origin of variations in organic matter abundance and

Origin of variations in organic matter abundance andcomposition in a lithologically homogeneous maar-type oil

shale deposit (GeÂ rce, Pliocene, Hungary)

Sylvie Derenne a,*, Claude Largeau a, Alice Brukner-Wein b,Magdolna Hetenyi c, GeÂ rard Bardoux d, AndreÂ Mariotti d

aLaboratoire de Chimie Bioorganique et Organique Physique, UMR CNRS 7573, ENSCP, 11 rue P. et M. Curie,

75231 Paris cedex 05, FrancebGeological Institute of Hungary, StefaÂnia uÂ t 14, H-1143 Budapest, Hungary

cInstitute of Mineralogy, Geochemistry and Petrography, JoÂzsef Attila University, PO Box 651, H-6701 Szeged, HungarydLaboratoire 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 (GeÂ rce oil shale, Pliocene) haspreviously 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 GeÂ rce well-6 core. Based on the above bitumen data, four samples were selected that were representativeof the whole set which exhibit contrasting features. Scanning and transmission electron microscopy showed theoccurrence 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 variablebacterial input. The relative abundance of these two contributions in the four selected samples is also re¯ected by dif-ferences in FTIR and solid-state 13C NMR spectra of the isolated kerogens. Curie point pyrolysis/GC/MS of thesekerogens revealed a relatively high terrestrial contribution in one sample and con®rmed the variable input of algae and

bacteria. The above di�erences in relative contributions account for the variations in organic matter quantity andquality 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 inHungary in the last 25 years: Pula, GeÂ rce, VaÂ rkeszoÈ andEgyhaÂ zaskeszoÈ (Ravasz and Solti, 1987) (Fig. 1). These

organic-rich deposits are the result of intense volcaniceruptions 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 29�C, as shown by a higharagonite content) due to periodic heating by post-vol-

canic geysers. Favourable conditions for planktonic lifedeveloped in these lakes thanks to the important nutri-ent supply provided by the weathering of the craterwalls. Based on palynological observations, these four

oil shales contain an abundant contribution of fossilcolonies of Botryococcus microalgae, especially in thecase of the Pula deposit (Nagy, 1978). A sample from

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

PI I : S0146-6380(00 )00093-0

Organic Geochemistry 31 (2000) 787±798

www.elsevier.nl/locate/orggeochem

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

1-4325-7975.

E-mail address: [email protected] (S. Derenne).

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 thatBotryococcus braunii provided a major input to this

massive alginite, both via selective preservation of algae-nan (resistant biomacromolecule building up the thickouter walls of Botryococcus) and via incorporation of

some high molecular weight lipids of Botryococcus intomacromolecular structures. In addition, this study spe-ci®ed the nature of the B. braunii races that contributedto 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,

GeÂ rce (Fig. 1), which is the largest Hungarian maar-type

oil shale deposit. Indeed, it covers 2.1 km2 with amaximum alginite thickness of ca. 70 m. In contrastwith Pula where alternations of massive and laminated

alginite occur, the lithology of the GeÂ rce deposit hasbeen shown to be homogeneous and to exclusively con-sist of laminated alginite, with lamination thicknessranging from 0.1 to 0.5 mm (JaÂ mbor and Solti, 1975).

Nevertheless, a previous study (Brukner-Wein andHetenyi, 1993) performed on 23 samples cored at var-ious depths (between 16.3 and 65 m) in GeÂ rce well-6,

revealed substantial di�erences in Rock-Eval para-meters between these samples. The aim of the presentwork was therefore to (i) understand the origin of the

above di�erences in spite of a similar lithology exhibitedby all these samples and (ii) to derive information on thefactors 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) wasdetermined for the same set of 23 samples as previously

studied. From previous Rock-Eval data and the presentresults on bitumen, four samples representative of thewhole set were selected for further studies. The latter

involved gas chromatographic/mass spectrometric (GC/MS) analysis of the saturate fraction of bitumens, stablecarbon isotope analysis of individual alkanes in thisfraction by GC-C-irMS, examination of isolated kero-

gens via Fourier transform infra-red (FTIR) and solidstate 13C nuclear magnetic resonance (NMR) spectro-scopies, Curie point pyrolysis coupled with GC/MS

(CuPy-GC/MS), scanning and transmission electronmicroscopy (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 byBrukner-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 (length25 m, i.d. 0.32 mm, ®lm thickness 0.4 mm). The oven washeated from 100 to 300�C at 4�C minÿ1. For GC/MS

analyses, the chromatograph was coupled with a HP5989A mass spectrometer operated at 70 eV. Isotopicanalyses 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 0.25 mm; heating program 100 to 350�C at3�C minÿ1, splitless injector at 320�C) coupled to a

combustion (CuO) furnace (850�C), a cryogenic watertrap, 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 andNicaise, 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 100.62MHz on a Bruker MSL400 spectrometer using high

power decoupling, cross polarization (contact time 1ms) and magic angle spinning (spinning rate 4 kHz) in adouble 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 610�C in a Fisher 0316M pyrolyser. A pyr-

olysis time of 10 s was used and the reactor wasmaintained at 250�C to prevent condensation of pyr-olysis products. The pyrolyser was directly connected to

Fig. 1. Map of Hungary showing the location of the GeÂ rce and

Pula deposits.

788 S. Derenne et al. / Organic Geochemistry 31 (2000) 787±798

the GC injector (heated at 280�C) of the same GC/MSsystem as above. The GC oven was heated from 50 to300�C at 3�C minÿ1.The kerogens were ®xed by 1% glutaraldehyde in

cacodylate bu�er (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

CO2 critical point technique and coating with gold priorto 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 aPhilips 300 microscope.

3. Results and discussion

3.1. Bitumen

3.1.1. Abundance and group compositionThe total amount of bitumen and the relative abun-

dances of the di�erent fractions (saturates, aromatics,resins and asphaltenes) are shown for the 23 samples ofthe core in Table 1. As previously observed for Rock-

Eval parameters, substantial di�erences can be noteddown the core. However, no simple relationship couldbe 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 selectedsamples (located at 28, 37.3, 46 and 54.5 m depth) werefurther 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, substantialpercentage 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 aromaticand resin contributions are relatively high and (iv) 54.5m, 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 foursamples was analysed by GC and GC/MS. As shown inFig. 2, all these fractions are dominated by a homo-

logous series of n-alkanes, ranging from C21 to C33 witha 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 asre¯ecting a strong terrestrial input with the long-chainn-alkanes derived from epicuticular waxes of higher

plants. However, a similar distribution was previouslyobserved in the case of Pula oil shale, where a signi®cantterrestrial input was not supported by palynological

observations. Indeed, stable carbon isotope ratios ofthese individual n-alkanes do not match with those ofeither C3 or C4 higher plants (Lichtfouse et al., 1994).Such values thus show that the above long chain n-

alkanes, in the Pula deposit, in fact result from thediagenetic reduction of B. braunii alkadienes. However,in the case of the GeÂ rce 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). d13Cmeasurements were therefore performed on the n-alkanes present in the extracts of the 28, 37.3 and 54.5 msamples (Table 2). The values thus obtained for the

long, C28 to C33, n-alkanes are in the same range asthose previously reported for Pula (Lichtfouse et al.,1994). Given this similarity, it is likely that terrestrialinput was minor for these long chain hydrocarbons and

that Botryococcus contribution predominated.In addition to n-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 themcorrespond to branched alkanes: C22 to C30 iso alkaneswith a strong even over odd predominance, C24 to C30

even-carbon-numbered anteiso alkanes and C24 to C30

even-carbon-numbered alkanes characterized by intensefragments at m/z 57, 85 and (Mÿ57)+ and thus assigned

to 5-methylalkanes. All these series likely re¯ect a bac-terial contribution since bacterial lipids are generallyconsidered as characterized by the occurrence of bran-ched components (Kolattukudy, 1976). This bacterial

contribution is also evidenced by the presence of somehopanoid 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 bran-ched alkanes eluting just before the 5-methylalkanes

were also detected. Their mass spectra are characterizedby a peak at m/z 168 or 196 but no precise structurecould be established. All these branched series are char-acterized by a substantially higher d13C values (ranging

from ÿ22 to ÿ25%) thus suggesting that they are not ofalgal origin.When comparing the four TIC traces (Fig. 2A±D), it

appears that bacterial contribution is lower in the 46 mand 54.5 m samples and negligible in the 37.3 m onesince the GC trace of the latter only shows the n-alkane

peaks (Fig. 2B). Moreover, this sample is characterizedby the highest HI value and the lowest OI one in thewhole set and hence could be classi®ed as a type I

kerogen (Tables 1 and 3). In addition, when comparedto the other three samples, the n-alkane distribution inthe 37.3 m sample is more markedly dominated by odd-carbon-numbered compounds (CPI of 11.7 against 3.7,

5.5 and 5.3 for the 28, 46 and 54.5 m samples, respec-tively) with a clear maximum at C27 and C29 and muchlower levels of the shortest, C21 to C25, n-alkanes. In the

S. Derenne et al. / Organic Geochemistry 31 (2000) 787±798 789

other samples, the latter alkanes are likely due to anadditional source. This is con®rmed by the d13C values

of the C22±C24 n-alkanes which are higher, for a givencompound, in the 28 and 54.5 m samples than in the37.3 m one (Table 2). Indeed, for example, the C23

alkane exhibits d13C values of ÿ25.5 and ÿ27% in the 28and 54.5 m samples, respectively, whereas it isÿ30.6% inthe 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 analysisElemental 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 andhigh oil potential (Brukner-Wein and Hetenyi, 1993). Inaddition, a good correlation can be noted between H/Cratios 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 GeÂ rce 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 C21 to C33 n-alkanes from the extracts of the 28.0, 37.3 and 54.5 m

samples from GeÂ rce 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 ÿ30.028.0 ÿ23.9 ÿ26.5 ÿ25.5 ÿ27.5 ÿ27.0 ÿ28.2 ÿ28.2 ÿ29.0 ÿ30.0 ÿ35.4 ÿ30.937.3 ÿ29.4 ÿ30.6 ÿ30.1 ÿ27.9 ÿ29.4 ÿ29.1 ÿ30.9 ÿ31.1 ÿ32.654.5a 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 ÿ33.5a Analysis of the 54.5 m sample was performed in duplicate.


Fig. 2. Total ion current chromatogram showing the composition of the saturated hydrocarbon fraction of the bitumen from GeÂ rce

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.


ization was not an important process for OM preserva-tion in these oil shales. TOC, HI and H/C values tend toincrease when the bacterial contribution (assessed fromthe abundance of branched and hopanoid saturated

hydrocarbons in the bitumen) decreases.

3.2.2. FTIR

All the FTIR spectra (Fig. 3) are characterized byintense 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 720cmÿ1 due to (CH2)n with n 5 4. This high aliphaticity isin 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 thehighest H/C and HI values is characterized by the mostintense 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 chainlength or the branching level in the alkyl chains. As a

result, a higher contribution of methyl groups and/orthe occurrence of shorter chains is noted in the 28 and46 m samples when compared to the other two. The

relative abundances of the OH (3400 cmÿ1), C�O (1710cmÿ1) and ole®nic C�C bands (1617 cmÿ1) with respectto the aliphatic ones (2920 and 2850 cmÿ1) are also

higher in the 28 and 46 m samples.

3.2.3. Solid state 13C NMRThe spectra of the four kerogens (Fig. 4) exhibit peaks

at the same chemical shifts. They are dominated by anintense 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 methylgroups and substituted carbons, respectively. Theseshoulders are much more signi®cant in the 28 and 46 m

samples thus resulting in a broader peak (width at halfheight of ca. 14 ppm against ca. 6 ppm in the other twosamples). These observations are consistent with therelative abundance of the CH3 and CH2 groups derived

from FTIR spectra. Signals at 130 ppm correspondingto unsaturated carbons and at 175 ppm (esters and/oramides) are observed with very low intensities in the

37.3 and 54.5 m kerogens whereas their intensities aresubstantially higher in the other two samples, in agree-ment with FTIR data.

3.2.4. CuPy-GC/MSCurie point pyrolysis was performed on the four

kerogens so as to obtain (i) more precise information onthe nature and length of the alkyl chains and (ii) betterinsight into the building blocks of these geomacromole-cules. The pyrochromatograms (Fig. 5) all show an

abundant homologous series of doublets correspondingto n-alkanes and n-alk-1-enes up to C27. These doubletsresult from the homolytic cleavage of long alkyl chains.

Fig. 3. FTIR spectra of GeÂ rce 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 state 13C NMR spectra of GeÂ rce 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).


They are relatively more intense with respect to theother 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/Cratios, FTIR and NMR data. Several minor series ofproducts containing n-alkyl chains are also identi®ed in

these pyrolysates; they are C7 to C29 n-alkan-2-ones andn-alken-2-ones, C7 to C29 n-alkylbenzenes. The relativeabundance of these series with respect to the n-alkanes is

similar in all the samples. n-Alkanones were previouslyshown to be derived from the thermal cleavage of etherlinkages between alkyl chains and are commonly

observed in kerogen pyrolysates (e.g. van de Meent etal., 1980). n-Alkylbenzenes likely result from cyclizationand aromatization upon pyrolysis since no aromatic

moieties could be detected in the FTIR spectra (Mulikand 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 variousspecies of microalgae and, as a result, alkane/alkenedoublets, alkanones and alkylbenzenes are detected in

algaenan pyrolysates (Largeau et al., 1984, 1986; Kadouriet al., 1988; Derenne et al., 1991, 1992a). These algaenanswere shown to provide an important input to kerogens in

a number of organic-rich deposits via the selective pre-servation pathway (Largeau et al., 1986; Derenne et al.,1991). In this mechanism, while the labile compoundsand classical biomacromolecules (like proteins and

polysaccharides) are rapidly degraded during the ®rststeps of biomass fossilization, such resistant biomacro-molecules are selectively preserved and hence selectively

enriched in kerogen (Tegelaar et al., 1989; Largeau andDerenne, 1993). The ®rst evidence of the involvement ofthis pathway in kerogen formation was obtained from a

comparative study of the algaenan isolated from theextant microalga B. braunii and an immature, Botryo-coccus-derived oil shale (Torbanite) (Largeau et al.,

1984). Since Botryococcus, from palynological studies, isknown to contribute to the GeÂ rce deposit, the abovealkyl-containing pyrolysis products are likely to bederived from Botryococcus algaenan. Moreover, three

di�erent chemical races, termed A, B and L, were dis-tinguished in extant B. braunii on the basis of the natureof the hydrocarbons they produce: alkadienes, terpenic

CnH2n-10 botryococcenes with n ranging from 30 to 37and a lycopadiene, respectively. The chemical structure

of the algaenans from the A and B races is based onlong polymethylenic chains whereas that of the L racealso comprises C40 isoprenoid chains with a lycopaneskeleton thus yielding a number of isoprenoid com-

pounds upon pyrolysis (Derenne et al., 1990a). The onlyisoprenoid hydrocarbon which signi®cantly contributesto the pyrolysates of the four GeÂ rce samples is prist-1-

ene. However, pristenes are ubiquitous pyrolysis pro-ducts of kerogens and several assumptions have beenput forward to account for their origin, including the

phytyl chain of chlorophyll (Didyk et al., 1978). Thelack of other isoprenoid compounds rules out a sig-ni®cant contribution of the L race of B. braunii in the

GeÂ rce deposit in contrast with what was observed in thecase of Pula deposit (Derenne et al., 1997). Moreover, asstressed above, the long chain n-alkanes of the bitumenindicate a contribution from the A race of B. braunii. As

a result, the n-alkane/n-alk-1-ene doublets in GeÂ rcepyrolysates likely originate from the algaenan of the Arace of B. braunii.

Phenol and higher substituted homologues with totalcarbon number up to C23 are present in the pyrolysatesof the 28, 37.3 and 46 m GeÂ rce kerogens. Phenols and

methoxyphenols substituted by short alkyl chains (4C3) in kerogen pyrolysates are usually considered to berelated to lignin-derived compounds (Saiz-Jimenez andde Leeuw, 1986) and therefore to re¯ect a terrestrial

input. Such low molecular weight phenols are especiallyabundant in the 28 m sample thus indicating a relativelyhigh terrestrial input and they signi®cantly contribute to

the pyrolysates of the 46 and 37.3 m samples, as shownby 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. Thehigher terrestrial contribution in the 28 m sample is inagreement with its low HI and high OI values and with

the relatively high OH, C�O and C�C levels indicatedby FTIR and 13C NMR. Long chain (C10+) 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 GeÂ rce well-6 kerogens and Rock-Eval data of the corresponding crude oil shales

Depth C H N S Ash H/C HI OI Tmax

(m) (%) (%) (%) (%) (%) (mgHC/gTOC) (mgCO2/gTOC) (�C)

28.0 60.2 7.2 2.61 0.95 9 1.44 441 72 402

37.3 63.9 8.8 1.58 0.87 11 1.66 748 44 431

46.0 52.3 6.3 1.91 1.75 21 1.44 570 54 408

54.5 61.8 8.1 1.75 0.80 32 1.57 637 56 430


Fig. 5. Curie point Py-GC/MS (610�C) of GeÂ rce 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.


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 GeÂ rce well-6 (37.3 m sample); well preserved Botryococcus colony typical of the Pula deposit (C). Trans-

mission electron microscopy of Botryococcus in kerogen from GeÂ rce 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).


chie¯y composed of fossil remains of Gloeocapsomorphaprisca (Klesment, 1974; Klesment and Nappa, 1980;Derenne et al., 1990b). These fossil microorganismswere suggested to be related to the microalga B. braunii

which can adapt to large salinity variations althoughthis relationship is still a matter of debate (Stasiuk andOsadetz, 1990). Under saline conditions, (i) the mor-

phology of the colony is markedly modi®ed, with thick,multilayered, outer walls entirely surrounding the cellswhereas 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 bothin the biosynthesized lipids and in the pyrolysis products

of the algaenan (Derenne et al., 1992b). Examination ofthe organic matter from GeÂ rce deposit by SEM revealedthe occurrence of lignous debris (Fig. 6B) and showedthe predominance of Botryococcus colonies. However,

the latter underwent extensive morphological diageneticalterations thus leading to hardly recognizable colonies(Fig. 6A) especially when compared to the well-pre-

served ones occurring in Pula deposit (Fig. 6C). As aresult, it is not possible from these observations toderive 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 importantmorphological alteration of Botryococcus colonies mayre¯ect evaporitic events. Indeed, when the ultrastructureof Coorongite (a Recent rubbery material formed from

Botryococcus biomass on the shores of some lakes or indried up basins) is examined, coalescence of the outerwalls and partial fusion of the colonies is noted

(Dubreuil et al., 1989). TEM observations of the 37.3 msample con®rmed the high level of alteration of themorphology of Botryococcus colonies and revealed the

accumulation of outer walls around cell voids (Fig. 6D).Coalescence of these walls was clearly observed viaTEM at high magni®cation (Fig. 6E).Hopanes ranging from C27 to C31 are present in the

pyrolysates along with C27 and C29 hopenes. Suchpolycyclic 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 ofkerogens (Tannenbaum et al., 1986; van Graas, 1986;

Eglinton and Douglas, 1988; Boreham et al., 1994;Innes et al, 1997; Salmon et al., 1997). They originatefrom bacterial lipid incorporation during diagenesis and

thus re¯ect bacterial input. As shown on the pyr-ochromatograms (Fig. 5), the relative amount of thehopanoids in the four samples exhibits strong varia-tions. It is rather high in the 46 and 28 m samples (rela-

tive abundance of the C27 hopene with respect to the C15

n-alkane of 0.8 and 0.6, respectively) and, to a lesserextent, in the 54.5 m one (C27 hopene/C15 alkane ratio

of 0.3) whereas hopanoid abundance is very low in the37.3 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 inagreement with the branching level deduced from FTIRand NMR spectra. Moreover, bitumen analysis sug-

gested a lower bacterial contribution for the 37.3 msample which is fully con®rmed by hopane abundancein the pyrolysates.

4. Conclusion

The largest Hungarian maar-type deposit, GeÂ rce oilshale, is known to exhibit substantial variations inorganic matter quantity and quality with depth

although its lithology is homogeneous (laminated algi-nite). The heterogeneity revealed by bulk geochemicalparameters 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. Thenature and isotopic composition of the saturatedhydrocarbons of the bitumens, along with the spectro-

scopic features of the isolated kerogens and identi®ca-tion of the products released upon kerogen pyrolysisshow that the observed di�erences 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 GeÂ rce oil shale, as re¯ected by HIvalues, therefore appears closely correlated to the algalcontribution. In contrast, the relative increase in bac-

teria and higher plant contributions, especially pro-nounced in the case of the 28 m sample, is associatedwith HI lowering. The above features are consistentwith the extremely highly aliphatic nature of Botryo-

coccus algaenan which accounts, owing to selective pre-servation, for the bulk of the algal-derived material.Moreover, they indicate that accumulated organic mat-

ter of bacteria and higher plant origin was characterizedby a rather low oil potential and hence was not domi-nated by waxy components. In addition, scanning and

transmission electron microscopy revealed an extensivealteration of the morphology of the colonies, possiblyrelated to arid periods and associated increases in the

salinity of the crater lake.

Acknowledgements

This study was partly supported by the Action InteÂ -greÂ e Franco-Hongroise (Balaton Programme). J.Maquet


(UPMC, Paris) is acknowledged for technical assistancein solid state NMR and B. Rousseau (ENS, Paris) forultrathin section preparation. Scanning electron micro-scopy was performed at the CIME Jussieu, Paris.

Associate Editor Ð M.G. Fowler

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