Xanthones—novel aromatic oxygen-containing compounds in crude oils

Xanthones—novel aromatic oxygen-containingcompounds in crude oils

Thomas B.P. Oldenburg*,a,1, Heinz Wilkesa,2, Brian Horsfielda,2,Adri C.T. van Duinb, Daniel Stoddartc, Arnd Wilhelmsc

aInstitute of Petroleum and Organic Geochemistry, Research Centre Julich, D-52425 Julich, GermanybFossil Fuels and Environmental Geochemistry (NRG), Newcastle upon Tyne NE1 7RU, UK

cNorsk Hydro A.S., Research Centre, Sandsliveien 90, N-5020 Bergen, Norway

Abstract

Xanthone and its alkylated homologues were determined to be present in 64 of 69 investigated oils from offshoreNorway (Central Graben, Viking Graben, Haltenbanken). This is the first description of xanthones in crude oils. Thesecompounds were identified by comparison with authentic standards by coinjection, based on mass spectra and relative

retention times on two different GC columns. The elution order of the four methylxanthones was established as 1-4-2-3 byusing a BPX-5 column. About 2/3 of the oils contain concentrations of xanthone lower than 5 mg/g oil, but some oils areclearly enriched in the parent compound. The highest amount of xanthone in the sample set was 38 mg/g oil. The relativeabundance of xanthone, the sum of the methylxanthones and the sum of the C2-xanthones is mainly controlled bymaturity.Partitioning processes may effect changes in the distribution of methylxanthones as observed for a biodegradation sequencefrom the Gullfaks field. Molecular dynamics calculations support the observation of a better preservation of the shieldedisomers (1- and to a lesser extent 4-methylxanthone) in the oil phase compared to the non-shielded isomers (2- and 3-

methylxanthone). The ratio of these two different isomer groups may be useful as an indicator of secondary migrationdistances, as demonstrated for an oil sequence from the Tampen Spur and Haltenbanken oils. However, biodegrada-tion could also cause an increase of the shielded isomers relative to the non-shielded isomers due to sterical hindrance

by the methyl groups restricting access to the oxygen functionalities. The origin of xanthones in crude oils and sourcerocks is not known but they could be generated as diagenetic products, formed by oxidation of xanthenes in thereservoir, or originate by geosynthesis from aromatic precursors. # 2002 Elsevier Science Ltd. All rights reserved.

1. Introduction

Oxygen compounds are well known constituents ofcrude oils and source rocks. The first detailed investiga-tion of oxygen compounds in crude oils was described

by Snyder et al. (1968). Apart from aliphatic carboxylicacids, phenols, benzo-, dibenzo- and benzonaphthofu-

rans and phenylketones they also identified compoundclasses with an additional heteroatom (sulfur or nitro-gen) such as sulfoxides, pyridones and quinolones. Since

then many detailed characterisations of oxygen com-pound classes in crude oils and source rocks have beencarried out at the molecular level, including carboxylic

acids (e.g. Seifert, 1975; Grimalt et al., 1990; Jaffe andGardinali, 1990; Jaffe and Gallardo, 1993; Azevedo etal., 1994; Barakat and Rullkotter, 1995), chromans(Sinninghe Damste et al., 1987, 1993) and aliphatic

ketones and aldehydes (Anders et al., 1975; Grimalt etal., 1990; George and Jardine, 1994; Leif and Simoneit,1995; Wang and Rullkotter, 1997).

0146-6380/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.

PI I : S0146-6380(02 )00015-3

Organic Geochemistry 33 (2002) 595–609

www.elsevier.com/locate/orggeochem

* Corresponding author at current address. Tel.: +44-191-

222-5932; fax: +44-191-222-5431.

E-mail address: [email protected]

(T.B.P. Oldenburg).1 Current address: Fossil Fuels and Environmental Geo-

chemistry (NRG), Newcastle upon Tyne NE1 7RU, UK.2 Current address: GeoForschungsZentrum Potsdam, Sec-

tion 4.3: Organic Geochemistry and Hydrocarbon Systems,

Telegrafenberg, D-14473 Potsdam, Germany.

Aromatic ketones and aldehydes are rarer con-stituents of fossil fuels. Latham et al. (1962) identifiedalkylated fluoren-9-ones in Wilmington petroleum. Thiscompound class occurred also in Athabasca oil sand

bitumen (Mojelsky and Strausz, 1986) and PosidoniaShale bitumen (Wilkes et al., 1998a). A detailed reser-voir study involving both fluids and core extracts carried

out by Bennett and Larter (2000) showed that the con-centrations of fluorenones in reservoir core extracts areoften very high, which may reflect sorption processes

after fluorenone formation. However, fluorenone abun-dance in flowed oils seems to be sporadic and may berelated to sample age and storage history. 1-Indanones

and 1-tetralones have been identified in the Green RiverFormation oil shale (Anders et al., 1975). Costa Neto etal. (1980) detected 1-indanones, naphthaldehydes andacetonaphthones in Brazilian oil shales from the Irati,

Tremembe and Marau Formations. A detailed char-acterisation of several types of aromatic ketones andaldehydes, such as indanones, tetralones, naphthalde-

hydes, acetylnaphthalenes, benzaldehydes and aceto-phenones in the low-polarity nitrogen, sulfur and oxygen(NSO) compound fractions of the Posidonia Shale bitu-

mens, has been provided by Wilkes et al. (1998b).Xanthones are aromatic oxygen-containing com-

pounds. These compounds consist of a tricyclic aro-

matic structure including an ether and a keto functionwithin the ring system (see Appendix). The unsub-stituted xanthone is not known to occur in nature.However, this ring system forms the basic structure of

different natural pigments of plants, e.g. gentian root(Falbe and Regitz, 1999). Highly substituted xanthones(with hydroxy-, methoxy- and alkylgroups) occur in

many different terrestrial plants (e.g. Tomasek andCrawford, 1986; Peres and Nagem, 1997) especially thosethat prefer tropical or subtropical climates. Xanthones are

synthesized by fungi, lichens, ferns, and many higherplants (Berti and Bottari, 1968; Culberson, 1969; Chexalet al., 1975; Devan and Scott, 1975). Many shrubs andherbs, which occur widely in temperate regions of the

world, contain xanthones (Wu et al., 1998).Xanthone is one of the predominant oxygenated

polycyclic aromatic compounds found in the mutagenic

fraction of extracts of diesel and gasoline engine exhaustparticles (Strandell et al., 1994); consequently, thiscompound has also been identified in urban aerosols

(Moriske and Ruden, 1988; Fernandez and Bayona,1992). In addition, xanthone was detected in watersamples from the North Sea and from the estuary of the

river Elbe, Germany (Bester and Theobald, 2000), andin sewage sludges in Upper Silesia (Bodzek andJanoszka, 1995).In this communication we report the occurrence of

xanthones in crude oils from offshore Norway. This isthe first description of these compounds in fossil organicmatter.

2. Materials and methods

2.1. Samples

Sixty-nine crude oils from the offshore area of Nor-way have been analysed in this study by geochemicalmethods. These samples include oils from the Central

Graben (7), the VikingGraben (39) and the Haltenbankenarea (23) (Fig. 1). For details of the geological history thereader is referred to Berglund et al. (1986), Gage and

Dore (1986), Heum et al. (1986) and Ziegler et al. (1986).The major source intervals for the Norwegian petroleumprovinces are contained in the Jurassic formations (Bar-

nard and Cooper, 1981; Goff, 1983; Field, 1985; Fjaeranand Spencer, 1991). Of these formations, the KimmeridgeClay Formation and its lateral equivalents are the sour-ces of the major oil accumulations in these areas (Schou

et al., 1985; Cornford et al., 1986; Karlsen et al., 1995).These Jurassic formations exhibit significant verticalheterogeneities in kerogen (Huc et al., 1985) and kerogen

pyrolysate composition (Bailey et al., 1989). Lateralorganic-facies heterogeneities are also reported by Gran-tham et al. (1979), Bailey et al. (1989) and Scotchman

(1991).Importantly for the study of migration and biode-

gradation, a suitable biodegradation sequence from the

Gullfaks field, as well as crude oils from migrationroutes collected from the Tampen Spur and the Halten-banken area, were available, as illustrated in Fig. 1.

2.2. Authentic standards

Xanthone was purchased from Aldrich (Steinheim,

Germany). Professor Dr. A. W. Frahm from the Uni-versity of Freiburg, Germany, provided the standards ofthe four methylxanthones and the 2,4- and 3,4-dime-

thylxanthones. Briefly, the alkylated xanthones weresynthesised by a two step reaction, the first step being acondensation reaction of a benzoic acid and a cresolunder Ullmann conditions (Pickert and Frahm, 1998).

The alkylated xanthones were then formed in a secondstep by intramolecular Friedel-Crafts acylation of thephenoxybenzoic acid as described by Pickert and Frahm

(1998).Known amounts of an internal standard (9-phenyl-

carbazole) were added to the low-polarity fraction for

quantification.

2.3. Liquid chromatographic separation

The crude oils were separated into seven fractionsaccording to the method of Willsch et al. (1997). Briefly,the oils were passed through neutral, acidic and basic

silica gel columns to remove high-polarity NSO com-pounds, bases and acids respectively. Chromatographyon the main silica gel column separated the medium-

596 T.B.P. Oldenburg et al. / Organic Geochemistry 33 (2002) 595–609

polarity NSO compounds (e.g. alcohols) and the low-polarity fraction. The latter was then separated via med-ium pressure liquid chromatography (MPLC) (Radke et

al., 1980) on silica gel into the saturated hydrocarbons,aromatic hydrocarbons and low-polarity NSO com-pounds. The xanthones eluted in the low-polarity NSO

compound fraction.

2.4. Gas chromatography—mass spectrometry (GC–MS)

For GC–MS measurements, a Finnigan MAT 95SQmass spectrometer (MS) was used that was coupled to aHewlett Packard 5890B gas chromatograph (GC). The

GC was equipped with a temperature-programmableinjection system (Gerstel KAS 3) and a BPX-5 fusedsilica capillary column (SGE) of 50 m length, 0.22 mm

inner diameter and 0.25 mm film thickness. Helium wasused as the carrier gas with electronic pressure control(EPC 1 ml/min). The oven temperature was pro-

grammed from 60 to 340 �C (final hold time 23 min) at3 �C/min.In electron impact (EI) mode, the MS was operated at

an electron energy of 70 eV and a source temperature of260 �C. EI mass spectra were recorded over the massrange of 50–600 Da at a scan rate of 0.7413 s/decade, an

inter scan time of 0.2 s and a scan cycle time of 1.0 s.While the identification of xanthones was performedusing GC–MS under EI conditions, for quantitationdata obtained from chemical ionisation (CI)-MS were

evaluated. Using i-butane as the reactant gas, massspectra were obtained which almost exclusively exhib-ited the corresponding pseudomolecular ions [M+H]+.

Fig. 1. Location maps illustrating the oil fields from offshore Norway sampled for this study.

T.B.P. Oldenburg et al. / Organic Geochemistry 33 (2002) 595–609 597

In CI mode the MS was operated at an electron energyof 50 eV and a source temperature of 260 �C. CI massspectra were recorded over the mass range of 100–600Da at a scan rate of 1.0281 s/decade, an inter scan time

of 0.2 s and a scan cycle time of 1.0 s. The quantitativeamounts were determined relative to 9-phenylcarbazoleas an internal standard. No corrections were made for

specific response factors of individual compounds. Wehave previously shown that CI–MS provides data com-parable to those obtained from GC with flame ionisa-

tion detection with respect to specific response ofindividual compounds (Wilkes et al., 1998a).For unequivocal identification of xanthone, the

methylxanthones and 2,4- and 3,4-dimethylxanthone,standard compounds were coinjected with a suitablenatural sample under the GC–MS conditions describedabove. In addition, coinjection experiments were per-

formed using a HP-1 fused silica capillary column of50 m length, 0.2 mm inner diameter and 0.33 mm filmthickness under otherwise identical GC–MS conditions.

2.5. Molecular dynamics calculations

The oil/water partition coefficients of xanthone andthe methylxanthone isomers were derived through theuse of molecular dynamics simulations to determine the

relative heats of solvation of these compounds in thewater and a cyclohexane phase. The solvent phases weresimulated using 200 water molecules and 50 cyclohexanemolecules, respectively. Each substrate/solvent system

was equilibrated at temperature/pressure conditions ofT=298 �K and P=10 bar until monotonic oscillatingvalues for the system energy were obtained, using a

time-step of 2 fs. Temperature and pressure were con-trolled during these simulations using the algorithmdescribed by Berendsen et al. (1984), using relaxation

times for the temperature and pressure coupling of 100and 1000 fs, respectively. A cutoff radius of 9.0 A wasused and the system energy was corrected for van derWaals interactions beyond this cutoff range (Allen and

Tildesley, 1987). A 7th order taper interaction was used,as described by de Vos Burchart (1992) to ensure astable simulation of the Coulomb interactions. Mole-

cular bonds were constrained using the RATTLE-approach (Andersen, 1983).After equilibration, relative heat of solvation values

for the methylxanthone isomers and xanthone in thewater and cyclohexane phases were obtained by aver-aging system energies over an interval of 2 ns. These

relative heats of solvation were used to calculate themethylxanthone and xanthone oil/water partitioncoefficients.Charge distributions and forces for xanthone and the

methylxanthones were calculated using the methodsdescribed by van Duin and Larter (1997). The water-phase was simulated using the TIP3P water force field

from Jorgensen et al. (1983). The cyclohexane phase wassimulated using the united atom approach (in which theCH2-groups are described as a single atom) using theparameters described by DeBolt and Kollman (1995).

3. Results and discussion

3.1. Identification of xanthones

The xanthones were detected in the low-polarity NSOcompound fraction, the same fraction in which carba-zoles and fluorenones occur. The parent compound,

xanthone, and the four methylxanthones as well as twodimethylxanthones were identified by comparison withauthentic standards by coinjection on two different GCcolumns and from their mass spectra. The elution order

of the four methylxanthones, reported here for the firsttime, was established as 1–4-2–3 on a BPX-5 column(Fig. 2); the same elution order was obtained on a HP-1

column. This elution order reflects the shielding effect ofmethyl substituents to the polar groups, the keto andthe ether function. Especially the methyl group close to

the keto-function (1-methylxanthone) but also to a les-ser extent the methyl group close to the ether group (4-methylxanthone) decreases the polarity of the methyl-

xanthones, resulting in an earlier elution. The retentiontimes of two dimethylxanthones (the 2,4- and 3,4-dime-thylisomers) were also determined as illustrated in Fig. 2.However, coelution of other C2-xanthones can not be

excluded. Further C2-xanthones (shaded peaks) wereidentified by MS evidence alone and used to determinethe sum of C2-xanthones.

In comparison with carbazole, fluorenone and theirmethylated homologues, xanthone and its alkylatedderivatives elute relatively late. The earlier elution of

fluorenone is perhaps due to its lower boiling point(342 �C) relative to carbazole (355 �C) and xanthone(351 �C) (Falbe and Regitz, 1999) and to its lowerpolarity. The influence of polarity on the elution order is

recognisable by the comparison of xanthone and carba-zole. Although carbazole has the higher boiling point,xanthone elutes later than carbazole due to its higher

polarity. In this regard, carbazole has a dipole momentof 2.09 D (Cowley and Partington, 1936) whereas xan-thone has a dipole moment of 3.11 D (Le Fevre and Le

Fevre, 1937).

3.2. Occurrence of xanthones, offshore Norway

Xanthone and its alkylated homologues were identifiedin 64 of the 69 investigated oils from offshore Norway.The exceptions, where xanthones were absent or occur-

red in very low amounts, are two oils from the CentralGraben (an oil generated from a hypersaline, anoxicsource environment; and a high maturity condensate), a


condensate from the Gullfaks Gamma field and twobiodegraded oils from the Grane and Troll area. For all

other oils the quantitative occurrence of xanthone, thesum of methylxanthones and the sum of the C2 xan-thones is illustrated as histograms in Fig. 3. Most of the

oils contain all three compound groups at concentra-tions lower than 5 mg/g oil but some oils are clearlyenriched in xanthones. Xanthone occurred at con-

centrations of up to 38 mg/g oil. Benzoxanthones couldnot be detected.

The relative abundance of xanthone, the sum of themethylxanthones and the sum of the C2-xanthones in all

investigated oils from offshore Norway is shown in theternary plot of Fig. 4. The crude oils show large varia-tions in relative abundances of xanthones, delineated by

the curve drawn on the figure. For most oils, especiallythose from Haltenbanken and many from the TampenSpur, the xanthone to methylxanthone ratio remains

essentially constant, while the relative abundance of theC2-xanthones varies appreciably. Three oils from the

Fig. 2. Mass chromatograms (electron ionisation) showing the elution order of xanthone, the four methylxanthones and two of the

C2-xanthones in a crude oil, offshore Norway.

Fig. 3. Histograms showing the quantitative occurrence of xanthone, the sum of the methylxanthones and the sum of the C2-xan-

thones in crude oils, offshore Norway.


Tampen Spur area are clearly enriched in xanthone. TheCentral Graben oils also contain a higher proportion of

xanthone than do most of the other oils.Fig. 5 shows the correlation of three xanthone ratios

(xanthone to the sum of the methylxanthones, xanthone

to the sum of the C2-xanthones and the sum of themethylxanthones to the sum of the C2-xanthones) with thematurity-related parameter of diahopane to the sum of

diahopane and normoretane, established by Cornford etal. (1986), Horstad et al. (1995) and Karlsen et al. (1995)

for oils from the Tampen Spur area. The three plots revealthe maturity dependence of the relative distribution of

xanthones. With increasing maturity the concentration ofxanthone increases relative to both the methylxanthonesand the C2-xanthones, and the sum of methylxanthones

increases relative to the sum of C2-xanthones. Oneremarkable oil from the Gullfaks Sør field (not depictedin Fig. 5) exhibits the highest values of the three ratios

described above (3.07, 7.90 and 2.57, respectively) whichcan not be explained by maturity effects only.

Fig. 4. Ternary plot showing the C0-, C1- and C2-xanthones distribution of all investigated oils from offshore Norway.

Fig. 5. Plots illustrating the maturity dependence of the relative abundances of xanthone, the sum of C1-xanthones and the sum of

C2-xanthones of oils from the Tampen Spur area.


3.3. Influences of partitioning processes, secondarymigration and biodegradation on the distribution ofxanthones

3.3.1. Partitioning processesPartitioning processes are important for understanding

the behaviour of polar compounds in petroleum systems,

since their distribution between different phases plays anessential role in geochemical processes such as second-ary migration or biodegradation. As described above,

xanthones are characterised by unusually high dipolemoments due to the two polar positions in the structure,the keto and the ether function. Therefore, the methyl

groups in the methylxanthones shield the polar posi-tions in a different way. Isomers with shielded func-tional groups, 1-methylxanthone and to a lesser extent4-methylxanthone, should show a partitioning beha-

viour different from that of the other isomers duringcontact of the oil with water in reservoirs through par-tition processes. As the xanthones have low solubility in

water, experimental measurements of their oil/waterpartition coefficients (Ko/w) are unreliable. Conse-quently, molecular modelling calculations were carried

out in order to establish the partitioning behaviour ofxanthone and the methylxanthones.Molecular dynamics (MD) calculations (Allen and

Tildesley, 1987; Hale, 1992; Frenkel and Smit, 1996; vanDuin and Larter, 1997) describe the movement ofmolecules in time, including both intermolecular move-ments, such as the rearrangement of solvent molecules

due to interaction with a substrate molecule, and intra-molecular movement, describing the conformationalfreedom of the molecules. Due to its ability to calculate

interactions between a solvent and a substrate moleculeMD has found applications in predicting the partition-ing behaviour of organic compounds between solvent

phases (e.g. Jorgensen and Nguyen, 1993; DeBolt andKollman, 1995; van Duin and Larter, 1997).In our study we used cyclohexane to simulate the oil

phase because MD calculations on solvation energies

work best if the solvent consists of similar, relativelysmall compounds. Cyclohexane has a density (778.55kg/m3) and boiling point (81 �C) comparable to that of

oils. Although cyclohexane is probably slightly moreapolar than most oils, qualitative differences betweenthe cyclohexane/water and oil/water partition coeffi-

cients of compounds should be similar. The MDsimulation of the various xanthones surrounded byeither 200 water molecules or 50 cyclohexane mole-

cules provided us with the solvation energies of thesecompounds in the water and ‘oil’ phase. As described byvan Duin and Larter (1997) the differences betweenthese solvation energies (E‘oil’ �Ewater) are directly rela-

ted to the compounds partition coefficient (Ko/w) by thefollowing equation (assuming E is measured in kcal/mol):

E 0‘oil � Ewater

� ��4:184

RT¼ ln Ko=w

� �

This theoretical definition of a partition coefficient

describes that a higher energy difference between the‘oil’ (i.e. cyclohexane) and water phase stability (E 0

‘oil’

�Ewater) indicates a higher water phase affinity for that

compound, which is equivalent to a lower Ko/w.Fig. 6 and the table therein show the results of the

MD simulations. MD calculations taking the difference

in the phase stability of the methylxanthone in oil andwater resulted in a decreasing oil/water partitioningcoefficient trend of 1>4>2>3. 1-Methylxanthone hasthe lowest affinity for the water phase due to the shield-ing effect of the methyl group with regard to the mostpolar keto oxygen. The shielding effect of the methylgroup in position 4 closest to the second polar function,

the ether oxygen, has a smaller effect on the water solu-bility than for the methyl group close to the keto-groupbut causes a lower affinity to water than the other two

isomers (2- and 3-methylxanthone) with methyl groupsmore distant from these two oxygen functionalities. Themethyl group in position 3 does not seem to alter the par-

titioning behaviour in comparison to the unsubstitutedxanthone.Based on these considerations, we have tested various

methylxanthone ratios, in particular the (1+4)/(2+3)-methylxanthone ratio as possible indicators of secondarymigration distances and biodegradation.

3.3.2. Secondary migrationIf partitioning processes affect the distribution of

xanthones between the oil and the water phase, xan-

thones might be useful as an indicator of secondarymigration distances. The (1+4)/(2+3) methylxanthoneratio would be expected to increase with increasing

migration distances, due to greater partitioning of 2-and 3-methylxanthone into the formation water.Unfortunately, the migration route and therefore themigration distance of many of the oils of our study is not

well known. An estimation of some migration distancesis possible, especially for the oils fromHaltenbanken anda part of the Tampen Spur oils.

For the Tampen Spur area two different fill-spill routeshave been suggested (Horstad et al., 1995; Karlsson,1986). Utilising the migration distance estimates from this

route established by Horstad et al. (1995), four of the sixsamples show progressively increasing (1+4)/(2+3)values with increasing migration distances. An explana-

tion of why the two oils from Tordis and Tordis Øst donot fall on the trend can be taken from the work ofKarlsson (1986), who proposed an additional migrationroute to the east of the main fill-spill route (Fig. 7). For

these oils, our results suggest a migration distance ofapproximately 65 km indicating that they probably didnot migrate along the main fill-spill route.

’


The second migration distance sequence is from theHaltenbanken area. The (1+4)/(2+3) methylxanthoneratio of most of these oils correlates also positive withincreasing migration distances as illustrated in Fig. 8. A

few oils with shortest and longest migration distancesdeviate most of this trend. One explanation of this obser-vation could be that the estimated migration distances

of these oils may be incorrect. However, additional fac-tors can affect the oil/water partitioning like differentlocal hydrodynamic conditions or the proximity of asampled oil to an oil/water contact. Two oils with high-

est values of the (1+4)/(2+3) ratio are strongly biode-graded. The influence of biodegradation and/or waterwashing will be discussed in the following section.

Fig. 6. Molecular dynamics calculations of the partitioning behaviour of xanthone and the four methylxanthones.

Fig. 7. Plot showing a possible influence of secondary migration on the methylxanthone distribution for oils from the Tampen Spur.


3.3.3. BiodegradationA suite of oils representing a biodegradation sequence

from the Gullfaks field was used to test the influence ofbiodegradation on the distribution of xanthones. Theextent of biodegradation increases from the east to thewest from non-degraded to 3–4 according to the scale of

Peters and Moldowan (1993) (Fig. 1). Based on quanti-tative modelling calculations, Horstad et al. (1991) pro-posed that the degradation process occurred in the

reservoir and not during migration into the reservoir.One of the more useful parameters illustrating the dif-ferent grades of (early-stage) biodegradation is the phy-

tane to n-octadecane ratio, which increases withincreasing biodegradation. Fig. 9a–f displays plots ofdifferent methylxanthone ratios against this parameter.Good correlations are observed for 1-methylxanthone

to the sum of 1-methylxanthone and each of the othermethylxanthones (Fig. 9a, c and e). In addition, theratio of 4-methylxanthone to the sum of 4-methyl-

xanthone and 3-methylxanthone (Fig. 9d) as well as theratio of the sum of 1- and 4-methylxanthone to the sumof 2- and 3-methylxanthone (Fig. 9f) fits well with the

biodegradation parameter. All these plots indicate a bet-ter preservation of 1-methylxanthone relative to the othermethylxanthones and to a lesser extent a better preserva-

tion of 4-methylxanthone relative to 3-methylxanthone,which results in the good correlation of the (1+4)/(2+3)-methylxanthone ratio with the phytane to n-octadecane ratio. Only the 4/(4+2) ratio of the two

compounds which have partition coefficients close toeach other (see Fig. 6) shows no correlation withincreasing biodegradation (Fig. 9b).

These results may indicate an effect of partitioningprocesses as described above, since biodegraded oilsmust have been exposed to water. As the non-shieldedisomers have a higher affinity to water, these com-

pounds are more bioavailable than the shielded isomers.Alternatively, preferential biodegradation of particularmethylxanthones could influence the distribution chan-

ges of the methylxanthones as illustrated in Fig. 10.Many aerobic bacteria prefer initial dioxygenolyticattack at positions 3 and 4 or 1 and 9a (see Appendix) of

xanthone, as described by Tomasek and Crawford(1986) and Wilkes (1993). Regioselective dioxygenationof biaryl ether structures is known to prompt cleavage

of ether bonds which corresponds to an initial oxyge-nation in position 4 and 4a for xanthones (Fortnagel etal., 1989; Wittich et al., 1992). These proposed xanthonecatabolic pathways of aerobic bacterial degradation

indicate that methyl groups in positions 1 and 4 mightprovide a sterical hindrance for enzymatic attackaround the oxygen functionalities. Initial dioxygenation

in positions 2 and 3 are not known, which supports theassumption that the (1+4)/(2+3)-methylxanthone ratioshould increase with increasing biodegradation. Possible

mechanisms of anaerobic xanthone metabolism havenot been described so far. The possible fate of xan-thones during crude oil biodegradation in anoxic reser-

voirs therefore remains an open question.In summary, two different mechanisms may influence

the distribution of methylxanthones in the same direc-tion: 1. water washing, which causes preferential

removal of 2- and 3-methylxanthone from the oil, or 2.biodegradation, which may result in a preferentialdegradation of 2- and 3-methylxanthone due to sterical

hindrance of 1-and 4-methylxanthone.

3.4. Possible origins of xanthones

We can speculate that xanthones may originate viaseveral possible mechanisms, including the diagenetictransformation of biogenic xanthones, pseudo Friedel-

Crafts reactions in carrier systems (geosynthesis) andthe oxidation of xanthenes either in sub-surface envir-onments or as laboratory artefacts. In the following, the

relevance of these possibilities is briefly discussed in thelight of our current observations on the presence orabsence of xanthones in different petroleum systems. A

qualitative overview is given in Table 1; detailed data forstudy areas other than the North Sea will be publishedelsewhere.

We have not detected xanthones in either sourcerocks or crude oils from the Sonda de Campeche, Mex-ico. As described by Santamaria-Orozco et al. (1998),this petroleum system comprises brecciated carbonate

reservoirs (Paleocene) fed from local generative carbon-ate source rocks (Tithonian) via faults. Xanthones couldnot be detected in similar carbonate petroleum systems

Fig. 8. Plot showing a possible influence of secondary migration

on the methylxanthone distribution for oils from Haltenbanken

(arrows illustrate possible incorrect migration distances resulting

in higher and lower methylxanthone ratios, respectively).


of the Gulf of Suez, Egypt (M. M. Y. Bakr, H. Wilkes,

unpublished results). In the same regard, it is note-worthy that recovered rock extracts and expelled oilsfrom hydrous pyrolysis experiments with an immaturelimestone from the Ghareb Formation do not contain

xanthones. All these observations are consistent with alack of potential precursors (terrestrial plants) and theoccurrence of migration pathways where water–rock–oil

interactions are small because migration distances are

short and permeability is ostensibly high.In contrast to the carbonate systems above, potential

source rocks of the Jurassic Khatatba Formation (Wes-tern Desert, Egypt) contain a significant contribution of

terrestrial organic matter. Xanthones are abundant (M.M. Y. Bakr, H. Wilkes, unpublished results). Our data-base for humic coals from England (Westphalian,

Fig. 9. Cross plots showing the influence of biodegradation on the methylxanthone distribution, using the degradation sequence from

the Gullfaks field.


Northumberland Basin) and Russia (Vise, MoscowBasin) supports the correlation between terrestrialorganic matter and the occurrence of xanthones, but

only in the rank range Ro=0.5–1.0%; outside of thisrange xanthones are absent (A. Armstroff, H. Wilkes, B.Horsfield, unpublished results). This may indicate that

xanthones are released or generated during coalificationat maturities between 0.5 and 1.0% vitrinite reflectancefollowed by destruction at higher temperatures and

pressures which could explain why xanthones could notbe detected in the very immature and very mature coalysamples from the Are Formation, offshore Norway. In

contrast, based on results for the Posidonia Shale (L.Jurassic, Lower Saxony Basin, Germany), xanthones areabsent over a similarly wide maturity range (Ro=0.48 –1.45%) from organic-rich clastic source rocks in which

the organic matter is derived primarily from algal pre-cursors. Our observations therefore support the assump-tion that xanthones may be generated as diagenetic

products from functionalised biogenic xanthones whichlose their functional groups. As described above highlysubstituted xanthones occur in many different terrestrial

plants.Our current database for Norwegian petroleum sys-

tems points to the occurrence of high concentrations of

xanthones in crude oils and an essential absence insource rocks. Were the analysis of an expanded (statis-tically relevant) dataset for source rocks to substantiate

this inference, then the formation of xanthones afterpetroleum expulsion, rather than being directly inher-ited from terrestrial precursors, has to be given serious

consideration. We speculate that the xanthones mightoriginate by geosynthesis from suitable aromatic pre-cursors in carrier systems. As described above for thepreparative synthesis of alkylated xanthones, these

compounds can be synthesized at high temperatures(T=180–200 �C) and in the presence of catalysts frombenzoic acid derivatives and phenols, compounds which

Fig. 10. Different proposed initial reactions of the xanthone catabolic pathway of aerobic bacteria described by Tomasek and Craw-

ford (1986) and Wilkes (1993) as well as for dibenzo-p-dioxin (Fortnagel et al., 1989; Wittich et al., 1992).

Table 1

Occurrence of xanthones in different study areas

Study area Source rock type SRa Oils

Offshore Norway Clastic (+) +

Posidonia Shale, Germany Marl – n.a.

Sonda de Campeche Carbonate – –

Ghareb Formation, Jordan Limestone – –

Western Desert, Egypt Clastic + +

Gulf of Suez, Egypt Carbonate n.a. –

Carboniferous coals, northern England Coals + n.a.

Moscow Basin, Russia Coals – n.a.

a SR, source rocks; +, present; (+), present in low amounts; –, absent; n.a., not available.


occur widely in sedimentary organic matter. Formationwater, which is more available in carrier rocks than insource rocks exerts a beneficial influence on the condi-tions for xanthone generation (e.g. lower pH for intra-

molecular Friedel–Crafts acylation). The mechanism forgenerating xanthones in subsurface environments wouldthen have the following essential steps:

1. generation and expulsion of precursor benzoicacids and phenols in oil phase;

2. partition of the precursors into the aqueousphase in the carrier system;

3. reaction of precursors in the aqueous phase, espe-

cially at clay mineral surfaces, to form xanthones;4. partition back into the oil phase.

Xanthones, therefore, show great promise as indica-

tors of the degree of organic–inorganic interactions incarrier systems.Following the procedures of Bennett and Larter

(2000) on fluorenes/fluorenones, we have consideredwhether xanthones may simply result from the oxida-tion of xanthenes. Window-sill experiments (oil filled in a

white glass was exposed to the sun and air over 6 months)showed that significant changes in the concentration ofxanthone and its derivatives, as well as changes in the

isomer distribution of alkylated xanthones did not occur.Furthermore the results of screening analysis on the oilsfrom offshore Norway showed that xanthene is absentor present in very low amounts while alkylxanthenes are

generally absent. These observations suggest either a fastand complete oxidation of xanthenes in petroleum sys-tems, or that xanthenes are not the precursors of xan-

thones in crude oils.

4. Conclusions

This is the first description of xanthones in crude oils.Xanthone was identified by comparison with an

authentic standard, based on the mass spectrum andrelative retention times on two GC columns of differentpolarity. The elution order of the four methylxanthones

was established for the first time as 1–4–2–3 by using aBPX-5 column. This elution order reflects the shieldingeffect of the alkylgroups on the polar groups, the keto

and the ether functions. Our results show that parti-tioning processes are a crucial control on the distribu-tion of the methylxanthones. Molecular dynamics

calculations reveal better preservation of the shieldedisomers (1- and to a lesser extent 4-methylxanthone) incomparison to the non-shielded isomers (2- and 3-methylxanthone) in the oil phase. We suggest that the

(1+4)/(2+3)-methylxanthone ratio might be useful asan indicator of secondary migration and biodegrada-tion. As the non-shielded isomers have a higher affinity

for the water phase, these compounds are more bioavail-able than the shielded isomers. In addition, proposedxanthone dioxygenation mechanisms in aerobic bacteriasuggest that the non-shielded isomers are probably pre-

ferentially biodegraded in comparison to the shielded iso-mers, due to sterical hindrance from the methyl groupsaround the oxygen functionalities in the shielded isomers.

The origin of xanthones in crude oils and source rocks isnot known; they could be diagenetic products, formed byoxidation of xanthenes in the reservoir or may originate

by geosynthesis from aromatic precursors.

Acknowledgements

We thank Norsk Hydro A.S., Norway, for financialsupport and providing the crude oils and source rocks.

We are grateful to A. W. Frahm from the PharmacyDepartment of the University of Freiburg for providingsamples of the alkylated xanthone standards. We thank

M. Lewan (USGS) and D. Santamaria (PEMEX) forproviding the sample sets from Jordan and Mexico,respectively. The technical staff of the IGC-4 is gratefully

acknowledged. This paper benefitted from the criticalreviews of B. Bennett and S. George.

Appendix

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Xanthones—novel aromatic oxygen-containing compounds in crude oils