Nitrogen isotopic geochemical characteristics of crude oils in several basins of China

Transcription

1 Science in China Ser. D Earth Sciences 2005 Vol.48 No Nitrogen isotopic geochemical characteristics of crude oils in several basins of China CHEN Chuanping 1,2, MEI Bowen 1 & CAO Yacheng 3 1. Jianghan Petroleum University, Jingzhou , China; 2. Huazhong University of Science and Technology, Wuhan , China; 3. Nanjing Institute of Soil Science, Chinese Academy of Sciences, Nanjing , China Correspondence should be addressed to Chen Chuanping ( chchuanp2002@etang.com) Received May 12, 2003 Abstract Nitrogen isotopic ratios of crude oils with different physical and chemical properties in several basins of China have been measured and their geochemistry has been studied elementally combined with molecular organic geochemistry methods. The data indicate that δ 15 N is in the range of They vary with the oils in different sedimentary types of basins. The oils forming in the limnic environment have lower δ 15 N with the value of about 1-5 whereas the oils in saline or semi-saline environment have higher δ 15 N, with most samples being above 10 and some exceeding 17. The average nitrogen isotopic ratio of oils from the Ordovician formation in the Tarim Basin is the lowest, with most samples having δ 15 N below zero, which reflects possibly the characteristic of the oils originating mainly from the organic matter formed in marine carbonate sedimentary environment. The δ 15 N distribution can be altered by fractionation. It increases obviously because of biodegradation and decreases during hydrocarbon migration. However, the effect of nitrogen isotopic fractionation in the short distance migration is unconspicuous. Keywords: nitrogen isotope, crude oil, sedimentary environment, isotopic fractionation, China. DOI: /03yd0180 Nitrogen occupies a high content in crust and in atmospheric circle. It is one of the main elements in organism and an important element in sedimentary circle. Although nitrogen is little in crude oil, to a certain degree, it influences the physical and chemical properties of oil, such as viscosity and density [1]. In reservoir the nitrogen-bearing compounds can form ion bonds or hydrogen bonds with substances on rock and form van der Vaals force among moleculae so they affect and alter the surface wettability of rock [2], which has to be considered in oil exploration, development, storage and transportation. Moreover, nitro- gen-bearing compounds are received attention in oil refinement, because they poison catalyzer and influence the quality of products directly [3]. Much work has been done to analyze and identify the molecular structures of organic nitrogen-bearing compounds in sedimentary matter and oil [4]. In petroleum exploration, researchers have used the parameters of nitrogen-bearing molecular geochemistry to trace the path and direction of hydrocarbon migration successfully for more than one decade [3,5,6]. However, geochemistry of nitrogen isotope has not Copyright by Science in China Press 2005

2 1212 Science in China Ser. D Earth Sciences been received much attention for a long time [7,8], especially in petroleum exploration. In China, no work on nitrogen isotope in oil and sedimentary organic matter has been reported yet except few researches in natural gas [9,10]. The situation changed somewhat in recent years. Talbot (2001) studied several nitrogen isotopic fractionations of sedimentary organic matter during formation and diagenism in some lakes and combined other researchers work [11]. He deemed that fractionation might be affected by the growth rate of organism or other environmentally determined factors so sedimentary organic matter in some lakes might contain N derived from more than one reservoir with different isotopic compositions. Having measured the nitrogen isotopic ratios of kerogen, bitumen, formation water samples, a few of oils and organic and inorganic nitrogen-bearing compounds in rock from Fordoche field in America, Williams et al. (1995) studied the relationship between digenesis and fluid migration [12]. The work started the study of geochemistry of nitrogen isotope in reservoir. Reservoir geochemistry is a new borderline subject in petroleum science developed quickly in the 1990s and has demonstrated a great applied importance in petroleum exploration and development [13]. It will be of momentous significance both in science and in application to probe the mechanism of nitrogen isotopic fractionation in oil formation, alteration and during migration, ascertain the orientation of hydrocarbon source, trace the path of migration and predict the property variation of oil in biodegradation with the technology and method of nitrogen isotopic geochemistry combined with nitrogen-bearing molecular geochemistry. In the paper we measured the oil nitrogen isotopic ratios of several basins of China and studied their distribution characteristics combining with molecular organic geochemistry method elementally. 1 Analytical method Crude oil contains very little nitrogen but very much hydrocarbon, therefore, few isotopic analyses were successful [12]. This situation has become a bottle neck of the study in oil nitrogen isotopic geochemistry. Having compared and tested existing methods, we select Kjeldahl s method to prepare nitrogen isotopic samples. The analytical data show a satisfying reproducibility and exactitude [14]. We have used the data to evaluate sedimentary environment [15] and trace the path of hydrocarbon migration [16] elementally. The results are consistent with those educed by molecular organic geochemistry methods in principle. Oil is heated in strong sulfate acid. The organic nitrogen-bearing compounds in oil are decomposed and the nitrogen is transformed into ammonium stayed in the acid solution. Nitrogen gas gives out by adding NaBrO into the solution and then goes into MS instrument. The isotopic ratio is measured and calculated under the criterion of atmospheric N 2. The method and experimental steps can be seen in ref. [14]. The MS instrument in the paper is Finnigan-MAT251 made in USA. 2 δ 15 N distribution δ 15 N values of the oil and kerogen measured are in the range of 6-20 and no sample having δ 15 N >20 has been found. They are close to δ 15 N of organic matter in soils of China [17]. The part above zero of the δ 15 N falls into the range of sedimentary organic matter(6) and oil(7) in refs. [7, 9] (see Fig. 1). It is notable that sedimentary organic matter and oils in refs. [7, 9] have no δ 15 N below zero. Fig. 1. δ 15 N distributions of natural substances (from Zhang, 1988). 1, Pyrogenic rock; 2, volcanic gas; 3, organism (a, plant, b, animal); 4, mudstone; 5, coal; 6, sedimentary organic matter; 7, crude oil; 8, natural gas; midline, atmospheric N 2.

3 Nitrogen isotopic geochemical characteristics of crude oils in several basins of China Nitrogen isotopic characteristics in different basins The nitrogen isotopic ratio distribution varies with different basins in China. But δ 15 N of the oil forming in the similar environment is very convergent and close to that of relevant kerogen. The distribution can be divided into three types mainly based on the range of δ 15 N: limnic, basic saline to high salinity and marine carbonate sedimentary environments. 3.1 Terrestrial limnic and saline-semi-saline environments A set of thick dingy mudstone layer rich in organic matter developed in western sag of Liaohe oil field, in northeast of China, which was in Sahejie4 and Sahejie3 Formations mainly. Geological and organic geochemical study 1) on two hydrocarbon source rocks has made it clear that Sahejie4 Formation deposited in a semi-saline to saline environment of semi-isolated arc lake or shallow lake but Sahejie3 Formation in the limnic environment mainly. The nitrogen isotopic ratios of normal oils and their relevant kerogens are listed in Table 1. The oils and kerogens in Table 1 can be divided into two groups based on their δ 15 N, which is consistent with previous results. The fact that oil δ 15 N is close to relevant kerogen δ 15 N suggests that oil and relevant source rock can be correlated according to nitrogen isotopic ratio. δ 15 N values of the kerogen and oils forming in the limnic environment in Sahejie3 Formation are lower with δ 15 N of about 3-5. Whereas the values of the samples in saline or semi-saline environment in Sahejie4 Formation are higher, with most being above 10 and some even exceeding 15. The distribution is similar to the characteristic of the late Pleistocene sedimentary organic matter with high δ 15 N in Bosumtwi Lake, Ghana, West Africa (δ 15 N > 10, with the maximum approaching to 17 ). Sedimentological and mineralogical evidence indicated that the lake used to be a shallow and alkaline tropical environment [11]. Two kinds of distributions in Liaohe oil field represent those in most basins in Eastern China. For example, δ 15 N values of 14 oils forming in the limnic environment in Shahejie Formation of Shengli oil field [18] are within , with the average being 5.08 ; and δ 15 N values of 7 oils in Minghuazheng and Guantao Formations of Dagang oil field average However, the values of most oils and bitumens forming in the typical saline lake environment in Jianghan field [19] and Jiangsu field are above 10, with the highest δ 15 N being The reason causing the δ 15 N difference in two environments may be that saline environment is favorable for bacteria s denitrification. An anaerobic environment can form in the bottom of high saline water system easily. The bacteria transform NO 3 in water into N 2 under the anaerobic environment so 15 NO 3 is enriched in saline water [20,21]. Talbot (2001) thought that the combination of sediments rich in organic matter and high ph water would have favoured the pro- Table 1 δ 15 N values of two types of oils and their kerogens in Liaohe Basin Sample Well Depth/m Formation Environment δ 15 N( ) Kerogen Lei Es 3 limnic 2.75 Oil Len Es Oil Len Es Oil Len Es Kerogen Lei Es 4 basic mild-salinized 7.50 Oil Lei Es 4 lake Oil Lei Es Oil Lei Es ) Mei Bowen et al., Study on the cause and characteristics of heavy crude in Lengdong zone of western sag, Liaohe Basin, 1994.

4 1214 Science in China Ser. D Earth Sciences duction of abundant ammonia and its subsequent loss through volatilization. The latter involves a strong fractionation effect and would lead to the development of lake water with a very high δ 15 N [11]. 3.2 δ 15 N in different zones of Tarim Basin The Tarim Basin has a vast area. There are diversified formations in the basin which spanned different geological ages and deposited in various environments, such as terrestrial limnic or lacustrine environments in the foreland basin in the Kuqa zone, marine carbonate sedimentary environment in Tazhong and Lunnan zones; even Paleozoic carbonate formation exists in the bottom of Kuqa sag [22]. Because the geological structure in the Tarim Basin is very complicated various physical and chemical interactions could occur among the oils and gases in different zones and different formations, which makes it very difficult to study whether with molecular geochemistry or with isotopic geochemistry. The sources of oils in different formations and different zones have been studied for many years by geologists and geochemists who seem to have reached a basic identity of views [23]. δ 15 N values of several kerogens in the Tarim Basin are shown in Table 2. Table 2 δ 15 N values of several kerogens in Tarim Basin Well Fang1 TZ23 TZ10 LX2 Yinan2 Formation E O C T J δ 15 N( ) From Table 2 we can find that there is a trend of nitrogen isotopic ratio varying with geological age. The older the age is, the lighter the δ 15 N is. Especially the source rock of Cambrian has δ 15 N below zero. The values of oils are in Table 3. They are close to δ 15 N of corresponding kerogen in age roughly and the trend varying with age is similar to that of the kerogen. Moreover, the distribution of δ 15 N varies with different zones. The average δ 15 N in Tazhong is the lowest compared with those of most oils in other zones and in other basins in China; δ 15 N in Lunnan is higher appreciably but it is still below zero on the whole, which reflects possibly the characteristic of the oil originating mainly from the organic matter in marine carbonate sediment two oils in Dawanqi with δ 15 N 1.76 and 1.32 respectively and an oil in T101 well with δ 15 N 2.79, they are from coal measure strata; the δ 15 N falls into the range of coal ( 3-7 ) in ref. [7] and Fig. 1(5). The oils from lacustrine source rock in Tertiary reservoir in the Yaha structural zone have a dispersed distribution. The maximum δ 15 N is and minimum 2.77, with a difference being The average δ 15 N is higher than the values of most normal oils in other zones of the basin obviously. Zhou (2003) believed that the differentiation occurred during oil and gas migration in the Yaha zone because of gravity and differential accumulation [24]. Condensate is a main composition in hydrocarbons. It occupies 78.9% of the whole reserves in equivalency of the zone whereas normal oil occupies only 21.1% of the reserves. At the same time, differentiation might result in fractionation of nitrogen isotopes [14], in which light nitrogen isotope was enriched in condensate and more heavy compositions in normal oil. It can be seen from Table 3 that δ 15 N of oil in the Tertiary reservoir in Yaha decreases gradually along YH1, YH6, YH401 (from deep to shallow). The trend is just the same as the direction of lacustrine oil migration from west to east [22], which seems to explain that the differentiation occurred in the reservoir. The problem of whether the difference between palaeo- and current atmospheric nitrogen isotopic compositions or biology in different ages which consume different nitrogen-bearing compounds in metabolism or the diversity of environments results in the lower δ 15 N of oil from marine carbonate organic matter than that of oil from organic matter deposited in terrestrial limnic and saline-semi saline water is not clear now, which needs to be studied further. 3.3 Distinguishing mixed oil from different origins It is possible to distinguish the mixed oil from different origins in reservoir based on the characteristic of nitrogen isotopic distribution varying with the types of environments. In Table 3 the isotopic ratios of two oils in different reservoirs (O, E) of the same well

5 Nitrogen isotopic geochemical characteristics of crude oils in several basins of China 1215 Table 3 Oil δ 15 N in Tarim Basin Zone Well Depth/m Formation δ 15 N ( ) Zone Well Depth/m Formation δ 15 N( ) TZ O 2.96 T K 2.79 TZ C 1.26 DW N 1.76 Tazhong TZ C 2.85 DW N 1.32 TZ O 2.36 YH O 2.77 TC O 0.47 Kuqa YH E 6.69 LN O 3.83 YH E 9.27 Lunnan LN O 1.84 YH E LN J 1.70 YH E LN T 0.64 YH E YH5 are lower than those of other oils in the same structural zone. The δ 15 N of deeper oil ( m) is 2.77, which is lower largely than that of upper oil ( m) with δ 15 N The phenomenon suggests whether there were two types of hydrocarbon sources in the zone and whether two different oils were mixed in the reservoir. Having studied the oils in Luntai block dome and Yaha structural zone with the organic geochemical method, Zhang et al. (1997) suggested that there were two directions of hydrocarbon migration in the reservoir based on the geochemical properties and heterogeneous compositions of the oil and distribution of formation water [22]. One direction was the oil with normal density which migrated from west to east and started at the southwest of Yaha structural zone; the other was condensate mixed with coalbed oil from east to west which started at the northeast of the zone. Analysis result of the oil δ 15 N is consistent with the above facts, which supports the viewpoint that two different oils were mixed in the reservoir. 4 Fractionation of nitrogen isotopes 4.1 Fractionation in oil biodegradation Natural series of oils suffering different degrees of biodegradation in Es 3 Formation of the Lengdong zone, Liaohe oil field, are selected as the example. In Table 1 the nitrogen isotopic ratio of normal oil in Es 3 is about 4.0 and the karegen is Whereas δ 15 N of heavy oil from the same source rock is much higher, most samples with δ 15 N above The cause of the heavy oil in Liaohe oil field has been studied widely. According to the parameters of molecular geochemistry 1) it is bacteria which lead to the degradation. Take two oils from the same source rock and in the same well Leng118 but in different depths as an example. Their group compositions and δ 15 N are in Table 4. The normal oil in deep reservoir ( m) has a higher content of hydrocarbon and a lower content of non-hc+asphaltene. Its δ 15 N is lower too (4.22 ). Contrarily, the hydrocarbon content of biodegraded oil in the shallow reservoir ( m) decreases and its non-hc+asphaltene increases; its δ 15 N goes up to The example indicates that the nitrogen isotopic ratio increased obviously when oil suffered biodegradation. Furthermore, δ 15 N increases gradually with the degree of biodegradation. In GC graphs of 3 oils in different wells (Fig. 2) the peaks of n-paraffin hydro- Table 4 Group concentrations and δ 15 N of two oils in Leng118 well Depth/m Formation Group composition (%) Sat A non-hc Asp N-HC+Asp δ 15 N( ) (shallow) Es (deep) Es Sat, Saturations; A, aromatics; non-hc, non hydrocarbons; Asp, asphaltene. 1) See footnote 1) on page 1213.

6 1216 Science in China Ser. D Earth Sciences The phenomenon that δ 15 N varies in biodegradation is supported by the nitrogen isotopic ratio of oil in Turpan-Hami field, in northwest China. The δ 15 N values of heavy oils in the Triassic reservoir and light oils in Jurassic are shown in Table 5. Organic geochemical study has indicated that heavy oils in the Triassic reservoir suffered biodegradation [25]. Average δ 15 N of heavy oils is higher over 13 than that of light oils. Fig. 2. GC graphs and δ 15 Ns of three oils in natural different biodegradation series. carbons decrease gradually in order of Leng118 (deep), Leng88 and Leng37; the distribution of n-paraffin hydrocarbons is full in Leng118 (deep) normal oil but it disappears nearly in Leng37 oil degraded gravely, which indicates that the degradation degree of three oils boosts up gradually. Three oils have δ 15 N 4.22, 8.64 and respectively. Corresponding to the variation of their chemical compositions the δ 15 N increases with the degree of biodegradation. The mechanism of nitrogen isotopic fractionation caused by biodegradation is very complicated. Usually it is believed that biodegradation is caused by aerobic bacteria especially in the shallower reservoir [1]. The rate of biodegradation in the reservoir is fast under the aerobic condition. Bacteria need abundant nitrogen to synthesize proteins and other body substances in metabolism. They make use of nitrogen-bearing compounds in water mainly because there is an adequate nitrogen source in water from ground. The organic nitrogen-bearing compounds in remains of dead bacteria and produced in metabolism come into oil directly or become a part of oil after a series of alterations [26]. However, when degradation goes along to some extent a layer of asphaltene is produced on the interface between water and oil which baffles the supply of nitrogen in water for bacteria. If there is no sufficient nitrogen to metabolize normally the bacteria switch to make use of nitrogen in oil, which will change the original structures and compositions of nitrogen-bearing compounds in oil and cause the fractionation of nitrogen isotopes. The chemical reaction rate is different for reactants with different N-isotopes. In general, the reaction rate of a molecular containing lighter N-isotope is faster because light isotope has a higher zero energy so breaking its chemical bond needs less energy [7]. Therefore, bacteria prefer to use the compounds with light nitrogen isotope in metabolism [11], which causes the enrichment of the compounds with heavy nitrogen isotope in oil. Table 5 Oil δ 15 N in Turpan-Hami oil field Sample Yudong1 Yudong4 Yudong201 Yu1 Lu2 Aishen1 Ha2 Ha3 Tai1 Pubei1 Formation T T T T T T J J J J Property H-oil H-oil H-oil H-oil H-oil H-oil L-oil L-oil L-oil L-oil δ 15 N( ) H, Heavy; L, light.

7 Nitrogen isotopic geochemical characteristics of crude oils in several basins of China Fractionation during hydrocarbon migration Fractionation of the compositions of nitrogenbearing compounds with different structures can occur along the path of hydrocarbon migration. The geochemical parameters of nitrogen-bearing moleculae in oil have been used to trace the direction and distance of fluid migration in reservoir [3]. The fractionation of the compounds with different nitrogen isotopes will occur simultaneously. Chen and Mei (2002) [16] selected Lunnan14 well in the Tarim Basin and the Lianhua reservoir of the Liaohe Basin as two examples of hydrocarbon migration in vertical and horizontal directions respectively to probe the effect and mechanism of isotopic fractionation. δ 15 N values of 5 oils in different reservoirs of Lunnan14 well decrease gradually along the vertical direction from 0.24 at bottom (C-O, m) to 2.95 in top (T, m) in thickness 753 m; wax content in oil decreases and the ratio of gas to oil (G/O) increases 1) in the same thickness (see Fig. 3, Tables 6 and 7). The variation of δ 15 N accords with that of G/O and the content of wax, which suggests that the migration occurred vertically. In the Lianhua reservoir two organic nitrogenbearing molecular parameters of oil (1) BCa-[a]/[c] and (2)1-EtCa/ca used to evaluate migration [2,3] both exhibit an increasing trend from south to north in the horizontal section, which indicates the direction of hydrocarbon migration. δ 15 N in the same section apart about 10 km decreases from 12.8 to gradually. The varying trend of δ 15 N accords with that of organic nitrogen-bearing molecular parameters. Two examples above demonstrate that the fractionation of nitrogen isotopes will occur during hydrocarbon migration accompanied with the fractionation of nitrogen-bearing moleculae. The varying trend is that δ 15 N decreases along the path of migration. Though the oils in the same basin have similar nitrogen isotopic compositions, the fractionation will Fig. 3. Reservoir section of Lunnan14 well in S-N (from Huang et al., 1995). Table 6 G/O and wax in different depths Depth/m Gas/oil (m 3 /m 3 ) Wax (%) Table 7 δ 15 N of oils in different depths Depth/m Reservoir formation δ 15 N( ) T C-T C C C-O 0.24 alter the range of δ 15 N distribution. Biodegradation often causes an increase of δ 15 N obviously. However, the effect of isotopic fractionation in the short distance migration is not unconspicuous. 1) Huang Difan, Formation and occurrence of the petroleum in Tarim Basin, 1995.

8 1218 Science in China Ser. D Earth Sciences 5 Conclusions (1) The δ 15 N values of oils from several basins in China are in the range of (2) The δ 15 N distribution varies with different sedimentary types of basins. Oils forming in the limnic environment have lower δ 15 N with the ratio of about 1 5, whereas the oils from the saline or semi-saline environment have higher δ 15 N, with most samples being above 10, even some exceeding 17. Average δ 15 N in the Ordovician Formation of the Tarim Basin is the lowest, with most samples having the δ 15 N below zero, which reflects probably the characteristic of the oil from marine organic matter in carbonate rock mainly. (3) The distribution range of δ 15 N is affected by fractionation. It increases obviously because of biodegradation whereas decreases during hydrocarbon migration. However, the effect of nitrogen isotopic fractionation in the short distance migration is unconspicuous. Acknowledgements The work was supported by the National Natural Science Foundation of China (Grant No ). All nitrogen isotopic ratios of samples in the paper were measured in Nanjing Institute of Soil Science, the Chinese Academy of Sciences. Particular thanks to Dr. Zhang Shuichang, Dr. Zhou Fengying in Beijing Petroleum Exploration and Development Science Institute and Dr. Bao Jianping, Dr. Zhu Yangming in Jianghan Petroleum University for supplying part of samples and discussing the work. References 1. Tissot, B. P., Welte, D. H., Petroleum Formation and Occurrence (2nd ed.), Berlin: Springer-Verlag, Larter, S. R., Aplin, A. C., Reservoir geochemistry: Methods, applications and opportunities, in The Geochemistry of Reservoirs (eds. Cubtt, J. M., England, W. A.), London: The Geological Society, 1995, Li, M., Larter, S. R., Stoddart, D. et al., Fractionation of pyrrolic nitrogen compounds in petroleum during migration: derivation of migration-related geochemical parameters, in The Geochemistry of Reservoirs (eds. Cubtt, J. M., England, W. A.), London: The Geological Society, 1995, Wilhelms, A., Patience, R. L., Larter, S. et al., Nitrogen fractionality distributions in asphaltenes isolated from several oils from different source rock types, Geochim. Cosmochim. Acta, 1992, 56: [DOI] 5. Maowen, L., Huanxin, Y., Stasiuk, L. D. et al., Effect of maturity and petroleum expulsion on pyrrolic nitrogen compound yields and distributions in Duvernay Formation petroleum source rocks in central Alberta, Canada, Org. Geochem., 1997, 26(11-12): [DOI] 6. Wang, T. G., Zu, D., Lu, H. et al., High molecular weight (C + 35 ) n-alkanes of high-waxy condensate and its source kitchen orientation in the Qianmiqiao burial-hill zone, Bohai Gulf Basin, Science in China, Series D, 2004, 47(3): Zheng, Y. F., Chen, J. F., Geochemistry of Stable Isotopes, Beijing: Science Press, Ader, M., Boudou, J. P., Javoy, M. et al., Isotope study on organic nitrogen of Westphalian anthracites from the Western Middle field of Pennsylvania (USA) and from the Bramsche Massif (Germany), Org. Geochem., 1998, 29(1-3): Zhang, Z. S., Geology and geochemistry of nitrogen in gas reservoir, Geology Geochemistry (in Chinese), 1988, (2): Zhu, Y. N., Shi, B. Q., Analysis on origin of molecular nitrogen in natural gases and their geochemical features, Geology Geochemistry (in Chinese), 1998, 28: Talbot, M., Nitrogen isotopes in palaeolimnolgy, in Tracking Environmental Change Using Lake Sediments, Physical and Geochemical Methods, Volume 2 (eds. Last, W. M., Smol, J. P.), Dordrecht: Kluwer Academic Publishers, 2001, Williams, L. B., Ferrell, Jr. R. E., Hutcheon, I. et al., Nitrogen isotope geochemistry of organic matter and minerals during diagenesis and hydrocarbon migration, Geochim. Cosmochim. Acta, 1995, 59: [DOI] 13. Cubitt, J. M., England, W. A., Geochemistry of reservoirs: and introduction. in The Geochemistry of Reservoirs (eds. Cubtt, J. M., England, W. A.), London: The Geological Society, 1995, Chen, C. P., Cao, Y. C., Mei, B. W., Preparing for the simple on nitrogen isotopic in crude oil and analyzing its δ 15 N ratio, Chinese Journal of Analytical Chemistry (in Chinese), 2002, 30(5): Chen, C. P., Mei, B. W., Characteristics of nitrogen isotope in different depositional environments in China, Oil and GAS Geology (in Chinese), 2001, 22(3): Chen, C. P., Mei, B. W., Nitrogen isotopic fractionation of crude oil during hydrocarbon secondary migration, Geochimica (in Chinese), 2002, 31(2): Zhu, Z. L., Wen, Q. X., Nitrogen in Soils of China (in Chinese), Nanjing: Jiangsu Science and Technology Press, Wu, J. C., Wu, S. H., Yin, W., 3-D modeling of sedimentary microfacies of Minghuazhen formation of Neogene in Gangxi development area in Huanghua depression, Journal of Palaeogeography (in Chinese), 2002, 4(4): Zhang, Y. S., Yang, Y. Q., Qi, Z. X. et al., Sedimentary characteristics and environments of the salt-bearing series of Qianjiang formation of the Paleogene in Qianjiang sag of Jianghan basin, Journal of Palaeogeography (in Chinese), 2003, 5(1): Han, G. X., Li, N. H., Huang, J. H., An Introduction to Biogeo-

9 Nitrogen isotopic geochemical characteristics of crude oils in several basins of China 1219 chemistry (in Chinese), Beijing: China Higher Education Press; Heideberg: Springer-Verlag, Xiao, H, Y., Liu, C. Q., Wang, S. L., Organic degradation by nitrification and denitrification before diagenesis in the Hongfeng lake, SW China, Geochemica (in Chinese), 2003, 32(4): Zhang, M., Lin, R. Z., Mei, B, W., Reservoir Geochemistry- Approach to Kuqa Petroleum Systems of Tarim Basin, China (in Chinese), Chongqing: Chongqing University Press, Wang, F. Y., Bian, L. Z., Zhang, S. C. et al., Two types of hydrocarbon precursors in Ordorician marine source rock in Tarim Basin, Science in China, Series D (in Chinese), 2001, 31(2): Zhou, X. X., Differentiation process inside oil and gas fields: Example from the Yaha oil and gas condensate field in the Tarim Basin, Geological Review (in Chinese), 2003, 49(5): Zhao, W. Z., Li, W., Zhang, Y., A preliminary study of the forming process of Lukqun heavy oil pool and its exploratory significance to Turpan-Hami Basin, Petroleum Exploration and Development (in Chinese), 1998, 25(2): Chen, C. P., Mei, B. W., Xiong, T. et al., Experimental research on the causation ofcrude oil δ 15 N variation in biodegradation, Geochimica (in Chinese), 2004, 33(1):