The origin of crude oils from the Shuguang-Huanxiling Buried Hills in the Liaohe Basin, China: evidence from chemical and isotopic compositions

Applied Geochemistry 18 (2003) 445–456 www.elsevier.com/locate/apgeochem

The origin of crude oils from the Shuguang-Huanxiling Buried Hills in the Liaohe Basin, China: evidence from chemical and isotopic compositions Yongqiang Xionga,*, Ansong Genga, Chunjiang Wangb, Guoying Shenga and Jiamo Fua a

The State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, PR China b The University of Petroleum, Beijing 102200, PR China Received 20 August 2001; accepted 18 March 2002 Editorial handling by B.R.T. Simoneit

Abstract Since the Shu103 well was successfully drilled in 1995, the Buried Hill reservoir is receiving a new exploration emphasis in the Liaohe Basin, China. The Buried Hill oils can be divided into 3 main types in the Shuguang-Huangxiling area based on their chemical and isotopic compositions. The first type is collected mostly from the Shuguang area. The similarity to the Es4 oils and the Es4 source rock extracts indicates that they were mainly expelled from the fourth member of the Shahejie formation (Es4) in the Chenjia Sag and/or Panshan Sag, deposited in a stratified paleolake system. The second is charactized by a relative high Pr/Ph ratio, low gammacerane content and depletion of 12 C in individual n-alkanes. These characteristics represent a typical origin from dominantly freshwater paralic lacustrine sediments. Various biomarker indices of the other Buried Hill oils fall between the former two types. The authors infer that these oils may be derived from the adjacent Qingshui Sag and/or Panshan Sag, and were likely generated from transitional sedimentary facies between well-circulated freshwater paralic lacustrine and relatively closed saline lacustrine. # 2002 Published by Elsevier Science Ltd.

1. Introduction The Liaohe Basin is one of the most important Cenozoic sedimentary basins in NE China. It can be divided into 7 substructural units: Western uplift, Western depression, Central uplift, Eastern depression, Eastern uplift, Damingtun depression, and Shenbei depression (Fig. 1). Since the first exploration well was drilled in 1964, the Liaohe Basin has become the third largest oilproducing province in China. With the development of petroleum exploration, Buried Hill, as a special type of oil-bearing trap, has become an important exploration

* Corresponding author. Fax: +86-20-8529-0706. E-mail address: [email protected] (Y. Xiong).

target in the Liaohe Basin. The exploration for the Buried Hill oil reservoir may date from 1972. However, during the initial several years, progress was slow due to limited knowledge on the Buried Hill oil reservoir and inadequate seismic exploration techniques. In 1979, with the enlightenment from the discovery of the Renqiu oilfield in the northern China, Well Shugu-1, the first exploration well for Buried Hill, was drilled in the Western depression of the Liaohe Basin and found the Shuguang limestone Buried Hill oil reservoir. The success at Shu103 block in 1995 marked an important breakthrough in the exploration of the Buried Hill reservoir in the Western depression, Liaohe Basin. By the end of 1997, nearly 30 wells had been drilled and a series of Buried Hill reservoirs were found in the ShuguangHuanxiling buried hill belt. High-production oil and gas

0883-2927/03/$ - see front matter # 2002 Published by Elsevier Science Ltd. PII: S0883-2927(02)00093-8

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flows have also been obtained in the Shu103, Shu112, Shu107, Shu110 and Shu111 wells, among which the production of Shu103 well is the highest, approximately 254t/d of oil and 11343m3/d of gas. Therefore, the Buried Hill belt has a significant oil exploration perspective in the Western depression, Liaohe Basin. The Liaohe Basin is a Tertiary rift developed on a paleouplift. The so-called Buried Hill is referred to in this study as a pre-Tertiary paleomorphologic high and covered by Paleogene sediments. It consists mainly of Middle and Upper Proterozoic carbonates (Pt), Archaeozoic migmatitic granites (Ar), and Mesozoic

Fig. 1. Map showing location of oil and rock core samples. Open symbols refer to rock samples. Solid symbols refer to oil samples. =Shuguang buried hill, =Dujiatai buried hill, =Qijia buried hill, =Huanxiling buried hill, 1=Chenjia Sag, 2=Panshan Sag, 3=Qingshui Sag.

clastic rocks and volcanics (Fig. 2). These pre-Tertiary basement rocks had been exposed on the surface for a long geological time. Weathering and tectonic activities formed secondary pores and fissures, thus creating good reservoir space. These pre-Tertiary basement rocks have no potential of oil and gas generation, thus the overlying Tertiary mudstone not only can act as a source rock, but also as a cap rock. In addition, faults and unconformity surfaces around Buried Hills provide pathways for hydrocarbon migration. Therefore, Buried Hills have good geological and geochemical conditions for oil reservoir formation. At present, the extent of exploration is relatively high in the Liaohe Basin, 50% of the resources of hydrocarbons have been proven. So it is necessary to clarify the origin of Buried Hill oils in order to reduce the risk in exploration drilling for petroleum. Table 1 shows the general Cenozoic stratigraphic units for the study area. Previous studies have indicated that the fourth (Es4) and third (Es3) members of the Eocene-Oligocene Shahejie Formation are the main source rocks in this basin (Huang et al., 1991; Editorial Committee for Petroleum Geology of China, 1993; Li et al., 1995). The sedimentary facies of the members Es3 and Es4 in the Western depression are displayed in Fig. 3. Briefly, during the sedimentation of the Es4 member, the area on the NE of Shuguang buried hill and Xingrongtai uplift was a semi-closed lacustrine bay without inflow of water. Whereas, the SW was a freshwater paralic lacustrine environment, where sediments were mainly sourced from the NW mountainous area. In the early Es3 member, drastic subsidence resulted in the development of an extensive deep lacustrine environment. Large clastics from the high on both sides slid into the bottom of the lake, forming turbidite and mudstone of deep lacustrine facies. However, abatement of rift activity and uplift of the north of the Western depression in the late Es3 member ultimately led to the shrinking of the water body in the lacustrine basin. The purpose of this study is (1) to characterize the chemical and stable isotopic compositions of the Buried Hill oils in the Liaohe Basin; and (2) to reveal whether

Fig. 2. Cross section AB (see Fig. 1 for location) showing the structure of buried hills in the western depression, the Liaohe Basin (modified after Editorial Committee of ‘‘Petroleum Geology of China’’, 1993). Pt=Proterozoic carbonates; Ar=Archaeozoic migmatitic granites; Ef=Fangshenpao Formation of Eogene; Es=Shahejie Formation of Eogene; Ed=Dongying Formation of Eogene; N+Q=Neogene+Quaternary.

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hydrocarbon generation and expulsion has a considerable effect on individual n-alkane isotope compositions; and (3) to determine the origin of the Buried Hill oil reservoirs in the Shuguang-Huanxiling area.

2. Experimental

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quartz sand, 0.5 cm thick. The flows generated during the heating can be expelled through the sand layer and a hole under the cylinder. The samples were heated for 72 h at temperatures ranging from 200 to 450  C in 50  C stages. Pressure was kept at 80 MPa during the whole experiment. In this paper, the residual oils refer to the extracts from the residuum after each heating stage,

In order to investigate the origin of the Buried Hill oils, 16 oil and 15 source rock samples were selected from the Shuguang-Huanxiling buried hill belt in the Western depression. Basic geochemical data of these samples are summarized in Tables 2 and 3. Three oils, L35, G16 and D57, of these were considered to be typical for generation from the Es3 and the Es4 source rocks, respectively (Xiong and Geng, 2000). As shown in Table 3, all source rocks analyzed are immature in the Shuguang-Huanxiling area. Vitrinite reflectance values (Ro, %) are less than 0.5% with two exceptions (S110–2 and S112–3). In order to reveal the hydrocarbon generation potentials of Es3 and Es4 source rocks, a simulation experiment on hydrocarbon generation and expulsion was performed using a dry pyrolysis system described by Lu (1990). Briefly, source rock samples were crushed into powder, then were pressed into the cylinder with a preloaded layer of Table 1 Cenozoic stratigraphic units in the Liaohe Basin Strata System Quaternary Neozoic

Eogene

Series

Formation

Thickness Age Member (m)

Pingyuan (Qp)

(Ma)

0–417

Miocene Minghuazhen (Nm) Guantao (Ng)

107–823

Oligocene Dongying (Ed)

0–1828

24.7 28.9 30.8

0–945

36.9 38.4

First (E3d1) Second (E3d2) Third (E3d3) Shahejie First (Es) (E3S1) Eocene Second (E3S2) Third (E3S3) Fourth (E3S4) Fangshenpao Upper (Ef) (E1-2f1) Paleocene Lower (E1-2f2)

0–304

0–380 0–1861

39.5

0–813.5

45.4

0–1204

46.4 56.4 65.0

Fig. 3. The sedimentary facies of the members Es3 and Es4 in the Western depression (modified after Editorial Committee of ‘‘Petroleum Geology of China’’, 1993). (a)—Es4 member; (b)— Middle and lower Es3 member; (c)—Upper Es3 member. 1— Erosion area; 2—source direction; 3—facies boundary; I— alluvial fan facies; II—flood plain facies (II1—river channel and interchannel subfacies; II2—flood basin swamp subfacies); III—delta facies (III1—delta plain facies; III2—delta front facies); IV—fan delta facies (IV1—submerged distributary channel subfacies; IV2—interdistributary shoal subfacies); V— lacustrine facies; VI—turbidite facies.

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e.g. remaining extracts. The expelled oils consist of the liquid hydrocarbons expelled into the sand layer, pipeline, and collection tube. After the removal of asphaltenes, the crude oils and the oil-like pyrolysates were separated into saturate and aromatic hydrocarbons and a polar NSO fraction by column chromatography. Saturated hydrocarbon fractions, obtained by elution with petroleum ether, were then analyzed by gas chromatography (GC), gas chromatography–mass spectrometry (GC–MS) and gas chromatography–isotope ratio mass spectrometry (GC– IRMS). Source rocks were Soxhlet extracted using chloroform for 72 h. Similar to the above process, the

extracts were subjected to column chromatography, and the saturated hydrocarbon fractions obtained were then analyzed by GC, GC–MS and GC–IRMS. GC–IRMS analyses were performed with a VG Isochrom II instrument. The GC was fitted with a 50 m  0.32 mm i.d. HP-5 column. Helium (12 psi) was used as carrier gas. The GC was held isothermally for 5 min at 70  C, programmed from 70 to 290  C at 3  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 CO2 as a reference gas that was automatically introduced into the IRMS at the beginning and end of each analysis, and the data were reported in per mil (%) relative to the PDB standard. A standard mixture of n-alkanes (nC12–nC32) and isoprenoid alkanes 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. Chemical composition of Buried Hill oils and extracts from source rocks Fig. 4. Pr/Ph ratio vs Ts(Ts+Tm) ratio of the buried hill oils.

Fig. 5. Pr/Ph ratio vs Grammacerane/C30 hopane ratio of the buried hill oils.

Fig. 6. Pr/nC17 ratio vs Ph/nC18 ratio of the buried hill oils.

In the Shuguang-Huanxiling area, most Buried Hill oils are found in the Proterozoic carbonates. Their API gravities range from 21 to 41 at reservoir depths of 1316–3843 m, and the wax content is relatively high, ranging from 11.14 to 23.70%. Various biomarker parameters calculated from GC and GC–MS analyses of saturated hydrocarbon fractions of the Buried Hill oils are given in Table 2. Ratios of pristane/phytane (Pr/Ph), Ts/(Ts+Tm) and Gammacerane/C30Hopane are affected by source or depositional environment. Their remarkable variation in Buried Hill oils, as shown in Figs. 4 and 5, suggests that these oils were probably derived from multiple sources. Ratios of Pr/nC17 and Ph/nC18 can be used to evaluate maturity as well as depositional environment. The oils from one organic facies generally have a single trend in the plot of Pr/nC17 against Ph/nC18 (Connan, 1980; Le Tran and Philippe, 1993). However, it is shown in Fig. 6 that these Buried Hill oils do not have a single maturity trend, which further implies that they were generated from different sources. These Buried Hill oils can be separated approximately into 3 groups, as indicated in Figs. 4 and 5: (1) Group A oils are mainly produced from the Shuguang buried hills, including S112, S125, S107, SG101, S114 and S2530. These oils are characterized by a relatively high concentration of gammacerane, a predominance of C29 steranes

Pr/Ph=pristane/phytane; OEP=(nC21+6nC23+nC25)/4(nC22+nC24); Ts/(Ts+Tm)=18(H)-trisnorneohopane/[18(H)-trisnorneohopane+17a(H)-trisnorhopane]; n.d., no data.

449

a

0.34 0.23 0.27 0.46 0.58 0.23 0.38 0.48 0.37 0.51 0.58 0.54 0.30 0.43 0.33 0.45 0.32 0.28 0.34 0.40 0.42 0.36 0.37 0.39 0.40 0.38 0.45 0.46 0.31 0.36 0.33 0.49 48.0 n.d. 45.0 49.8 56.6 47.8 48.4 58.7 49.3 34.5 58.6 57.5 46.6 61.4 45.6 47.8 12.3 n.d. 34.0 27.4 23.3 28.6 27.4 23.1 30.0 31.0 23.0 25.5 27.7 21.8 23.8 16.7 39.7 n.d. 21.0 22.8 20.0 23.6 24.2 18.2 20.7 34.5 18.4 17.0 25.7 16.8 30.6 35.5 0.63 0.50 0.58 0.62 0.60 0.60 0.60 0.63 0.60 0.61 0.59 0.61 0.62 0.61 0.59 0.61 <0.01 0.26 0.24 0.73 0.41 0.73 0.41 0.27 0.44 0.15 0.40 0.38 0.24 0.18 0.23 0.09 0.44 0.27 0.28 0.26 0.46 0.20 0.25 0.12 0.22 0.53 0.52 0.50 0.41 0.45 0.43 0.62 1.14 1.11 1.13 1.07 1.06 1.06 1.02 1.07 1.04 1.04 1.02 1.06 1.05 1.09 1.09 1.05 0.83 2.09 4.24 0.70 0.54 0.65 0.48 0.70 0.52 0.33 0.36 0.32 0.46 0.65 0.39 0.38 0.70 0.56 2.11 0.36 0.34 0.32 0.31 0.33 0.29 0.29 0.31 0.29 0.35 0.32 0.41 0.37 1.19 0.50 0.75 0.63 0.70 0.64 0.77 0.56 0.69 0.91 0.98 1.01 0.83 1.08 1.15 1.41 8.22 5.50 10.18 20.47 15.03 23.70 11.14 17.90 n.d. 14.82 14.99 15.44 15.98 n.d. 15.96 n.d. 30 22 21 30 30 30 34 32 n.d. 41 36 33 31 n.d. 32 n.d. 3249–3323 1316–1332 1347–1396 3678–3722 3398–3464 3555–3633 1778–1796 3663–3693 3362–3364 3805–3843 3359–3400 3208–3290 2175–2272 3001–3149 2796–2879 2310–2390 Leng35 Es3 Gao1–6 Es4 Du57 Es4 Shu112 Pt Shu125 Pt Shu107 Pt SG101 Pt Shu114 Pt Shu2–5-30 Pt Shu123 Pt Shu103 Pt Shu103–3 Pt DG4 Pt Du126 Pt QG16 Pt Huan612 Pt L35 G16 D57 S112 S125 S107 SG101 S114 S2530 S123 S103 S1033 DG4 D126 QG16 H612

C29 Pr/ Gammacerane / C31 hopanes Ph/ OEP Ts/ Steranes nC17 nC18 (Ts+Tm) C30 Hopane 22S/(22S+22R) C28 C27 Pr/ Ph Density Wax (g/cm3) (%) Stratum Depth (m) Sample Well

Due to the effect of organic source input, the Ts/ (Ts+Tm) ratio is not an effective maturity parameter for the Buried Hill oils (Peters and Moldowan, 1993). In addition, the C3117a(H)- homohopane 22S/(22S+22R) ratios of the analysed samples have reached equilibrium (0.57–0.60) (Seifert and Moldowan, 1986), indicating that the maturity of these oils is at or past the threshold of oil generation. Therefore, the C3117a(H)-homohopane 22S/(22S+22R) ratio also is not useful for evaluating the maturity of oils. In this study, typical sterane maturity parameters such as C29 sterane abb/ (abb+aaa) and C29 aaa sterane 20S/(20S+20R) ratios (Peters and Moldowan, 1993) are calculated to estimate the thermal maturities of the oils. The C29sterane ratios of 20S/(20S+20R) and abb/(abb+aaa) are cross plotted in Fig. 7. The OEP values vary from 1.02 to 1.11.

Table 2 Geochemical data of the oil samples from the Western depression, Liaohe Basina

and low Pr/Ph ratio (Table 2). Gammacerane is considered as an indicator for water column stratification (Sinninghe Damste´ et al., 1995). The Pr/Ph ratio is lower than 0.8 confirming an anoxic/reducing depositional environment for the source rock of group A oils. This indicates that the related source rock was probably deposited in a stratified paleolake system. Additionally, with one exception, the oils have Ts/(Ts+Tm) ratios less than 0.3. All these characteristics are in agreement with those of the typical Es4 oils, e.g. oils G16 and D57 (Table 2). (2) The L35 oil is produced from the center of the Chenjia Sag, and is considered to be typical of oils generated from the Es3 source rock. Similar to the L35 oil, the H612 oil from the Huanxiling buried hill (Table 2) shows many characteristics commonly observed in the Es3 oils and related source rocks, typically with relatively high Pr/Ph ratio (> 1) and low gammacerane content (Li et al., 1995). Besides, the Ts/(Ts+Tm) ratio of H612 oil is up to 0.62, obviously higher than that of group A oils. In comparison with group A oils, the H612 oil is possibly expelled from the related source rock deposited in a relatively well-circulated paleolake system. (3) Group C oils include S103, S103–3 and S123 from the Panshan Sag located between the Shuguang and the Xingrongtai oilfields, and QG16, D126 from the Qijia and the Dujiatai buried hills, respectively. Various biomarker indices of these oils fall within the former two types, respectively; such as Pr/Ph ratios approaching 1, gammacerane/C30hopane ratios range from 0.15 to 0.40, and Ts/(Ts+Tm) ratios varying between 0.41 and 0.53.

C29 steranes C29 steranes 20S/(20S+20R) bb/(dd+bb)

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Based on the results of the biomarkers, the Buried Hill oils are believed to be generated from the adjacent marginally mature to mature source rocks. Although these oils are distinct in maturity, no systematic variation is observed. As shown in Table 3, the total organic C contents (TOC) of the Es3 and the Es4 source rocks range from 1.73 to 6.74%, and their Rock-Eval hydrogen indices (HI) from 267 to 679 mg/g TOC, indicating that both the members Es3 and Es4 have relatively high potentials for hydrocarbon generation. The organic matter in these source rocks is mostly attributed to type II. Table 3 shows that the most Es4 source rocks are characterized by the relatively high concentration of gammacerane and low Pr/Ph ratio. In contrast, the Es3 source rocks generally have a very low gammacerane content. However, maturity parameters, e.g. OEP, C31 17a(H)– homohopane 22S/(22S+22R), C29sterane ratios of 20S/ (20S+20R) and abb/(abb+aaa), suggest that the maturity of these source rocks is very low, especially for

Fig. 7. Crossplots of %C2920S/(20S+20R) and %C29abb/ (aaa+abb) values from GC–MS of the buried hill oils.

the Es3 source rocks. As most analysed source rock samples in the study are immature or low maturity, it prevents the direct correlation between the Buried Hill oils and the related source rock extracts based on their chemical compositions. 3.2. Effect of hydrocarbon generation and expulsion on n-alkane isotope profile GC–IRMS has become an useful tool for making oil/ oil and oil/source correlations (Bjorøy et al., 1991, 1994), and elucidating the origin of the source organic matter (Freeman et al., 1990; Hayes et al., 1990). A previous study has discussed the oil/source correlation for the biodegraded oils from the Liaohe Basin using the C isotopic composition of individual n-alkanes from oils and their asphaltene pyrolysates (Xiong and Geng, 2000). In this study, the authors try to reveal the origin of the Buried Hill oils by comparing stable C isotopic compositions of individual n-alkanes from the Buried Hill oils with those from potential source rocks. When chemical and isotopic compositions of various biomarkers from crude oils and source rock extracts are used for oil/oil and oil/source correlation, two preconditions generally need be satisfied. One is that a similar maturity is required between oil and related source rock; another is that hydrocarbon expulsion has no obvious effect on the chemical and isotopic composition of various biomarkers. For the Buried Hill reservoir in this study, hydrocarbon expulsion and migration have occurred. But also the inconsistency of maturities between the Buried Hill oils and the analysed source rocks has been demonstrated based on various maturity parameters. Simulation experiments on hydrocarbon generation and expulsion from the typical immature source rocks

Fig. 8. Variations of the yields of pyrolysates with heating temperature.

Table 3 Biomarker data for Es3 and Es4 source rocks from the Liaohe Basina Well

Depth (m)

Strata

TOC (%)

S1 (mg/g)

S2 (mg/g)

HI (mg/g)

Bitumen (mg/g)

Organic matter type

Ro (%)

S90–1 S112–1 P1 S110–1 D124–1

Shu90 Shu112 Pan1 Shu110 Du124

2305 2530–2540 2741 2690–2710 2363

Es3 Es3 Es3 Es3 Es3

2.43 3.68 2.98 2.40 3.59

0.13 0.52 0.29 0.25 0.63

6.50 20.5 13.74 8.40 17.53

267 557 461 350 488

2.14 9.31 2.39 2.26 3.91

IIb IIa IIa IIb IIa

0.32 0.33 0.32 0.34 0.31

S90–2 S112–2 S112–3 S110–2 S54 S103 S101 S52 D124–2 D22

Shu90 Shu112 Shu112 Shu110 Shu54 Shu103 Shu101 Shu52 Du124 Du22

2735 3096 3356–3374 3340–3360 2766 3105 2116 2716 3005 1340

Es4 Es4 Es4 Es4 Es4 Es4 Es4 Es4 Es4 Es4

4.47 6.74 4.86 3.23 1.73 3.79 4.80 3.18 3.74 5.83

1.67 8.43 0.62 0.88 0.23 0.95 1.17 0.80 1.19 0.53

18.00 41.09 23.81 16.58 5.49 19.01 32.59 12.01 23.00 32.70

403 610 490 513 317 502 679 378 615 561

8.99 n.d. 3.40 3.70 2.04 2.76 8.61 4.02 9.21 1.80

IIb IIa n.d. IIa n.d. IIb IIa IIb-III IIa n.d.

0.32 0.49 0.59 0.53 n.d. 0.49 0.29 0.34 0.43 0.21

Sample

Pr/Ph

Pr/nC17

Ph/nC18

OEP

Ts/(Ts+Tm)

C31 hopanes 22S/(22S+22R)

Gammacerane/ C30abhopane

Steranes C27

C28

C29

C29 steranes 20S/(20S+20R)

C29 steranes bb/(aa+bb)

S90–1 S112–1 P1 S110–1 D124–1

0.85 0.69 2.37 1.19 1.03

0.43 1.75 1.98 1.15 0.48

0.22 3.65 1.01 1.32 0.37

1.86 1.72 1.36 1.31 1.22

0.33 0.31 0.36 0.34 0.40

0.36 0.41 0.54 0.51 0.43

0.01 0.03 0.02 0.02 0.05

40.1 48.1 54.7 48.4 38.6

21.4 22.1 15.8 19.6 28.5

38.5 29.8 29.5 32.0 32.9

0.04 0.05 0.08 0.06 0.02

0.22 0.23 0.18 0.18 0.17

S90–2 S112–2 S112–3 S110–2 S54 S103 S101 S52 D124–2 D22

0.47 0.91 0.75 1.19 0.88 0.91 0.22 0.80 1.00 1.10

1.66 1.28 0.36 1.15 0.39 0.38 0.18 1.13 1.76 1.98

0.44 1.81 0.61 1.32 0.24 0.58 4.26 0.94 1.75 3.03

1.30 1.16 1.10 1.45 1.05 1.10 1.33 1.09 1.07 3.45

0.13 0.13 0.50 0.46 0.59 0.66 0.11 0.53 0.53 0.13

0.52 0.61 0.61 0.63 0.60 0.58 0.50 0.60 0.60 n.d.

0.09 0.05 0.53 0.32 0.78 0.27 0.48 0.06 0.29 n.d.

48.2 34.8 26.1 24.8 15.3 24.7 18.2 32.6 10.3 42.2

19.6 23.2 25.5 22.6 38.6 25.9 32.9 19.1 26.4 13.9

32.2 42.0 48.4 52.6 46.2 39.4 48.9 48.3 63.3 43.9

0.14 0.28 0.35 0.41 0.24 0.45 0.08 0.29 0.29 0.20

0.16 0.19 0.32 0.50 0.23 0.26 0.17 0.35 0.34 n.d.

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Sample

a Pr/Ph=pristane/phytane; OEP=(nC21+6nC23+nC25)/4(nC22+nC24); Ts/(Ts+Tm)=18a(H)-trisnorneohopane/[18a(H)-trisnorneohopane+17a(H)-trisnorhopane]; n.d., no data.

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Table 4 d13C values (%) for n-alkanes of the oils from the Liaohe Basina ID n-C13 n-C14 n-C15 n-C16 n-C17 n-C18 n-C19 n-C20 n-C21 n-C22 n-C23 n-C24 n-C25 n-C26 n-C27 n-C28 n-C29 n-C30

L35

G1614 D57

20.9 n.d. 20.5 30.6 20.7 28.6 21.0 28.6 21.8 28.9 22.8 28.3 23.8 29.1 23.8 28.5 23.6 29.3 24.2 29.2 24.4 29.1 24.1 29.2 24.3 29.5 25.2 28.9 25.6 29.5 25.8 29.7 26.4 30.7 26.5 n.d.

n.d. 27.0 28.2 28.0 28.6 29.1 29.2 29.0 28.9 28.9 29.3 29.1 28.9 29.0 29.1 29.4 29.6 n.d.

S112 29.6 29.6 29.3 29.6 29.4 29.8 30.2 29.7 30 30.4 30.4 30.5 30.0 29.8 30.4 30.2 29.8 29.9

S125 29.0 28.9 28.9 28.8 28.8 28.8 29.1 28.9 29.2 29.4 29.3 29.0 29.4 28.9 29.6 29.6 29.7 30.7

S107 28.9 28.3 29.2 28.9 28.6 29.3 29.2 29.2 29.4 29.5 30.3 29.5 29.8 29.8 29.2 29.0 29.3 29.5

SG101 S114 27.1 27.7 27.9 27.8 28.4 28.1 28.6 28.6 28.6 29.0 28.9 28.8 29.3 29.2 29.0 29.1 29.2 29.1

29.4 29.8 29.8 30.2 28.7 29.4 31.2 29.7 30.3 30.4 30.9 32.6 31.8 32.0 30.4 30.6 n.d. n.d.

S2530 S123 28.8 29.1 28.7 28.9 29.3 29.2 29.4 29.1 29.4 29.6 29.7 29.8 29.7 29.9 29.6 29.6 29.4 30.3

26.2 26.3 26.2 26.7 26.6 27.0 26.1 26.5 26.6 26.0 27.1 26.4 26.7 27.1 28.5 28.7 29.2 29.5

S103 26.4 26.8 26.7 27.0 27.1 27.8 28.1 27.7 27.9 27.9 28.0 27.7 27.8 27.6 28.7 28.5 28.1 29.0

S1033 DG4 26.5 n.d. 26.8 26.9 26.5 27.1 26.7 27.2 27.1 25.8 27.4 27.5 27.5 31.5 27.5 28.7 28.0 29.2 27.8 29.3 28.2 29.6 27.9 29.2 27.9 29.1 28.2 29.8 28.5 30.5 28.4 31.2 28.7 30.3 28.8 31.94

D126 25.4 25.3 25.4 26.0 26.2 26.3 27.0 26.6 26.9 27.0 26.8 26.9 27.2 27.4 27.4 28.3 28.7 28.9

QG16 H612 25.0 24.9 24.7 25.3 25.9 26.0 26.6 26.2 26.5 26.8 27.4 27.2 27.8 27.5 28.3 28.2 28.8 29.2

22.7 22.6 22.6 23.2 23.9 24.1 25.1 24.7 24.9 25.3 25.1 25.1 25.4 25.4 26.4 26.2 26.6 26.8

n.d., no data. a A standard mixture of n-alkanes (nC12–nC32) and isoprenoid alkanes with known isotopic composition were used daily to test the performance of the GC–IRMS instrument. The reproducibility, run at different times, was typically less than 0.3%.

Fig. 9. Carbon isotopic compositions of the n-alkanes in the P1 pyrolysates: (a) residual n-alkanes; (b) expelled n-alkanes.

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Fig. 10. Correlation of C isotopic compositions of the expelled n-alkanes with the residual n-alkanes from the P1 pyrolysis products: (a) 250  C, (b) 300  C, (c) 350  C, (d) 400  C.

of members Es3 (P1) and Es4 (D22) were carried out in order to test the applicability of individual n-alkane C isotope compositions for oil/source correlation of the Buried Hill oil reservoirs in the Liaohe Basin. The pyrolysis results (Fig. 8) indicate that the main stage of hydrocarbon generation and expulsion occurred during  heating between 300 and 400 C (e.g. Ro=0.33 1.30%). Information from GC and GC/MS of saturated hydrocarbons showed that hydrocarbon generation and expulsion has obvious effects on some ratios of biomarkers both in the expelled oils and in the remaining extractables (residuals) (Xiong et al., 1999), e.g. Pr/Ph, Pr/nC17, Ph/nC18, and Ts/(Ts+Tm). Fig. 9 shows the C isotopic profiles of the n-alkanes in the P1 pyrolysates (expelled oils and residual oils) at different heating temperatures. During the early stage of hydrocarbon generation, the liquid n-alkanes are mainly derived from the primary cracking of kerogen, so whether in the expelled oils or in the residual oils, the isotopic compositions of individual n-alkanes have no obvious variation (less than 2%) with increasing pyrolysis temperature, and display a similar distribution with that of the unheated source rock extracts (Fig. 9). The behavior of sample D22 is also similar. This suggests that the n-alkane isotope profile can be used for oil/oil and oil/source correlation within the conventional oil window. However, at and past the peak of hydrocarbon generation, the secondary cracking of those heavy components formed at the early stage, such as asphaltenes, NSO fraction and high C number

Table 5 d13C values (%) for n-alkanes of the source rocks from the Liaohe Basina ID

S90–1 S112–1 P1

n-C12 n-C13 n-C14 n-C15 n-C16 n-C17 n-C18 n-C19 n-C20 n-C21 n-C22 n-C23 n-C24 n-C25 n-C26 n-C27 n-C28 n-C29 n-C30

n.d. n.d. n.d. 24.1 24.7 23.7 24.9 25.1 25.8 26.1 26.9 27.3 n.d. 27.1 n.d. 27.7 27.7 n.d. n.d.

26.5 26.8 26.7 27.5 29.1 25.6 24.9 23.9 24.8 24.6 24.8 26.5 25.6 26.0 26.8 n.d. n.d. n.d. n.d.

S110–1 D124–1 S90–2 S112–2 S112–3 S110–2 S54

n.d. n.d. 23.4 27.1 24.1 26.2 23.3 26.3 24.2 25.9 24.8 26.4 25.9 27.1 27.4 28.1 27.7 28.2 28.2 29.0 28.3 28.9 28.4 28.7 28.3 28.5 28.1 28.0 27.9 28.2 28.8 27.6 27.7 28.3 27.7 28.7 n.d. 30.5

27.1 28.1 26.2 24.3 25.2 24.6 26.4 26.7 27.4 28.3 28.4 27.4 25.8 26.4 25.2 n.d. n.d. n.d. n.d.

n.d. n.d. n.d. 29.7 29.1 30.0 29.6 31.2 28.7 29.9 27.8 29.3 28.5 29.4 28.9 30.8 28.4 29.5 28.8 29.6 30.4 29.3 30.8 29.4 30.6 28.6 30.2 28.5 29.7 28.7 29.4 29.1 28.4 30.4 n.d. 27.1 n.d. 30.4

n.d. 29.2 29.0 28.9 29.3 28.9 29.0 30.1 29.1 29.6 29.8 29.5 29.2 29.7 29.1 29.4 30.2 30.5 30.9

n.d. n.d. n.d. 28.2 29.0 27.5 29.4 30.4 30.2 30.1 30.2 30.5 30.4 30.0 30.4 30.9 31.2 29.3 30.5

S103

n.d. n.d. n.d. n.d. 27.6 28.0 27.7 28.2 27.3 28.4 27.0 27.5 27.8 28.9 29.5 30.0 29.2 29.9 29.1 29.8 29.8 30.8 29.4 31.3 29.0 31.3 28.2 30.5 28.4 30.5 27.8 30.5 28.2 33.7 26.2 29.7 n.d. 30.9

S101

S52

D124–2 D22

n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 28.8 n.d. 29.2 28.7 n.d. 29.3 28.3 30.2 29.4 27.7 30.2 31.1 28.8 29.8 30.0 28.9 30.4 31.1 29.0 30.8 31.2 29.2 31.5 31.3 29.5 33.0 30.8 29.2 32.3 30.8 29.2 32.1 30.9 29.4 31.2 31.0 29.7 31.5 31.5 30.0 n.d. 31.1 28.9 n.d. 30.5 26.9 n.d. 31.9 31.2

n.d. n.d. n.d. n.d. n.d. n.d. 31.0 30.6 n.d. 30.1 31.0 30.3 32.0 33.3 30.6 32.1 29.8 n.d. n.d.

n.d., no data. a A standard mixture of n-alkanes (nC12–nC32) and isoprenoid alkanes with known isotopic composition were used daily to test the performance of the GC–IRMS instrument. The reproducibility, run at different times, was typically less than 0.3%.

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n-alkanes, makes the C isotope of residual n-alkanes markedly enriched in 13C, especially for the high molecular weight fraction. Significant differences (up to 4%) in the C isotopic compositions are observed between the n-alkanes in the remaining extractables or the expelled

oils generated at this stage and those formed at the early stage. In addition, comparison of the isotopic profiles of individual n-alkanes from the expelled oils and related residuals shows that the hydrocarbon expulsion has no

Fig. 11. Carbon isotopic compositions of individual n-alkanes from Es3 and Es4 source rock extracts.

Fig. 12. Carbon isotopic compositions of individual n-alkanes from Shuguang-Huanxiling buried hill oils.

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considerable effect on the C isotopic composition of the liquid n-alkanes (Fig. 10). In other words, at the threshold of the conventional oil window (Ro% 0.5– 0.8), the n-alkane isotopic composition could provide additional constraints on oil/oil and oil/source correlations in the Liaohe Basin. 3.3. Origin of the oils from different buried hills Stable C isotopic values of individual n-alkanes from the Buried Hill oils and their potential source rocks are summarized in Tables 4 and 5. Fig. 11 displays the C isotopic distribution of individual n-alkanes from the Es3 and the Es4 source rock extracts. For the Es3 member, the n-alkane d13C values range from 23 to 28%, obviously enriched in 13C relative to those derived from bacteria and algae (Collister et al., 1994). Whereas, the n-alkane d13C values of the Es4 member range from 28 to 32%, relatively depleted in 13C. This obvious difference between the two source rocks suggests that the n-alkane isotope profile can be used to differentiate sources of organic matter in the Liaohe Basin. Isotopic evidence also supports the paleoenvironmental interpretations that the Es3 source beds in the Liaohe Basin were deposited in a relatively well circulated paleolake system with abundant terrestrial organic matter, and the Es4 source beds were deposited in a stratified paleolake system with organic matter bearing a relatively high amount of planktonic inputs. This is in agreement with the conclusion by Li et al. (1995). As shown in Fig. 12, the n-alkane isotopic values of group A oils show less variation in the n-C13 to n-C30 alkanes, ranging from 28.0 to 30.5%. A relatively flat shape indicates that either these n-alkanes with different C numbers have a homogenous precursor or n-alkanes with C numbers less than 21 are derived from algae that happen to produce isotopically similar alkanes as the land plants. The similar isotope profile to the Es4 oils further supports that the group A oils possibly derived from the Es4 source rocks. Similar to the L35 oil, the n-alkanes of the H612 oil are very enriched in 13C. The d13C values range from 22 to 26% and become gradually depleted in 13C with increasing C number (Fig. 12). The isotope profile is consistent with an oil derived from terrestrial organic matter (Bjorøy et al., 1991; Zhao and Huang, 1995), inferring that terrestrial organic matter may be a major contributor for the group of oils. This statement is supported by the data on sedimentary facies of the potential source rocks in the Huanxiling area (Fig. 3). For the group C oils, the isotope compositions of the n-alkanes fall between these of the group A and group B oils, ranging from 24.5 to 29.0%. Chemical and isotopic compositions indicate that group C oils simultaneously possess characteristics of both the Es3 and the Es4 source. Therefore, a possible explanation is that these oils may be the mixture of

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the above two sources. However, combined with the information on sedimentary facies, another reasonable explanation is that these oils were likely generated from a transitional sedimentary facies between well-circulated freshwater paralic lacustrine and relatively closed saline lacustrine. Various biomarker parameters and Ro values (Table 3) indicate that the source rocks in the Shuguang-Huanxiling area are immature to low maturity. According to the results of simulation pyrolysis experiments (Fig. 8), the Buried Hill oils were less likely expelled from the local Es3 source rocks, and the amounts generated from the Es4 source rocks are also limited in the area. However, the geological data show that in some sag centers, e.g. the Chenjia Sag, the Panshan Sag and the Qingshui Sag, the maturity of source rocks are relatively high. As shown in Fig. 3, the Es4 source rocks in Chenjia Sag and Panshan Sag were mainly deposited in a relatively closed saline lacustrine environment. Therefore, the group A oils may be generated mainly from the Es4 source rocks in the Chenjia Sag and/or the Panshan Sag close to the Shuguang buried hills. Whereas, the group B and C oils were mainly from the Qingshui Sag and/or the Panshan Sag. Due to the relatively deeper subsidence in the south of the Western depression, the Es3 source rocks had entered the oil-generating window in the Qingshui Sag. As a result, the Es3 source rocks also become one of the potential hydrocarbon contributors for the group B and C oils. 4. Conclusions Based on the results of biomarker and GC–IRMS analyses, the Shuguang-Huanxiling buried hill oils can be classified into 3 main groups.  Group A oils, mainly produced from the Shuguang buried hill, were possibly generated from the adjacent marginally mature to mature Es4 source rocks in the Chenjia Sag and/or the Panshan Sag. The characteristics of the oils infer that their source rocks were possibly deposited in a stratified paleolake environment.  Group B oils mainly refer to the H612 oil from the Huanxiling buried hill. Similar to the L35 oil, the H612 oil can be considered typical as generated from dominantly freshwater paralic lacustrine sediment.  Group C oils are from the buried hills between the Shuguang and the Huanxiling oilfield. Chemical and isotopic compositions suggest that they may be derived mainly from the Qingshui Sag and/or the Panshan Sag, and likely generated from transitional sedimentary facies between well-circulated freshwater paralic lacustrine and relatively closed saline lacustrine.

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Acknowledgements The project was supported by the Chinese Academy of Sciences (Grant no. kzcx2–110) and the State Key Laboratory of Organic Geochemistry, CAS (Grant no. 001107). We are grateful to Professor J.L. Lu for his help in the simulation experiment. Drs. B.R.T. Simoneit, M.W. Li and an anonymous reviewer also are acknowledged for their helpful comments and suggestions on this manuscript.

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