Carbon isotopic composition of individual
n
-alkanes in
asphaltene pyrolysates of biodegraded crude oils from the
Liaohe Basin, China
Yongqiang Xiong, Ansong Geng *
The State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Wushan, Guangzhou 510640, PR China
Abstract
Biodegraded oils are widely distributed in the Liaohe basin, China. In order to develop eective oil-source correla-tion tools speci®cally for the biodegraded oils, carbon isotopic composicorrela-tions of individualn-alkanes from crude oils and their asphaltene pyrolysates have been determined using the gas chromatography±isotope ratio mass spectrometry technique. No signi®cant fractionation in the stable carbon isotopic ratios of n-alkanes in the pyrolysates of oil asphaltenes was found for anhydrous pyrolysis carried out at temperatures below 340C. This suggests that the stable
carbon isotopic distribution ofn-alkanes (particularly in the C16±C29range) in the asphaltene pyrolysates can be used
as a correlation tool for severely biodegraded oils from the Liaohe Basin. Comparison of then-alkane isotopic com-positions of the oils with those of asphaltene pyrolysates shows that this is a viable method for the dierentiation of organic facies variation and post-generation alterations.#2000 Elsevier Science Ltd. All rights reserved.
Keywords:GC±IRMS; Biodegraded oil; Asphaltene pyrolysis;n-alkane;d13C; Oil correlation; Liaohe basin
1. Introduction
The Liaohe basin is one of the most important Cen-ozoic sedimentary basins in NE China. Since the ®rst oil discovery from the Xinglongtai ®eld in 1975, the Liaohe basin has become the third largest oil-producing pro-vince in China, with 22 oil®elds in production. Previous studies have shown that the fourth (Es4) and third (Es3) members of the Eocene-Oligocene Shahejie formation are the main source rocks in this basin (Editorial Com-mittee for Petroleum Geology of China, 1993). In addi-tion to normal gravity oils, heavy to extra-heavy oils (<10API) are produced in the Western Depression of
the Liaohe basin. It was suggested that these heavy oils were formed by biodegradation of originally normal gravity oils (e.g. Huang et al., 1991). Because biode-gradation signi®cantly altered the chemical
composi-tions of the oils, it is very dicult to ascertain oil-source relationships for these biodegraded oils using conven-tional ``®ngerprinting'' techniques involving hopanes and steranes.
Gas chromatography±isotope ratio mass spectro-metry (GC±IRMS) has been widely applied in the various ®elds of organic geochemistry, e.g. identifying organic source (Freeman et al., 1990; Hayes et al., 1990; Rieley et al., 1991), correlating oil with possible source rocks (Bjorùy et al., 1991, 1994), and reconstructing paleoenvironment and paleoclimate (Hayes et al., 1990; Schoell et al., 1994; Ruble et al., 1994). Thed13C values
ofn-alkanes reported in slightly biodegraded oils are not signi®cantly dierent from those of the related, non-biodegraded oils, indicating that isotopic fractionation of these compounds during biodegradation is not a cause for serious concerns (Bakel et al., 1993; Boreham et al., 1995). Therefore, the GC±IRMS technique can be used to make oil±oil and oil±source correlation, and to elucidate the origin of source organic matter for slightly biodegraded oils. For severely biodegraded oils, how-ever, the application of this technique is limited due to
0146-6380/00/$ - see front matter#2000 Elsevier Science Ltd. All rights reserved. P I I : S 0 1 4 6 - 6 3 8 0 ( 0 0 ) 0 0 0 8 3 - 8
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* Corresponding author. Fax: +86-85290706.
the analytical diculties related to the low abundance of
n-alkanes and acyclic isoprenoids.
Asphaltenes are ubiquitous components of crude oils, particularly in high abundance in biodegraded oils because of the preferential removal of hydrocarbon components. Statistically, asphaltene pyrolysates show chemical compositions somewhat similar to those of kerogens in associated source rocks (Behar et al., 1984; Pelet et al., 1986). Numerous studies indicate that biodegradation has much less impact on the composi-tion of asphaltenes than on the hydrocarbons (Rubin-stein et al., 1977, 1979; Cassani and Eglinton, 1986). As asphaltenes are generally formed before or during hydrocarbon generation, the formation `locked' therein is more representative of the source rock characters than the free hydrocarbons in the oil which are more suscep-tible to secondary alteration processes (Rubinstein et al., 1979). As pyrolysis of the asphaltenes can potentially recover much of the important geochemical informa-tion, it has been increasingly used in oil±oil and oil± source rock correlation studies, especially for biodegraded oils (Rubinstein et al., 1979; Behar et al., 1984; Cassani and Eglinton, 1986). Among others, useful information has been previously presented by Wilhelms et al. (1994) who compared the isotopic composition of n-alkanes and isoprenoids alkanes in oils and associated asphaltene pyrolysates derived from dierent types of source rocks. Oil±source correlation for heavy and extra-heavy oils has been one of the most dicult geochemical problems in the Liaohe basin. The purpose of this study was to investigate the carbon isotopic composition of indivi-dualn-alkanes from asphaltene pyrolysates, in order to develop correlation techniques for the biodegraded oils in the Liaohe basin.
2. Experimental
2.1. Samples
The oil®eld locations in the Liaohe basin are shown in Fig. 1. Based on the oil biodegradation sequence estab-lished previously (Huang et al., 1991), seven heavy oil samples with dierent levels of biodegradation and two normal gravity oils were selected in this study. Basic geochemical parameters of these oil samples are sum-marized in Tables 1 and 2.
2.2. Methods
The asphaltene fractions were separated and puri®ed (Huang et al., 1991) using a modi®ed method originally used by Cassani and Eglinton (1986). Asphaltenes were precipitated out of the oils by adding excessive petro-leum ether (b.p. 30±60C). After the removal of
asphal-tenes, the remaining oils were separated into saturate
and aromatic hydrocarbons and a polar NSO fraction by column chromatography using a silica gel±alumina column. Saturated hydrocarbon fractions, obtained by elution with petroleum ether, were then analyzed by gas chromatography±¯ame ionization detection (GC±FID), gas chromatography±mass spectrometry (GC±MS) and GC±IRMS. The asphaltenes were puri®ed, placed into Pyrex glass ampoules (10015 mm i.d.), and then sealed
under vacuum after repeated ¯ushings with nitrogen and evacuation. The sealed glass ampoules were heated in a mue furnace at 280, 300, 320 (or 330), 340 and 360C for 72 h respectively. After cooling, the ampoules
were opened, and then extracted with dichloromethane. The extractable pyrolysates were subjected to column chromatography, and the saturate fractions obtained were then analyzed by GC±FID, GC±MS and GC± IRMS.
GC±FID analyses were performed on an HP 5880A gas chromatography ®tted with a 25 m0.32 mm i.d.
OV-1 fused silica capillary column. Helium was used as the carrier gas. The temperature program used was iso-thermal for 2 min at 60C, programmed at 4C minÿ1to
290C, and then isothermal at 290C for 20 min.
GC±MS analyses were conducted with a Platform II mass spectrometer. The GC was ®tted with a 50 m0.32
mm i.d. fused silica HP5 column. The GC oven was held isothermally for 5 min at 60C, programmed from 60 to
120C at 8C minÿ1 and from 120 to 300C at 2C
minÿ1, with a ®nal holding time of 30 min. The
ion-source temperature was 150C, and the instrument was
operated in the electron impact (EI) mode with an elec-tron energy ofÿ70 ev.
GC±IRMS analyses were performed in a VG Iso-chrom II instrument. The GC was ®tted with a 50 m0.32 mm i.d. non-polar fused silica AT5 column.
Helium (12 psi) was used as the carrier gas. The GC was
held isothermally for 5 min at 70C, programmed from
70 to 290C at 3C minÿ1and then held isothermally for
40 min at 290C. The combustion furnace was run at
850C. Carbon isotope ratios for individual alkanes
were calculated using CO2 as a reference gas that was
automatically introduced into the IRMS at the beginning and end of each analysis, and the data were reported in per mil (%) relative to the PDB standard. A
standard mixture of n-alkanes (nC12±nC32) and
iso-prenoid alkanes (provided by Malvin Bjorùy) with known isotopic composition were used daily to test the performance of the instrument. Replicate analyses of
this mixture show that the standard deviation for each compound is less than 0.3%.
3. Results and discussion
3.1. Oil classi®cation and biodegradation ranking
Based on results of GC±FID and GC±MS analyses, relative ranking in the extent of biodegradation for each of the oils included in this study were assigned (Table 1), according to the scale reported in Peters and Moldowan Table 1
General data of the oil samples from the Liaohe basin
Well Oil®eld Depth (m) Stratum Biodegradation
Ranka Characteristic
Leng-35 Lengjiabao 3249±3323 Es3 0 Non-biodegradation Tuo-31-35 Niuxingtuo ± ± 0 Non-biodegradation Tuo-5 Niuxingtuo ± ± 1 n-Alkanes slightly
degradaded Gao-1-6 Gaosheng 1328 ± 2 General depleted of
n-alkanes
Du-57 Shuguang 1347±1396 Es4 2 General depleted of n-alkanes
Huan-618 Shuguang 1817±1826 Es4 2 General depleted of n-alkanes
Leng-80 Lengjiabao 1383±1483 ± 3b Only tracts of n-alkanes remain, Streranes partly degraded Qi-40 Huanxiling ± Es3 6 Steranes partly
degraded Leng-38 Lengjiabao ± ± 6 Steranes partly
degraded
a Based on Peters and Moldowan (1993).
b The rank of biodegradation is determined based on tracts of n-alkanes remaining. Details see text.
Table 2
Biomarker parameters of the oils from the Liaohe basin
Sample Pr/Ph Pr/nC17 Ph/nC18 OEPa Ts/ (Ts+Tm)
C31Hopanes 22S/(22S+22R)
C29
Steranes 20S/ (20S+20R)
C29Steranes
bb/(aa+bb)
Gammacerane /C30abHopane
Leng-35b 1.19 0.70 0.83 1.14 0.44 0.63 0.32 0.34 <0.01 Tuo-31-35 0.87 0.95 1.33 1.10 0.27 0.54 0.27 0.23 0.13 Tuo-5 0.81 1.04 1.79 1.16 ± ± ± ± ± Gao-1-6 0.50 0.56 2.09 1.11 0.27 0.50 0.28 0.23 0.26 Du-57 0.75 2.11 4.24 1.13 0.28 0.58 0.34 0.27 0.24 Leng-80 0.75 1.65 4.07 ± 0.32 0.56 0.36 0.34 0.14 Huan-618b 0.76 5.10 7.90 ± 0.20 0.60 0.23 0.23 0.29 Qi-40 ± ± ± ± 0.44 0.57 0.64 0.51 0.24
a OEP=[(Ci+6Ci
(1993). In the later discussion, slightly biodegraded oils refer to the oils with biodegradation rank 1±3, whereas severely biodegraded oils refer to oils with rank 4 or higher.
The Leng-80 oil shows contradicting geochemical signatures in terms of its relative ranking in biode-gradation. Based on the presence of trace amounts ofn -alkanes, the level of biodegradation of this oil corre-sponds to rank 3. However, the clear alteration in the distribution of regular steranes in this oil would suggest a higher rank (6). This inconsistency was most likely
caused by the mixing of a late arrived, less or non-bio-degraded oil, with an early implaced, severely biode-graded oil.
With the exception of the Leng-35 oil, the sterane and hopane distributions are almost identical for the non-biodegraded and slightly non-biodegraded oils. These data, together with available geological evidence, suggest that these oils were derived from a single source kitchen. Although steranes are clearly aected by mild biode-gradation for the Leng-80, Qi-40 and Leng-38 oils, the close similarity in the hopane distributions indicates that these oils were from a common source. Comparison of biomarker compositions in these oils with those repor-ted previously for all possible source rocks in the region (Li et al., 1995; Wang et al., personal communication) indicate that these oils were mainly derived from the Es4 member of the Shahejie Formation. In contrast, the Leng-35 oil shows many characteristics commonly observed from the Es3 source rocks, typically with
relatively high pristane/phytane ratios (>1) and low gammacerane abundance. Based on all available data, the Leng-35 oil can be considered as a typical Es3 source derived oil.
3.2. Eects of pyrolysis temperature on carbon isotopic composition of individualn-alkanes in asphaltene pyrolysates
The eects of pyrolysis temperature on the distribu-tion of biomarkers released from asphaltenes have been discussed (e.g. Cassani and Eglinton, 1986). As was shown previously (Rubinstein et al., 1979), pyrolysis at low temperature over a long period of time or at high temperature but over a short period of time could pro-duce similar biomarker distributions but repro-duced overall hydrocarbon yields. Pyrolysis at high temperature over an extended period of time often causes the signi®cant breakdown of some biomarker molecules. A detailed discussion on the sterane and hopane compositional dierences in the asphaltene pyrolysates and related oils was presented earlier (Xiong et al., in press), including the relative enrichment of 17a(H)-22,29,30-trisnorho-pane, and C29and C30moretanes in the pyrolysates.
Asphaltene isolated from the Huan-618 oil was used to test the eects of pyrolysis temperature on the carbon isotopic composition of individualn-alkanes from pyr-olysates (Fig. 2). The asphaltene was pyrolyzed for 72 h at ®ve dierent temperatures (i.e. 280, 300, 320, 340 and 360C). As shown in Table 3, the yields of total extracts
and saturate fractions initially increase with pyrolysis temperature. Once a maximum was reached at 340C,
they decrease at higher temperature possibly due to a higher rate of destruction than generation ofn-alkanes.
GC±IRMS results obtained from the saturate frac-tions of the Huan-618 oil and its asphaltene pyrolysates are shown in Fig. 3. The isotopic compositions of indi-vidualn-alkanes from the pyrolysates obtained at tem-peratures below 340C display similard13C distributions
over the n-C15±n-C28 range (ÿ29.5 to ÿ27.3%). This
suggests that pyrolysis temperature, if it is below the maximum hydrocarbon release from asphaltene, does not appear to impact strongly on the isotopic distribu-tions of individualn-alkanes in the pyrolysates. This is in sharp contrast with the pyrolysates obtained at 360C, which shows an enrichment of13C inn-alkanes
by approximately 1±2%, withd13C values ranging from
ÿ27.9 to ÿ26.5%. This apparent isotope kinetic
fractionation eect is likely caused by preferential
cracking of C12±C12 bonds of heavy hydrocarbons to
form light hydrocarbons including gaseous components (Galimov, 1985).
Asphaltene isolated from the Leng-80 oil was also pyrolyzed for 72 h at 300 and 330C, respectively, in
order to verify the temperature eect observed above. As was expected, relatively higher yield ofn-alkanes was obtained at 330C (112 mg/g asphaltene) than that
obtained at 300C (68 mg/g asphaltene) (Table 3). Gas
chromatograms of the saturate fractions of the Leng-80 oil and its asphaltene pyrolysates are presented in Fig. 4. As shown in Fig. 5, there is only a minor variation (less than 0.7%) observed in thed13C values of then-C
16and
n-C29 alkanes. However, large variation in the d13C
values (1±3%) are observed for lighter (C13±C15) and
heavier (C30±C32)n-alkanes, possibly due to interference
from co-eluting compounds. Whatever the reasons were for this discrepancy, these results indicate that it is bet-ter to use thed13C values of then-alkanes in the range of
Table 3
Yields of various fractions from pyrolysates at dierent temperatures
Sample Temperature (C) Yield, %w asphaltene
Total extract
Sat Aro NSO
Huan-618 280 15.2 2.5 3.8 8.9
300 20.6 7.9 5.3 7.4
320 23.5 13.3 4.9 5.3
340 26.5 17.4 4.3 4.8
360 14.6 9.8 2.7 2.1
Leng-80 300 26.9 6.8 4.6 15.5
330 27.6 11.2 5.2 11.2
C16±C29for geochemical applications, in order to
mini-mize the in¯uence of analytical artifacts. Therefore, 330C/72 h was adopted as the optimal pyrolysis
condi-tions in this study.
3.3. Carbon isotopic composition of individualn-alkanes from oils
The stable carbon isotopic composition of individual
n-alkanes in oils is a useful tool in oil to oil and oil to source rock correlation (Bjorùy et al., 1991). It was also found quite eective for correlating non-biodegraded oils to their prospective source rocks in the Liaohe basin (Li et al., 1995).
Fig. 6 displays the carbon isotopic compositions of individualn-alkanes in the non-biodegraded and slightly biodegraded oils from the Western Depression of the Liaohe basin. Consistent with the isotopic signatures reported previously for the Es3 and Es4 source rocks and related oils (Li et al., 1995), the Leng-35 oil clearly falls within the range characteristic of the Es3 source rocks, re¯ecting an origin from the Es3 source rocks. In contrast, the isotopic compositions of individual
n-alkanes in other four oils (i.e. Tuo-31-35, Gao-1-6, Du-57 and Tuo-5) are similar to those of the Es4 source rocks, suggesting a dierent source in the Es4 member. The isotopic compositions are in clear agreement with the molecular compositions of the steranes and hopanes. The lack of signi®cant variation in d13C values of the
C15±C29 n-alkanes observed among the four oils
indi-cates that the oils were likely derived from a rather uniformed source rock system.
Then-alkane isotopic signatures of the other two oils, Huan-618 and Leng-80 fall between the values of typical Es3 and Es4 oils (Fig. 6). Similar to the Leng-35 oil, the Fig. 4. Gas chromatograms of saturated hydrocarbon fractions
from the Leng-80 oil and its pyrolysates.
n-alkanes of Huan-618 oil have an isotopically heavy lighter-endn-alkanes and display a trend that gradually depletes the 13C with increasing carbon number. The
isotope variation between homologues of n-alkanes in the Huan-618 oil (up to 4%) is signi®cantly larger than the maximum range observed for dierent n-alkyl car-bon skeletons from a singe organism, 1.6% (Monson
and Hayes, 1982). The maturity level of the Huan-618 oil is relatively low, judging from a number of bio-marker ratios [e.g. Ts/(Ts+Tm), 20S/(20S+20R) and bb/(aa+bb) for C29 steranes]. Thus, it is possible that
the homologous n-alkanes in the Huan-618 oil were derived from dierent precursors or similar precursor living in dierent environments of deposition. The
n-C14±n-C32alkanes in the Leng-80 oil is also relatively
enriched in the13C, withd13C values being 4±6%more
positive than the Es4 sourced oils. The presence of sev-eral components that are depleted in 13C relative to
othern-alkanes (Fig. 6) may be used as evidence for the possible mixing of oils from more than one source rock, if analytical artifacts could be independently ruled out.
From the preceding discussion, we conclude that the characteristic carbon isotopic signatures of individual
n-alkanes identi®ed from non-biodegraded oils can be potentially extended to the oil±source correlation stu-dies for mildly biodegraded oils, as along as the con-centrations of n-alkanes remaining in the oils are sucient for accurate determinations of thed13C values.
Fig. 6. The carbon isotopic compositions of individualn-alkanes from the Liaohe slightly biodegraded oils.
3.4. Comparison between thed13C values of then -alkanes from the oils and their asphaltene pyrolysates
For severely biodegraded oils, e.g. Leng-38 and Qi-40, the GC±IRMS approach discussed above is of limited usefulness, due to the lack ofn-alkanes and acyclic iso-prenoid alkanes. Obviously, biodegradation has also changed the distributions of steranes that are commonly used in oil±source correlation studies. As many previous studies have indicated that asphaltene pyrolysates usually contain useful biomarker signatures that can be used for correlative purpose, we intend to suggest here that this can also be achieved isotopically.
Fig. 7 shows that the individual n-alkanes in the asphaltene pyrolysates of the Leng-80, Leng-38 and Qi-40 oils have similarn-alkanes carbon isotopic compositions. Thed13C values display limited variation in then-C
16±n
-C27alkanes, ranging fromÿ28.0 toÿ29.1%. This
sug-gests that these asphaltenes probably have rather similar sources, as pyrolysates involving diverse source inputs usually show a much wider isotopic spread. On the other hand, asphaltene pyrolysates generated from the Leng-80, Leng-38 and Qi-40 oils exhibit n-alkane iso-topic compositions similar to those of non-biodegraded or slightly biodegraded oils [e.g. Tuo-31-35, Gao-1-6 and Du-57 (Fig. 6)]. This demonstrates that these severely biodegraded oils were most likely derived from the same source as the non-biodegraded or slightly biodegraded oils. Consequently, we contend that the carbon isotopic composition of individual n-alkanes released by asphaltene pyrolysis may be very useful for oil±oil and oil±source correlation of severely biode-graded oils.
Signi®cant dierences (1±4%) in the carbon isotopic compositions of individual n-alkanes between the oil and related asphaltene pyrolysates are observed for the Huan-618 and Leng-80 oils. Although the n-alkanes in these two oils dier isotopically from the Es4 source rocks and related oils, the isotopic pro®les of their asphaltene pyrolysates indicate a close genetic link to the Es4 source rocks. This discrepancy was likely caused by post-generation alterations, such as maturity and mixing. Although increasing maturity usually leads to isotopically heavy products, the isotopic shift is gen-erally very small (Bjorùy et al., 1992; Collister et al., 1994). A variation over 2±3% suggests that these oils may have had inputs with d13C-enriched signature, in
addition to the Es4 contributions preserved in the asphaltene. After detailed examination of n-alkane isotopic compositions for all the possible source units in the region, we postulate that the additional hydro-carbons may be derived from the Es3 member of the Shahejie Formation. This suggestion is consistent with the saturate gas chromatographic data, i.e. the co-occurrence of n-alkanes with partially degraded ster-anes.
4. Conclusions
This study demonstrates that carbon isotopic sig-natures of individualn-alkanes from source rocks and related, non-biodegraded oils can be extended to oil-source rock correlation studies for slightly biodegraded oils. When GC±IRMS is coupled with o-line asphal-tene pyrolysis, this approach has signi®cant potential for typing of severely biodegraded oils. In combination with conventional biomarker compositions and sensible geological interpretations, comparison ofn-alkaned13C
values in the free oils and asphaltene pyrolysates provides data that are potentially very useful for dier-entiating organic source variation even for biodegraded oils.
Acknowledgements
This work was supported by grants from the Natural Science Foundation of China (Grant: 49972039) and the presidential fund of the Chinese Academic of Science. We are grateful to T.S. Xiang, J.Z. Liu and M. Shao for the GC±IRMS analysis. Drs. A. Wilhelms, D. Karlsen and an anonymous reviewer are also acknowledged for their useful comments and suggestions. Dr. Maowen Li of Geological Survey of Canada is thanked for improv-ing the manuscript.
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