Carbon isotopic composition of individual n-alkanes in asphaltene pyrolysates of biodegraded crude oils from the Liaohe Basin, China

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1 Organic Geochemistry 31 (2000) 1441± 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 , PR China Abstract Biodegraded oils are widely distributed in the Liaohe basin, China. In order to develop e ective oil-source correlation tools speci cally for the biodegraded oils, carbon isotopic compositions of individual n-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 340 C. This suggests that the stable carbon isotopic distribution of n-alkanes (particularly in the C 16 ±C 29 range) in the asphaltene pyrolysates can be used as a correlation tool for severely biodegraded oils from the Liaohe Basin. Comparison of the n-alkane isotopic compositions of the oils with those of asphaltene pyrolysates shows that this is a viable method for the di erentiation of organic facies variation and post-generation alterations. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: GC±IRMS; Biodegraded oil; Asphaltene pyrolysis; n-alkane; d 13 C; Oil correlation; Liaohe basin 1. Introduction * Corresponding author. Fax: addresses: xiongyq@gig.ac.cn (Y. Xiong), asgeng@gis. sti.gd.cn (A. Geng). The Liaohe basin is one of the most important Cenozoic 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 province 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 Committee for Petroleum Geology of China, 1993). In addition to normal gravity oils, heavy to extra-heavy oils (<10 API) 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 biodegradation signi cantly altered the chemical compositions of the oils, it is very di cult to ascertain oil-source relationships for these biodegraded oils using conventional `` ngerprinting'' techniques involving hopanes and steranes. Gas chromatography±isotope ratio mass spectrometry (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). The d 13 C values of n-alkanes reported in slightly biodegraded oils are not signi cantly di erent from those of the related, nonbiodegraded 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, however, the application of this technique is limited due to /00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S (00)

2 1442 Y. Xiong, A. Geng / Organic Geochemistry 31 (2000) 1441±1449 the analytical di culties 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 composition of asphaltenes than on the hydrocarbons (Rubinstein 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 susceptible to secondary alteration processes (Rubinstein et al., 1979). As pyrolysis of the asphaltenes can potentially recover much of the important geochemical information, 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 di erent types of source rocks. Oil±source correlation for heavy and extra-heavy oils has been one of the most di cult geochemical problems in the Liaohe basin. The purpose of this study was to investigate the carbon isotopic composition of individual n-alkanes from asphaltene pyrolysates, in order to develop correlation techniques for the biodegraded oils in the Liaohe basin. 2. Experimental 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 mu e furnace at 280, 300, 320 (or 330), 340 and 360 C 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 isothermal for 2 min at 60 C, programmed at 4 C min 1 to 290 C, and then isothermal at 290 C 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 60 C, programmed from 60 to 120 C at 8 C min 1 and from 120 to 300 C at 2 C min 1, with a nal holding time of 30 min. The ionsource temperature was 150 C, and the instrument was operated in the electron impact (EI) mode with an electron energy of 70 ev. GC±IRMS analyses were performed in a VG Isochrom 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 2.1. Samples The oil eld locations in the Liaohe basin are shown in Fig. 1. Based on the oil biodegradation sequence established previously (Huang et al., 1991), seven heavy oil samples with di erent levels of biodegradation and two normal gravity oils were selected in this study. Basic geochemical parameters of these oil samples are summarized in Tables 1 and 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 petroleum ether (b.p. 30±60 C). After the removal of asphaltenes, the remaining oils were separated into saturate Fig. 1. Locations of oil elds in the Liaohe basin (a) Huanxiling, (b) Shuguang, (c) Lengjiabao, (d) Gaosheng, (e) Niuxingtuo.

3 Table 1 General data of the oil samples from the Liaohe basin Y. Xiong, A. Geng / Organic Geochemistry 31 (2000) 1441± Well Oil eld Depth (m) Stratum Biodegradation Rank a Characteristic Leng-35 Lengjiabao 3249±3323 Es3 0 Non-biodegradation Tuo 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 ± 3 b 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. held isothermally for 5 min at 70 C, programmed from 70 to 290 Cat3 C min 1 and then held isothermally for 40 min at 290 C. The combustion furnace was run at 850 C. Carbon isotope ratios for individual alkanes were calculated using CO 2 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 (nc 12 ±nc 32 ) and isoprenoid 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 2 Biomarker parameters of the oils from the Liaohe basin Sample Pr/Ph Pr/nC 17 Ph/nC 18 OEP a Ts/ (Ts+Tm) C 31 Hopanes 22S/(22S+22R) C 29 Steranes 20S/ (20S+20R) C 29 Steranes bb/(aa+bb) Gammacerane /C 30 abhopane Leng-35 b <0.01 Tuo Tuo ± ± ± ± ± Gao Du Leng ± Huan-618 b ± Qi-40 ± ± ± ± a OEP=[(C i +6C i+2 +C i+4 )/4(C i+1 +C i+3 )] ( 1)i+1,i =23. b Data provided by Wang Chunjiang.

4 1444 Y. Xiong, A. Geng / Organic Geochemistry 31 (2000) 1441±1449 (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 biodegradation. Based on the presence of trace amounts of n- alkanes, the level of biodegradation of this oil corresponds 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-biodegraded oil, with an early implaced, severely biodegraded oil. With the exception of the Leng-35 oil, the sterane and hopane distributions are almost identical for the nonbiodegraded and slightly biodegraded oils. These data, together with available geological evidence, suggest that these oils were derived from a single source kitchen. Although steranes are clearly a ected by mild biodegradation 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 reported 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 E ects of pyrolysis temperature on carbon isotopic composition of individual n-alkanes in asphaltene pyrolysates The e ects of pyrolysis temperature on the distribution 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 produce similar biomarker distributions but reduced 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 di erences 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-trisnorhopane, and C 29 and C 30 moretanes in the pyrolysates. Asphaltene isolated from the Huan-618 oil was used to test the e ects of pyrolysis temperature on the carbon isotopic composition of individual n-alkanes from pyrolysates (Fig. 2). The asphaltene was pyrolyzed for 72 h at ve di erent temperatures (i.e. 280, 300, 320, 340 and 360 C). As shown in Table 3, the yields of total extracts Fig. 2. Total ion current of saturated hydrocarbons from Huan-618 oil.

5 Y. Xiong, A. Geng / Organic Geochemistry 31 (2000) 1441± Table 3 Yields of various fractions from pyrolysates at di erent temperatures Sample Temperature ( C) Yield, %w asphaltene Total extract Sat Aro NSO Huan Leng and saturate fractions initially increase with pyrolysis temperature. Once a maximum was reached at 340 C, they decrease at higher temperature possibly due to a higher rate of destruction than generation of n-alkanes. GC±IRMS results obtained from the saturate fractions of the Huan-618 oil and its asphaltene pyrolysates are shown in Fig. 3. The isotopic compositions of individual n-alkanes from the pyrolysates obtained at temperatures below 340 C display similar d 13 C distributions over the n-c 15 ±n-c 28 range ( 29.5 to 27.3%). This suggests that pyrolysis temperature, if it is below the maximum hydrocarbon release from asphaltene, does not appear to impact strongly on the isotopic distributions of individual n-alkanes in the pyrolysates. This is in sharp contrast with the pyrolysates obtained at 360 C, which shows an enrichment of 13 Cinn-alkanes by approximately 1±2%, with d 13 C values ranging from 27.9 to 26.5%. This apparent isotope kinetic fractionation e ect is likely caused by preferential cracking of C 12 ±C 12 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 330 C, respectively, in order to verify the temperature e ect observed above. As was expected, relatively higher yield of n-alkanes was obtained at 330 C (112 mg/g asphaltene) than that obtained at 300 C (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 the d 13 C values of the n-c 16 and n-c 29 alkanes. However, large variation in the d 13 C values (1±3%) are observed for lighter (C 13 ±C 15 ) and heavier (C 30 ±C 32 ) n-alkanes, possibly due to interference from co-eluting compounds. Whatever the reasons were for this discrepancy, these results indicate that it is better to use the d 13 C values of the n-alkanes in the range of Fig. 3. The carbon isotopic composition of individual n-alkanes from Huan-618 oil and its asphaltene pyrolysates at di erent temperatures.

6 1446 Y. Xiong, A. Geng / Organic Geochemistry 31 (2000) 1441±1449 C 16 ±C 29 for geochemical applications, in order to minimize the in uence of analytical artifacts. Therefore, 330 C/72 h was adopted as the optimal pyrolysis conditions in this study Carbon isotopic composition of individual n-alkanes from oils Fig. 4. Gas chromatograms of saturated hydrocarbon fractions from the Leng-80 oil and its pyrolysates. 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 e ective 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 individual n-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 di erent 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 d 13 C values of the C 15 ±C 29 n-alkanes observed among the four oils indicates that the oils were likely derived from a rather uniformed source rock system. The n-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. 5. The carbon isotopic composition of individual n-alkanes from Leng-80 pyrolysates at 300 and 330 C.

7 Y. Xiong, A. Geng / Organic Geochemistry 31 (2000) 1441± Fig. 6. The carbon isotopic compositions of individual n-alkanes from the Liaohe slightly biodegraded oils. n-alkanes of Huan-618 oil have an isotopically heavy lighter-end n-alkanes and display a trend that gradually depletes the 13 C 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 di erent n-alkyl carbon 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 biomarker ratios [e.g. Ts/(Ts+Tm), 20S/(20S+20R) and bb/(aa+bb) for C 29 steranes]. Thus, it is possible that the homologous n-alkanes in the Huan-618 oil were derived from di erent precursors or similar precursor living in di erent environments of deposition. The n-c 14 ±n-c 32 alkanes in the Leng-80 oil is also relatively enriched in the 13 C, with d 13 C values being 4±6% more positive than the Es4 sourced oils. The presence of several components that are depleted in 13 C relative to other n-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 studies for mildly biodegraded oils, as along as the concentrations of n-alkanes remaining in the oils are su cient for accurate determinations of the d 13 C values. Fig. 7. The carbon isotopic compositions of individual n-alkanes in the pyrolysates of asphaltenes from the biodegraded oils at 330 C.

8 1448 Y. Xiong, A. Geng / Organic Geochemistry 31 (2000) 1441± Comparison between the d 13 C values of the n- 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 of n-alkanes and acyclic isoprenoid 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 similar n-alkanes carbon isotopic compositions. The d 13 C values display limited variation in the n-c 16 ±n- C 27 alkanes, ranging from 28.0 to 29.1%. This suggests 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 isotopic 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 biodegraded oils. Signi cant di erences (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 di er 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 generally very small (Bjorùy et al., 1992; Collister et al., 1994). A variation over 2±3% suggests that these oils may have had inputs with d 13 C-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 hydrocarbons may be derived from the Es3 member of the Shahejie Formation. This suggestion is consistent with the saturate gas chromatographic data, i.e. the cooccurrence of n-alkanes with partially degraded steranes. 4. Conclusions This study demonstrates that carbon isotopic signatures of individual n-alkanes from source rocks and related, non-biodegraded oils can be extended to oilsource rock correlation studies for slightly biodegraded oils. When GC±IRMS is coupled with o -line asphaltene pyrolysis, this approach has signi cant potential for typing of severely biodegraded oils. In combination with conventional biomarker compositions and sensible geological interpretations, comparison of n-alkane d 13 C values in the free oils and asphaltene pyrolysates provides data that are potentially very useful for di erentiating organic source variation even for biodegraded oils. Acknowledgements This work was supported by grants from the Natural Science Foundation of China (Grant: ) and the presidential fund of the Chinese Academic of Science. 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