Advances in Organic Geochemistry 1987 Org. Geochem. Vol. 13, Nos 4-6, pp. 1011-1021, 1988 Printed in Great Britain. All rights reserved

0146-6380/88 $3.00 + 0.00 Copyright © 1988 Pergamon Press plc

Organic geochemistry and petrography of Tertiary coals and carbonaceous shales from Argentina

HI~CTOR J. VILLAR*, WILHELM PUTTMANN and MONIKA WOLF Lehrstuhl f/ir Geologic, Geochemie und Lagerstfitten des Erd61s und der Kohle, RWTH Aachen,

Lochnerstr. 4-20, D-5100 Aachen, F.R.G.

Abstract--A series of eight Tertiary coal and carbonaceous shale samples with vitrinite reflectance values between 0.50 and 0.58% were extracted, fractionated and the saturated and aromatic hydrocarbons analysed for characteristic components by GC and GC-MS. Additionally, a microscopical study was undertaken in order to obtain a more precise picture of the samples under investigation.

The saturated hydrocarbon fractions displayed the typical n-alkane distribution for coals of this rank, with CPI values between 2.0 and 3.1. Among the branched/cyclic compounds, pristane and ~, ~-homohopane were recognised as relevant components point!ng to an oxic depositional environment. Detection of benzohopanes (C32-C35) in the aromatic hydrocarbon fractions suggests that bacte- riohopanetetrol was a significant constituent of the coal biomass. Taking into consideration the Pr/Ph ratios, ash contents and microscopical characteristics of the samples, aspects of the possible degradation of hopanetetrol to homohopane are discussed. Resin-derived diterpenoids with the phyllocladane and kaurane skeleton were tentatively identified and, although minor compounds, they arc interpreted to be a sign of the contribution of Podocarpaceae and Araucareaceae to the coal swamp.

Aromatic compounds were dominated by alkylnaphthalene derivatives, presumably formed by C-ring cleavage and aromatisation of higher plant-derived pentacyclic triterpenois, which were main components in the high-boiling range of the fractions investigated. Angiosperms (especially Fagaceae) are postulated as source for these polycyclic compounds and, hence, for some of the polyalkylated aromatic bicyclics detected.

Key words: coal, diagenesis, organic petrography, hopanoids, pentacyclic triterpenoids, alkylnaphthalenes, aromatisation, angiosperms, gymnosperms

INTRODUCTION

The study of the soluble organic matter extractable from coals may provide valuable information about the biological materials contributing to the swamp depositional environment and their early trans- formation products during the first stages of di- agenesis. The detection of specific biological markers becomes more difficult with increasing rank from peat to bituminous coals and, with a few exceptions, is virtually impossible at higher maturities. The tran- sition from sub-bituminous to high-volatile bitu- minous coals (R, ~, 0.5%) constitutes a maturation stage in which the biological/facial fingerprint may be still recognised and linked to the new generated compounds under mild temperature conditions. With regard to this concept, we present here the results of our investigation directed toward a preliminary geochemical and petrographical characterisation of Argentine Tertiary coals and carbonaceous shales.

Six of the investigated samples (86004-86009; Table 1) were obtained from Rio Turbio Coal Mine, SSW of Santa Cruz Province. The lower section of the Eocene Rio Turbio Formation (Russo et al.,

*Present address: Centro de Investigaciones en Recursos Geolbgicos-Consejo Nacional de Investigaciones Cientificas y T~nicas, J. R. Velazco 847, 1414 Buenos Aires, Argentina.

1980; Riccardi and Rolleri, 1980) consists of giau- conitic sandstones and conglomerates bearing marine invertebrates, abundant flora and two intercalated coal seams; "Lower ' and "Upper". The upper section includes sandstones and pelites also containing in- vertebrates, fossil plants and the coal seams "B", "A" and "Dorotea". The whole sequence conforms with a typical marine depositional environment with tran- sitions from infralitoral to limnic deposition (coastal areas, barrier islands/lagoons and intertidal flats). Samples 86002 and 86003 (Table 1) were taken from coal layers in recently opened exploration areas to the west of the Santa Cruz Province. They are thought to correlate with the Rio Turbio formation to the north of the Province.

EXPERIMENTAL

Analytical methods

The powdered coal and carbonaceous shale sam- ples were extracted with dichloromethane (24h; 50°C). The resulting extracts were separated by col- umn chromatography into saturated hydrocarbon, aromatic hydrocarbon and N,S,O-compound frac- tions. The hydrocarbon fractions were subsequently analysed by gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). Stan- dard extraction and fractionation procedures, and

I011

1012 HI~CTOR J. VILLAR et al.

Table 1. Sample location and age Geological Borehole/

Sample No. formation Coal seam depth (m) Age 86002 Rio Leona -- - Eocene/Oligocene 86003 Rio Lista -- Eocene/Oligocene 86004 Rio Turbio Dorotea ...... Eocene 86005 Rio Turbio Dorotea x/329 Eocene 86006 Rio Turbio A x/345 Eocene 86007 Rio Turbio B x/358 Eocene 86008 Rio Turbio Upper x/627 Eocene 86009 Rio Turbio Lower x/646 Eocene

GC/GC-MS methods have been described elsewhere (Piittman and Villar, 1987).

Maceral composition was determined by random point count analysis on a Zeiss Universal microscope under white reflected light and blue light-induced fluorescence mode. For the vitrinite reflectance mea- surements, A Zeiss MPM 01K microscope was used.

Identification o f aromatic hydrocarbons

Identification of 1,2,5-trimethylnaphthalene (l,2,5- tri-MN) and cadalene were respectively achieved by coinjection with commercially available standards and that of 1,2,5,6-tetramethylnaphthalene (l,2,5,6- tetra-MN), by coinjection with an authentic com- pound synthesised at our laboratories for a previous study. 1,2,7-trimethylnaphthalene (I ,2,7-tri-MN) was identified on the basis of coinjection experiments with commercially available 1,2,6-trimethylnaphthalene on SE-54 and OV-I capillary columns. It is seen to elute immediately before the reference isomer (Rowland et al., 1984). Tentative assignments of the respective peaks to l,l,5,6-tetramethyl-l,2,3,4-tetra- hydronaphthalene (1,1,5,6-tetra-MTHN) and 2,2,7,8- tetramethyl- 1,2,3,4-tetrahydronaphthalene (2,2,7,8- tetra-MTHN) were based, for the first isomer, on comparison of its mass spectrum with published data (Anders et al., 1973; Zalkow et al., 1979) and, for the latter, on mass spectra interpretation.

Diterpenoid- and triterpenoid-related structures were assigned by comparing GC retention behaviour and mass spectrometric fragmentation of the respective peaks with previously published data (Spyckerelle et al., 1977; Laflamme and Hites, 1979; Wakeham et al., 1980; Simoneit and Mazurek, 1982; Radke et al., 1982; Chaffee and Johns, 1983; Chaffee et al., 1984; Hussler et al., 1984). The proposed structure for the secotriterpenoid 14 (Table 4) was based on mass spectra interpretation (Piittmann and Villar, 1987).

RESULTS

Saturated hydrocarbons

The distribution patterns of saturated hydro- carbons showed a suite of n-alkanes ranging from n-C20 to n-C33, with strong predominance of odd chain lengths maximising at n-C27 or n-C29 (Fig. 1). Short chain n-alkanes (C15-C19) were present as

secondary constituents. Pristane was the major iso- prenoid in all eight samples and, occasionally, the most abundant compound in the whole saturated hydrocarbon fraction. The calculated pristane/ phytane ratios (Table 2) resulted in anomalously high values. Further regular acyctic isoprenoids (i-Cl4 to i-C18) were also detected, the most prominent peak after pristane being the C~6-homologue.

Two C~5H28 bicyclic sesquiterpenoids were recog- nised as significant components. One of them ex- hibited an identical MS-fragmentation pattern as 4fl (H)-eudesmane (Alexander et al., 1983). The other compound was tentatively identified as trans- cadinane (Katayama and Marumo, 1983). This assignment was preferred to that of trans-guaiane, which displays an almost identical mass spectrum (Katayama and Marumo, 1983), in view of the apparently widespread presence of cadinanes in coals and because of the conspicuous occurrence of cadalene--its very possible aromatis-ation product-- in all the aromatic hydrocarbon fractions.

Conifer resin-derived tricyclic and tetracyclic diterpenoids were identified in the m / z 123 mass chromatograms. For the tricyclic compounds, the most relevant peaks were adscribed to noriso- pimarane, rimuane and abietane, according to pub- lished mass spectra (Noble et al., 1986; Philp, 1985). Retention behaviour and distinctive fragmentation patterns described by Noble et al. (1985) provided the clues to identify the tetracyclics as 16B(H)-phyllo- cladane, ent-16B(H)-kaurane and 16~(H)-phyllo- cladane.

The distributions of hopanoids did not vary quali- tatively from sample to sample and were dominated by 7, B-components. However, their relative abun- dance and the proportions of ~,B-homohopanes S + R with respect to total hopanes were seen to differ (Table 2). Two extreme examples are illustrated by the m/z 191 mass chromatograms of samples 86008 and 86009 in Fig. 1. The relative amounts of ~, B-homohopanes decrease markedly with the ash content and increase with pristane/phytane ratio (Fig. 2). Moreover, the higher percentages of • ,B-homohopane corresponded to augmented values of total hopanes as estimated by hopanes/n-alkanes quotient, and to lower ~,B-bishomo-/~,B-homo- hopane ratios. Estimation of the isomerisation at position C22 indicated higher values of the isomer S

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Aromatic hydrocarbons

All samples showed similar features: alkyl- naphthalene derivatives as major compounds, lower amounts and simple distribution of phenanthrenes, and a complex assemblage of triterpenoid hydro- carbons, the most of which were identified as aromatisation products of fl-amyrin. Three chro- matograms are shown in Fig. 3, as example. From them, it can be readily seen that alkylnaphthalenes are dominated either by i,2,5- (sample 86004) or 1,2,7-tri-MN (sample 86008), or both isomers (sample 86003). Other main polyalkylnaphthalenes are 1,2,5,6-tetra-MN, cadalene and two tetra- methylnaphthalenes (peaks 6 and 7 in Fig. 3) which could not be identified. Among the alkyltetralins, 1,1,5,6-tetra-MTHN predominated in all the samples; 2,2,7,8-tetra-MTHN was also recognised as a further relevant C4-tetralin in most of the fractions. Pi- manthrene, simonellite and retene were detected as the major components of the phenanthrene deriva- tives, with minor quantities of other lower substituted fully aromatised alkylphenanthrenes being present. In particular, methytphenanthrenes and phenan- threne itself exhibited very low relative concen- trations. A set of oleanane-derived aromatised triterpenoids were found to be dominated either by 2,2,4a,9-tetramethyl- 1,2,3,4,4a,5,6,14b-octahydro- picene or 2,2,9-trimethyl-l,2,3,4-tetrahydropicene. Further identified triterpenoid derivatives consisted of C-ring cleaved triaromatics showing prominent m/z 169 and m/z 183 fragments in their mass spectra. Two of the major compounds (13 and 14, Fig. 3) are thought to be fl-amyrin derivatives and have been discussed in previous papers (Chaffee et al., 1984; Piittmann and Villar, t987). A plausible genetic relationship between two of the above mentioned alkylnaphthalenes and the C-ring cleaved tri- terpenoids may be inferred from Fig. 4. In fact, an increase in the relative amounts of 1,2,5,6-tetra-MN with respect to 1,2,5-tri-MN parallels that of the C:8- with respect to the C27-secotriterpenoid.

Despite their relative low abundance, benzo- hopanes ranging from C32 to C35 (Hussler et al., 1984) were also recognised. Their distribution is exemplified and compared to that of saturated hopanes in the m/z 191 mass chromatogram of sample 86004 (Fig. 5). Pentacyclic hydroaromatic hopanoids of the type described by Greiner et al. (1976, 1977) could not be detected.

Organic petrography

Maceral groups and maceral/submaceral analysis are summarised in Table 3. Common features for all the samples were the high contents of liptinite, reach- ing up to 52% volume m.m.f, in sample 86009. A combination of sporinite, cutinite and liptodetrinite predominated over other macerals of the liptinite

Organic geochemistry of Tertiary coals 1015

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group. The volume percent of resinite was generally low, except for sample 86005. Sample 86002 revealed an elevated proportion of fluorinite (7%), a frequent characteristic of cutinite-rich coals. Mineral matter occurred mainly as finely dispersed clay particles, acting as a matrix and embedding the organic remains in the high-ash samples. None of the samples contained significant amounts of pyrite. The preservation of liptinites (as seeen by structure of tissues and signs of corrosion or transport) was better in the low mineral matter layers. Samples with very high ash contents showed a significant influence of hypautochtonous organic material.

D I S C U S S I O N

From the geochemical and microscopical data it is obvious that these Tertiary coals have formed from higher plants. However, the liptinite content, maceral association, intermixing of the mineral matter and fine stratification precludes to classify them as typical humic coals. A subaquatic and rather oxygenated environment, with slight changes in the water table but permanently under moist conditions, was proba- bly a more likely environment. Additionaly, it can be presumed that the mineral content increases in layers being formed in deeper waters. The scarcity of identified alginite is taken as possible evidence of strong acidic conditions that may have inhibited their expected development. The most relevant geochem- ical characteristics associated to this coal depositional environment will be discussed in the following text.

Diterpanes. The occurrence of phyllocladanes and kauranes lends support to the findings of Noble et al. (1985), who proposed that these tetra- cyclic diterpanes are biomarkers for gymnosperms, particularly Podocarpaceae and Araucareaceae, in Australian sediments and crude oils. It is well docu- mented (Romero, 1977) thai the vegetation of the area at the time of deposition of Rio Turbio

Formation was alternatively dominated by forests of gymnosperms and angiosperms, being Podo- carpaceae and Araucareacea the sole representatives among the gymnosperm flora. Furthermore, rimuane and norisopimarane, may be specifically traced to Dacrydium (Noble et al., 1986 and refences therein), one of the dominating genera of the Rio Turbio gymnosperm association.

Hopanoids and acyclic isoprenoids. Several studies have reported a,p-homohopane to be a major com- pound of the extractable hydrocarbons of coals and peats. Van Dorsselaer et al. (1977a) have found the C31-hopane as the almost unique homologue in the branched and cyclic alkane fraction of an Australian pollen lignite. Similar results have been observed by Quirk et al. (1980) in their study of sphagnum peat. The authors concluded that a,/~-homohopane was probably derived from bacterial products under aero- bic decay of the peat. Reports on the dark lithotypes of a German Eocene brown coal deposit (Winkler, 1986) and on a sub-bituminous coal of Miocene age (Mycke and Michaelis, 1986) have shown analogous observations. Van Dorsselaer et al. (1977h) have postulated an oxidative cleavage at C32 and sub- sequent decarboxylation of a precursor such as bacte- riohopanetetrol to account for its conspicuous presence in the Yallourn and Morwell Australian lignites. Identical pathways have been recently pro- posed by Chaffee et al. (1986) for coals in general.

The hopane distribution of the coals investigated here differ in several features with respect to the above reference examples. Primarily, isomerisation at position 22 in a a,//-homohopane is almost completed in all samples (Table 2). Furthermore, moretanes are easily recognisable as seen in the 191 m / z mass chro- matograms (Figs 1 and 5). In some aspects, the pattern resembles that obtained from the Yaliourn lignite after heating experiments (Van Dorsselaer et al., 1977a). Although the C32-hopanoic acid could not be detected in the extracts, the occurrence of

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b e n z o h o p a n e s ( C 3 2 - - - C 3 5 ) i n all the aromatic hydro- carbon fractions suggests that bacteriohopanetetrol was a significant constitutent of the coal biomass. The fate of its side chain may have been dependant on the early diagenetic conditions which favoured oxidation and cleavage, or preservation through dehydration and cyclisation. Thus, both degradation pathways of bacterial C35-hopanoids presumably occured simultaneously during the maturation of the organic matter, since it seems improbable that hop- anetetrol itself or related C32- to C35-derivatives could have been preserved from degradation (i.e. linked to the kerogen matrix) and, in later diagenesis, released to yield benzohopanes and almost no C32- to C35- saturated hopanes.

Pristane/phytane ratios of the samples afford fur- ther information regarding the diagenesis of hop- anoids. From Fig. 2 it can be seen that the samples with the higher Pr/Ph ratios show a relative enrich- ment in a,/~-homohopanes. It is normally viewed (Brooks et al., 1969; Didyk et al., 1978) that the Pr/Ph ratio may reflect the redox potential of the deposi- tional environment in which chlorophyll decayed.

Table 4. Aromatic compounds identified in the samples studied. Numbers refer to peaks labelled in Fig. 3

Compound

1 2,2,7,8-Tetramethyl- 1,2,3,4-tetrahydronaphthalene 2 1,1,5,6-Tetramethyl- 1,2,3A-tetrahydronapbthalene 3 1,2,7-Trimethylnaphthalene 4 1,2,5-Trimethylnaphthalene 5 1,6-Dimethyl-4-isopropylnaphthalene (cadalene) 6 Tetramethylnapthalene + Cs-naphthalene 7 Tetramethylnaphthalene 8 1,2,5,6-Tetramethylnaphthalene 9 1,7-Dimethylphenanthrene (pimanthrene)

10 1,1 -Dimethyl-7-isopropryl- 1,2,3,4-tetrahydropbenanthrene (simonellite)

I 1 I-Methyl-7-isopropylphenanthrene (retene) 12 3,3,7-Trimethyl- 1,2,3,4-tetrahydrochrysene 13 C27-Triaromatic-8,14-secotriterpenoid 14 C~-Triaromatic-8,14-secotriterpenoid 15 2,2,4a,9-Tet ramethyl- 1,2,3,4Aa,5,6,1 4b-octahydropicene 16 2,2,9-Trimet hyl- 1,2,3,4-tetrahydropicene 17 2,9-Dimethylpicene

1018 HI~CTOR J. V1LLAR et aL

.C3.! , x2

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~-amyrin ' Ill IV

2,, 2,,

Fig. 6. Postulated degradation pathways of fl-amyrin in the samples investigated. 1 and ll, after Spycker¢ll¢ et aL (1977), I, aflamme and Hitcs (1979), Wakeham et aL (1980), III and IV, partly from Chaffc¢ et al. (1984), Piittmann and Villar (1987) and this work, Numbers refer to Table 4 and Fig. 3.

Organic geochemistry of Tertiary coals 1019

Consequently, if pristane is derived in these coal samples from chlorophyll decomposition, initial oxi- dation during diagenesis must have been severe, thus forcing the phytyl rest to degrade into pristane and lower isoprenoids and producing only residual amounts of phytane. Since the maturation stage of all the samples is rather similar, the variation of Pr/Ph ratio can be considered as an indicator of the oxi- dative fluctuations in the swamp which might in turn have influenced the formation of homohopane from hopanetetrol. Whether the variations of the hopanoid distribution with the ash content (Fig. 2) has only a facial implication or reflects a further superimposed catalytic effect of the mineral matter, remains un- clear. In the first case, it can be concluded that with increasing subaquatic conditions the coals become poorer in total hopanoids and, in particular, in homohopane (sample 86009). In samples approach- ing the humic coal characteristics, the opposite effect is observed (sample 86008). Microscopically, no specific maceral/submaceral content could be cor- related with the relative changes of the hopane pat- tern. However, it could be assessed that samples with lower ash content showed better preserved tissues than the high-ash samples, in which the liptinites displayed signs of transport and corrosion. This apparent contradiction--well preserved tissues in the coals relatively enriched in hopanes--was also re- ported by Winkler (1986) in the study of the light and dark lythotypes of brown coals from Helmstedt, F.R.G. In the present work, it seems likely that different bacterial flora could have influenced the hopane input, depending mainly on the variations of the water table level.

Alkylnaphthalenes and aromatised triterpenoids. One of the most striking features of the extracts is the specific distribution of the aromatic constituents, mainly alkylnaphthalenes and polycyclic components conceptually related to triterpenoids precursors (Fig. 3). Some of the bicyclic aromatic hydrocarbons, essentially 2,2,7,8-tetra-MTHN (1), 1,2,7,-tri-MN (3), 1,2,5-tri-MN (4) and 1,2,5,6-tetra-MN (8), are presumed to be formed via C-ring cleavage and aromatisation of amyrin related compounds.

Several authors have suggested that polyring triterpenoids are likely to be degraded to bicyclic compounds on maturation (Anders et al., 1973; Bendoraitis, 1974; Rullkttter and Wendisch, 1982; Hussler et al., 1984; Strachan et al., 1986; Hayatsu et al., 1987; Pfittmann and Villar, 1987). In the series of coals and shales investigated here, the detection of relative high amounts of components expected to be produced by the C-ring opening of oleanane struc- tures, together with hydroaromatic- chrysene and picene derivatives carrying the oleanane substitution pattern (Figs 4 and 6), reinforces the idea of ~-amyrin as a plausible precusor. These last compounds have been widely reported to occur in recent and fossil sediments (Spyckerelle et al., 1977; Laflamme and Hites, 1979; Wakeham et al., 1980) and a micro-

biological alteration process of the biogenic precursor has been inferred to explain their rapid diagenetic formation. Their presence in brown coals has been documented in detail by Chaffee and Johns (1983). In addition, the occurrence of terpanes with an oleanane carbon skeleton in geological materials can be consid- ered an indication of a dicotyledonous angiosperm input (Chaffee et al., 1986). Since palynological stud- ies have demonstrated that the Fagaceae family was a taxon that periodically dominated the floral associ- ations of Rio Turbio Formation (Romero, 1977), it is likely that plants of this group were a source of ~-amyrin in the coals. Therefore, assuming that the seco-compounds 13 and 14 are ~-amyrin-derived, it follows an identical origin for 1,2,5-tri-MN, 1,2,7-tri-MN and 1,2,5,6-tetra-MN. However, it should be pointed out that only sample 86008 (Fig. 3) approaches to the ideal relative proportions of 1,2,5-tri-MN, 1,2,7-tri-MN and 1,2,5,6-tetra-MN that are expected from the hypo- thetical degradative scheme of Fig. 6. Indeed, we believe that several other secondary sources can affect the distribution of alkylnapthalenes. For instance, in sample 86004 (Fig. 3), 1,2,5-tri-MN largely domi- nates over 1,2,7-tri-MN and 1,2,5,6-tetra-MN. It can be reasoned that only a part of 1,2,5-tri-NM is obtained from the clevage of the C27-secotriterpenoid (13) and that a significant portion is derived from resin compounds via full aromatisation of compound 2 without rearrangement. In fact, 1,1,5,6-tetra- MTHN is present in relatively high concentrations in that sample. On the other hand, additional minor secotriterpenoids detected in most of the samples, with a structure not related to oleanane, may serve as precursors of further alkylnaphthalenes such as the tetramethylnaphthalenes 6 and 7 (Fig. 3).

A point of interest seems to be the different de- gradation pathways that plant-derived pentacyclic triterpenoids may follow in the geosphere. They are summarised in Fig. 6 for //-amyrin; the numbered compounds are the ones detected in the samples. Pathways I and II imply early aromatisation from the A- to the E-ring. C-ring opening (pathways III and IV) and a shift from the geminal methyl group at CA could be mediated by acid catalysis. The further clevage of the 11(12) bond is probably caused by mild thermal maturation.

CONCLUSIONS

The characteristics of the hydrocarbon fractions of the Tertiary coals and carbonaceous shales in- vestigated seem to be strongly facies dependant.

Variations of the water level in an oxic environ- ment may have affected the development and type of bacterial flora and, thus, the relative amounts of hopanoids and their distribution.

The conifer- and angiosperm-derived material contributing to the coal swamp is geochemically expressed in two different groups of diagenetic

1020 HI~CTOR J. VILLAR et at.

compounds. Conifer resins from Podocarpaceae and Araucareaceae may have formed the diterpenoid- related pimanthrene, simonellite and retene (9, 10,11) and the sesquiterpenoid-related cadalene (5), as well as their saturated counterparts norisopimarane, rim- uane, abietane, phyllocladane and kaurane, fl-amyrin aromatisation compounds and their cleavage prod- ucts (1,2,5-tri-MN, 1,2,7-tri-MN, 1,2,5,6-tetra-MN) can be considered as an indication of angiosperm input (mainly from the Fagaceae family) to these coals and shales in an acidic environment.

Acknowledgements--We are grateful to Yacimientos Car- boniferos Fiscales, Argentine Rep., for supplying the samples used in this study. One of us (H.J.V.) thanks the Consejo Nacional de Investigaciones Cientificas y T6cnicas, Argentine Rep., for a research fellowship at Aachen Technical University. Fruitful comments and linguistic revision by M.Sc. J. Tayler is gratefully acknowledged.

REFERENCES

Alexander R., Kagi R. I. and Noble R. A. (1983) Identification of the bicyclic sesquiterpenes Drimane and Eudesmane in petroleum. J. C. S. Chem. Commun. 226-228.

Anders D. E., Doolittle F. G. and Robinson W. E. (1973) Analysis of some aromatic hydrocarbons in a benzene- soluble bitumen from Green River shale. Geochim. Cos- mochim. Acta 37, 1213-1228.

Bendoraitis J. G. (1974) Hydrocarbons of biogenic origin in petroleum--aromatic triterpenes and bicyclic sesqui- terpenes. In Advances in Organic Geochemistry 1973 (Edited by Tissot B. and Bienner F.), pp. 209-224. Editions Technip, Paris.

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