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Sedimentary processes for the geosynthesis of heterocyclic aromatichydrocarbons and fluorenes by surface reactions

Muhammad Asif a,c,*, Robert Alexander b,*, Tahira Fazeelat a, Kliti Grice c

a Department of Chemistry, University of Engineering and Technology, G.T. Road, Lahore, Pakistanb Department of Chemistry, Curtin University of Technology, GPO Box U1987, Perth WA 6845, Australiac WA Organic and Isotope Geochemistry Centre, The Institute for Geoscience Research, Department of Chemistry, Curtin University of Technology, GPOBox U1987, Perth WA 6845, Australia

a r t i c l e i n f o

Article history:Received 23 September 2009Received in revised form 2 December 2009Accepted 4 December 2009Available online 11 December 2009

a b s t r a c t

Laboratory experiments have shown that activated carbon catalyses the reactions of biphenyls (BPs) withsurface adsorbed reactants incorporating either S, O, N or methylene, leading to the formation of somecommon constituents reported in sedimentary organic matter (OM) namely, dibenzothiophene (DBT),dibenzofuran (DBF), carbazole (C) and fluorene (F). A relationship between the % abundance of the heteroelement in the kerogen and the abundance of the related heterocyclic compound in the associated solubleOM supports the hypothesis that these reactions occur in Nature. More specific supporting evidence wasobtained from the good relationship observed between methyl and dimethyl isomers of the reactant BPsand the methyl and dimethyl isomers of the proposed heterocyclic products.

It is suggested that these aromatic compound distributions reported in sediments and crude oils fromthe Upper Indus Basin (Pakistan) and the Carnarvon Basin (NW Australia) are the result of a surface reac-tions of compounds with BP rings and surface adsorbed species of the hetero element on the surface ofcarbonaceous material.

Crown Copyright � 2009 Published by Elsevier Ltd. All rights reserved.

1. Introduction

Heterocyclic aromatic compounds occur widely in sedimentsand petroleum (Fan et al., 1990; Clegg et al., 1997; Radke et al.,2000; Bakr and Wilkes, 2002; Zhang et al., 2008). The most com-mon constituents of this group are dibenzothiophenes (DBTs), dib-enzofurans (DBFs) and carbazoles (Cs). These compounds comprisea BP cyclic ring system, with the incorporation of a hetero atomforming a third five-membered ring. It is convenient for the pur-poses of this paper to include the fluorenes (Fs) in this group,although in this case it is carbon which forms the third ring. Themechanism of formation of these structurally similar compoundshas not been reported. The abundance of heterocyclic aromatichydrocarbons in sediments and crude oils has been related to pal-aeoenvironment of deposition (Fan et al., 1990; Fenton et al., 2007;Zhang et al., 2008), thermal maturity (Ramanampisoa and Radke,1995; Li et al., 1997, 1999) and organic facies source (Bakel andPhilp, 1990; Radke et al., 2000; Bakr and Wilkes, 2002), as wellas to migration effects (Larter et al., 1996).

The changes in relative abundances of benzothiophenes (BTs)and DBTs has been the basis for proposed maturity indicators forOM (Radke, 1988; Radke and Willsch, 1991; Santamaria-Orozcoet al., 1998). The high abundances of DBF and alkylated DBFs incoals led to the suggestion of their relationship with oxidative deg-radation (Hayatsu et al., 1978) and/or their terrestrial origin fromlower vascular plants, fungi and lichens (Radke et al., 2000). Thedifferences amongst the distributions of DBF and alkylated DBFshave also been reported and have been attributed to changes inlithology and thermal maturity (Willsch and Radke, 1995; Raman-ampisoa and Radke, 1995). Relative abundances of DBFs and DBTshave been reported in different depositional environments, wheremarine carbonates show higher relative amounts of DBT, whilefreshwater/lacustrine sediments show higher relative amounts ofDBFs (Fan et al., 1990; Jinggui et al., 2005). Pyrrolic N-containingcompounds in petroleum have been suggested as indicators ofmigration (Larter et al., 1996). Little work has been carried outon the occurrence and distribution of Cs in sedimentary OM. Thesource of pyrrolic N has been shown to be related to facies, andthe concentrations of Cs and benzocarbazoles (BCs) have also beenshown to increase with thermal maturity (Bakel and Philp, 1990;Clegg et al., 1997; Zhang et al., 2008). Contrasting results have beenreported for Canadian oils, reflecting thermal maturity and the pal-aeoenvironment of deposition (Li et al., 1999). The geochemicalsignificance of Fs is not understood, except for those reported in

0146-6380/$ - see front matter Crown Copyright � 2009 Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.orggeochem.2009.12.002

* Corresponding authors.E-mail addresses: asifchaudhrry@gmail.com (M. Asif), R.Alexander@curtin.

edu.au (R. Alexander).

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different oil and source correlation studies (Fan et al., 1990; Jingguiet al., 2005), while alkylated Fs have not been reported in sedimen-tary OM. These contrasting concentrations and the relative distri-butions of heterocyclic aromatic hydrocarbons in sediments andcrude oils may be due to lack of knowledge about their source(s)and mechanisms of formation.

In this study, we have extended our recent publication (Asifet al., 2009b). We report evidence for the geosynthesis of DBTs insedimentary OM by way of catalysis of surface S incorporation intothe BP cyclic system. The formation of DBFs, Cs and Fs is shown tooccur under laboratory conditions through carbon surface cataly-sis, forming a third ring in BPs. Evidence that these conversionsalso occur in sediments is provided by relative abundance datashowing precursor–product relationships between BPs and theheterocyclic compounds in crude oil and sediments reported fromthe Upper Indus Basin (Pakistan) and the Carnarvon Basin (NWAustralia).

2. Experimental

2.1. Reference compounds and laboratory heating experiments

Reference compounds were obtained from commercial suppli-ers: BP (Acros Organics), 3-methylbiphenyl (TCI chemicals), 1,7-dimethylfluorene (Chiron, Norway), sodium azide (Sigma–Aldrich),nonylamine, diisopropyl amine, acetonitrile (Sigma–Aldrich),1,2,3,4-tetramethylbenzene (TeMB; Sigma–Aldrich). Activated car-bon (technical grade, Asia Pacific Suppliers) was used for heatingexperiments and was conditioned at 340 �C (minimum 2 h) beforeuse. The coal (sub-bituminous, Permian in age) from Collie in Wes-tern Australia (Le Blanc-Smith, 1990) was obtained from the Hebeseam and sieved to pass 90 mesh.

Heating experiments were carried out using glass tubes deacti-vated with dichlorodimethylsilane (Alltech) after soaking in HCl(1 M). In a typical experiment 1 mg of BP with 1 mg of each reac-tant selected from the following [acetonitrile, nonylamine,1,2,3,4-tetramethylbenzene, molecular oxygen (air), sodium azide]were added to 10 mg activated carbon (or coal). The reactants wereflushed with N2 and sealed under vacuum [except in the case of themolecular oxygen (air) experiment] before heating in a thermostatat selected temperatures between 270 �C and 300 �C. Heating timewas 15–16 h for all experiments. The reaction products were ex-tracted with dichloromethane and chromatographed by passingthe extract through a small-scale silica column. Blank experimentswithout carbon (or coal) were carried out in parallel. The reactionproducts were analysed using gas chromatography–mass spec-trometry (GC–MS).

2.2. Sediments and crude oils

Some of the sediment samples, a crude oil from the Upper IndusBasin, Pakistan and two crude oils from the Carnarvon Basin, NWAustralia have been used and extend our previous study (Asif et al.,2009b). Lithology, depth and biomarker thermal maturity valuesfor the sediments and crude oil from Upper Indus Basin are shownin Table 1. The biomarker ratios show small contradictions in thethermal maturity of sediment samples. Where the C32 22S/(22S + 22R) hopane (0.41–0.50) and C29 bb/(bb + aa) ratios (0.45–0.61) indicate a low to moderate range of maturity, while the C29

20S/(20S + 20R) ratio (0.42–0.52) indicates a higher relative maturity(Peters et al., 2005). The available Tmax values (427–433 �C) for thesediments from a depth range of 4290–4510 m indicate an immatureto early oil generation window of maturity. The Upper Indus Basincrude oil shows a higher relative maturity, whereby the biomarker

Table 1Data for sediments and crude oil from the Upper Indus Basin (Pakistan).

No.a Depth (m) Lithology S/(S + R) C32 hopane S/(S + R) C29 sterane bb/(bb + aa), C29 sterane Tmax (�C)

1 4290 Marl 0.42 0.43 0.58 4272 4310 Marl 0.42 0.48 0.61 4313 4345 Shaley limestone 0.43 0.43 0.61 4314 4510 Limestone 0.41 0.50 0.51 4335 4650 Sandstone 0.43 0.52 0.56 –6 4680 Sandstone 0.42 0.42 0.58 –7 4710 Shaley siltstone 0.44 0.49 0.52 –8 4834 Shaley clay stone 0.50 0.42 0.55 –9 4860 Shaley clay stone 0.44 0.52 0.55 –

10 4940 Claystone 0.43 0.44 0.45 –11 2687 n.d. 0.57 0.48 0.61 –

n.d.: Not determined.a Nos. 1–10 are sediments; No. 11 is a crude oil (Chaknaurang).

Table 2Concentrations of compounds and elemental kerogen composition for the Upper Indus Basin (Pakistan) sediments.a

Samples Concentration (lg/g TOC) Elemental composition (%)

No. Depth BP DBT DBF C F S O N

1 4290 2.88 8.52 0.51 – 1.05 4.3 12.53 0.122 4310 1.06 2.66 0.19 0.12 0.33 2.1 11.11 0.103 4345 2.23 6.78 0.50 0.80 1.14 n.d. n.d. n.d.4 4510 0.22 0.74 0.05 0.07 0.12 0.3 27.28 0.015 4650 1.80 3.67 0.38 0.25 0.71 n.d. n.d. n.d.6 4680 1.35 1.81 0.31 0.28 0.33 1.1 7.51 0.377 4710 7.26 19.49 1.87 0.23 – n.d. n.d. n.d.8 4834 1.52 0.68 0.19 – 0.18 0.4 10.75 <0.019 4860 2.01 3.29 0.47 0.19 0.89 1.7 14.37 0.11

10 4940 0.27 0.31 0.10 – 0.14 1.7 3.39 0.03

–: Below detection limit.n.d.: Not determined.

a BP, biphenyl; DBT, dibenzothiophene; DBF, dibenzofuran; C, carbazole; F, fluorene; S: sulfur; O: oxygen; N: nitrogen.

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maturity parameters have reached equilibrium (cf. Peters et al.,2005; Asif et al., 2009a). The Griffin and Barrow crude oils from theCarnarvon Basin (NW Australia) have been described as typical ma-ture crude oils (e.g. van Aarssen et al., 1997; Bastow et al., 2003).

2.3. Sample preparation and liquid chromatography

The liquid chromatography separation for crude oils andsediment extracts was carried out using established procedures

28 32 36 40 28 32 36 40

Blanks (a) AC, BP , S

(b) AC, BP, Oxygen (Air)

(d) AC, BP, TeMB

(c) AC, BP, sodium azide

Relative retention time20 24 28 32 20 24 28 32

28 32 36 40 28 32 36 40

28 32 36 40 28 32 36 40

BP

DBT

DBF

2-ABP

C

TeMBBP

F

TeMB BP

MBPs

Fig. 1. Total ion chromatograms (TICs) of extracts from laboratory heating experiments. Samples were heated at 300 �C for16 h. Each blank experiment was identical incomposition, temperature and time but without activated carbon. See data for the S reaction in Asif et al. (2009b). AC, activated carbon; BP, biphenyl; S, sulfur; DBT,dibenzothiophene; DBF, dibenzofuran; 2-ABP, 2-aminobiphenyl; C, carbazole; F, fluorene; TeMB, 1,2,3,4-tetramethylbenzene; MBPs, methylbiphenyls; see Appendix A forstructures.

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(Bastow et al., 2003; Asif et al., 2009b). Extracted sediments wereused for elemental analysis (see below).

Deuteriated phenanthrene (phenanthrene d10) was used asinternal standard in the aromatic fraction for quantification ofthe individual isomers of DBTs, DBFs, Fls, Cs and BPs.

2.4. GC–MS

GC–MS was performed using a Hewlett–Packard (HP) 5973Mass Selective Detector (MSD) interfaced to an Agilent 6890 gaschromatograph. A 60 m, 0.25 mm i.d. column with a 0.25 lm 5%phenyl 95% methyl polysiloxane stationary phase (DB-5MS, J&WScientific) was used. Samples (1 mg/ml in n-hexane) were intro-duced into the split/splitless injector using an Agilent 6890 auto-sampler. The injector was operated in pulsed splitless mode at280 �C and the helium carrier gas was maintained at constant flow(1.1 ml/min). The GC column was programmed from 40 �C to310 �C at 3 �C/min (10 �C/min for heating experiment extracts)with initial and final hold times of 1 and 30 min, respectively.

2.5. Identification and quantification

Aromatic hydrocarbons were identified using relative retentiontimes and mass spectra data reported in the literature (Li et al.,1992; Trolio et al., 1999; Radke et al., 2000; Bakr and Wilkes,2002; Bastow et al., 2003). Quantification was performed by com-paring peak area of the internal standard with those of the desiredcompounds, using the following ions: m/z 188 (deuteriated phen-anthrene), m/z 154 [biphenyl (BP)], m/z 168 [methylbiphenyls(MBPs)], m/z 182 [dimethylbiphenyls (DMBPs)], m/z 166 [fluorene(F)], m/z 180 [methylfluorenes (MFs)], m/z 167 [carbazole (C)], m/z181 [methylcarbazoles (MCs)], m/z 195 [dimethylcarbazoles(DMCs)], m/z 168 [dibenzofuran (DBF)], m/z 182 [methyldibenzofu-rans (MDBFs)], m/z 184 (DBT). Table 2 shows the concentrations(lg/g TOC) of heterocyclic aromatic compounds and amount of Fin the extract of the sediments and the elemental compositions(%) of heteroatoms (N, S and O) in the kerogen from the Upper In-dus Basin, Pakistan.

2.6. Elemental analysis

Analysis was performed with a Carlo Erba NA1500 elementalanalyser. Samples were weighed into tin capsules and dropped intoa combustion tube at 1000 �C through which a constant stream ofhelium was maintained. Just prior to sample introduction the he-lium stream was dosed with a precise volume of pure oxygen.The sample was instantaneously burned, followed by intense oxi-dation of the tin capsule at 1800 �C (flash combustion). The result-ing combustion gases were passed over catalysts to ensurecomplete oxidation and absorption of halogens, S and other inter-ferences. Excess oxygen was removed as the gases were sweptthrough a reduction tube containing Cu at 650 �C. Any N oxideswere reduced to nitrogen. The gases were separated on a chro-matographic column into nitrogen (N), carbon dioxide (C) andwater vapour (H) and quantitatively measured with a thermal con-ductivity detector (TCD). The system response was calibrated withknown calibration standards.

For oxygen analysis samples were precisely weighed into silvercapsules and dropped at preset times into a combustion tube (at1050 �C) through which a constant stream of helium was main-tained. The resulting pyrolysis gases were passed over catalysedcarbon to ensure complete conversion of oxygen in the sample tocarbon monoxide. The carbon monoxide was separated from otherpyrolysis gases on a chromatographic column and quantitativelymeasured using a TCD. The system response was calibrated withknown calibration standards.

For sulfur analysis samples were precisely weighed into tin cap-sules and dropped into a combustion tube (at 1000 �C) throughwhich a constant stream of helium was maintained. Just prior tosample introduction the helium stream was dosed with a precisevolume of pure oxygen. The sample was instantaneously burned,followed by intense oxidation of the tin capsule at 1800 �C (flashcombustion). The resulting combustion gases were passed overcatalysts to ensure complete oxidation. The gas stream was thendried by means of a water scrubber. Excess oxygen was removedas the gases are swept through a reduction tube containing Cu at650 �C. Finally the sulfur dioxide was separated from other inter-fering gases on a chromatographic column and quantitatively mea-sured using a thermal TCD. The system response was calibratedwith known calibration standards.

3. Results and discussion

Evidence that the solid-state carbonaceous material promoteschemical reactions in sediments has been suggested from data ob-tained from hydrogen exchange reactions between hydrocarbons(Alexander et al., 2009) and S incorporation into BP compounds(Asif et al., 2009b). Here we extend these data, showing that labo-ratory reactions of O, N and C species on carbonaceous surfaceswith BPs form the heterocyclic and F ring systems.

28 30 32 34

(a) AC, BP, nonyl amine

(b) AC, BP, TeMB

(c) AC, BP, Acetonitrile

F

F

F

MBPs

2-

3- 4-

BP

Relative retention time

2-HBP

Fig. 2. TICs of extract from heating experiments of BP with activated carbon usingdifferent alkyl precursor compounds. Heating temperature and duration were thesame for all experiments, i.e. 300 �C and 16 h. AC, activated carbon; BP, biphenyl;MBPs, methylbiphenyls; 2-HBP, 2-hydroxybiphenyl; TeMB, 1,2,3,4-tetramethyl-benzene; F, fluorene, see Appendix A for structures.

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3.1. Laboratory experiments with activated carbon

Laboratory experiments have shown that carbon surfaces cata-lyse the reaction between BP and surface reactants. Mass chro-matograms (Fig. 1) of the reaction products obtained from theheating experiments with activated carbon at 300 �C in deactivatedand evacuated glass tubes show significant concentrations of eachof the compounds of interest. Blank experiments without carbon(Fig. 1 blanks) showed no products and when activated carbonwas heated alone no products were formed.

The carbon-catalysed formation of DBT from BP and elemental Sunder these reaction conditions has been reported by Asif et al.(2009b) and is reproduced in Fig. 1a. The study of surface reactionshas now been extended by providing sources of reactive O, N and Cfor reaction with BP. Fig. 1b shows a chromatogram indicating for-mation of DBF. The surface O was provided to the activated carbonby molecular oxygen (Bansal and Goyal, 2005). This experimentwas carried out after sealing the reaction tube without evacuationto provide a source of O from air. Reaction of BP with surface N wasmade possible by using sodium azide (Tasis et al., 2003). The reac-tion product contained both aminobiphenyl and carbazole asshown in Fig. 1c. The reaction of 1,2,3,4-tetramethylbenzene withBP under the same reaction conditions resulted in the formationof the heterocyclic products from O and N donors yielding F andMBP isomers as shown in Fig. 1d.

A similar set of experiments was carried out with crushed coalinstead of carbon. The formation of DBT under these reaction con-ditions has been reported (Asif et al., 2009b) and we report herethat the reaction reported above that afforded C was repeated un-der these reaction conditions at a reaction temperature of 270 �C.Again a significant amount of C was observed in the chromato-grams of the reaction products vs. from the blank experiment.

The results indicate that carbonaceous surfaces other than acti-vated carbon facilitate these reactions.

3.1.1. Probable mechanism of reactionCatalysis by carbon surfaces is a known process (Boehm, 1994;

Bansal and Goyal, 2005) and the formation of active adsorbed sur-face reactants involves free radical reactions. Since carbon surfaceshave low polarity but are electrical conductors, the reactions theyfacilitate are more likely to involve radical rather than ionic inter-mediates. More details about the mechanism of these reactions isbeyond the scope of this paper; however, it is interesting to ob-serve that both methylation of BP and methylene substitution ofBP to yield MBP isomers and F (Fig. 2) could involve surface carb-enoid (or carbene) species (Smith, 1994). A similar distribution ofthese products resulted when TeMB was substituted with nonylamine (Fig. 2a) or acetonitrile (Fig. 2c), indicating that the reactivemethylene species can be formed on the solid carbon surface bymethylene abstraction from different compound types. The forma-tion of C from BP could similarly involve a N radical species such asnitrene (Gilchrist, 1978) by direct insertion or via 2-aminobiphenyl(Fig. 1c). A similar set of radical processes appears to be responsiblefor formation of DBF from BP and adsorbed oxygen. The proposedreaction sequence for formation of these compounds is shown inFig. 3.

3.2. Distributions in sediments and crude oils

In order to assess the likelihood that the precursor–productrelationships observed in the laboratory experiments have also oc-curred in Nature, i.e. in sedimentary OM. Thus, relative abundancesof the parent compounds (non-alkylated) and their proposedproducts were examined in sediments and crude oils. Both the

NH

O

:CH2

(NaN3)

O2 (Air)

AC

OH

NH2

CH3

BP

DBF

C

F

:NH2

AC

AC

Fig. 3. Proposed reaction pathways on activated carbon for formation of heterocyclic aromatic compounds and F from BP. AC, activated carbon; BP, biphenyl; F, fluorene; DBF,dibenzofuran; C, carbazole.

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unsubstituted and methyl substituted isomers of these compoundsare common constituents in sediments and crude oils.

3.2.1. Parent compoundsA sequence of sediments from the Upper Indus Basin, Pakistan

have been analysed for their % N, S and O kerogen composition.The results are shown in Table 2 together with the concentrationof BP, DBT, DBF, C and F (see Appendix A for structures) in the

soluble organic matter (SOM) from each sample. Plots of the % ele-mental composition from the kerogen for each element matchesthe hetero atom for each of the compounds formed (Fig. 4). Theseresults suggest a probable relationship between the solid stateabundance of the hetero atom in the kerogen and the organiccompound formed from it after its reaction with BP (or a relatedprecursor). Fig. 5 shows the relationships between the concentra-tions of the BP and the heterocyclic aromatic hydrocarbons and Fin these sediments. Formation of DBT, DBF, C and F support theinterpretation that BP is related to the formation of this group ofcompounds by way of its reaction with a species containing thehetero element within the kerogen.

3.2.2. Methylated homologuesIn order to facilitate easy recognition of heterocyclic com-

pounds with substituents on similar positions in the carbon ringsystem to the BP structural systems, Table 3 has been includedto show these relationships for both methyl and dimethylcompounds.

The representative mass chromatograms in Fig. 6 obtained fromthe aromatic fraction of the Upper Indus Basin sediment (depth4345 m) show the relative abundances of MBPs, MDBFs, MCs andMFs. The sample is immature (i.e. early oil generation window),supported by the C32 hopane and C29 sterane isomerization ratios(0.43 and 0.43, respectively) and Tmax value (431 �C). The mostabundant methyl substituted isomer for all heterocyclic com-pounds has the substituent in position 1, indicating a structuralrelationship with the most abundant MBP isomer (3-MBP). Simi-larly, the least abundant methyl substituted heterocyclic isomers(4-methyl) shows some association with the least abundant MBPisomer, i.e. 2-MBP. The reactant–product relationships of the com-pounds are represented by symbols in Fig. 6. It is noteworthy thatthe relative abundance results for the methyl isomers of the BPreactant and the products indicate that the heteroatomic elements(O, N) and methylene insertion occurred into MBPs in sediments toproduce corresponding methyl homologues of DBF, C and F,respectively. While there is some evidence for methylation of thearomatic rings during these surface catalysed reactions, the

0

2

4

6

8

10

Elemental S (%)

Con

c. D

BT

(µ

g/g

TO

C)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Elemental O (%)

Con

c. D

BF

(µ

g/g

TO

C)

0.0

0.1

0.2

0.3

0.4

0 1 2 3 4 5

0 5 10 15 20 25 30

0.0 0.1 0.2 0.3 0.4Elemental N (%)

Con

c, C

(µ

g/g

TO

C)

Fig. 4. Relationship between compounds in SOM and the N, S and O concentrationof kerogen from each sample from Upper Indus Basin sediments (data in Table 2).

R2 = 0.95

R2 = 0.76

R2 = 0.970

2

4

6

8

10

12

14

16

18

20

0 1 2 3 4 5 6 7 8Concentration, Biphenyl (µg/g TOC)

Con

cent

ratio

ns (

µg/g

TO

C

DBT

F

DBF

Fig. 5. Relationship of reactant (BP)–product (DBT, DBF and F) for Upper IndusBasin sediments (data in Table 2).

Table 3Ring position relationships between BP and related heterocyclic compounds.

12

34

56

2'3'

4'

5'6'

1'

12

345

6

78 9

X

X = O = DBF = NH = C = CH2 = F

Biphenyl Substituted DBF, C, F

2 43 1; 34 22,20 4,52,30 3,5; 1,5a

2,5 1,42,4 2,42,40 2,52,3 3,43,5 1,33,30 1,8; 3,6; 1,6a

3,40 1,7; 2,64,40 2,73,4 1,2; 2,3

a After rotation of phenyl ring along single bond.

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majority of methyl heterocyclics are derived from the methylsubstituted BPs rather than by methylation of the parent heterocy-clic compounds (see Fig. 2).

These relationships between methyl substituted BPs andmethyl homologues of DBF, C and F are also shown for the crudeoils. Fig. 7 shows the distribution of these compounds in two dif-ferent crude oils from Pakistan and Australia. The symbols repre-sent the relationships between the MBP isomers and the productheterocyclic compounds derived from them. While the peak pat-terns are a little different from those from the sediment extract(Fig. 6), the relative abundances of isomers supports a reactant–product relationship consistent with that discussed above for thesediment samples.

3.2.3. Dimethyl homologuesThe product–precursor relationship can also be extended to the

compounds with two methyl substituents. This approach is, how-ever, limited by the availability of GC–MS data to enable reliableidentification of these isomers. Data are available for DMBPs andDMCs (Trolio et al., 1999; Bastow et al., 2003); however, no dataare available for DMFs or DMDBFs although we were able to obtainan authentic sample of 1,7-DMF. The relationships between DMBPisomers and isomers of DMC and DMF are apparent in the chro-matograms shown in Fig. 8 for a sediment extract from the UpperIndus Basin and the Griffin crude oil from Australia.

In both samples, the two most abundant DMBP isomers are 3,30-and 3,40-, showing structural association with the two most abun-dant DMC isomers, 1,8- and 1,7- (refer to Table 3 for comparison ofrelated substitution patterns in BP and the heterocyclic com-pounds). Again, this relationship is consistent with a carbon cata-lysed derivation of the DMCs from DMBPs. Some preference forthe position of substitution into the BP ring system is indicated

by the relative abundances of reaction products. The nextmost abundant isomer is 1,6-DMC. It is a co-product of 3,30-DMBPand, like 2,6-DMC, which is a co-product of 3,40-DMBP, it has alower abundance than the alternative product with a methyl atposition 1, indicating a preference for substitution of the heteroatom adjacent to a methyl substituent. This is again the case forpreferential formation of 1,2-DMC, rather than 2,3-DMC, from3,4-DMBP.

In the case of DMFs only the 1,7- isomer was identified using anauthentic sample. The other peak assignments were made assum-ing that the effect of changing the position of ring substitution ofmethyl groups on Cs has the same effect on retention time as ob-served for DBFs and therefore can be used to predict the retentiontime of the DMF isomers relative to the 1,7-DMF reference com-pound. Again, the relationships in relative abundance of these iso-mers to those of DMBP support the proposed formationrelationship.

MFs

1

3

2

492

3

4

MBPs

1

3+2

4

MDBFs

MCs1

2

3 4

Relative retention time

Fig. 6. Representative ion chromatograms showing relative distributions of MBPs(m/z 168), MDBFs (m/z 182), MFs (m/z 180), MCs (m/z 181) from the Upper IndusBasin, Pakistan sediment (depth, 4345 m). Symbols on peaks relate precursor–product compounds.

2

3

4

2

3

4

9

2

3

1

4 92

3

1

4

1

3+24

1

3+2

4

1

3

2

4

(a) Upper Indus Basin, Pakistan (b) Carnarvon Basin, Australia

MBPs

MFs

MDBFs

MCs

Relative retention time

1

32

4

Fig. 7. Relative distributions of MBPs and methyl homologues of DBF, C and F incrude oils from two different basins. (a) Chaknaurang, Upper Indus Basin, Pakistan;(b) Barrow, Carnarvon Basin, NW Australia. MBPs (m/z 168), MFs (m/z 180), MDBFs(m/z 182) and MCs (m/z 181). Symbols on peaks relate precursor–productcompounds.

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4. Conclusions

DBT, DBF, C and F have been shown to form by way of reactionsof BP with surface-active S, O, N and methylene species on carbonsurfaces when heated at 300 �C. Evidence that similar reactions oc-cur in sediments was shown by the enhanced formation of the het-erocyclic compounds relative to BPs when the appropriate heteroelement was present in the kerogen. More specific evidence for areactant–product relationship between BPs and heterocyclics(and Fs) was obtained from a comparison of the methylated com-pounds in sediments and crude oils. Methyl substituted BPs (bothmono and dimethyl) were shown to have an isomer abundanceprofile that matched that predicted for the product methylatedheterocyclics (and Fs).

Acknowledgements

The authors thank G. Chidlow for GC–MS maintenance. TheHigher Education Commission, Islamabad, Pakistan, is thankedfor an IRSIP fellowship and a travel award grant (IRSIP-5-Ps-20)to M.A. We also thank the Oil and Gas Development CooperationLtd (OGDCL), Islamabad, for providing sediment samples. K.G.acknowledges ARC support from a QEII Discovery grant supportingelemental analysis and associated consumable costs. W. Meredithand E. Tegelaar are acknowledged for constructive reviews.

Appendix A

2,3'

2,5

2,4+2,4' 2,33-E

3,5

3,3'3,4'

4,4'3,4

1,8

2,4

2,7+1,2

2,52,6

1,3

1,6

1,7

1,4+4-E1,5+3-E

Relative retention times

DMBPs

DMCs

2,2' 2,6' 2,3'

2,5

2,4+2,4' 2,33-E

3,5

3,3'3,4'

4,4'

3,4

1,8

2,42,72,5

2,6

1,3

1,6

1,7 1,4+4-E1,5+3-E

1,2

Carnarvon Basin, AustraliaUpper Indus Basin, Pakistan

3,9 2,9

1,9

4,9

EF

1,8

1,3

1,7

1,6

DMFs

3,9 2,9

1,9

4,9

EF

1,8 1,3 1,7

1,6

ΔΔ

Δ

X

X

2,2'

Δ

Fig. 8. Relative distribution of DMBPs (m/z 182), DMCs (m/z 195) and DMFs (m/z 194) in the Upper Indus Basin sediment (depth 4940 m) and the Carnarvon Basin Griffincrude oil. Numbers on peaks indicate dimethyl substituted isomers. Symbols show precursor–product relationships (Table 3).

S

12

3

456

78

NH

O

12

3

456

78 99

12

3

456

78 9

12

3

45

6

2'3'

4'5'

6'

1'

NH2

OH

BP: biphenyl DBF: dibenzofuran

C: carbazole F: fluorene

DBT: dibenzothiophene 2-ABP: 2-aminobiphneyl

TeMB: 1,2,3,4-tetramethylbenzene 2-HBP:2-hydroxybiphenyl

M. Asif et al. / Organic Geochemistry 41 (2010) 522–530 529

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Associate Editor—C.C. Walters

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