Identification of long-chain 1,2-di-n-alkylbenzenes in Amposta crude oil from the Tarragona Basin, Spanish Mediterranean: Implications for the origin and fate of alkylbenzenes

Ga,ch;micn efCosmochrmico AclaVol.55, pp.3677-3683 Copyright 0 1991Pergamon Press plc. Pnntedin U.S.A.

0016.7037/91/$3.00 t .OO

Identification of long-chain 1,2-di-n-alkylbenzenes in Amposta crude oil from the Tarragona Basin, Spanish Mediterranean: Implications for the origin and fate of alkylbenzenes* JAAP S. SINNINGHE DAMST~,’ A. C. KOCK-VAN DALEN,’ PIERRE A. ALBRECHT,~ and JAN W.

DE

LEEUW’

’ Organic Geochemistry Unit, Delft University of Technology, de Vries van Heystplantsoen 2, 2628 RZ Delft, The Netherlands 2 Laboratoire de Chimie Organiques des Substances Naturelles, URA 31 du CNRS, Institut de Chimie, UniversitC Louis Pasteur. 67008 Strasbourg, France (Received November 7, 1990: accepted in

revisedform September 25, 1991)

Abstract-Homologous series of CIs-C40 1,2-di-n-alkylbenzenes (with alkyl side chains containing 2 or more carbon atoms) were identified in the Amposta crude oil (Tarragona Basin, Spain). Structural assignments were confirmed by synthesis of the Cl8 members of this series. These 1,2_dialkylbenzenes in combination with monoalkylbenzenes and 2-alkyltoluenes dominate the alkylbenzene distribution in this “immature” oil. This phenomenon lends support to the hypothesis that alkylbenzenes are formed in the subsurface by cyclisation and aromatisation reactions of linear, functionalised precursors. In other, more mature, oils other alkyltoluenes isomers are present as well and 1,2_dialkylbenzenes have not been reported. Isomerisation reactions of initially formed 2-alkyltoluenes during catagenesis may lead to the formation of thermodynamically more stable isomers (i.e., ortho and para alkyltoluenes) encountered in these more mature crude oils. oil. Structural assignments were confirmed the CIR members of this series.

INTRODUCTION ALTHOUGH PRESENT in most crude oils, long-chain alkylbenzenes have only occasionally been identified probably due to the complexity of their aromatic fractions. Rigorous identification of these compounds is, however, a prerequisite to assess their applicability for palaeoenvironmental, maturity, and oil-source rock and oil-oil correlation studies. The identification of a number of long-chain alkylbenzenes in crude oils has been reported. Homologous series of C12C39 n-alkylbenzenes and ortho, meta, and para n-alkyltoluenes have been identified in a number of oils (SOLLI et al., 1980; OSTROUKHOV et al., 1983; ALBAIG~S et al., 1986; WILLIAMS et al., 1988; SINNINGHE DAMST& et al., 1988). In a detailed study OSTROUKHOV et al. (1983) assigned homologous series (C12-C23) of all possible n-alkyldimethylbenzene and n-alkylethylbenzene isomers in Samotlor crude oil. Their concentration relative to the alkylbenzenes and alkyltoluenes is 3 to 5 times lower and certain isomers dominate over other isomers. A number of long-chain alkylbenzenes with an isoprenoid side-chain have also been recognised. SCHAEFLE et al. (1977) reported the presence of aromatic carotenoids in an extract of a sample from the Paris basin. OSTROUKHOV et al. (1982) and SUMMONS and POWELL (1987) identified a series of Cl3 to CxI aryl isoprenoids (2-alkyl- I ,3,4-trimethylbenzenes) in the Bucharsky oil (USSR) and in oils from the Michigan and Alberta Basins, respectively. These data enabled XINKE et al. ( 1990) to identify a series of “novel biomarkers” consisting of the same series of aryl isoprenoids in oils and sediments from the South Florida Basin. SINNINGHE DAMST!? et al. (1988) assigned a series of methylated phytanylbenzenes in a number of immature crude oils. In this paper we report the identification of a novel class of alkylbenzenes (1,2-di-n-alkylbenzenes) in Amposta crude

by synthesis

of

EXPERIMENTAL Sample The Amposta oil is from the Tarragona Basin (Spanish Mediterranean offshore). The source rock of this oil has probably been deposited during marginal hypersaline (carbonate-evaporite) episodes in the Miocene (ALBAIGBS et al., 1986). Data on the biological markers present in this oil have been described by ALBAIGBSand TORRADAS (1974), ALBAIG~S (1980), ALBAIGBSet al. (1985, 1986). BARBE et al. (1988), and SINNINGHEDAMST~ et al. (1988, 1989b).

Fractionation The crude oil was fractionated by chromatography on a column (25 X 2 cm: V, 37 mL) packed with alumina (activated for 2 h at 150°C) into a saturated hydrocarbon fraction by elution with 45 mL hexane and an “aromatic hydrocarbon” fraction by elution with 140 mL hexane!dichloromethane (9: 1, v/v). An aliquot (1 I. 1 mg) of the latter fraction (25.5% of the total oil) was subsequently separated by preparative argentatious thin layer chromatography (SiO*; 25 X 25 cm; thickness 0.25 mm) with hexane as the developer (SINNINGHE DAMSTI? et al., 1989a). The band (Rf = 0.5-0.8) corresponding to alkylbenzenes was scraped off the TLC plate and ultrasonically extracted with ethyl acetate (X3). This fraction (2.4 mg) was analysed by GC and W-MS.

Syntheses Dodecylbenzene, I-methyl-2-undecylbenzene, I-decyl-2-ethylbenzene, I-nonyl-2-propylbenzene, 1-butyl-2-octylbenzene, I-heptyl2-pentylbenzene, and 1,2-dihexyl-benzene were prepared by desulphurisation with Raney Ni of small amounts (ca. 0.5 mg) of previously synthesised (PERAKIS,1986) 2-decylbenzo(b]thiophene, 4-methyl-2-

nonylbenzo[b]-thiophene,4-ethyl-2-octylbnzo[b]thiophene,2-methyl-4-nonylbenzo[b]thiophene, 4-butyl-2-hexylbenzo[b]thiophene, 4dipentylbenzo[b]thiophene and 2-butyl-4-hexylbenzo[b]thiophene, respectively. I-Methyl-3-undecylbenzene and 1-methyl-4-undecylbenzene were prepared by a Grignard reaction of meta and para tolylmagnesiumbromide with n-undecanal. The obtained secondary alcohols were hydrogenolysed with triethylsilane in trifluoroacetic acid and BFj etherate as a catalyst (KURSANOVet al., 1975; PARNES

* Delft Organic Geochemistry Unit Contribution 2 12. 3677

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et al., 1977), yielding the desired products. The purity and identity of the obtained standards was checked by GC and ‘H-NMR. Mass spectra of all the synthetic compounds were obtained by W-MS. Gas Chromatography The conditions have been described previously (SINNINGHE DAM& et al., 1989b). The pseudo Kovlts retention indices of the synthetic compounds on two different stationary phases (CP-SiI 5 and DB 170 I ) are listed in Table 1. Mass Spectrometry Gas chromatography-mass spectrometry (GC-MS) was performed on a Hewlett-Packard 5840 gas chromatograph equipped with an oncolumn injector connected to a VG-70s mass spectrometer operated at 70 eV with a mass range m/z 40-800 (cycle time 1.8 s) at a resolution of 1000. The gas chromatograph was equipped with a fused silica capillary column (25 m X 0.32 mm) coated with CP-Sil 5 (film thickness = 0.12 pm) and was operated with helium as the carrier gas. RESULTS During a study of the so-called low-molecular-weight aromatic fractions of a number of sediment extracts and crude oils (SINNINGHE DAMSTB et al., 1988, 1989b), homologous series of alkylbenzenes were encountered in the Amposta crude oil from the Tarragona Basin (Spain). The main homologous series present were n-alkylbenzenes and I-n-alkyl2-methylbenzenes as reported previously (ALBAIG~S et al., 1986; SINNINGHE DAMST~ et al., 1988). Mass chromatograms of m/z 105 and 119 revealed a number of other homologous series which were previously tentatively identified as branched alkyltoluenes and I-n-alkyl-2,6_dimethylbenzenes, respectively (ALBAIG~S et al., 1986). The identity of these series of alkylbenzenes remained unclear, and consequently a study was undertaken to firmly establish their structures. The alkylbenzene fraction (6% of the total oil by weight) was isolated from the Amposta crude oil by column and argentatious thin layer chromatography and analysed with GC and GC-MS. Figure 1a-c shows partial mass chromatograms of m/z 9 1, 105, and 119. Coinjections with authentic dodec-

Table 1: Pseudo K&ts retention for two stationary phases.

Compound

dodecylbenzene I-methyl-2-undecylbenzene I-methyl-3-undecylbenzene I-methyl-4-undecylbenzene 1-decyl-2-ethylbenzene I-propyl-2-nonylbenzene l-butyl-2-octylbenzene I-heptyl-2-pentylbenzene 1,2-dihexylbenzene

indices of alkylbenzenes

pseudo Koviits retention index. CP-Sil Sb DB 1701’ 1857 1939 1860 1947 lS42 1920 1852 1930 1830 1910 1802 1876 1786 1867 1777 1856 1775 18.53

I, = 100~1 + IOO.(t, - tm)/(tn+, - t.) where t, is the retention time of the compound for which the txeudo Kovats retention index is determined, 1, and t.+t are the retention times of the n-alkanes which bracket the compound of interest, and n is the number of carbon atoms of the n-alkane that elutes just prior to the compound of interest. b I = 25m.id. = 0.32 mm, film thickness = 0.12 km, 130°C to 300°C at 4’C.min~‘, carrier gas He ’ I = 30 m, i.d. = 0.25 mm, film thickness = 0.25 em. 130°C to 300°C at 4T.min.t, carrier gas H,

1’

il -cl -E

+ scan nu!m!

F

FIG. 1. Partial, accurate (window 0.02 amu) mass chromatograms of(a) m/z 97.78, (b) m/z 105.22, (c)m/z 119.25, (d) m/z 218.51, (e) m/z 246.57, (f) m/z 274.64, showing the distribution of the CIs-CZO alkylbenzenes in the Amposta crude oil. Key: I = alkyl-benzenes; 2 = 1-alkyl-2-methylbenzenes; 3 = I-alkyl-4-methylbenzenes; 4 = Ialkyl-3-methylbenzenes; 5 = 1-alkyl-2-ethylbenzenes; 6 = 1-alkyl-2propylbenzenes; 7 = I-alkyl-2-butylbenzenes; 8 = I-alkyl-2-pentylbenzenes; 9 = I-alkyl-2-hexylbenzenes; 10 = I-alkyl-2-heptylbenzenes.

ylbenzene, 1-methyl-2-undecylbenzene, 1-methyl-3-undecylbenzene, and 1-methyl-4-undecylbenzene indicated that only the first two of these four compounds are major alkylbenzenes in the Amposta oil. These four compounds are often the most abundant Cl8 alkylbenzenes in sediment extracts, crude oils, and pyrolysates of kerogens and coals (e.g., RADKE, 1987). The mass chromatogram of m/z 105 (Fig. 1b) reveals the occurrence of the unknown homologous series (indicated in black) previously identified as branched alkyltoluenes (ALBAI& et al., 1986). Mass chromatography of m/z 218, 246, and 274 (Fig. ld,e), the molecular ions of the C16, Cl*, and CZo alkylbenzenes, respectively, reveal that these alkylbenzenes elute earlier than the major I-alkyl-2-methylbenzenes. The m/z 119 mass chromatogram shows the presence of the remaining homologous series. The mass spectra of four of the five unknown C18 alkylbenzenes (Fig. 2f-i; mass spectra of authentic standards are shown) all show a base peak at m/z 105. This fact in combination with their elution behaviour probably led to their

3679

Long-chain alkylbenzenes in oil from the Tarragona

h

91 91

L-L 18

L-l_ 18

FIG. 2. Mass spectra of authentic (a) dodecylbenzene, (b) I-methyl-2-undecylbenzene, (c) 1-methyl-3-undecylbenzene, (d) I-methyl-4-undecvlbenzene, (e) I-decyl-2-ethyl-benzene, (f) I-propyl-2-nonylbenzene, (g) 1-butyl-2-octylbenzene, (hj I-he&l-2-pentyldenzene, and (i) 1,2-hihexylbenzene

tentative identification as isoalkylbenzenes by ALBAIG~S et al. (1986). However, the observed homology of the various series (Figs. lb and 3) indicates that the major alkyl side chain is linear and not branched. A closer look at the mass spectra of the various unknown C,a alkylbenzenes reveals apart from the molecular ion at m/z 246 and the base peak at m/z 105 a number of other ions caused by loss of CnHZn+, (m/z 133 + 14.m) and C,H*, (m/z 134 + 14.m). These ions seem to occur in pairs in the various mass spectra: e.g., m/z 1331134 and 217 in Fig. 2f; m/z 1471148 and 2031204 in Fig. 2g; and m/z 161/162 and 189/190 in Fig 2h. The evennumbered fragment ions seem to be highly characteristic for these compounds as shown by their partial mass chromatograms. The occurrence of these pairs of ions seem to indicate a dialkyl substitution pattern of these alkylbenzenes. The base peak at m/z 105 results from loss of both alkyl side chains and should consequently be considered as a secondary fragment ion. The mass spectra of the unknown C,a alkylbenzenes are virtually identical with those of 1,2-dialkylbenzenes formed upon Raney Ni desulphurisation of C,8 2,4-dialkylbenzo[b]thiophenes in Rozel Point oil as reported previously (SINNINGHE DAM!& et al., 1987). Therefore, these compounds were synthesised by desulphurisation of individual previously obtiined C18 2,4-dialkylbenzo[b]thiophenes (PERAKIS, 1986). Their mass spectra are identical to those of

the unknown C,a alkylbenzenes, and they coelute on a capillary column coated with CP-Sil 5 (as monitored by GCMS) with the geological alkylbenzenes. The C,a alkylbenzene with a base peak at m/z 119 shows a similar mass spectrum as the synthesised 1-decyl-2-ethylbenzene, also a member of the series of C,a 1,2-dialkylbenzenes, and also coelutes with this compound. Detection of daughter ions of selected parent ions by linked scanning with constant B/E ratio for two authentic 1,2-dialkylbenzenes provided further information concerning their fragmentation pathways. Major daughter ions of the molecular ion (primary fragments) of 1-butyl-2-octylbenzene and 1-heptyl-2-pentylbenzene are observed at m/z 147, 16 1, 162, 175, 189, and 190 and m/z 147, 148, 189, 203, and 204, respectively. This indicates that P-cleavage and McLafferty rearrangement of both alkyl side chains occurs but, surprisingly, cleavage of the (Ycarbon-carbon bond also takes place, probably by a complicated sequence of reactions (cf. BUDZIKIEWICZ et al., 1964; p. 165). Ions resulting from this latter fragmentation reaction are also observed in the main beam spectra (Fig. 2g, h). Daughter ions of the above-mentioned primary fragments were primarily m/z 105 for odd mass (odd electron) species and m/z 105 and 106 for even mass (odd electron) species, confirming that the base peak at m/z 105 in the main beam mass spectra (Fig. 2g, h) is indeed a secondary fragment ion. Comparison of the mass spectra of the

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m/z 105

29

-

scan

number

FIG.3. Accurate (window 0.02 amu) mass chromatogram of m/z 105.22 for the Amposta crude oil. The peak labelled I is l-methyl-3-phytanylbenzene (SINNINGHE DAMST~ et al., 1988). Total number of carbon atoms of l-alkyl-2-methylbenzenes are indicated in a few cases.

Cl8 1,2_dialkylbenzenes (Fig. 2) indicates that the secondary fragment ion at m/z 105 dominates the mass spectrum if the shorter n-alkyl side chain contains at least three carbon atoms. The identification of the other members of these series of 1,2-di-n-alkylbenzenes was established by the linear relationship between the pseudo Kov&ts retention indices and the number of carbon atoms. With increasing number of carbon atoms the mixtures of 1,2-di-n-alkylbenzenes contain an increasing number of structural isomers as is evident from a comparison of the mass chromatograms of the molecular ions ofthe C16, C18, and CZO(Fig. Id-f). With the total number of carbon atoms greater than 19, some of these isomers (peaks indicated with f in Fig. 3) start to coelute with each other and with the major I-alkyl-2-methylbenzenes, leading to an overestimation of the abundance of I-alkyl-2-methylbenzenes as determined from the m/z 105 mass chromatogram.

chain alkyl &substituted benzenes (with a base peak in their mass spectra at m/z 119) in the Amposta and Samotlor crude oils further demonstrates this point. In the Amposta crude oil these compounds are dominated by 1-alkyl-2-ethylbenzenes whereas this compound is only a minor isomer in the Samotler crude oil (OSTROUKHOV et al., 1983). The other relatively abundant 1,2_dialkylbenzene isomers present in the Amposta crude oil have to our knowledge never been encountered in other crude oils. The abundance of 1,2-dialkylbenzenes is significant in view of the formation of alkylbenzenes in the subsurface. In this respect it is worthy to note that the Amposta crude oil is a relatively immature crude oil (ALBAG& et al., 1986). A number of possible origins for the formation of longchain alkylbenzenes can be envisaged.

1) Biosynthesis.

DISCUSSION The long-chain alkylbenzenes in the Amposta crude oil are dominated by alkylbenzenes and 1,2_dialkylbenzenes (including the ortho alkyltoluenes). This type of alkylbenzene distribution is quite distinct from those observed in other oils. In the Samotlor crude oil (USSR; OSTROUKHOV et al., 1983), Salmonete crude oil (Spain; ALBAIG~S et al., 1986), West Texas and Michigan oils (USA; WILLIAMS et al., 1988), and an oil from British Columbia (RADKE, 1987), all three alkyltoluene isomers are present in approximately equal abundance. A comparison of the distribution of the long-

2)

CONNAN et al. (1986) considered the predominance of pentadecylbenzene in the alkylbenzene distributions of anhydrites to be a result of bacterial contribution to the organic matter deposited in a hypersaline environment, although neither these components nor their possible precursors have yet been reported to be present in living organisms. Furthermore, ERDMAN ( 196 1) was unable to find detectable quantities of CI-C10 alkylbenzenes in unpolluted, Recent sediments from typical present-day aquatic environments. LANGWORTHY et al. (1982) only reported the presence of branched alkylbenzenes in archaebacteria. Diagenesis offinctionalised precursors. o-Cyclohexylalkanoic acids are known in nature but are restricted to two

Long-chain alkylbenzenes in oil

types of bacteria living under extreme conditions (DE ROSA et al., 197 1; SUZUKI et al., 198 1). These compounds have been mentioned as possible precursors for n-alkylcyclohexanes occurring in sediments and crude oils (e.g., JOHNS et al., 1966; RUBINSTEIN and SRAUSZ, 1979; FOWLER and DOUGLAS, 1984; FOWLERet al., 1986; REED et al., 1986; HOFFMANN et al., 1987). Dehydrogenation of alkylcyclohexanes may, in turn, lead to the formation of alkylbenzenes. HOFFMANNet al. (1987), however, noted that “it is unlikely that these isolated examples could fully account for the geological diversity of the alkylcyclohexanes.” Functionalised monoaromatics (e.g., phenol, benzoic acid, 1,3-benzenediol) with long n-alkyl side chains do occur in nature (e.g., GELLERMANet al., 1976; REUSCH and SADOFF, 1983; ASAKAWAet al., 1987) and cannot be ruled out as possible precursors for certain long-chain alkylbenzenes. 3) Alkylation of low-molecular-weight aromatics. Alkylation of benzene and toluene by primary alcohols and wax esters has been demonstrated to occur under simulated subsurface conditions and are, therefore, proposed to explain the origin of alkylbenzenes in sediments and crude oils (RIGBY et al., 1986; WILLIAMSet al., 1988). 4) Cyclisation and aromatisation of linear precursors during diagenesis. This mechanism has been proposed for the formation of alkylcyclohexanes (e.g., RUBINSTEIN and SRAUSZ, 1979; HOFFMANNet al., 1987). The marked similarity between the carbon number distributions of n-alkanes and n-alkylcyclohexanes in sediments and crude oils suggested ring closure of either n-alkanes or their precursors (FOWLERand DOUGLAS, 1984; REED et al., 1986; HOFFMAN et al., 1987). Triunsaturated n-alkanoic acids can undergo cychsation after moderate heating to form unsaturated 1,2-dialkylcyclohexanes possessing a COOH group at the o-position of one of the alkyl side chains (e.g., AWL et al., 1986). Model experiments in which stearic and oleic acid were heated (150-300°C) in the presence of clay generated dodecylcyclohexane and undecylmethylcyclohexanes (RUBINSTEINand SRAUSZ, 1979) and undecylcyclohexane and cis- 1-methyl-2-undecylcyclohexane (HOFFMANN et al., 1987) amongst other products. OsTROUKHOVet al. (1983) demonstrated that alkylbenzenes are also produced with dodecylbenzene and 2-undecyl- lmethylbenzene as the major products in experiments with stearic and oleic acid, respectively. However, the reaction mechanisms leading to the formation of these cyclic products are poorly understood at present. 5) Thermal degradation of kerogen. Long-chain alkylbenzenes have been reported to occur in pyrolysates of kerogens (LARTERet al., 1978; ISHIWATARIand FIJKUSHIMA, 1979; SOLLIet al., 1980; PHILP and GILBERT, 1984; DEREWE et al., 1990; DOUGLASet al., 1990a,b). Therefore, it is not unlikely to propose that long-chain alkylbenzenes in crude oils are derived from thermal breakdown of kerogen. 6) Natural desulphurisation of organic sulphur compounds, The Amposta crude oil contains abundant C,,-r& 2,4dialkylbenzo[b]thiophenes (SINNINGHE DAMST~ et al., 1989b) which upon treatment with Raney Ni can be desulphurised to 1,2_dialkylbenzenes (SINNINGHEDAMST~

from the Tarragona Basin

3681

et al., 1987). Although loss of (poly)sulphide moieties with increasing catagenesis occurs readily, sulphur-containing aromatic moieties (such as benzothiophenes) are much more stable. A more likely possibility is that upon increasing catagenesis free or bound 2-alkyl- and 2,5-dialkylthiolanes and 2-alkyl- and 2,6_dialkylthianes, also present in the Amposta crude oil (SINNINGHEDAMST~et al., 1989b), generate alkadienes by loss of H$S, which subsequently may act as precursors for 1,2-dialkylbenzenes by ring closure and aromatisation reactions (i.e., possibility 4). In this respect, it is noteworthy that upon heating (300°C, l-30 days) of the Rozel Point and Maruejols crude oils, which both contain abundant cyclic sulphides, longchain alkylbenzenes were formed (SCHMID, 1986; CONNAN et al., 1989) with long-chain alkylbenzenes and alkyltoluenes (though not exclusively alkylated at position 2) dominating. However, PAYZANTet al. (1989) heated cyclic sulphides (350°C l-14 days) in the presence of CaCOj and did not report long-chain alkylbenzenes as major pyrolysis products. Therefore, natural desulphurisation seems as yet to be a controversial scenario for the production of 1,2-dialkyl-benzenes. For the origin of the n-alkylbenzenes in the Amposta oil, possibility (1) can be rejected, whilst possibility (2) seems unlikely. Furthermore, the low abundance of meta and para alkyltoluenes is in contradiction with the alkylation hypothesis (possibility 3), since upon alkylation of toluene three isomeric alkyltoluenes were formed in roughly equal amounts (WILLIAMSet al., 1988). Kerogen pyrolysates often also contain the three isomeric alkyltoluenes in equal amounts (e.g., DOUGLASet al., 199 1b), although in pyrolysates of a certain type of kerogen (i.e., that mainly derived from the fossilised algae Gloeocapsamorpha prisca) the linear alkyltoluenes also dominate (DOUGLASet al., 199la,b; DERENNEet al., 1990). This phenomenon was attributed to cyclisation and aromatisation reactions of the linear building blocks of the aliphatic biopolymer of the outer cell wall of G. prisca (DOUGLASet al., I99 1b). The dominance of the linear alkylbenzenes (including the novel isomers identified herein) in the Amposta crude oil is consistent with possibility (4); cyclisation and aromatisation of linear C r&,o precursors during diagenesis can generate these extended series of 1,2_dialkylbenzenes. These precursors may originate from natural desulphurisation of cyclic sulphides (i.e., possibility 6). Suitable precursors should possess linear carbon skeletons and functionalities (e.g., double bonds and alcohol or acid groups) which enable the precursors to cyclisize during diagenesis. It is also feasible that these linear precursors were part of macromolecules with the linear alkylbenzenes generated by thermal degradation of the kerogen which sourced this oil (i.e., a combination of possibilities 4 and 5). The question remains how the meta and para alkyltoluenes, often abundant alkylbenzenes in crude oils, have been formed. HOFFMANNet al. (1987) suggested cyclisation of iso and anteiso fatty acids for the formation of 3- and 4-methylI-alkylcyclohexanes. An alternative explanation could be isomerisation by methyl shifts catalysed by Lewis acids (e.g., clays) of the initially formed, thermodynamically less stable (COX and PIXHER, 1970) ortho alkyltoluenes during

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catagenesis. This kind of isomerisation reaction has been proposed for polycyclic aromatic components in the subsurface (e.g., naphthalenes and phenanthrenes; RADKE, 1987, 1988). Since the Amposta crude oil is a relatively immature crude oil (ALBAIG!& et al., 1986), such reactions obviously did not take place. In more mature crude oils from the Tarragona Basin meta and para alkyltoluenes are much more abundant relative to ortho alkyltoluenes (ALBAIG~S et al., 1986), probably due to this isomerisation reaction. On an empirical basis ALBAIG~~Set al. (1986) already defined and successfully applied a molecular marker ratio (3-methyl- laIkylbenzene/2-methyl- 1-alkylbenzene) derived from this process in their study of the petroleum geochemistry of the Tarragona Basin. Acknowledgments-We thank Dr. J. AlbaigCs for provision of the Amposta crude oil sample; Dr. J. M. A. Baas for information concerning the thermodynamic stability of alkylbenzenes; Mrs. W. 1. C. Rijpstra and Mr. W. Pool for analytical assistance; and Drs. R. E. Summons, M. Radke, and J. Grimalt for useful comments on an earlier draft of this paper. Editorial handling: S. G. Wakeham REFERENCES ALBAIG!%J. (1980) Identification and geochemical significance of long chain acyclic isoprenoid hydrocarbons in crude oils. I. In Advances in Organic Geochemistry 1979 (eds. A. G. DOUGLAS and J. R. MAXWELL),pp. 19-28. Pergamon. ALB~~GBSJ. and TORRADASJ. M. (1974) Significance of an even nparaffin predominance of a Spanish crude oil. Nature 250, 567568. ALBAIGBSJ., BORBONJ., and WALKERW., II (1985) Petroleum isoprenoid hydrocarbons derived from catagenetic degradation of Archaebacterial lipids. Ora. Geochem. 8. 293-297. ALBAIGBSJ., ALGA~AJ., &AVELL E., and GR~MALTJ. (1986) Petroleum geochemistry of the Tarragona Basin (Spanish Mediterranean off-shore). In Advances in Organic Geochemistry 1985 (eds. D. LEYTHAEUSER and J. RULLK~TTER);Org. Geochem. lo,44 I450. ASAKAWAWA Y., MASUYAT., TORI M., and CAMPBELLE. 0. (1987) Long chain alkyl phenols from the liverwort Schistochila appandiculata. Phytochem. 26, 735-738. AWL R. A., NEFF W. E., FRANKELE. N., PLATTNERR. D., and WE~SLEDERD. (1986) Cyclic fatty esters: hydroperoxides from photosensitised oxidation of methyl 9-(6-propyl-3-cyclohexenyl)(Z)8-nonenoate. Chem. Phys. Lipids 39, l-23. BARBEA., GR~MALTJ. O., and ALBAIG~SJ. (1988) Novel cyclohexyl isoprenoid hydrocarbons in carbonate-evaporite oils. Naturwiss. X,624-625. BUDZIK~EWICZ H., DJERAS~~C., and WILLIAMSD. H. (1964) Interpretation of Mass Spectra of Organic Compounds. Holden-Day. CONNAN J., BOUROULLEC J., DESSORTD., and ALBRECHTP. (1986) The microbial input in carbonate-anhydrite facies of a sabkha palaeoenvironment from Guatemala: A molecular approach. In Advances in Organic Geochemistry I985 (eds. D. LEYTHAEUSER and J. RULLK~TTER);Org. Geochem. 10,29-50. CONNAN J., DESSORTD., and ALBRECHTP. A. (1989) Thermal evolution of the so-called aromatic fraction of immature sulphur-rich crude oils and source rocks. Abstract No. 3 17 in Abstract Book of the 14th International Meeting on Organic Geochemistry, Paris (France). COX J. D. and P~LCHERG. (19iv) Thermochemistry of Organic and Organometallic Compounds. Academic Press. DERENNES., LARGEAUC., CASADEVALLE., SINNINGHEDAM& J. S., TEGELAARE. W., and DE LEEUWJ. W. (1990) Characterisation of Estonian Kukersite by spectroscopy and pyrolysis: Evidence for abundant alkyl phenolic moieties in an Ordovician,

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