293

Geochemical Journal, Vol. 43, pp. 293 to 304, 2009

*Corresponding author (e-mail: duany@lzb.ac.cn)Copyright 2009 by The Geochemical Society of Japan.

Geochemical characteristics and genesis of crude oils from Gasikule oilfieldin 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 730000, China

(Received July 26, 2007; Accepted February 26, 2009)

The Gasikule oilfield, including a deep layer E31 and a shallow layer N21 pool, is the largest one in the Qaidam Basinand its crude oil output accounts for 80% of total oil output in the basin. In order to understand the genetic mechanism ofcrude oils from this oilfield, crude oils from both E31 and N21 pools in the Gasikule oilfield were collected and analyzedusing gas chromatography and gas chromatography-mass spectrometry. The geochemical characteristics of biomarkers inthe crude oils were synthetically studied. The biomarker compositions indicate that the crude oils were derived fromstrong anoxic and saline-hypersaline lacustrine environment with a dominance of algal organic matter. It was also ob-served that the ancient salinity of the sedimentary environments of the source rocks for the N21 crude oils was slightlyhigher than that for the E31 crude oils. The E31 crude oils were derived mostly from bacteria whereas the N21 crude oilsoriginated mostly from plankton. The maturity parameters of biomarkers reflect that the crude oils were low mature. Thematurity of the E31 crude oils was slightly higher than that of the N21 crude oils. These results, together with research ofcrude oil formation, show that the N2 and E31 crude oils were yielded from free biological lipids, extractable organicmatters and kerogen in the Tertiary source rocks deposited under an anoxic and saline-hypersaline environment at thestage of low maturity.

Keywords: Qaidam basin, Gasikule oilfield, E31 and N21 pools, crude oils, origin

The Qaidam Basin is an important Cenozoicpetroliferous basin in western China, in which sometwelve oilfields so far have been discovered including theGasikule oilfield as the largest one with a deep layer E31and a shallow layer N21 pool. The crude oil outputs ac-count for 80% of the total oil output in the basin. TheGasikule oilfield is located at a latency uplifting fromthe bedrock. The E31 pool structure is a integrate latencyanticline with a short axes extending from north to south,with two reverse faults (No. III and No. XI) and two nor-mal faults (No. 146 and No. 46) distributed in its rim (Fig.3a). The N21 pool exhibits a tectonic characteristic thatthe southern oil pool on the south of number II fault is anintegrate anticline with a long axes extending from northto south and the northern oil pool is a structural nose withan axes extending from the east to west. The E31 reser-voir rocks comprise fine-grained sandstones with the po-rosity and permeability of 10~25% and 10~50 103 m2,respectively. The N21 reservoir rocks are middle-grainedsandstones with the porosity and permeability of 15~28%and more than 10 103 m2, respectively (Qinghai Pe-troleum Administration, unpublished data). Traps of thetwo oil pools are controlled by the structure and lithol-ogy. However, the mechanism of hydrocarbons accumu-lation for this oilfield has not been well known. In thispaper, the crude oils from 15 oil wells in the Gasikule

INTRODUCTION

Qaidam Basin, located in the northeastern corner ofthe Tibetan plateau in northwest China, is a Tertiary sedi-mentary basin of an interior saline lake (Fig. 1). Duringthe Cenozoic Era, the climate of the Qaidam Basin areawas dry and water supply was quite limited to the lake,resulting in the formation and development of saline andhypersaline lakes (Huang et al . , 1993). The drypalaeoclimate and the high salinity of water had a pro-found effect on the abundance, nature and preservationof organic matters in the sediments. Lacustrine mudstones,marls and calcareous mudstones of the Oligocene andlower Pliocene Series deposited in a deep lacustrine en-vironment were widespread in the western Qaidam Ba-sin, which have been considered as the effective sourcerocks for the western Qaidam Basin (Ritts et al., 1999;Philp et al., 1991; Huang et al., 1991; Hanson et al., 2001;Fig. 2). These source rocks have an average TOC contentof about 0.56%, type II kerogen of low maturity (QinghaiPetroleum Administration, unpublished data). On the otherhand, oil and gas reservoirs occur from the Oligocene toPliocene Series in the western Qaidam Basin (Fig. 2).

294 Y. Duan et al.

oilfield were analyzed using gas chromatography and gaschromatography-mass spectrometry. The geochemicalcharacteristics of biomarkers in the crude oils were syn-thetically studied and the genetic mechanism of crude oilswas discussed in detail.

SAMPLES AND METHODS

The crude oil samples were collected from the

Gasikule oilfield in the Western Qaidam Basin (Figs. 1and 3). Most of them were obtained from these oil wellsexploited in recent years without any geochemical analy-sis of oils previously. The existence of abundant n-alkanes(Fig. 4) and the lack of C-10 demethylated hopanes(Seifert and Moldowan, 1978; Trendel et al., 1990) in allthe samples studied show the crude oils with less andwithout any biodegradation.

The asphaltenes were removed from the oils by pre-cipitation with n-hexane followed by filtration. Thedeasphalted crude oils were further fractioned by columnchromatography on silica gel and Al2O3. The saturatedhydrocarbons, aromatic hydrocarbons and non-hydrocar-bons were obtained by elution with n-hexane, benzeneand ethanol, respectively. The saturated hydrocarbonswere analyzed with a 6890N GC/5973N massspectrometry (GC-MS) equipped with a HP-5 capillarycolumn (30 m 0.32 mm i.d., 0.25 m film thickness).The GC oven temperatures are programmed from 80 to300C at 4C min1, and then maintained at this tempera-ture for 30 min. Helium was used as carrier gas. MS con-ditions were EI ionization at 70 eV with an ion sourcetemperature at 250C. The GC-MS system was operatedin the full scan mode and scanned from m/z 20 to m/z750.

The stable carbon isotope compositions of crude oilsand rocks (from which inorganic carbon was removed)were determined after the combustion of samples in quartz

Fig. 1. Studied area showing the location of the Gasikule oil-field. I, Gasi fault block; II, Mangya sag.

Fig. 2. Tertiary stratigraphy, deposition systems, potential source rocks and reservoir intervals, and source rock parameters ofthe western Qaidam Basin. MP, Mean C29 sterane 20S/(20R + 20S) ratio; Ro, Vitrinite reflectance in the Oligocene source rocksof Lucan 1 well. E3 further divided into two subsections from bottom to top (E31 to E32).

Geochemical characteristics and genesis of crude oils 295

tubes. The tubes were opened in a vacuum line (2 103torr) and the water was removed by trapping at 70C.The produced CO2 was collected in a sampling ampouleat 196C and transferred directly to the inlet system of aFinnigan MAT 252 mass spectrometer for carbon isotoperatio determination. Carbon isotope values are expressedrelative to PDB carbonate (0).

RESULTS AND DISCUSSION

Bulk characteristics of crude oilsThe gravities of the crude oils from the Gasikule oil-

field are from 0.8364 to 0.8442 g/cm3 for the E31 oil pooland 0.8462 to 0.8540 g/cm3 for the N21 pool, with a meanof 0.8394 and 0.8499 g/cm3, respectively (Table 1). Thesevalues are lower than those of the crude oils from mostof other low mature oilfields throughout China(0.8500~0.9400 g/cm3; Wang et al., 1995). One possibleexplanation of this phenomenon is that the crude oils fromthe two pools are without biodegradation and from sourcerocks deposited under anoxic and saline-hypersaline en-vironment. The viscosities of the crude oils range from10.49 to 18.87 mPas for the E31 oil pool and from 11.09to 18.40 mPas for the N21 one, averaging at 15.10 and14.10 mPas, respectively. These data show that the twooil pools have similar density and viscosity. However, thesalinity and sulfur contents in the crude oils from the E31oil pool are obviously lower than those from the N21 oilpool. As the crude oils from the two oil pools are lowmatured as discussed in the following text, they containrelatively low concentration of saturated hydrocarbon andhigh concentration of non hydrocarbon plus bitumen.

Property and formation environment of crude oil parentmaterial

The n-alkane chromatograms of the crude oils fromthe E31 and N21 oil pools are very similar and show n-alkane distributions in the range of C11~C39 with the maxi-mum around C17 to C22. The nC21/nC21+ ratios are morethan one (Table 2, Fig. 4). Generally, algae and bacteriacontain dominant short chain n-alkanes (Han and Calvin,1969). Therefore, the distributions of n-alkanes suggestthe crude oils originated principally from algal and bac-terial organic matters (Han and Calvin, 1969; Volkman etal., 1983; Duan, 2000; Duan and Ma, 2001). The steranesin the crude oils from the two oil pools are dominated byC27 and the composition of steranes shows C27 > C29 >C28 (Table 3; Figs. 4 and 5), indicating the crude oils aremainly derived from plankton (Moldowan et al., 1985).The crude oils contain 4-methyl steranes and the ratio of4-methyl steranes/regular steranes ranges from 0.08 to0.25 for the E31 pool and 0.10 to 0.31 for the N21 oil pool,indicating that this ratio do not differ significantly be-tween the two oil pools. 4-methyl steranes are usuallyconsidered to be derived from dinoflagellates (Robinsonet al., 1984), even though they may also exist in bacteria(Philp et al., 1991). The presence of abundant 4-methylsteranes indicates an important contribution fromdinoflagellates to the crude oils. The relative abundancesof bicyclic terpanes and alkylcyclohexanes exist in smallamounts and do not significantly differ in the crude oilsfrom the two oil pools. The bicyclic terpane/hopane ra-tios range from 0.01 to 0.21 for the E31 crude oils andfrom 0.03 to 0.20 for the N21 crude oils (Table 4, Fig. 4).The alkylcyclohexane/hopane ratios in the E31 and N21crude oils are in the range of 3.79~9.92 and 2.75~8.09,respectively. These ratios in the studied samples are lowerthan those (20.71~63.55) in the northern Qaidam oilsderived from the Jurassic source rocks that are dominatedby terrigenous higher plant materials (Duan et al., 2006).This comparison suggests only a small contribution fromhigher land plants to the studied crude oils. In general,the contents of tricyclic terpanes and tetracyclic terpanesare closely related to the nature of the organic matters insource rocks. Tricyclic terpanes is generally believed tobe derived mainly from algae and bacteria (Aquino Netoet al., 1983), while terrigenous organic matter containsrelatively high abundance of C24 tetracyclic terpane(Hanson et al., 2000). As shown in Fig. 6, the studiedsamples are in a narrow range and contain high tricyclicterpanes and low C24 tetracyclic terpane. These data re-flect that the oils from the two oil pools have a similarsource and were derived mainly from algae and bacteria.However, small difference in organic matter sources ofthe oils is observed between the E31 and N21 oil pools asshown in Fig. 7. It is well known that steranes are de-rived mainly from plankton (Moldowan et al., 1985;

Fig. 3. The E31 (a) and N21 (b) pool structures showing thesampling locations.

AlekRealce

AlekRealce

296 Y. Duan et al.

Fig. 4. Representative chromatograms of (a) n-alkanes, (b) steranes, (c) pentacyclic terpanes, (d) 4-methyl steranes, (e) bicyclicterpanes and (f) long chain alkylcyclohexanes for the studied N21 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 E31 oils from the Gasikule oilfield.

Geochemical characteristics and genesis of crude oils 297

Volkman, 1986; Philp et al., 1991), while hopanes origi-nated primarily from bacteria (Ourisson et al., 1979;Brault and Simoneit, 1988). Figure 7A shows that the E31crude oils contain more hopanes and the N21 crude oilscontain more steranes, suggesting that the former is de-rived mostly from bacteria and the latter mainly fromplankton. At the same time, Fig. 7B shows that the E31crude oils contain more C28 sterane compared to the N21crude oils. As C28 sterane are derived principally fromalgae (Volkman, 1986), the E31 crude oils are mostly fromalgae. This recognition is consistent with the studied re-sults of nitrogen compounds in the crude oils from thetwo oil pools (Duan et al., 2004a). The compositions ofnitrogen compounds indicate carbazoles in the E31 crudeoils are evidently different from those in the N21 crudeoils. The carbon isotopic ratios of the oils from these twooil pools range from 26.1 to 27.1, with an averageof 26.5 (Table 5). These values are within the rangeof lacustrine organic matters deposited in a saline-

Sample No. Well No. Oil pool Reservoir age Depth (m) Crange Cmax CPI C21/C21+ Pr/Ph Pr/nC17 Ph/nC18C-38 16-6 E31 E31 3329 11~37 22 0.98 1.02 0.66 0.71 1.09C-37 8-6 E31 3354 14~39 22 0.97 1.14 0.60 0.61 1.00C-36 9-36 E31 3356 15~36 22 0.95 1.04 0.48 0.46 0.91C-43 3-4 E31 3445 14~38 22 0.99 1.14 0.52 0.54 0.96C-40 9-38 E31 3510 14~38 22 0.95 1.01 0.48 0.58 1.08C-41 5-36 E31 3536 12~35 20 0.96 1.03 0.59 0.54 0.92C-42 3-34 E31 3600 14~39 22 0.98 1.17 0.52 0.57 1.06

C-48 662 N21 N21 1383 14~39 22 0.93 1.09 0.57 0.94 1.69C-44 742 N21 1499 14~34 17 0.94 1.20 0.41 0.68 1.72C-45 213 N21 1662 13~35 22 0.96 1.01 0.60 0.94 1.75C-49 493 N21 1716 11~38 22 0.94 1.08 0.54 0.82 1.58C-46 414 N21 1756 13~38 22 0.96 1.04 0.52 0.76 1.42C-50 X662 N21 1833 13~35 22 0.90 1.09 0.51 0.75 1.41C-51 475 N21 1924 14~37 17 0.97 1.16 0.67 1.02 1.77C-47 525 N21 2196 14~37 22 0.96 1.02 0.52 0.80 1.51

Table 1. Physical features and fraction components of the crude oils from Gasikule oilfield

Table 2. Analytical data of n-alkanes and isoprenoid alkanes in the studied crude oils

C21/C21+, C21 n-alkanes/>C21 homologues; CPI, [(Ci+6Ci+2 + Ci+4)/(4Ci+1 + 4Ci+3)](1)i+1, i = C24~C34.

hypersaline environment (Peters et al., 1996).The crude oils from the two oil pools have a low Pr/

Ph ratio (0.66; Table 2). The Pr/Ph ratio is generallyconsidered to be a guide to depositional conditions, wherethe values above one represent oxic conditions and val-ues below one reflect anoxic conditions or anoxichypersaline depositional conditions (Didyk et al., 1978;ten Haven et al., 1987, 1988). The low Pr/Ph ratios in theE31 and N21 crude oils indicate that these oils originatedfrom a strong reducing sedimentary environment. Furtherevidence is the studied oil samples lay in the area of areducing environment on the cross-plot of Pr/n-C17 ver-sus Ph/n-C18 (Connan and Cassou, 1980; Peters et al.,1999; Hanson et al., 2000; Duan et al., 2008) (Fig. 8).The crude oils contain high amount of -carotane (Table5). The -carotane is commonly related to anoxic and al-gal-rich lacustrine environments (Fu et al., 1986; Hansonet al., 2001), although it is also from blooms of purplehalophilic bacteria (Jiang and Fowler, 1986). A high con-

Bulk properties E31 oil pool N21 oil pool

Range Mean value Range Mean value

Gravity (g/cm3) 0.8364~0.8442 0.8394 0.8462~0.8540 0.8499Viscosity (mPas) 10.49~18.87 15.10 11.09~18.40 14.10Solidifying point (C) 31.5~35.9 34.4 32.5~38.0 35.3Salinity (ppm) 300.3~1084.0 651.8 462.7~1414.0 987.7Sulfur content (%) 0.16~0.22 0.19 0.38~0.62 0.47Saturated hydrocarbon (%) 31.39~77.37 49.38 44.40~75.53 59.57Aromatic hydrocarbon (%) 2.94~7.97 5.22 3.51~15.00 7.47Nonhydrocarbon + Asphaltene (%) 17.55~63.71 45.35 9.46~45.19 32.95Nonhydrocarbon/Asphaltene 6.44~15.93 10.80 1.52~17.90 5.55

298 Y. Duan et al.

tent of gammacerane and homohopane in source rocks oroils is generally associated with a hypersaline depositionalenvironment (Moldowan et al., 1985; Fu et al., 1986;Philp et al., 1991). The E31 and N21 crude oils containhigh gammacerane/C30 hopane ratios and high relativeabundance of C34+35 hopanes (Table 4), suggesting thecrude oils derived from hypersaline depositional environ-ment. Further evidence is that the crude oils are also lo-cated in the hypersaline depositional environment areain Fig. 8. However, a slight difference in formation envi-ronments of the parent materials also exists between theE31 and N21 crude oils in Fig. 9. We have studied in detailon the carbon isotopic compositions of crude oils fromthe Qaidam Basin (Duan et al., 2003) and revealed thatthe carbon isotopic compositions of crude oils from thewestern Qaidam Basin were mainly controlled by thepaleosalinity of the depositional environment for thesource materials. It was found that the high carbon iso-topic ratios corresponded with the high saline non-marine deposition environment. Most of the E31 crude oilshave lighter carbon isotopic compositions than the N21crude oils (Fig. 9), indicating that the depositionalpaleosalinity of the E31 oil source materials is slightlylower than the N21 oil one.

Crude oil maturityThe C29 steranes 20S/(20S + 20R) and /( + )

ratios, relative content of diasteranes and Ts/Tm ratio arematurity parameters that can been used to distinguishcrude oil maturation (Seifert and Moldowan, 1978; Mac-kenzie and MacKenzie, 1983; Mackenzie, 1984; Hansonet al., 2000; Duan et al., 2006). Huang et al. (1991) sug-gested that the value 0.25 of C29 steranes 20S/(20S + 20R)and /( + ) was a limit between immaturity andlower maturity and a value 0.42 was taken as the bound-

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C-48

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39.7

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C-44

39.6

24.9

35.5

4.13

0.06

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0.25

0.26

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37.7

25.2

37.1

3.61

0.09

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C-49

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Fig. 5. Ternary diagram of sterane C27, C28 and C29 composi-tions for the studied crude oil samples.

Geochemical characteristics and genesis of crude oils 299

C23 tricyclic terpane+

Sample No. Oil pool C34 + C35 (%) Ts/Tm Gammacerrane/-C30 hopane Tricyc./hop. Bicyc./hop. A BC-38 E31 8.7 0.29 0.60 0.58 0.19 0.21 0.28C-37 8.0 0.33 0.69 0.56 0.01 0.25 0.33C-36 10.6 0.31 0.70 0.63 0.04 0.24 0.27C-43 8.0 0.38 0.70 0.52 0.04 0.23 0.33C-40 12.2 0.35 0.80 0.54 0.03 0.24 0.31C-41 11.0 0.41 0.69 0.78 0.20 0.28 0.28C-42 9.7 0.39 0.78 0.57 0.03 0.25 0.31

C-48 N21 11.3 0.29 0.73 0.51 0.03 0.23 0.30C-44 11.9 0.30 0.72 0.42 0.03 0.28 0.29C-45 10.0 0.33 0.65 0.56 0.07 0.21 0.28C-49 12.5 0.28 0.68 0.55 0.20 0.21 0.26C-46 13.0 0.33 0.54 0.63 0.10 0.25 0.29C-50 10.9 0.32 0.64 0.62 0.13 0.25 0.28C-51 9.1 0.29 0.64 1.03 0.14 0.26 0.30C-47 10.1 0.28 0.65 0.52 0.05 0.22 0.33

Fig. 6. Cross-plots of tricyclic terpane/hopane ratios vs. relative content of C27 sterane (a) and C24 tatracyclic terpane/(C24tatracyclic terpane + C26 tricyclic terpane) ratios vs. C23 tricyclic terpane/(C23 tricyclic terpane + C30 hopane) ratios (b) for thestudied crude oil samples.

Table 4. Analytical data of terpanes in the studied crude oils

A, C23 tricyclic terpane/(C23 tricyclic terpane + C30 hopane); B, C24 tetracyclic terpane/(C24 tetracyclic terpane + C26 tricyclic terpane). Tricyc.,Tricyclic terpane; Hop., Hopane; Bicyc., Bicyclic terpane.

due to the absence of clay minerals to catalyze the steranerearrangement (Mello et al., 1988; Peters and Moldowan,1993). Philp et al. (1991) suggested that the amount ofdiasteranes in oils could decrease in highly reducing non-carbonate environments with increasing salinity. There-fore, the low diasterane/regular sterane ratios for the twooil pools indicate a lower maturity and saline-hypersalinecondition. The Ts/Tm ratio is commonly used to infer oilmaturation (Moldwan et al., 1986), but Ts/Tm ratios arealso low in carbonate source rocks (McKirdy et al., 1983,1984). The studied crude oils have lower Ts/Tm ratios(0.28~0.41), again, reflecting a lower maturity and sa-

ary between lower maturity and maturity. Based on theseparameters, the maturation of the studied crude oils ispresented in Fig. 10 and Table 3. As C29 steranes 20S/(20S + 20R) and /( + ) ratios in the crude oils arefrom 0.25 to 0.33 and from 0.26 to 0.32 (Table 3), re-spectively, the studied crude oils are low mature. The rela-tive contents of diasteranes in the crude oils from the twooil pools are very low and diasterane/regular sterane ra-tios range from 0.07 to 0.11. The ratio of diasterane/regu-lar sterane has been generally used as a maturation in-dex. However, previous studies showed that carbonate-derived oils contain a relatively low amount of diasteranes

300 Y. Duan et al.

Fig. 7. Cross-plot of regular sterane/hopane ratios vs. relative content of C27 sterane (a) and sterane C28/C29 ratios vs. C27/C29ratios (b) for the studied crude oil samples.

Table 5. Analytical data of alkycyclohexanes, -carotane and carbon isotopic compositions in the studiedcrude oils

Cyclohex, Alkycyclohexanes; hop., hopane; CPI, Odd carbon alkycyclohexanes/even carbon homologues; C21/C21+, C21 alkycyclohexanes/>C21 homologues; 13C value was determined by examination of whole oil.

line condition. A small difference in maturity is observedbetween the E31 and N21 crude oils. As shown in Fig. 10,most of the E31 crude oils are much more mature than theN21 crude oils.

Crude oil genesisAs shown in Tables 25 and Figs. 410 above, the

crude oils from Gasikule oilfield have a slight even car-bon predominance for n-alkanes, low Pr/Ph ratios, rela-tively high amounts of gammacerane, C34+35 hopanes andC27 steranes, low C29 steranes 20S/(20S + 20R) and /( + ) ratios and 13C-rich carbon isotope ratios. These

are consistent with those of the Tertiary source rocks inthe western Qaidam Basin (Ritts et al., 1999; Duan et al.,2006). For example, Pr/Ph ratios, gammacerane/C30 hopane ratios, relative contents of C27 steranes, C29steranes 20S/(20S + 20R) and /( + ) ratios andcarbon isotope ratios for the Tertiary source rocks are0.16~0.93, 0.31~1.13, 28.2~64.2% (Zhu et al., 2003),0.01~0.50 and 0.10~0.89 (Huang et al., 1991) and 21~26 (Duan et al., 2006), respectively. These data indi-cate that Tertiary source rocks were deposited under an-oxic and saline-hypersaline lacustrine environments, andhave algal organic matters dominated and a lower matu-

Sample No. Oil pool Crange Cmax CPI C21/C21+ Cyclohex/hop. -Carotane/hop. 13C ()C-38 E31 13~37 17 1.09 1.04 6.76 0.08 27.0C-37 14~38 17 1.03 0.80 4.30 0.06 26.2C-36 14~34 17 1.06 0.81 5.59 0.10 27.0C-43 14~38 17 1.02 0.72 4.01 0.06 26.9C-40 14~38 17 1.04 0.78 3.79 0.08 26.6C-41 13~37 16 1.06 1.45 9.92 0.09 26.8C-42 14~38 17 1.03 0.82 5.03 0.07 27.1

C-48 N21 14~38 17 1.06 0.86 2.75 0.07 26.2C-44 15~36 18 0.93 0.69 7.27 0.14 26.2C-45 13~36 17 1.02 0.81 3.93 0.06 26.3C-49 13~37 17 1.08 1.16 4.40 0.09 26.3C-46 13~37 17 1.03 1.03 5.15 0.11 26.1C-50 13~33 17 1.04 1.40 4.45 0.07 26.7C-51 13~36 17 1.12 2.90 8.09 0.05 26.4C-47 13~38 17 1.07 0.98 2.85 0.06 26.1

Geochemical characteristics and genesis of crude oils 301

rity. The consistency of the crude oils with the sourcerocks suggests that the E31 and N21 crude oils studied werederived most probably from the Tertiary source rocks. Werule out the possibility of pre-Tertiary source contribu-tions to the E31 and N21 crude oils in western Qaidambasin, because pre-Tertiary lacustrine source rocks weredeposited under freshwater conditions and had highermaturities (Qinghai Petroleum Administration, unpub-lished data). If the oils originated from pre-Tertiary sourcerocks, they would have low concentrations ofgammacerane and higher maturities. We also exclude thepossibilities that highly mature light oils with quite lessbiomarkers were migrated from deep layers to the E31 andN21 reservoir rocks and then contaminated by low-matu-rity bitumens. An example is that the northern Qaidam

N1 crude oils of Lenghu and Nanbaxian regions, derivedfrom Jurassic source rocks, all the same are mature (Duanet al., 2006). The previously studies showed that the E3,N1 and N2 source rocks had different maturities anddepositional palaeosalinity. Their mean steranes C29 20S/(20S + 20R) are 0.29, 0.26 and 0.22 (Fig. 2), respectively,and their average depositional palaeosalinity are 16wt%,19wt% and 20~60wt% (Duan et al., 2004b, 2006), re-spectively. As described above, the maturity of the crudeoils from the E31 oil pool is slightly higher than that fromthe N21 oil pool and the depositional paleosalinity of N21oil source materials is higher that of E31 oil source mate-rials. Compared with the maturity and depositionalpalaeosalinity of the E3, N1 and N2 source rocks, thesedata strongly suggest that the E31 crude oils are derivedmostly from the E3 source rocks, while the N21 crude oils

Fig. 8. Cross-plot of Pr/n-C17 ratios vs. Ph/n-C18 ratios for thestudied crude oil samples (Connan and Cassou, 1980; Peterset al., 1999; Hanson et al., 2000). The studied crude oil sam-ples are in the area of reduction and hypersaline lacustrineenvironment.

Fig. 9. Cross-plots of Pr/Ph ratios vs. 13C values for the stud-ied crude oil samples.

Fig. 10. Cross-plots of Ts/Tm ratios vs. C29 sterane 20S/(20S + 20R) ratios (a) and C29 sterane 20S/(20S + 20R) ratios vs. /(+ ) ratios (b) (Huang et al., 1991) for the studied crude oil samples.

302 Y. Duan et al.

are derived mostly from the N1N2 source rocks. Theseresults are well consistent with pyrrolic nitrogen com-pounds in the crude oils from these two oil pools (Table6; Duan et al., 2004a). For the E31 oil pool, absolute con-centrations of neutral nitrogen compounds decrease toboth northward and southward directions of the anticline.These reflect that the charging points of crude oils in theE31 oil pool are on the west and east sides of the anticlineand the crude oils migrated from the central to north andsouth of the anticline, also reflect the oils derived fromthe source rocks in the Mangya sag and Gasi fault block.For the N21 oil pool, absolute concentrations of neutralnitrogen compounds decrease to tectonically west andnorth directions. These indicate that the charging pointsof crude oils in the N21 oil pool are on the tectonic eastand south sides and the crude oils migrate from east towest and from south to north of the tectonics. Therefore,the Mangya sag and Gasi fault block provided sourcesfor the crude oils of the N21 oil pool. The mean carbazoleconcentration is 3.53 g/g for the E31 oil pool and 23.38g/g for the N21 oil pool (Table 2), reflecting that the crudeoil migration distance of the E31 oil pool is longer thanthat of the N21 oil pool. This means that the crude oilsfrom the E31 oil pool were derived mostly from the upperE32 source rocks whereas the crude oils from the N21 oilpool were derived mainly from the N1N21 source rockswith the feature of autogeny and autostorage. The gen-eration of large amounts of hydrocarbons from the Terti-ary source rocks deposited in saline-hypersaline lacustrineenvironments in the Qaidam Basin at the stage of lowmaturity was demonstrated by pryrolysis experiment (un-published data). An E32 mudstone sample with low matu-rity (Ro = 0.32%) was crushed to 100 mesh, and thenextracted by BMA (benzene:methanol:acetone =70:15:15.v/v) solvent. The extracted organic matter andsolid remnant material were pyrolyzed individually for72 h in a sealed stainless steel reactor at temperaturesranging from 150 to 450C in temperature steps of 50C.This simulating experiment revealed that abundant liq-uid hydrocarbons were formed at heating temperature of200~300C corresponding to low maturate stage of or-ganic matter evolution. The generation and drainage ofliquid hydrocarbons reach the peak values at heating tem-

perature of 250C. At the same time, some important con-tribution of extractable organic matter from the sourcerocks to the formation of liquid hydrocarbons was con-firmed (unpublished data). The studied results of the abun-dant non-hydrocarbon fraction of immature-low maturecrude oils from the western Qaidam Basin show that thenon-hydrocarbons are composed primarily of fatty acids,alkanols, fatty acid glycerol monoesters, amide and ster-ols (Duan et al., 2004b), indicating the crude oils derivedmainly from biological lipids. The distribution of n-alkanes in the studied crude oils exhibits an obviouslyeven carbon predominance and a maximum at C16 to C22.Such distribution of n-alkanes are similar to those of fattyacid, alkanols, fatty acid glycerol monoesters and amidein the crude oils from the Gasikule oilfield reported pre-viously (Duan et al., 2004b). This suggests that n-alkanesin the crude oils were derived probably from the reducedproducts of these compounds. Thus, the crude oils fromthe E32 and N21 oil pools were formed possibly by evapo-rating and reducing free biological lipids, thermal trans-lating of extractable organic matters that were survivedwithout bonding to kerogen and thermal cracking ofkerogen in the Tertiary source rocks deposited under sa-line to hypersaline environments at the stage of low ma-turity.

CONCLUSIONS

The geochemical characteristics of crude oils from theE31 and N21 pools in Gaskule oilfield were syntheticallystudied for a better understanding of their organicmatter-formation environments, matrix types, maturityand genesis. The studied results show that n-alkanes inthe crude oils have the obvious even-odd predominance,low Pr/Ph ratios and certain amount of -carotane. Allthese geochemical parameters indicate that the oils origi-nated from strong anoxic environments. High abundanceof gammacerane and C34+35 hopanes in the crude oils re-flect that they were derived from the potential source rocksdeposited under saline to hypersaline lacustrine environ-ments. It was observed that the ancient salinity of the sedi-mentary environment for the source rocks generating theN21 crude oils was slightly higher than that forming the

C, Carbazole; MC, Methylcarbazole; DMC, Dimethylcarbazole; TMC, Trimethylcarbazole; BC, Benzocarbazole; 1,8-DMC, 1,8 Dimethylcarbazole;PSNs, Partially shielded isomers; ENs, Exposed isomers; W, Total content of carbazole compounds.

Table 6. Mean contents of pyrrolic nitrogen compounds in the studied crude oils (Duanet al., 2004a)

Oil pool C MC DMC TMC BC W 1,8-DMC PSNs ENs(%) (%) (%) (%) (%) (g/g) (%) (%) (%)

E31 12.85 21.22 38.58 23.02 4.33 3.59 14.48 65.78 19.75N21 14.62 26.68 37.48 17.94 3.29 23.38 13.15 64.42 22.44

Geochemical characteristics and genesis of crude oils 303

E31 crude oils. The crude oils contain high abundance ofC27 steranes, regular steranes, 4-methyl steranes and longchain tricyclic terpanes, and low abundance of bicyclicsesquiterpanes and alkylcyclohexanes, showing that thecrude oils were mainly from bacteria and algae. How-ever, the E31 crude oils were mostly from bacteria wherethe N21 crude oils were derived mostly from plankton.Low C29 sterane 20S/(20S + 20R) and /( + ) ra-tios, Ts/Tm ratios and low abundance of rearrangedsteranes in the crude oils show their low maturity. It wasfound that the maturity of the E31 crude oils was slightlyhigher than that of the N21 crude oils. All these resultsshow that the N2 and E31 crude oils originated from freebiological lipids, extractable organic matters and kerogenin the Tertiary source rocks deposited under a saline-hypersaline environment at the stage of low maturity.

AcknowledgmentsThis study was supported by the NationalNatural Science Foundation of China (Grant No. 40872092,40772069) and the 973 Program of China (Grant No.2005CB422105). We are grateful to Yoshikazu Sampei and JordiTritlla Cambra for their reviews which significantly improvedthe manuscript. G. D. Zheng acknowledges the financial sup-port from Chinese Academy of Sciences as 100-Tallent pro-gram to perform research work in 20062009.

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