Geochemical characteristics and genesis of crude oils from Gasikule oilfield in Western Qaidam Basin, China

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1 Geochemical Journal, Vol. 43, pp. 293 to 304, 2009 Geochemical characteristics and genesis of crude oils from Gasikule oilfield in Western Qaidam Basin, China YI DUAN,* JIANGONG WANG, BAOXIANG WU, CHAOYANG ZHENG, WENXIU YU and GUODONG ZHENG Key Laboratory of Gas Geochemistry, Institute of Geology and Geophysics, Chinese Academy of Sciences, 382 West Donggang Road, Lanzhou , China (Received July 26, 2007; Accepted February 26, 2009) The Gasikule oilfield, including a deep layer and a shallow layer N 2 pool, is the largest one in the Qaidam Basin and its crude oil output accounts for 80% of total oil output in the basin. In order to understand the genetic mechanism of crude oils from this oilfield, crude oils from both pools in the Gasikule oilfield were collected and analyzed using gas chromatography and gas chromatography-mass spectrometry. The geochemical characteristics of biomarkers in the crude oils were synthetically studied. The biomarker compositions indicate that the crude oils were derived from strong anoxic and saline-hypersaline lacustrine environment with a dominance of algal organic matter. It was also observed that the ancient salinity of the sedimentary environments of the source rocks for the N 2 crude oils was slightly higher than that for the crude oils. The crude oils were derived mostly from bacteria whereas the N 2 crude oils originated mostly from plankton. The maturity parameters of biomarkers reflect that the crude oils were low mature. The maturity of the crude oils was slightly higher than that of the N 2 crude oils. These results, together with research of crude oil formation, show that the N 2 and crude oils were yielded from free biological lipids, extractable organic matters and kerogen in the Tertiary source rocks deposited under an anoxic and saline-hypersaline environment at the stage of low maturity. Keywords: Qaidam basin, Gasikule oilfield, pools, crude oils, origin INTRODUCTION Qaidam Basin, located in the northeastern corner of the Tibetan plateau in northwest China, is a Tertiary sedimentary basin of an interior saline lake (Fig. ). During the Cenozoic Era, the climate of the Qaidam Basin area was dry and water supply was quite limited to the lake, resulting in the formation and development of saline and hypersaline lakes (Huang et al., 993). The dry palaeoclimate and the high salinity of water had a profound effect on the abundance, nature and preservation of organic matters in the sediments. Lacustrine mudstones, marls and calcareous mudstones of the Oligocene and lower Pliocene Series deposited in a deep lacustrine environment were widespread in the western Qaidam Basin, which have been considered as the effective source rocks for the western Qaidam Basin (Ritts et al., 999; Philp et al., 99; Huang et al., 99; Hanson et al., 200; Fig. 2). These source rocks have an average TOC content of about 0.56%, type II kerogen of low maturity (Qinghai Petroleum Administration, unpublished data). On the other hand, oil and gas reservoirs occur from the Oligocene to Pliocene Series in the western Qaidam Basin (Fig. 2). *Corresponding author ( duany@lzb.ac.cn) Copyright 2009 by The Geochemical Society of Japan. The Qaidam Basin is an important Cenozoic petroliferous basin in western China, in which some twelve oilfields so far have been discovered including the Gasikule oilfield as the largest one with a deep layer and a shallow layer N 2 pool. The crude oil outputs account for 80% of the total oil output in the basin. The Gasikule oilfield is located at a latency uplifting from the bedrock. The pool structure is a integrate latency anticline with a short axes extending from north to south, with two reverse faults (No. III and No. XI) and two normal faults (No. 46 and No. 46) distributed in its rim (Fig. 3a). The N 2 pool exhibits a tectonic characteristic that the southern oil pool on the south of number II fault is an integrate anticline with a long axes extending from north to south and the northern oil pool is a structural nose with an axes extending from the east to west. The reservoir rocks comprise fine-grained sandstones with the porosity and permeability of 0~25% and 0~ µm 2, respectively. The N 2 reservoir rocks are middle-grained sandstones with the porosity and permeability of 5~28% and more than µm 2, respectively (Qinghai Petroleum Administration, unpublished data). Traps of the two oil pools are controlled by the structure and lithology. However, the mechanism of hydrocarbons accumulation for this oilfield has not been well known. In this paper, the crude oils from 5 oil wells in the Gasikule 293

2 oilfield were analyzed using gas chromatography and gas chromatography-mass spectrometry. The geochemical characteristics of biomarkers in the crude oils were synthetically studied and the genetic mechanism of crude oils was discussed in detail. SAMPLES AND METHODS The crude oil samples were collected from the Fig.. Studied area showing the location of the Gasikule oilfield. I, Gasi fault block; II, Mangya sag. Gasikule oilfield in the Western Qaidam Basin (Figs. and 3). Most of them were obtained from these oil wells exploited in recent years without any geochemical analysis of oils previously. The existence of abundant n-alkanes (Fig. 4) and the lack of C-0 demethylated hopanes (Seifert and Moldowan, 978; Trendel et al., 990) in all the samples studied show the crude oils with less and without any biodegradation. The asphaltenes were removed from the oils by precipitation with n-hexane followed by filtration. The deasphalted crude oils were further fractioned by column chromatography on silica gel and Al 2 O 3. The saturated hydrocarbons, aromatic hydrocarbons and non-hydrocarbons were obtained by elution with n-hexane, benzene and ethanol, respectively. The saturated hydrocarbons were analyzed with a 6890N GC/5973N mass spectrometry (GC-MS) equipped with a HP-5 capillary column (30 m 0.32 mm i.d., 0.25 µm film thickness). The GC oven temperatures are programmed from 80 to 300 C at 4 C min, and then maintained at this temperature for 30 min. Helium was used as carrier gas. MS conditions were EI ionization at 70 ev with an ion source temperature at 250 C. The GC-MS system was operated in the full scan mode and scanned from m/z 20 to m/z 750. The stable carbon isotope compositions of crude oils and rocks (from which inorganic carbon was removed) were determined after the combustion of samples in quartz Fig. 2. Tertiary stratigraphy, deposition systems, potential source rocks and reservoir intervals, and source rock parameters of the western Qaidam Basin. MP, Mean C 29 sterane 20S/(20R + 20S) ratio; Ro, Vitrinite reflectance in the Oligocene source rocks of Lucan well. further divided into two subsections from bottom to top ( to 2 ). 294 Y. Duan et al.

3 tubes. The tubes were opened in a vacuum line (2 0 3 torr) and the water was removed by trapping at 70 C. The produced CO 2 was collected in a sampling ampoule at 96 C and transferred directly to the inlet system of a Finnigan MAT 252 mass spectrometer for carbon isotope ratio determination. Carbon isotope values are expressed relative to PDB carbonate (0 ). RESULTS AND DISCUSSION Bulk characteristics of crude oils The gravities of the crude oils from the Gasikule oilfield are from to g/cm 3 for the oil pool and to g/cm 3 for the N 2 pool, with a mean of and g/cm 3, respectively (Table ). These values are lower than those of the crude oils from most of other low mature oilfields throughout China (0.8500~ g/cm 3 ; Wang et al., 995). One possible explanation of this phenomenon is that the crude oils from the two pools are without biodegradation and from source rocks deposited under anoxic and saline-hypersaline environment. The viscosities of the crude oils range from 0.49 to 8.87 mpa s for the oil pool and from.09 to 8.40 mpa s for the N 2 one, averaging at 5.0 and 4.0 mpa s, respectively. These data show that the two oil pools have similar density and viscosity. However, the salinity and sulfur contents in the crude oils from the oil pool are obviously lower than those from the N 2 oil pool. As the crude oils from the two oil pools are low matured as discussed in the following text, they contain relatively low concentration of saturated hydrocarbon and high concentration of non hydrocarbon plus bitumen. Fig. 3. The (a) (b) pool structures showing the sampling locations. Property and formation environment of crude oil parent material The n-alkane chromatograms of the crude oils from the oil pools are very similar and show n- alkane distributions in the range of C ~C 39 with the maximum around C 7 to C 22. The nc 2 /nc 2+ ratios are more than one (Table 2, Fig. 4). Generally, algae and bacteria contain dominant short chain n-alkanes (Han and Calvin, 969). Therefore, the distributions of n-alkanes suggest the crude oils originated principally from algal and bacterial organic matters (Han and Calvin, 969; Volkman et al., 983; Duan, 2000; Duan and Ma, 200). The steranes in the crude oils from the two oil pools are dominated by C 27 and the composition of steranes shows C 27 > C 29 > C 28 (Table 3; Figs. 4 and 5), indicating the crude oils are mainly derived from plankton (Moldowan et al., 985). The crude oils contain 4-methyl steranes and the ratio of 4-methyl steranes/regular steranes ranges from 0.08 to 0.25 for the pool and 0.0 to 0.3 for the N 2 oil pool, indicating that this ratio do not differ significantly between the two oil pools. 4-methyl steranes are usually considered to be derived from dinoflagellates (Robinson et al., 984), even though they may also exist in bacteria (Philp et al., 99). The presence of abundant 4-methyl steranes indicates an important contribution from dinoflagellates to the crude oils. The relative abundances of bicyclic terpanes and alkylcyclohexanes exist in small amounts and do not significantly differ in the crude oils from the two oil pools. The bicyclic terpane/hopane ratios range from 0.0 to 0.2 for the crude oils and from 0.03 to 0.20 for the N 2 crude oils (Table 4, Fig. 4). The alkylcyclohexane/hopane ratios in the crude oils are in the range of 3.79~9.92 and 2.75~8.09, respectively. These ratios in the studied samples are lower than those (20.7~63.55) in the northern Qaidam oils derived from the Jurassic source rocks that are dominated by terrigenous higher plant materials (Duan et al., 2006). This comparison suggests only a small contribution from higher land plants to the studied crude oils. In general, the contents of tricyclic terpanes and tetracyclic terpanes are closely related to the nature of the organic matters in source rocks. Tricyclic terpanes is generally believed to be derived mainly from algae and bacteria (Aquino Neto et al., 983), while terrigenous organic matter contains relatively high abundance of C 24 tetracyclic terpane (Hanson et al., 2000). As shown in Fig. 6, the studied samples are in a narrow range and contain high tricyclic terpanes and low C 24 tetracyclic terpane. These data reflect that the oils from the two oil pools have a similar source and were derived mainly from algae and bacteria. However, small difference in organic matter sources of the oils is observed between the oil pools as shown in Fig. 7. It is well known that steranes are derived mainly from plankton (Moldowan et al., 985; Geochemical characteristics and genesis of crude oils 295

4 Fig. 4. Representative chromatograms of (a) n-alkanes, (b) steranes, (c) pentacyclic terpanes, (d) 4-methyl steranes, (e) bicyclic terpanes and (f) long chain alkylcyclohexanes for the studied N 2 oils, and (g) n-alkanes, (h) steranes, (i) pentacyclic terpanes, (j) 4-methyl steranes, (k) bicyclic terpanes and (l) long chain alkylcyclohexanes for the studied oils from the Gasikule oilfield. 296 Y. Duan et al.

5 Table. Physical features and fraction components of the crude oils from Gasikule oilfield Bulk properties oil pool N 2 oil pool Range Mean value Range Mean value Gravity (g/cm 3 ) ~ ~ Viscosity (mpa s) 0.49~ ~ Solidifying point ( C) 3.5~ ~ Salinity (ppm) 300.3~ ~ Sulfur content (%) 0.6~ ~ Saturated hydrocarbon (%) 3.39~ ~ Aromatic hydrocarbon (%) 2.94~ ~ Nonhydrocarbon + Asphaltene (%) 7.55~ ~ Nonhydrocarbon/Asphaltene 6.44~ ~ Table 2. Analytical data of n-alkanes and isoprenoid alkanes in the studied crude oils Sample No. Well No. Oil pool Reservoir age Depth (m) C range C max CPI C 2 /C 2+ Pr/Ph Pr/nC 7 Ph/nC 8 C ~ C ~ C ~ C ~ C ~ C ~ C ~ C N 2 N ~ C N ~ C N ~ C N 2 76 ~ C N ~ C-50 X662 N ~ C N ~ C N ~ C 2 /C 2+, C 2 n-alkanes/>c 2 homologues; CPI, [(C i +6C i+2 + C i+4 )/(4C i+ + 4C i+3 )] ( )i+, i = C 24 ~C 34. Volkman, 986; Philp et al., 99), while hopanes originated primarily from bacteria (Ourisson et al., 979; Brault and Simoneit, 988). Figure 7A shows that the crude oils contain more hopanes and the N 2 crude oils contain more steranes, suggesting that the former is derived mostly from bacteria and the latter mainly from plankton. At the same time, Fig. 7B shows that the crude oils contain more C 28 sterane compared to the N 2 crude oils. As C 28 sterane are derived principally from algae (Volkman, 986), the crude oils are mostly from algae. This recognition is consistent with the studied results of nitrogen compounds in the crude oils from the two oil pools (Duan et al., 2004a). The compositions of nitrogen compounds indicate carbazoles in the crude oils are evidently different from those in the N 2 crude oils. The carbon isotopic ratios of the oils from these two oil pools range from 26. to 27., with an average of 26.5 (Table 5). These values are within the range of lacustrine organic matters deposited in a saline- hypersaline environment (Peters et al., 996). The crude oils from the two oil pools have a low Pr/ Ph ratio ( 0.66; Table 2). The Pr/Ph ratio is generally considered to be a guide to depositional conditions, where the values above one represent oxic conditions and values below one reflect anoxic conditions or anoxic hypersaline depositional conditions (Didyk et al., 978; ten Haven et al., 987, 988). The low Pr/Ph ratios in the crude oils indicate that these oils originated from a strong reducing sedimentary environment. Further evidence is the studied oil samples lay in the area of a reducing environment on the cross-plot of Pr/n-C 7 versus Ph/n-C 8 (Connan and Cassou, 980; Peters et al., 999; Hanson et al., 2000; Duan et al., 2008) (Fig. 8). The crude oils contain high amount of β-carotane (Table 5). The β-carotane is commonly related to anoxic and algal-rich lacustrine environments (Fu et al., 986; Hanson et al., 200), although it is also from blooms of purple halophilic bacteria (Jiang and Fowler, 986). A high con- Geochemical characteristics and genesis of crude oils 297

6 Table 3. Analytical data of steranes in the studied crude oils Sample No. Oil pool C 27 C 28 C 29 Reg.ster./ hop. Dia./ Reg.ster. 4-Meth.ster./ Reg.ster. C 29 20S/(20S + 20R) C 29 ββ/(ββ + αα) (%) (%) (%) C C C C C C C C-48 N C C C C C C C Reg. ster., Regular sterane; 4-Meth. ster., 4-Methyl sterane; Dia., Diasterane; hop, hopane. Fig. 5. Ternary diagram of sterane C 27, C 28 and C 29 compositions for the studied crude oil samples. tent of gammacerane and homohopane in source rocks or oils is generally associated with a hypersaline depositional environment (Moldowan et al., 985; Fu et al., 986; Philp et al., 99). The crude oils contain high gammacerane/c 30 hopane ratios and high relative abundance of C hopanes (Table 4), suggesting the crude oils derived from hypersaline depositional environment. Further evidence is that the crude oils are also located in the hypersaline depositional environment area in Fig. 8. However, a slight difference in formation environments of the parent materials also exists between the crude oils in Fig. 9. We have studied in detail on the carbon isotopic compositions of crude oils from the Qaidam Basin (Duan et al., 2003) and revealed that the carbon isotopic compositions of crude oils from the western Qaidam Basin were mainly controlled by the paleosalinity of the depositional environment for the source materials. It was found that the high carbon isotopic ratios corresponded with the high saline nonmarine deposition environment. Most of the crude oils have lighter carbon isotopic compositions than the N 2 crude oils (Fig. 9), indicating that the depositional paleosalinity of the oil source materials is slightly lower than the N 2 oil one. Crude oil maturity The C 29 steranes 20S/(20S + 20R) and ββ/(ββ + αα) ratios, relative content of diasteranes and Ts/Tm ratio are maturity parameters that can been used to distinguish crude oil maturation (Seifert and Moldowan, 978; Mackenzie and MacKenzie, 983; Mackenzie, 984; Hanson et al., 2000; Duan et al., 2006). Huang et al. (99) suggested that the value 0.25 of C 29 steranes 20S/(20S + 20R) and ββ/(ββ + αα) was a limit between immaturity and lower maturity and a value 0.42 was taken as the bound- 298 Y. Duan et al.

7 Table 4. Analytical data of terpanes in the studied crude oils Sample No. Oil pool C 34 + C 35 (%) Ts/Tm Gammacerrane/αβ-C 30 hopane Tricyc./ hop. Bicyc./ hop. A B C C C C C C C C-48 N C C C C C C C A, C 23 tricyclic terpane/(c 23 tricyclic terpane + C 30 hopane); B, C 24 tetracyclic terpane/(c 24 tetracyclic terpane + C 26 tricyclic terpane). Tricyc., Tricyclic terpane; Hop., Hopane; Bicyc., Bicyclic terpane. C 23 tricyclic terpane+ Fig. 6. Cross-plots of tricyclic terpane/hopane ratios vs. relative content of C 27 sterane (a) and C 24 tatracyclic terpane/(c 24 tatracyclic terpane + C 26 tricyclic terpane) ratios vs. C 23 tricyclic terpane/(c 23 tricyclic terpane + C 30 hopane) ratios (b) for the studied crude oil samples. due to the absence of clay minerals to catalyze the sterane rearrangement (Mello et al., 988; Peters and Moldowan, 993). Philp et al. (99) suggested that the amount of diasteranes in oils could decrease in highly reducing noncarbonate environments with increasing salinity. Therefore, the low diasterane/regular sterane ratios for the two oil pools indicate a lower maturity and saline-hypersaline condition. The Ts/Tm ratio is commonly used to infer oil maturation (Moldwan et al., 986), but Ts/Tm ratios are also low in carbonate source rocks (McKirdy et al., 983, 984). The studied crude oils have lower Ts/Tm ratios (0.28~0.4), again, reflecting a lower maturity and saary between lower maturity and maturity. Based on these parameters, the maturation of the studied crude oils is presented in Fig. 0 and Table 3. As C 29 steranes 20S/ (20S + 20R) and ββ/(ββ + αα) ratios in the crude oils are from 0.25 to 0.33 and from 0.26 to 0.32 (Table 3), respectively, the studied crude oils are low mature. The relative contents of diasteranes in the crude oils from the two oil pools are very low and diasterane/regular sterane ratios range from 0.07 to 0.. The ratio of diasterane/regular sterane has been generally used as a maturation index. However, previous studies showed that carbonatederived oils contain a relatively low amount of diasteranes Geochemical characteristics and genesis of crude oils 299

8 Fig. 7. Cross-plot of regular sterane/hopane ratios vs. relative content of C 27 sterane (a) and sterane C 28 /C 29 ratios vs. C 27 /C 29 ratios (b) for the studied crude oil samples. Table 5. Analytical data of alkycyclohexanes, β-carotane and carbon isotopic compositions in the studied crude oils Sample No. Oil pool C range C max CPI C 2 /C 2+ Cyclohex/ hop. β-carotane/ hop. δ 3 C ( ) C-38 3~ C-37 4~ C-36 4~ C-43 4~ C-40 4~ C-4 3~ C-42 4~ C-48 N 2 4~ C-44 5~ C-45 3~ C-49 3~ C-46 3~ C-50 3~ C-5 3~ C-47 3~ Cyclohex, Alkycyclohexanes; hop., hopane; CPI, Odd carbon alkycyclohexanes/even carbon homologues; C 2 /C 2+, C 2 alkycyclohexanes/ >C 2 homologues; 3 C value was determined by examination of whole oil. line condition. A small difference in maturity is observed between the crude oils. As shown in Fig. 0, most of the crude oils are much more mature than the N 2 crude oils. Crude oil genesis As shown in Tables 2 5 and Figs. 4 0 above, the crude oils from Gasikule oilfield have a slight even carbon predominance for n-alkanes, low Pr/Ph ratios, relatively high amounts of gammacerane, C hopanes and C 27 steranes, low C 29 steranes 20S/(20S + 20R) and ββ/ (ββ + αα) ratios and 3 C-rich carbon isotope ratios. These are consistent with those of the Tertiary source rocks in the western Qaidam Basin (Ritts et al., 999; Duan et al., 2006). For example, Pr/Ph ratios, gammacerane/c 30 αβ hopane ratios, relative contents of C 27 steranes, C 29 steranes 20S/(20S + 20R) and ββ/(ββ + αα) ratios and carbon isotope ratios for the Tertiary source rocks are 0.6~0.93, 0.3~.3, 28.2~64.2% (Zhu et al., 2003), 0.0~0.50 and 0.0~0.89 (Huang et al., 99) and 2~ 26 (Duan et al., 2006), respectively. These data indicate that Tertiary source rocks were deposited under anoxic and saline-hypersaline lacustrine environments, and have algal organic matters dominated and a lower matu- 300 Y. Duan et al.

9 rity. The consistency of the crude oils with the source rocks suggests that the crude oils studied were derived most probably from the Tertiary source rocks. We rule out the possibility of pre-tertiary source contributions to the crude oils in western Qaidam basin, because pre-tertiary lacustrine source rocks were deposited under freshwater conditions and had higher maturities (Qinghai Petroleum Administration, unpublished data). If the oils originated from pre-tertiary source rocks, they would have low concentrations of gammacerane and higher maturities. We also exclude the possibilities that highly mature light oils with quite less biomarkers were migrated from deep layers to the and N 2 reservoir rocks and then contaminated by low-maturity bitumens. An example is that the northern Qaidam N crude oils of Lenghu and Nanbaxian regions, derived from Jurassic source rocks, all the same are mature (Duan et al., 2006). The previously studies showed that the, N source rocks had different maturities and depositional palaeosalinity. Their mean steranes C 29 20S/ (20S + 20R) are 0.29, 0.26 and 0.22 (Fig. 2), respectively, and their average depositional palaeosalinity are 6wt%, 9wt% and 20~60wt% (Duan et al., 2004b, 2006), respectively. As described above, the maturity of the crude oils from the oil pool is slightly higher than that from the N 2 oil pool and the depositional paleosalinity of N 2 oil source materials is higher that of oil source materials. Compared with the maturity and depositional palaeosalinity of the, N source rocks, these data strongly suggest that the crude oils are derived mostly from the source rocks, while the N 2 crude oils Fig. 8. Cross-plot of Pr/n-C 7 ratios vs. Ph/n-C 8 ratios for the studied crude oil samples (Connan and Cassou, 980; Peters et al., 999; Hanson et al., 2000). The studied crude oil samples are in the area of reduction and hypersaline lacustrine environment. Fig. 9. Cross-plots of Pr/Ph ratios vs. δ 3 C values for the studied crude oil samples. Fig. 0. Cross-plots of Ts/Tm ratios vs. C 29 sterane 20S/(20S + 20R) ratios (a) and C 29 sterane 20S/(20S + 20R) ratios vs. ββ/(ββ + αα) ratios (b) (Huang et al., 99) for the studied crude oil samples. Geochemical characteristics and genesis of crude oils 30

10 Table 6. Mean contents of pyrrolic nitrogen compounds in the studied crude oils (Duan et al., 2004a) Oil pool C MC DMC TMC BC W,8-DMC PSNs ENs (%) (%) (%) (%) (%) (µg/g) (%) (%) (%) N C, Carbazole; MC, Methylcarbazole; DMC, Dimethylcarbazole; TMC, Trimethylcarbazole; BC, Benzocarbazole;,8-DMC,,8 Dimethylcarbazole; PSNs, Partially shielded isomers; ENs, Exposed isomers; W, Total content of carbazole compounds. are derived mostly from the N N 2 source rocks. These results are well consistent with pyrrolic nitrogen compounds in the crude oils from these two oil pools (Table 6; Duan et al., 2004a). For the oil pool, absolute concentrations of neutral nitrogen compounds decrease to both northward and southward directions of the anticline. These reflect that the charging points of crude oils in the oil pool are on the west and east sides of the anticline and the crude oils migrated from the central to north and south of the anticline, also reflect the oils derived from the source rocks in the Mangya sag and Gasi fault block. For the N 2 oil pool, absolute concentrations of neutral nitrogen compounds decrease to tectonically west and north directions. These indicate that the charging points of crude oils in the N 2 oil pool are on the tectonic east and south sides and the crude oils migrate from east to west and from south to north of the tectonics. Therefore, the Mangya sag and Gasi fault block provided sources for the crude oils of the N 2 oil pool. The mean carbazole concentration is 3.53 µg/g for the oil pool and µg/g for the N 2 oil pool (Table 2), reflecting that the crude oil migration distance of the oil pool is longer than that of the N 2 oil pool. This means that the crude oils from the oil pool were derived mostly from the upper 2 source rocks whereas the crude oils from the N 2 oil pool were derived mainly from the N N 2 source rocks with the feature of autogeny and autostorage. The generation of large amounts of hydrocarbons from the Tertiary source rocks deposited in saline-hypersaline lacustrine environments in the Qaidam Basin at the stage of low maturity was demonstrated by pryrolysis experiment (unpublished data). An 2 mudstone sample with low maturity (Ro = 0.32%) was crushed to 00 mesh, and then extracted by BMA (benzene:methanol:acetone = 70:5:5.v/v) solvent. The extracted organic matter and solid remnant material were pyrolyzed individually for 72 h in a sealed stainless steel reactor at temperatures ranging from 50 to 450 C in temperature steps of 50 C. This simulating experiment revealed that abundant liquid hydrocarbons were formed at heating temperature of 200~300 C corresponding to low maturate stage of organic matter evolution. The generation and drainage of liquid hydrocarbons reach the peak values at heating temperature of 250 C. At the same time, some important contribution of extractable organic matter from the source rocks to the formation of liquid hydrocarbons was confirmed (unpublished data). The studied results of the abundant non-hydrocarbon fraction of immature-low mature crude oils from the western Qaidam Basin show that the non-hydrocarbons are composed primarily of fatty acids, alkanols, fatty acid glycerol monoesters, amide and sterols (Duan et al., 2004b), indicating the crude oils derived mainly from biological lipids. The distribution of n- alkanes in the studied crude oils exhibits an obviously even carbon predominance and a maximum at C 6 to C 22. Such distribution of n-alkanes are similar to those of fatty acid, alkanols, fatty acid glycerol monoesters and amide in the crude oils from the Gasikule oilfield reported previously (Duan et al., 2004b). This suggests that n-alkanes in the crude oils were derived probably from the reduced products of these compounds. Thus, the crude oils from the 2 oil pools were formed possibly by evaporating and reducing free biological lipids, thermal translating of extractable organic matters that were survived without bonding to kerogen and thermal cracking of kerogen in the Tertiary source rocks deposited under saline to hypersaline environments at the stage of low maturity. CONCLUSIONS The geochemical characteristics of crude oils from the pools in Gaskule oilfield were synthetically studied for a better understanding of their organic matter-formation environments, matrix types, maturity and genesis. The studied results show that n-alkanes in the crude oils have the obvious even-odd predominance, low Pr/Ph ratios and certain amount of β-carotane. All these geochemical parameters indicate that the oils originated from strong anoxic environments. High abundance of gammacerane and C hopanes in the crude oils reflect that they were derived from the potential source rocks deposited under saline to hypersaline lacustrine environments. It was observed that the ancient salinity of the sedimentary environment for the source rocks generating the N 2 crude oils was slightly higher than that forming the 302 Y. Duan et al.

11 crude oils. The crude oils contain high abundance of C 27 steranes, regular steranes, 4-methyl steranes and long chain tricyclic terpanes, and low abundance of bicyclic sesquiterpanes and alkylcyclohexanes, showing that the crude oils were mainly from bacteria and algae. However, the crude oils were mostly from bacteria where the N 2 crude oils were derived mostly from plankton. Low C 29 sterane 20S/(20S + 20R) and ββ/(ββ + αα) ratios, Ts/Tm ratios and low abundance of rearranged steranes in the crude oils show their low maturity. It was found that the maturity of the crude oils was slightly higher than that of the N 2 crude oils. All these results show that the N 2 and crude oils originated from free biological lipids, extractable organic matters and kerogen in the Tertiary source rocks deposited under a salinehypersaline environment at the stage of low maturity. Acknowledgments This study was supported by the National Natural Science Foundation of China (Grant No , ) and the 973 Program of China (Grant No. 2005CB42205). We are grateful to Yoshikazu Sampei and Jordi Tritlla Cambra for their reviews which significantly improved the manuscript. G. D. Zheng acknowledges the financial support from Chinese Academy of Sciences as 00-Tallent program to perform research work in REFERENCES Aquino Neto, F. R., Trendel, J. M., Restle, A., Connan, J. and Albrecht, P. (983) Occurrence and formation of tricyclic and tetracyclic terpanes in sediments and petroleum. Advances in Organic Geochemistry 98 (Bjorøy, M. et al., eds.), , Wiley, Chichester. Brault, M. and Simoneit, B. R. T. (988) Steroid and triterpenoid distributions in Bransfield Strait sediments: Hydrothermally-enhanced diagenetic transformations. Org. Geochem. 3, Connan, J. and Cassou, A. M. 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