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Qualitative and quantitative analysis of dibenzofuran, alkyldibenzofurans, and benzo[b]naphthofurans in crude oils and source rock extracts Meijun Li, and Geoffrey S. Ellis Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef502558a • Publication Date (Web): 10 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015
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Qualitative and quantitative analysis of dibenzofuran, alkyldibenzofurans,
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and benzo[b]naphthofurans in crude oils and source rock extracts
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Meijun Li∗, and Geoffrey S. Ellis
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‡
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§
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University of Petroleum, Beijing 102249, China
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‡
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USA
State Key Laboratory of Petroleum Resources and Prospecting, College of Geosciences, China
Central Energy Resources Science Center, U.S. Geological Survey, Denver, Colorado 80225,
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ABSTRACT: Dibenzofuran (DBF), its alkylated homologues and benzo[b]naphthofurans
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(BNFs) are common oxygen-heterocyclic aromatic compounds in crude oils and source rock
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extracts. A series of positional isomers of alkyldibenzofuran and benzo[b]naphthofuran were
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identified in mass chromatograms by comparison with internal standards and standard retention
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indices. The response factors of dibenzofuran in relation to internal standards were obtained by
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gas-chromatography mass-spectrometry analyses of a set of mixed solutions with different
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concentration ratios. Perdeuterated dibenzofuran and dibenzothiophene are optimal internal
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standards for quantitative analyses of furan compounds in crude oils and source rock extracts.
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The average concentration of the total DBFs in oils derived from the siliciclastic lacustrine rock
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extracts is 518 µg/g in the Beibuwan Basin, South China Sea, which is about five times that
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observed in the oils from carbonate source rocks in the Tarim Basin, Northwest China. The BNFs
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occur ubiquitously in source rock extracts and related oils of various origins. This paper suggests
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that the relative abundance of benzo[b]naphthofuran isomers, namely, benzo[b]naphtho[2,1-
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d]furan/{benzo[b]naphtho[2,1-d]furan
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BN21F/(BN21F + BN12F) ratio may be a potential molecular geochemical parameter to indicate
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oil migration pathways and distances.
+
benzo[b]naphtho[1,2-d]furan}
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INTRODUCTION
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Dibenzofuran (DBF), its alkylated homologs, and benzo[b]naphthofurans (BNFs) (Figure 1), are
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important oxygen-heterocyclic aromatic compounds in oils, coals, sediment extracts, and tar
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deposits. The presence of DBF in bituminous coals has been recognized . Dibenzofuran is one of
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the most ecotoxic heterocyclics (HETs) . Due to its comparatively high aqueous solubility, low
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octanol–water partition coefficient, high toxicity, and relatively long and persistent plumes, a
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number of key HETs including dibenzofuran, methyldibenzofurans, and dimethyldibenzofurans
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are recommended for the assessment of coal-tar contaminated aquifers . Dibenzofuran and
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benzo[b]naphtho[2,3-d]furan (BN23F) can react with chlorine atoms to yield polychlorinated
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dibenzofurans (PCDFs) during the combustion of municipal solid waste in incinerators . DBF and
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other heterocyclic aromatic compounds are also observed in combustion flue gases of lignite ,
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which can cause environment problems .
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1
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2
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Dibenzofuran,
its
alkylated
homologues
(collectively
abbreviated
as
DBFs)
and
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benzo[b]naphthofurans, have also been applied as important molecular markers in organic
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geochemistry. The occurrence and distribution of DBFs are mainly dependent on source rock
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types and/or depositional environments . DBFs seem to prevail in freshwater source rocks,
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terrestrial oils, and coals
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(DBTs) are more abundant in marine shales and carbonates
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dibenzothiophene to alkyl dibenzofuran (ADBT/ADBF) has been proposed to distinguish
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depositional environments
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concentrations of DBFs in petroleum are also affected by secondary migration processes . Thus,
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the DBFs in petroleum are potential molecular indices to indicate petroleum migration distances
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and filling pathways .
5-7
5-6,8-10
, whereas their sulfur-heterocyclic counterparts dibenzothiophenes
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10-12
. The relative abundance of alkyl
. More recently, researchers also observed that the absolute 9
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Four methyldibenzofuran isomers (MDBFs), i.e., 4-, 2-, 3- and 1-MDBF, were identified by
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co-injection of internal standards in gas chromatography–mass spectrometry (GC–MS) analyses
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of aromatic fractions of crude oils and rock extracts . The standard retention indices for seven
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dimethyldibenzofuran (DMDBF) isomers have also been reported . A total of eleven peaks were
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observed in the m/z 196 mass chromatograms that potentially correspond to nine DMDBF and
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two ethyldibenzofuran (EDBF) isomers . However, the specific molecular structures, i.e., the
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positions of the ethyl group or the two methyl groups for each isomer and the distribution patterns
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of DMDBF isomers in crude oils and rock extracts have not yet been reported.
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The origin of DBFs in oil and sedimentary organic matter remains a controversial issue.
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Previous studies suggest that DBFs may originate from dehydrated and condensed
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polysaccharides
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and biphenyl in Permian rocks (East Greenland) probably derived from phenolic compounds of
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lignin of the woody plants . The majority of natural products related to dibenzofuran are
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metabolites of lichens or of higher fungi. The lichen dibenzofurans appear to be formed by
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carbon-carbon oxidative coupling of orsellinic acid and its homologues . Thus, Radke et al.
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(2000) proposed that DBFs in crude oil and sediment extracts are potential biomarkers for
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lichens . However, further work is needed to confirm the precursor-product relationship between
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lichens and DBFs.
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or by the oxidative coupling of phenols . High abundances of DBF, DBT,
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Simulation experiments and geological observations showed that biphenyl and oxygen can
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form dibenzofuran . Similarly, methyl-substituted biphenyls can react to yield corresponding
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methylated DBFs. The derivatives of biphenyl are commonly used as reactants to synthesize
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dibenzofurans in the laboratory . More geological evidence and laboratory experiments are still
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needed to fully understand the origin and evolution of DBFs in the geosphere.
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Quantification of DBFs in oils and rock extracts are typically carried out by comparison of
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peak areas in mass chromatograms of selected ions in GC–MS with those of authentic standards.
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The commonly used internal standards in previous studies were squalane , 1,1’-binaphthyl , 1-
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phenlynaphthalene and 2-phenylnaphthalene . Some perdeuterated aromatic compounds, e.g.,
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phenanthrene-d10
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quantitative analyses of DBFs, PAHs, and DBTs.
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and dibenzothiophene-d8 , are also common internal standards for
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The use of internal standards can yield more accurate results compared to the external
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approach. The best internal standards are chemically similar to the target compound because any
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loss of the target compound during the experimental procedure is replicated by the loss of internal
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standard, making this a self-correcting system . Isotopically labeled versions of the target
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compounds are ideally suited for this purpose, and deuterated dibenzofuran (DBF-d8) is the
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compound of choice for an internal standard for quantitative analysis of DBFs in crude oils and
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rock extracts.
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In this paper, we identify the DBF, MDBFs, DMDBFs, EDBFs and BNFs in crude oils and
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rock extracts by comparison with co-injected internal standards in GC–MS analyses and
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correlation with standard retention indices reported in the literature. Furthermore, we report the
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response factors for DBFs and BNFs to deuterium-labeled internal standards DBF and DBT in
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GC–MS analyses. We also compared the distribution patterns of BNFs in source rocks and oils
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derived from typical marine carbonate, marine shale, lacustrine shale, and fluvial/deltaic source
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facies.
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EXPERIMENTAL
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Rock samples were ground in a crusher to
