocarbon in the lycopadiene-race of
Botryococcus braunii. Phytochemistry 29, 2187±2192. Derenne, S., Largeau, C., Casadevall, E., Sinninghe DamsteÂ,
J.S., Tegelaar, E.W., Leeuw, J.W. de., 1990b. Characteriza-tion of Estonian Kukersite by spectroscopy and pyrolysis: evidence for abundant alkyl phenolic moieties in an Ordovi-cian, marine, type II/I kerogen. Organic Geochemistry 16, 873±888.
Derenne, S., Largeau, C., Casadevall, E., Berkalo, C., Rous-seau, B., 1991. Chemical evidence of kerogen formation in source rocks and oil shales via selective preservation of thin resistant outer walls of microalgae: origin of ultralaminae. Geochimica et Cosmochimica Acta 55, 1041±1050.
Derenne, S., Largeau, C., Berkalo, C., Rousseau, B., Wilhelm, C., Hatcher, P., 1992a. Non-hydrolysable macromolecular constituents from outer walls ofChlorella fuscaand Nano-chlorum eucaryotum. Phytochemistry 31, 1923±1929. Derenne, S., Metzger, P., Largeau, C., van Bergen, P.F.,
Gatellier, J.P., Sinninghe DamsteÂ, J.S. et al., 1992b. Similar morphological and chemical variations ofGloeocapsomorpha prisca in Ordovician sediments and cultured Botryococcus brauniias a response to changes in salinity. Organic Geo-chemistry 19, 299±313.
Derenne, S., Largeau, C., HeteÂnyi, M., Brukner-Wein, A., Connan, J., Lugardon, B., 1997. Chemical structure of the organic matter in a Pliocene maar-type oil shale. Implicated
Botryococcusraces and formation pathways. Geochimica et Cosmochimica Acta 61, 1879±1889.
Didyk, B.M., Simoneit, B.R.T., Brassell, S.C., Eglinton, G., 1978. Organic geochemical indicators of
palaeoenviron-mental conditions of sedimentation. Nature (London) 272, 216±222.
Dubreuil, C., Derenne, S., Largeau, C., Berkalo, C., Rous-seau, B., 1989. Mechanism of formation and chemical struc-ture of coorongiteÐI. Role of the resistant biopolymer and of the hydrocarbons ofBotryococcus braunii. Ultrastructure of coorongite and its relationship with torbanite. Organic Geochemistry 14, 543±553.
Durand, B., Nicaise, G., 1980. Procedures for kerogen isolations. In: Durand, B. (Ed.), Kerogen, Technip, Paris, pp. 35±53. Eglinton, T.I., Douglas, A.G., 1988. Quantitative study of
bio-marker hydrocarbons released from kerogens during hydrous pyrolysis. Energy and Fuels 2, 81±88.
Innes, H.E., Bishop, A.N., Head, I.M., Farrimond, P., 1997. Preservation and diagenesis of hopanoids in Recent lacus-trine sediments of Priest Pot, England. Organic Geochem-istry 26, 565±576.
JaÂmbor, A., Solti, G., 1975. Geological conditions of the Upper Pannonian oil shale deposit recovered in the Balaton Highland and at KemeneshaÂt. Acta Miner. Petr. Szeged XXVII (1), 73±85. Kadouri, A., Derenne, S., Largeau, C., Casadevall, E., Berkal-o, C., 1988. Resistant biopolymer in the outer walls of
Botryococcus braunii, B Race. Phytochemistry 27, 551±557. Klesment, I., 1974. Application of chromatographic methods in
biogeochemical investigations. Determination of the struc-tures of sapropelites by thermal decomposition. Journal of Chromatography 91, 705±713.
Klesment, I., Nappa, L., 1980. Investigation of the structure of Estonian oil shale Kukersite by conversion in aqueous sus-pension. Fuel 59, 117±122.
Kolattukudy, P.E. 1976. In Kolattukudy, P.E. (Ed.), Chemistry and Biochemistry of Natural Waxes. Elsevier.
Largeau, C., Derenne, S., 1993. Relative eciency of the Selective Preservation and Degradation Recondensation pathways in kerogen formation. Source and environment in¯uence on their contributions to type I and II kerogens. Organic Geochemistry 20, 611±615.
Largeau, C., Casadevall, E., Kadouri, A., Metzger, P., 1984. Formation ofBotryococcus braunii kerogens. Comparative study of immature Torbanite and of the extant alga Botryo-coccus braunii.In Schenck, P.A., de Leeuw J.W., Lijmbach G.W.M. (Eds.), Advances in Organic Geochemistry 1983. Pergamon Press, Oxford (Organic Geochemistry 6, 327±332). Largeau, C., Derenne, S., Casadevall, E., Kadouri, A., Sellier, N., 1986. Pyrolysis of immature Torbanite and of the resis-tant biopolymer (PRBA) isolated from exresis-tant alga Botryo-coccus braunii. Mechanism of formation and structure of Torbanite. In: Leythaeuser, D., RullkoÈtter, J., (Eds.), Advances in Organic Geochemistry 1985. Pergamon Press, Oxford (Organic Geochemistry, 10, 1023-1032).
Lichtfouse, E., Derenne, S., Mariotti, A., Largeau, C., 1994. Possible algal origin of long chain oddn-alkanes in immature sediments as revealed by distributions and carbon isotope ratios. Organic Geochemistry 22, 1023±1027.
Mulik, J.D., Erdman, J.G., 1963. Genesis of hydrocarbons of low molecular weight in organic rich aquatic systems. Science 141, 806±807.
Nagy, E., 1978. Palynological investigations of alginites in Hungary. Journal of Palynology 14, 94±100.
Saiz-Jimenez, C., de Leeuw, J.W., 1986. Lignin pyrolysis pro-ducts : their structure and their signi®cance as biomarkers. Organic Geochemistry 10, 869±876.
Salmon, V., Derenne, S., Largeau, C., Beaudoin, B., Bardoux, G., Mariotti, A., 1997. Kerogen chemical structure and source organisms in a Cenomanian organic-rich black shale (Central Italy) Ð indications for an important role of the ``sorptive protection'' pathway. Organic Geochemistry 27, 423±438.
Stasiuk, L.D., Osadetz, K.G., 1990. The life cylce and phyletic anity of Gloeocapsomorpha prisca Zalessky 1917 from Ordovician rocks in the Canadian Williston Basin. Currents Research of the Geological Survey of Canada 89 (1D), 123± 137.
Tannenbaum, E., Ruth, E., Kaplan, I.R., 1986. Steranes and triterpanes generated from kerogen pyrolysis in the absence and presence of minerals. Geochimica et Cosmochimica Acta 50, 805±812.
Tegelaar, E.W., Leeuw, J.W.de, Derenne, S., Largeau, C., 1989. A reappraisal of kerogen formation. Geochimica and Cosmochimica Acta 53, 3103±3106.
van Graas, G., 1986. Biomarker distributions in asphaltenes and kerogens analyzed by ¯ash pyrolysis±gas chromatography± mass spectrometry. Organic Geochemistry 10, 1127±1135. van de Meent, D., Brown, S.C., Philp, R.P., 1980.
