Note
Inadequate separation of saturate and monoaromatic
hydrocarbons in crude oils and rock extracts by
alumina column chromatography
Chunqing Jiang
a, Maowen Li
a,*, Adri C.T. van Duin
baGeological Survey of Canada, 3303-33 Street, NW Calgary, AB, Canada T2L 2A7 bNRG, University of Newcastle Upon Tyne NE1 7RU, UK
Received 8 May 2000; accepted 8 June 2000 (returned to author for revision 25 May 2000)
Abstract
When compared to the classical silica gel/alumina column chromatographic method, the neutral alumina procedure, recently validated for the isolation of organic nitrogen fractions from crude oils/rock extracts, appears to yield severely altered distributions for many classes of monoaromatic hydrocarbons in the aromatic hydrocarbon fraction. The potential analytical sequestration, if not recognized, could lead to erroneous geochemical interpretations. # 2000 Published by Elsevier Science Ltd. All rights reserved.
Keywords:Aryl isoprenoids; Alkylbenzenes; Column chromatography; Fractionation techniques
1. Introduction
Some laboratories still routinely use open-column chromatography to isolate saturated and aromatic hydrocarbon fractions from crude oils and rock extracts, although more reliable medium pressure and high pressure liquid chromatographic methods have been developed in the 1980s. The traditional fractiona-tion procedure adopted by the Geological Survey of Canada, for example, utilizes a column packed with activated silica gel/alumina, and usedn-pentane andn -pentane/dichloromethane to obtain saturated and aro-matic hydrocarbon fractions, respectively. In order to isolate undistorted polar organic nitrogen fractions for their potential use as petroleum migration tracers (Li et al., 1995; Larter et al., 1996), a neutral alumina column procedure was modi®ed and validated (Li et al., 1992), on the basis of a method originally proposed for syn-thetic fuels (Later et al., 1981). This procedure has been
adopted recently in many petroleum geochemistry laboratories. Through this procedure, de-asphaltened oils/rock extracts can be separated into saturated and aromatic hydrocarbon fractions, and a nitrogen-enriched fraction, by sequential elution with n-hexane, toluene and chloroform/methanol, respectively. It remains unresolved whether the compositions of aromatic hydrocarbon fractions obtained by the Later method are quantitatively comparable to those from the classical procedure.
Monoaromatic hydrocarbons comprise between 25 and 35% of total aromatic hydrocarbons in the C12+ fractions of crude oils and rock bitumens (Tissot and Welte, 1984). These include predominantly alkylbenzenes together with many other polycyclic compounds. Frac-tionation of monoaromatic hydrocarbons represents a major analytical challenge in open column chromato-graphy, as these compounds could enter either the saturated or aromatic hydrocarbon fractions due to their relatively low polarity. This note compares the distributions of alkylbenzenes, alkyltoluenes and aryl isoprenoid hydrocarbons in rock extracts and related oils from the Uppermost Devonian-Lower Mississippian
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* Corresponding author. Fax: +1-403-292-7159.
Bakken Formation of the Canadian Williston Basin, obtained using dierent fractionation procedures. Pre-liminary results indicate that the application of the modi®ed Later method could result in severely biased distortion of monoaromatic hydrocarbon distributions. The inadequate separation of saturated and monoaro-matic hydrocarbons, if unrecognized, could lead to erroneous geochemical interpretations.
2. Experimental
Samples used in this study were taken from the Canadian Williston Basin in southeastern Saskatch-ewan, including organic-rich black shales and related drill stem test oils from the Bakken Formation, one of the most important petroleum source rocks in the basin (Osadetz et al., 1992). Two column chromatographic procedures were used to fractionate deasphaltened oils and rock extracts. In the classical procedure (Fowler et al., 1995), a 25 mm i.d. column was packed with a mix-ture of pre-activated silica gel (28±200 mesh) and alu-mina (80±200 mesh) (1:3, w/w). n-Pentane, n-pentane/ dichloromethane (1:1,v/v) and chloroform/methanol (98:2, v/v) were used to elute saturated and aromatic hydrocarbons and polar NSO fractions. In the modi®ed Later method, the column was packed with 8 g of neu-tral alumina; n-hexane (50 ml), toluene (50 ml) and chloroform/methanol (98:2, v/v. 70 ml) were used to elute saturated and aromatic hydrocarbons and a nitro-gen-enriched fraction. A constant loading of adsorbent/ sample (100:1, w/w) and ®xed solvent volume/adsorbent ratio were used. Saturated and aromatic hydrocarbons were analyzed by gas chromatography (GC) and gas chromatography±mass spectrometry (GC/MS), using a Packard 6890 series GC coupled to a Hewlett-Packard 5973 series Mass Selective Detector. G.C con-ditions: capillary column (J&W DB-5, 30 m0.32 mm
i.d., 0.25mm ®lm thickness); split injection at 250C; He
as carrier gas; oven temperature initially at 60C (1
min), programmed at 3C/min to 300C, then
main-tained for 30 min. The mass spectrometer was operated in both full scan and selected ion monitoring modes.
3. Results and discussion
The traditional silica gel/alumina procedure was spe-ci®cally designed to ensure a relatively clear-cut fractio-nation between saturated and aromatic hydrocarbons. It is our experience that most monoaromatic hydrocarbons commonly found in crude oils and rock extracts, such as alkylbenzenes, monoaromatic steroids, D-ring aroma-tized 8,14-secohopanes and benzohopanes, are routinely fractionated into the aromatic hydrocarbon fraction. This has been con®rmed in the present study, in which
these monoaromatic components were found in negli-gible amounts in the saturated fractions for a wide range of geological samples. In contrast, these compounds occur in both saturated and aromatic hydrocarbon fractions obtained using the modi®ed Later method, usually with signi®cantly higher concentrations in the saturated fraction than in the supposedly aromatic hydrocarbon fractions. Comparable results were obtained for the polycyclic di-, tri- and tetraaromatic hydrocarbons by both of the chromatographic proce-dures, thus this note will focus on the monoaromatic hydrocarbons.
Figs. 1±3 give the summedm/z91+92, 105+106 and 133+134 mass fragmotograms showing the molecular distributions of alkylbenzenes, alkyltoluenes and aryl isoprenoid hydrocarbons in a Bakken oil from the Canadian Williston Basin, obtained using the two chro-matographic fractionation procedures. Ellis et al. (1992) reported the shape-selective separation of alkylated monoaromatic hydrocarbons using dealuminated mor-denite. It was shown thatn-alkyl benzenes, toluenes and xylenes were selectively sorbed from a pentane solution by the molecular sieve, whereas l-alkyl-2,3,6-trimethyl benzenes were largely excluded by the molecular sieve. Similar fractionation eects were observed using the Later method in the present investigation. As indicated in Figs. 1 and 2, most n-alkylbenzenes and n -alkylto-luenes went into the saturated fraction, whereas the aryl isoprenoid analogs are more signi®cantly enriched in the aromatic hydrocarbon fractions. As only the relatively high-molecular-weight n-alkylbenzenes and n -alkylto-luenes remain in the aromatic hydrocarbon fractions, they could bias the molecular distributions of these compound classes in the original geological samples.
Trimethyl aryl isoprenoids in the C13±C31range have been identi®ed previously in source rock bitumens and oils from several Paleozoic-age petroleum systems in the Western Canada and Williston Basins. The principal trimethyl aryl isoprenoids possess the 1-alkyl-2,3,6-tri-methyl substitution pattern (2,3,6-TMAI;I) character-istic of diaromatic carotenoids found in the Chloro-biaceae family of photosynthetic sulfur bacteria [e.g. isorenieratane (II) and isorenieratene (III)] and could potentially indicate proli®c bacterial growth in the photic zone under euxinic conditions (Summons and Powell, 1986,1987; Requejo et al., 1992; Hartgers et al., 1993, 1994; Koopmans et al., 1996; Cliord et al., 1998). Another important class of trimethyl aryl isoprenoids belong to the 1-alkyl-3,4,5-trimethyl series (3,4,5-TMAI:
Fig. 1. M/z91+92 mass fragmentograms showing the distributions ofn-alkylbenzenes in the saturate or aromatic hydrocarbon fractions obtained by dierent separation procedures.
diaryl analog (VI) of isorenieratene] has not been iden-ti®ed. Because of their speci®c origin, the trimethyl aryl isoprenoids appear to be useful markers for oil±oil and oil±source correlation. In the Williston Basin, for example, the Bakken shale extracts are relatively enriched in 2,3,6-TMAI and 3,4,5-TMAI (Requejo et al., 1992). In contrast, these compounds are either absent or in extremely low concentrations in the bitumens of the Mississippian Lodgepole carbonates (Jiang et al., 2000). Therefore, the trimethyl aryl isoprenoid distributions, if characterized properly, could potentially provide addi-tional information for the dierentiation of Bakken and Lodgepole derived oils in this basin.
As shown by the summedm/z 133+134 mass frag-mentograms (Fig. 3), aromatic hydrocarbon fractions obtained by the Later method gave severely biased dis-tributions for trimethyl aryl isoprenoids. The 3,4,5-TMAI isomers eluted almost entirely into the saturate fractions, whereas dominantly 2,3,6-TMAI isomers remained in the aromatic hydrocarbon fractions. As the adsorbents used for both chromatographic procedures have little or no molecular sieving capacities, the large dierence in the aryl isoprenoid distributions obtained by the two dierent chromatographic methods cannot be explained adequately by size exclusion eect, as reported by Ellis et at. (1992). Alkyl groups are not adsorbed on the adsorbents and thus often make less contribution to the adsorption energy of the molecule than aromatic nuclei or functional groups. However,
alkyl substituents are well known to alter the adsorption energies more for silica than for alumina (Lee et al., 1981). Therefore, aromatic hydrocarbons usually form a relatively narrow band in a silica chromatographic col-umn, whereas an alumina column is often superior for the separation of aromatic hydrocarbon classes accord-ing to their number of double bonds, aromaticity and alkyl substituents. Charge distributions for 2,3,6-TMAI and 3,4,5-TMAI, calculated for the energetically favourable stretched conformations using the semi-empirical MOPAC method with PM3-parameters (Dewar and Thiel, 1977; Stewart, 1989) and the Mulli-ken-method, show that the protons attached to the unmethylated aromatic carbons have relatively high partial charges. In 3,4,5-TMAI, these charged sites are shielded from interaction with solvent and absorbents by the alkyl chain, while in 2,3,6-TMAI these charged sites are more accessible for such interactions. This could potentially explain the observed partitioning behavior of these two isomers as 2,3,6-TMAI might have a higher anity for the more polar aromatic fraction.
The results of the present investigation have far reaching implications for the cost-eective management of laboratory analytical program in a petroleum geo-chemical study and for the geogeo-chemical±geological interpretations of the aromatic hydrocarbon data. This indicates a clear need for a more careful validation of the Later method before it is used routinely to replace the classical silica/alumina methods in obtaining the
saturated and aromatic hydrocarbon fractions from geological samples. For example, the 3,4,5-TMAI and 2,3,6-/3,4-5-trimethyl diaryl isoprenoids are potential chemical markers for a strain of extinct photosynthetic green sulfur bacteria (Hartgers et al., 1993, 1994; Koopmans et al., 1996). GC/MS analysis of the aro-matic hydrocarbons resulting from the Later method would severely underestimate the abundance of these compounds in the samples. Moreover, inconsistent use of chromatographic procedures in the preparation of samples makes it dicult to compare dierent analytical data sets accumulated over time.
4. Conclusions
The modi®ed Later method (Li et al., 1992) is very attractive to many petroleum geochemists, as it could produce saturated and aromatic hydrocarbon fractions and organic nitrogen-enriched fractions in a single frac-tionation step. Results of the present study demonstrate that, because this method was designed to isolate undistorted organic nitrogen fractions, it potentially
yields a severely altered distribution of monoaromatic hydrocarbons in the aromatic hydrocarbon fractions obtained. Consequently, whether it can be used adequately and cost-eectively to replace the tra-ditional silica/alumina procedure or more advanced MPLC and HPLC methods depends on the speci®c objectives of individual geochemical applications. As operating companies increasingly contract out their analytical program, geochemical data-users should be warned of the potential problems when the short-cut method is used outside an area for which it was originally developed.
Associate EditorÐA.G. Douglas
Acknowledgements
We thank Huanxin Yao, Sneh Achal, Laura Mulder and Rachel Jones for laboratory technical assistance, Drs. Martin Fowler and Lloyd Snowdon for useful dis-cussions. This is a Geological Survey of Canada Con-tribution No. 2000029.
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