The distribution and origin of dimethyldibenzothiophenes in sediment extracts from the Liaohe Basin, East China

The distribution and origin of dimethyldibenzothiophenes in sediment extracts from the Liaohe Basin, East China

Organic Geochemistry 65 (2013) 63–73 Contents lists available at ScienceDirect Organic Geochemistry journal homepage: www.elsevier.com/locate/orggeo...

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Organic Geochemistry 65 (2013) 63–73

Contents lists available at ScienceDirect

Organic Geochemistry journal homepage: www.elsevier.com/locate/orggeochem

The distribution and origin of dimethyldibenzothiophenes in sediment extracts from the Liaohe Basin, East China Meijun Li a,⇑, Bernd R.T. Simoneit b, Ningning Zhong a, Ronghui Fang a a b

State Key Laboratory of Petroleum Resources and Prospecting, College of Geosciences, China University of Petroleum, Beijing 102249, China Department of Chemistry, Oregon State University, Corvallis, OR 97331, USA

a r t i c l e

i n f o

Article history: Received 5 March 2012 Received in revised form 8 October 2013 Accepted 8 October 2013 Available online 17 October 2013

a b s t r a c t The distributions of dimethyldibenzothiophenes (DMDBTs), the relationship between DMDBTs and dimethylbiphenyls (DMBPs), and the applications of DMDBTs as maturity indicators in source rocks have been investigated in a set of 21 lacustrine shales from the Eocene Shahejie Formation (Well SG1) in the Western Depression, Liaohe Basin, China. All source rock samples are characterized by total organic carbon contents of 1.37–3.27% and Type II (with minor Type III) kerogen. They were deposited in suboxic and brackish lacustrine environments and have maturities ranging from immature to mid-mature. The 3,30 -DMBP isomer can potentially react to yield 4,6-DMDBT; 2,6-DMDBT and 2,8-DMDBT by incorporating sulfur into biphenyl, which may be supported by a strong positive correlation between the absolute concentration changes of 3,30 -DMBP and those of 4,6-DMDBT, 2,6-DMDBT plus 2,8-DMDBT. The relative abundance of DMDBT isomers may be explained by the sulfur radical mechanism and are also controlled by steric hindrance and thermodynamic stability. The 4,6-/(1,4- + 1,6)-DMDBT ratio shows no regular trend with increasing maturity at the low stage, and it should be used with caution as a maturity indicator for immature sediments. However, within the oil generation window, the relative concentration of 4,6-DMDBT progressively increases with increasing maturity, which can be explained by its higher thermodynamic stability relative to the 1,4DMDBT isomer. The 4,6-/(1,4- + 1,6)-DMDBT ratio exhibits a linear increase with increasing thermal maturity of the sediments. Thus, this ratio can be applied as an effective maturity indicator for source rocks within oil generation window. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Dibenzothiophene (DBT) and its alkylated homologues are important sulfur heterocyclic aromatic compounds. They are widespread in sedimentary organic matter, crude oils and coals. Their distribution patterns, relative and absolute concentrations have been widely used to indicate thermal maturity (Radke et al., 1986, 1991; Radke and Willsch, 1994; Bao et al., 1996; Chakhmakhchev et al., 1997; Santamaría-Orozco et al., 1998; Kruge, 2000), depositional environments for sedimentation of organic matter (e.g. Hughes et al., 1995), kerogen types in source rocks (Wu et al., 1995), and oil migration pathways (Wang et al., 2004; Li et al., 2008). Some maturity parameters based on the isomerization of epimers of steranes and hopanes are restricted to immature levels through the early stage of the oil generation window (Peters et al., 2005). However, the drastic decrease in the concentrations of those biomarkers at elevated thermal maturity levels may influence the ⇑ Corresponding author. Present address: Energy and Resources Science Research Center, USGS, Denver, CO 80225, USA. Tel.: +1 303 236 9367, +86 108 973 1709; fax: +86 108 973 1109. E-mail address: [email protected] (M. Li). 0146-6380/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.orggeochem.2013.10.007

applicability of these maturity parameters in the maturation assessment of organic matter and related oils (ten Haven et al., 1986; Peters et al., 2005). Many aromatic maturity indicators, such as MPI-1 (methylphenanthrene index), MPR (methylphenanthrene ratio) and MDR (methyldibenzothiophene ratio) rely on differing thermodynamic stabilities in methyl group positions. The maturity indicator MPI1 has been shown to change in a regular fashion with increasing maturity up to overmature levels (Radke et al., 1986; Radke, 1988). Some parameters based on methylated dibenzothiophenes, such as the MDR, and the 4,6-/1,4-dimethyldibenzothiophene (4,6-/1,4-DMDBT) ratio, have been introduced as maturity indicators (Radke et al., 1982; Chakhmakhchev et al., 1997). Radke (1988) proposed that the MDR relies on the same chemical basis as the MPR, i.e. a shift in predominance from thermodynamically unstable towards more stable isomers with increasing maturity. The MDR has been calibrated against Rm% (mean vitrinite reflectance) with a high correlation in the 0.56–1.32% (Rm) range (Radke, 1988). Thus, the MDR can also be rated as an equivalent maturity parameter to vitrinite reflectance and the influence of organic facies appears to be negligible. However, MDR variations in the early maturation stage (0.4–0.7% Rm) exhibit significantly different maturation trends for Type II and III kerogens (Radke et al., 1986).

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Fig. 1. Location and structural sketch maps of the Liaohe Basin, East China (After Hu et al., 2005; Li et al., 2012a,b). Well SG1 is located in the Western Depression.

2. Samples and methods 2.1. Geological setting

Fig. 2. Identification of biphenyl (BP), methylbiphenyls (MBPs) and dimethylbiphenyls (DMBPs). Numbers on peaks represent the substitution patterns of methyl groups. DBF: dibenzofuran; DPM: diphenylmethane.

Laboratory simulation experiments have shown that biphenyl and sulfur can form dibenzothiophene (Xia and Zhang, 2002; Asif et al., 2009). Similarly, methylbiphenyls can yield the corresponding methyldibenzothiophenes, e.g. 2-methylbiphenyl (2-MBP) yields 1-methyldibenzothiophene (1-MDBT). The widespread distributions of DBT and alkylated DBTs in sediments and crude oils are likely the result of a catalytic reaction between biphenyl ring systems and surface adsorbed sulfur on carbonaceous materials (Asif et al., 2009). This mechanism was also supported by the geochemical relationship between the isomer distributions of the methylbiphenyls (MBPs) and methyldibenzothiophenes (MDBTs) in crude oils and sediment extracts. However, more geochemical evidence for DBTs substituted with multiple methyl groups, such as dimethyldibenzothiophene (DMDBT) is needed to further confirm this reaction mechanism. Here we report the distributions of MDBTs in a series source rock samples from the Tertiary lacustrine shales in the Liaohe Basin, East China. The maturation trend of the parameters based on the dimethyldibenzothiophenes in the early maturity stage was studied. The product-precursor relationship between DMDBTs and dimethylbiphenyls (DMBPs), along with reaction pathways, are discussed in relation to the distribution patterns of some of the DMDBT isomers with increasing maturity.

The geological setting of the petroleum reservoirs in the Western Depression of the Liaohe Basin has been summarized by numerous authors (e.g. Ge and Chen, 1993; Huang et al., 2003; Hu et al., 2005; Li et al., 2012a,b). The Liaohe Basin is located in Liaoning Province of northeast China, adjacent to the northeast end of the Bohai Sea (Fig. 1). The Western Depression is one of the main oil producing regions in the basin (Fig. 1). It is a wedge shaped rift basin with a Cenozoic sedimentary thickness of > 4 km (Hu et al., 2005). The Paleogene sequence in the basin can be divided into three formations: Fangshenpao, Shahejie and Dongying. The Shahejie Formation (Es) is widely distributed in the entire basin and contains the most important source rock beds and hydrocarbon producing zones. It can be further subdivided into four members (Es4, Es3, Es2, and Es1, oldest to youngest). During the early Es3, drastic subsidence resulted in the development of an extensive deep lacustrine environment and this member consists of dark mudstones and shales of a deep lacustrine origin. The Es3 member is the main source rock bed in the entire Liaohe Basin, ranging in thickness from < 500 m on the margins to > 2000 m in the basin center. The Es2 member was deposited during a contraction of the lake in the basin because of tectonic uplift. The Es1 is mainly composed of poorly sorted coarse sediments which comprise the second most important producing zone in the basin (Ge and Chen, 1993).

2.2. Samples, TOC, Rock–Eval, Ro and extraction analyses A total of 21 core and cutting samples were selected from the Eocene Shahejie Formation (Es3 and Es2) of Well SG1 in the Western Depression (Fig. 1) and ground in a crusher to < 80 mesh. The total organic carbon (TOC) content was measured on a LECO CS-230 Carbon/Sulfur Analyzer using the standard procedure (SY/ T 5116-1997). Rock–Eval analyses were carried out on approximately 50 mg of crushed core samples using a Rock–Eval II (DELSI, France) instrument (with S2 measured between 300 °C and 550 °C). The vitrinite reflectance values (%, Ro) were measured on polished rock blocks using a Leitz MPV-3 microscopic photometer. The maceral compositions of whole rocks were analyzed on a Zeiss MPM400 microphotometer using standard procedures (SY/T 6414-1999). The powdered samples were extracted for 24 h using a Soxhlet apparatus with 400 ml of dichloromethane:methanol (93:7, v:v) to

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Fig. 3. The identification of dimethyldibenzothiophenes (DMDBTs) in the m/z 212 mass chromatograms. Numbers on peaks represent the positions of the methyl groups.

obtain soluble bitumen. The extracts were deasphaltened using nhexane and then fractionated by liquid chromatography using silica gel:alumina columns (3:2, w:w) into saturated and aromatic hydrocarbon fractions using n-hexane:dichloromethane (50:50, v:v) and dichloromethane as respective eluents.

2.3. Gas chromatography-mass spectrometry (GC–MS) analysis GC–MS analyses of the saturated hydrocarbon fractions of extracts were performed using an Agilent 5975i mass spectrometer, coupled with an HP 6890 GC equipped with an HP-5MS fused silica

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Fig. 4. The distribution of some dimethyldibenzothiophene (DMDBT) isomers in lacustrine shales from the Liaohe Basin. Numbers on peaks represent the positions of the methyl groups.

column (60 m  0.25 mm, 0.25 lm coating). Helium was used as the carrier gas and the injector temperature was 300 °C. The GC oven temperature was initially set to 50 °C, and programmed to 120 °C at 20 °C/min, then to 310 °C at 3 °C/min with a final hold of 25 min. The mass spectrometer was operated in full scan mode with a scan range of 50–600 Da. Electron impact ionization was employed (70 eV). The aromatic hydrocarbon fraction of sediment extracts was analyzed using the same GC–MS system and column. The GC operating conditions were as follows: the temperature was held initially at 80 °C for 1 min, ramped to 310 °C at 3 °C/min, and then kept isothermal for 16 min. The MS was operated in the electron impact (EI) mode with an ionization energy of 70 eV, and a scan range of 50–600 Da. 2.4. Qualitative and quantitative analyses of biphenyls and dibenzothiophenes The identification of biphenyl (BP), all methylbiphenyl (MBP), ethylbiphenyl (EBP), and dimethylbiphenyl (DMBP) isomers were determined by comparison of their mass spectra and relative retention times with reported literature data (Alexander et al., 1986, 1991; Cumbers et al., 1987; Trolio et al., 1999). Fig. 2 shows the partial mass chromatograms (m/z 154, 168, 182) of an aromatic

fraction from a sediment extract in this study. The methyl substitution pattern on the biphenyl rings is indicated by the numbers on the corresponding peaks (Fig. 2). A known amount of phenanthrene-d10 (Phen-d10: molecular formula: C14D10; molecular mass: 188.30; purity = 98%, supplied by the laboratory of Dr. Ehrenstorfer, Augsburg, Germany) was added to each sample as internal standard prior to GC–MS analysis. The absolute concentrations of the DMBP isomers were determined by comparison with the peak area of Phen-d10. By comparison with previously published relative retention times and retention indices in m/z 184, 198 and 212 mass chromatograms (Chakhmakhchev et al., 1997; Mössner et al., 1999; Schade and Andersson, 2006), dibenzothiophene (DBT), methyldibenzothiophenes (MDBTs), and dimethyldibenzothiophenes (DMDBTs) can be identified (Li et al., 2012a,b). Correct identification of DMDBT isomers is key to the assessment of the product-precursor relationship between DMDBT and DMBP isomers. In order to firmly identify the DMDBT isomers on the HP-5MS column under the GC conditions in our laboratory, selected authentic DMDBT standards were co-injected in the GC–MS analyses (Fig. 3). The 1,2-, 1,3-, 1,4-, 2,3-, and 2,8-DMDBT standards were purchased from Chiron (Trondheim, Norway) and 2,7-DMDBT from Prof. Jan Andersson’s laboratory (University of Münster, Germany). The results show that the identification of some DMDBT isomers is different from those previously published. For example, 1,2-DMDB apparently has the highest retention index (Fig. 3) and does not co-elute with 1,9-DMDBT. The peak labelled 1,2 + 1,9 by Asif et al. (2009) is actually just 1,7-DMDBT. According to Mössner et al. (1999) and Schade and Andersson (2006) 2,8-DMDBT elutes slightly earlier than 2,7-DMDBT, which in turn elutes very slightly earlier than 3,7-DMDBT. When all three are present they co-elute, which seems reasonable considering their minor differences in retention index. However, in our study the comparison of mass chromatograms with co-injection of the authentic standards indicates that 2,8-DMDBT was separated from the 2,7-isomer (Fig. 3a–c). Chakhmakhchev et al. (1997) identified 4,6-, 2,4- and 1,4DMDBT by co-elution of these three internal DMDBT standards and defined the 4,6/1,4-DMDBT and 2,4-/1,4-DMDBT maturity indicators. However, more and more studies show that the peak of 1,4-DMDBT in the m/z 212 mass chromatogram identified by them should be 1,4-DMDBT and 1,6-DMDBT (Mössner et al., 1999; Schade and Andersson, 2006; Li et al., 2012a,b). Therefore, the thermal maturity parameters 4,6-/1,4-DMDBT and 2,4/1,4DMDBT should be 4,6-/(1,4- + 1,6)-DMDBT and 2,4-/(1,4- + 1,6)DMDBT, respectively. The 4 position on one aromatic ring is symmetric with the 6 position on the other aromatic ring. The 1,4-DMDBT and 1,6-DMDBT also have similar standard molal enthalpies of formation (Richard, 2001). The maturity indicators 4,6-/(1,4- + 1,6)-DMDBT and 2,4-/ (1,4- + 1,6)-DMDBT are based on the thermodynamic stability differences between these isomers. Thus, the co-elution of 1,4- and 1,6-DMDBT has no significant influence on the application of the 4,6-/(1,4- + 1,6)-DMDBT or 2,4-/(1,4- + 1,6)-DMDBT maturity parameters. Fig. 4 shows the distribution of DMDBTs in m/z 212 mass chromatograms for some of the sediment extracts in this study. A known amount of internal standard, dibenzothiophene-d8 (DBT-d8; molecular formula: C12D8S; molecular mass: 192.31; purity = 99.5%, supplied by the laboratory of Dr. Ehrenstorfer, Augsburg, Germany), was added to each sample prior to GC–MS analysis. By comparison with the peak area of C12D8S, the absolute concentrations of DBT and each isomer of MDBT and DMDBT were determined. The response factor of DMDBT to DBT-d8 is 0.97, which means that DMDBT has generally same response in GC– MS analysis (Li et al., 2012a,b).

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Table 1 Total organic carbon content, selected parameters of Rock–Eval pyrolysis, maceral compositions and fraction of extractable organic matter (EOM) for the Es3 and Es2 samples from well SG1. Sample No.

Depth (m)

TOC (%)

Tmax (°C)

Ro (%)

mean

dn1

SG04 SG05 SG06 SG07 SG08 SG09 SG11 SG12 SG13 SG14 SG15 SG16 SG17 SG18 SG19 SG20 SG21 SG22 SG25 SG26 SG27

2314 2352 2404 2459 2485 2518 2653 2672 2730 2783 2815 2888 2914 2956 3111 3239 3306 3372 3752 3819 4052

1.37 1.44 1.72 1.93 2.24 2.11 1.83 1.92 1.91 1.86 1.90 1.76 1.78 1.77 2.13 1.99 3.27 1.87 1.72 1.64 1.42

432 435 433 431 430 430 433 434 434 435 434 435 437 437 437 437 435 439 441 439 440

0.36 0.36 0.33 0.32 0.33 0.33 0.34 0.44 0.45 0.52 0.47 n.d. 0.49 0.44 0.61 0.61 0.58 0.59 0.65 0.68 0.73

0.05 0.05 0.06 0.04 0.03 0.04 0.05 0.03 0.03 0.05 0.05 0.04 0.03 0.03 0.04 0.05 0.03 0.04 0.02 0.03 0.04

S1 (mg/g)

HI

OI

Sap. (%)a

Exi. (%)a

Vitr. (%)a

Inert. (%)a

PI S1/ (S1 + S2)

EOM (mg/ g Corg)

SAT (mg/ g Corg)

ARO (mg/ g Corg)

NSO (mg/ g Corg)

2.37 2.37 4.06 6.34 7.97 8.65 5.42 7.51 6.85 6.57 6.37 6.23 5.57 5.27 7.35 6.34 8.95 5.49 4.02 3.48 2.86

172 164 235 329 356 409 296 391 359 354 336 354 312 298 345 319 274 294 233 213 202

100 93 73 52 53 61 83 75 83 120 74 78 56 99 80 91 64 86 108 122 113

7.4

42.1

3.8

46.7

14.8

51.3

9.5

24.4

6.6

43.3

16.1

33.8

53.0

15.8

7.1

24.4

0.09 0.08 0.05 0.04 0.04 0.03 0.04 0.04 0.04 0.08 0.05 0.05 0.04 0.07 0.06 0.10 0.28 0.13 0.22 0.18 0.15

55.11 40.80 25.72 59.38 42.99 45.97 41.51 61.70 66.43 60.89 66.73 74.46 62.46 67.38 84.32 138.26 203.93 170.68 200.81 151.90 109.2

5.13 3.05 3.83 18.90 8.34 9.14 7.31 18.41 18.91 16.12 17.38 18.51 16.29 17.20 24.73 40.92 9.27 35.92 58.32 18.85 30.70

2.47 2.91 3.01 9.03 4.33 4.67 3.68 6.56 7.30 6.75 7.74 5.75 6.71 6.16 7.34 17.66 4.16 12.00 23.91 9.47 15.10

6.46 9.16 9.71 19.66 12.88 15.60 14.47 21.43 20.22 18.31 20.44 16.80 15.30 18.56 16.18 21.25 3.21 16.00 25.38 9.55 17.15

a Mineral-matter-free basis; Sap: sapropelinite; Exi: exinite; Vitri: vitrinite; Inert: inertinite; S1: hydrocarbons generated by pyrolytic degradation of the kerogen in the rock (mg/g); HI: hydrogen index (mg/g Corg); OI: oxygen index (mg/g Corg); Ro (%): vitrinite reflectance; PI: production index (S1/(S1 + S2)); EOM: extractable organic matter; SAT: saturated fraction; ARO: aromatic fraction; NSO: N–S–O-Compounds; dn1: standard deviation.

Table 2 The absolute concentrations of selected DMDBT and DMBP isomers and selected biomarker parameters. Sample No.

Pr/Ph

G/C30H

Ts/Tm

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

4,6-/1,4DMDBT

4.6-DMDBT (lg/g Corg)

1,4-DMDBT (lg/g Corg)

(4,6 + 2,6 + 2,8)DMDBT (lg/g Corg)

3,30 -DMBP (lg/g Corg)

4,6-DMDBT (%)

2,8-DMDBT (%)

SG04 SG05 S06 SG07 SG08 SG09 SG11 SG12 SG13 SG14 SG15 SG16 SG17 SG18 SG19 SG20 SG21 SG22 SG25 SG26 SG27

0.73 0.71 0.74 0.53 0.44 0.43 0.56 0.72 0.75 1.09 1.02 1.02 0.99 1.40 1.18 1.43 1.11 1.30 1.20 1.09 1.20

0.09 0.10 0.13 0.33 0.40 0.35 0.29 0.27 0.22 0.11 0.16 0.21 0.15 0.13 0.14 0.14 0.13 0.11 0.16 0.15 0.17

0.34 0.34 0.39 0.44 0.45 0.41 0.38 0.48 0.47 0.65 0.67 0.56 0.64 0.75 0.75 0.85 1.11 1.45 2.26 2.45 1.98

0.30 0.28 0.38 0.49 0.46 0.47 0.44 0.49 0.51 0.54 0.54 0.56 0.56 0.56 0.56 0.57 0.55 0.59 0.55 0.54 0.56

0.98 0.78 0.72 0.69 0.53 0.64 0.65 0.78 0.87 0.72 0.92 0.88 0.87 0.83 0.90 0.92 1.02 1.10 1.29 1.50 1.67

0.43 0.25 0.18 0.28 0.18 0.16 0.24 0.68 1.00 0.61 0.88 1.22 0.90 0.86 1.52 3.09 4.62 5.02 6.67 4.68 4.65

0.44 0.32 0.25 0.26 0.34 0.25 0.37 0.87 1.15 0.85 0.96 1.38 1.04 1.04 1.68 3.37 4.53 4.55 5.16 3.12 2.78

0.81 0.48 0.40 0.40 0.51 0.45 0.68 1.60 2.11 1.48 1.75 2.54 1.88 1.93 2.63 5.30 8.31 8.84 11.44 8.06 7.83

0.20 0.12 0.15 1.01 0.65 0.71 0.92 1.36 2.13 1.55 1.58 3.22 2.40 2.72 2.44 4.65 5.99 7.75 9.71 8.70 8.18

52.25 51.51 43.83 35.38 34.45 36.37 35.40 36.43 47.56 41.11 50.49 47.97 47.61 44.61 57.65 58.25 55.61 56.77 58.34 58.04 59.3

28.99 30.24 38.13 40.79 44.75 40.63 38.68 28.03 20.00 29.27 23.79 18.40 17.30 22.92 14.27 8.73 15.77 8.47 10.71 9.74 5.8

Note: Pr/Ph: prystane/phytane ratio; G/C30 H: gammacerane/C30 hopane ratio; Ts/Tm: 18a-22,29,30-trinorneohopane/17a-22,29,30-trinorhopane ratio.

3. Results and discussion 3.1. Summary of geochemical properties The TOC (%) contents, maceral compositions, selected Rock–Eval pyrolysis parameters, extract yields, along with data on fraction compositions are summarized in Table 1. All source rock samples are characterized by total organic carbon contents of 1.37–3.27% with an average TOC content of 1.88%. HI values vary between 164 mg HC/g TOC and 409 mg HC/g TOC (Table 1). All source rocks of the Es3 and Es2 members have relatively low Pr/Ph ratios with an average of 0.92, and high gammacerane

contents with a gammacerane/C30 hopane about 0.19 (Table 2), which indicate suboxic to anoxic and brackish water depositional environments. 3.2. Thermal maturity profile of well SG1 Tmax is the temperature at which the maximum rate of hydrocarbon generation occurs during Rock–Eval pyrolysis (Espitalié, 1986; Burnham and Sweeney, 1989). It increases gradually with increasing vitrinite reflectance. The Tmax values for the rocks from Es3 interval of Well SG1 range from 430–441 °C, indicating that all the samples are low mature to the main phase of the oil generation

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Fig. 5. Variation of (a) Tmax and (b) organic carbon normalized EOM concentration (mg/g TOC) with depth for source rock samples from well SG1. EOM: extractable organic matter.

Fig. 6. Variation of (a) vitrinite reflectance (%, Ro), (b) Ts/Tm and (c) C31 hopanes 22S/(22S + 22R) and with depth for source rock samples from well SG1.

window. The Tmax of well SG1 shows a nearly straight line relationship with depth (Fig 5a). It reaches 435 °C at a depth of about 2850 m, corresponding approximately to the onset of the oil generation window. Vitrinite reflectance (%, Ro) is the most widely used maturity indicator in organic matter maturation assessment, despite uncertainties arising from the sample quality and measurement method. The Ro (%) value of well SG1 is linear with depth on the semilog coordinate (Fig. 6a). A reflectance value of 0.50% generally indicates the onset of the oil generation window. The burial depth corresponding to 0.50% (Ro) for well SG1 is also about 2850 m (Fig. 6a). The 22S/(22S + 22R) ratios of the C31–C35 17a(H)-hopanes are effective maturity indicators for immature to the early oil generation stages. A 22S/(22S + 22R) ratio in the range of 0.50–0.54 indicates that the source rocks barely entered the oil generation window, whilst ratios in the range of 0.57–0.62 suggest that the main phase of oil generation has been reached or surpassed (Peters

et al., 2005). The 22S/(22S + 22R) ratios for C31 17a,21b(H)-hopane show a progressive change with increasing depth and get to the equilibrium value of 0.55 (Zumberge, 1987) at a depth of 2783– 3111 m in well SG1 (Fig 6c). Then it remains relatively constant and the inflection point indicates the onset of oil generation for the Es3 source rocks in well SG1. Thus, the depth of 2850 m is the top of the oil generation window for these source rocks. The Ts/Tm ratios show a nearly straight line relationship with depth (Fig. 6b) and a coefficient of more than 0.95. Fig. 5b illustrates the variations of the normalized extractable organic matter (EOM) concentration (mg/g TOC) with increasing depth. It indicates that the EOM is present at a relatively low concentration (< 60 mg/g TOC) for rocks with a depth lower than 2850 m, and shows a slight increase to 84 mg/g TOC at a depth of 3100 m. Then it follows a drastic increase up to 200 mg/g TOC, which indicates the onset of intensive C15+ hydrocarbon generation in these source rocks.

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Fig. 7. Proposed scheme of reaction pathways of 3,30 -DMBP to 4,6-DMDBT, 2,8-DMDBT and 2,6-DMDBT (after Asif et al., 2009).

Fig. 8. Correlation of the absolute concentrations of (4,6 + 2,6 + 2,8)-DMDBT with those of 3,30 -DMBP (lg/g TOC) in sediments from well SG1.

3.3. Distribution relationship between dimethylbiphenyls and dimethyldibenzothiophenes Based on laboratory experiments and geological observations, Asif et al. (2009) reported that biphenyl and sulfur can react to yield DBT with the catalysis of activated carbon. A relationship between the substitution patterns in MBP and MDBT was observed in the natural system (Asif et al., 2009), and it was also proposed that dimethylbiphenyls (DMBPs) can react to yield the corresponding dimethyldibenzothiophene (DMDBT) isomers. As for example, 3,30 -DMBP can form 4,6-DMDBT, 2,8-DMDBT and 2,6-DMDBT (Fig. 7). The relative distributions of DMDBTs and DMBPs in the sediment extracts are shown in Figs. 2 and 3. The two most abundant DMBP isomers (3,30 - and 3,40 -DMBP) correspond to the most abundant DMDBT isomers (4,6- and 3,6-DMDBT). The absolute abundance relationship between the corresponding individual

Fig. 9. Variations of the total concentrations of 4,6-, 2,8-plus 2,6-DMDBT with depth for selected rock samples from well SG1.

isomers of MBP and MDBT in sediments has been previously reported (Asif et al., 2009). For example, a straight line relationship (R2 = 0.98) between the absolute concentrations of the most abundant isomer, 4-MDBT, to 3-MBP, indicates that the MDBTs have product-precursor links with the MBPs in the sediments. In our study we found that the summed absolute concentrations of 4,6-, 2,8- plus 2,6-DMDBT increase with those of 3,30 -DMBP (Fig. 8). This

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Fig. 10. The depth trend of (a) the relative content of 4,6-DMDBT or 2,8-DMDBT among the three isomers formed from 3,30 -DMBP, and (b) the absolute concentrations (lg/g TOC) of 4,6-DMDBT and 2,8-DMDBT.

straight line relationship (R2 = 0.95) may indicate a product-precursor link between DMDBTs and DMBPs in the sediments (Fig. 8). Fig. 9 shows the depth trend of the absolute concentrations of 3,30 -DMBP and the summed absolute concentrations of 4,6-, 2,8-, and 2,6-DMDBT. The absolute concentration of 3,30 -DMBP is generally < 2.0 lg/g TOC at a depth above 2800 m, with a slight increase up to 3.0 lg/g TOC at a depth of 3100 m. Then a dramatic increase follows to a value of 8.0 lg/g TOC at a depth of 3372 m with a subsequent nearly constant value. The concentrations of 4,6-, 2,8, plus 2,6-DMDBT also show a similar increase with depth, roughly parallel to that of 3,30 -DMBP (Fig. 9). This suggests that the formation reactions of DMBPs and concurrent formation of DMDBTs from DMBPs occurs along with the intensive C15+ hydrocarbon generation from the sedimentary organic matter. 3.4. Reaction pathways for dimethyldibenzothiophenes from dimethylbiphenyls Fig. 7 shows three possible reaction pathways from 3,30 -DMBP to 4,6-, 2,6- and 2,8-DMDBT. The 4,6-DMDBT isomer can be produced by inserting a sulfur atom into the biphenyl ring system through pathway (1) (Fig. 7). The 2,8-DMDBT is produced via pathway (2). The 2,6-DMDBT isomer is probably formed via pathway (3) after rotation across the single bond joining the two benzene rings followed by sulfur insertion. The formation of methyldibenzothiophene may be explained by the sulfur radical mechanism. The S atom could react as electrophiles. The two methyl groups in the 3,30 -isomer activate the ortho-positions (i.e. 2 and 20 ) para-positions (i.e. 6 and 60 ) toward electrophilic attack (Fig. 7). Therefore, the generally equal amount of 4,6-DMDBT and 2,8-DMDBT would be expected. However, due to the greater steric hindrance imposed by the two methyl groups at the 3- and 30 -positions of biphenyl (Fig. 7), the 2,8-DMDBT isomer would be formed preferentially and its relative abundance would be higher than that of 4,6-DMDBT. Fig. 10a illustrates the depth trend of the abundance percentage of 2,8-DMDBT or 4,6-DMDBT among the three possible isomers formed from 3,30 -DMBP. It shows that the percentage of 2,8DMDBT increases gradually with increasing burial depth at low maturity, and has the highest value at about 2600 m, with a gradual decrease with greater depth. The percentage of 4,6-DMDBT, however, has a completely opposite trend with depth (Fig. 10a).

This should result from the relatively higher thermodynamic stability of the 4,6-DMDBT isomer (Budzinski et al., 1993; Richard, 2001), because it should be more stable once formed. The simulation experiments by Asif et al. (2009) indicated that the transformation reactions at higher temperatures favor the generation of the DMDBT with the methyl substituent adjacent to the heteroatom. Thus, with progressive maturation, the predominant isomer should be the thermodynamically more stable 4,6-DMDBT. This suggests that in the low maturity stage the system is controlled by the kinetic process, and in the mature to highly mature stage the thermodynamically stable isomers dominate. Fig. 10b shows the variations of the absolute concentrations of 4,6-DMDBT and 2,8-DMDBT with increasing depth. Both isomers are present at very low concentrations (< 0.5 lg/g TOC) in the immature section (< 2600 m). Then, 2,8-DMDBT remains approximately constant at about 0.5 lg/g TOC below that depth, but 4,6DMDBT is higher than 2,8-DMDBT at depths > 2600 m, increases slightly to 1.5 lg/g TOC at  3100 m, and then shows a drastic increase below that depth. Thus, the concentrations of the different DMDBT isomers are controlled by various factors in sediments, such as the differences in thermal maturity, the amounts of the corresponding DMBPs (and their thermodynamic stability), as well as steric effects. The distributions of methylated PAHs in oils and sedimentary organic matter are controlled by complicated chemical processes, such as methylation, demethylation, methyl migration and disproportionation reactions (Alexander et al., 1985; Strachan et al., 1988; van Aarssen et al., 1999; Bastow et al., 2000). Such methylation reactions would also be expected to occur during the formation of methylated DBTs. This mechanism may explain why the more thermodynamically stable positions are favored and why the random distribution of isomers observed before the onset of catagenesis is replaced by a clear trend in the increasing absolute and relative abundances of the most thermodynamically stable isomers. Similarly, phenylation also may be one of the important mechanisms influencing the natural distributions of phenylated PACs, such as phenylphenanthrene, phenylanthracene, phenyldibenzothiophene and phenyldibenzofuran (Marynowski et al., 2001, 2002; Rospondek et al., 2007, 2009; Li et al., 2012a,b). Large amounts of methyl radicals liberated during the thermally mediated structural changes experienced by kerogen can provide adequate methyl groups. But an equivalent amount of parent

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Fig. 11. Variations of (a) 4,6-/(1,4- + 1,6)-DMDBT and (b) the absolute concentrations of 4,6-DMDBT and 1,4-DMDBT with depth for source rock samples from well SG1.

compounds (unsubstituted DBT for example) is also needed for both alkylation and arylation processes. Due to the insufficient amount of S atom in biota to produce the large amounts of sulfur compounds in petroleum and sedimentary organic matter, a thermal reaction between elemental sulfur and the organic matter of a sediment was proposed to be responsible for the genesis of the sulfur containing organic compounds (Hanson, 1960; Douglas and Mair, 1965). The sulfur bearing constituents of organisms from which the kerogen was formed are proteins. Due to the low content of protein sourced S, extra sulfur must have been incorporated into the organic matter during the formation of source rocks, especially for high sulfur source rocks (Gransch and Posthuma, 1974 and references therein). The relatively high concentrations of DBTs are always associated with anoxic depositional environments with a high sulfur content (e.g. Hughes et al., 1995). Li et al. (2012a) also observed that the concentrations of the total DBTs and benzonaphthothiophenes (BNTs) in oils from carbonate source rocks in the Tarim Basin (northwest China) are as much as five times greater than for the oils from lacustrine siliciclastic source rocks (Beibuwan Basin, South China Sea). Further study is needed to prove which mechanism is correct or both are possible.

from the a-position to the sterically less crowded b-position, may result in the regular change of the MPI-1 values with increasing maturity (Radke et al., 1986). The results of molecular modeling, maturation experiments and geological data indicate that a decrease in unstable isomer abundances is associated with an increase in the stable isomers beginning at the onset of oil generation (Rospondek et al., 2008; Szczerba and Rospondek, 2010). Fig. 11b illustrates the variations of the absolute concentrations of 4,6-DMDBT or 1,4-DMDBT + 1,6-DMDBT with burial depth. Both of isomers are present at very low and variable concentrations in the immature sediments (2200–2600 m). They gradually increase to 1.5 lg/g TOC at 3100 m, followed by an abrupt increase, corresponding approximately to the onset of intense oil generation. Apparently, the increase of the 4,6-/(1,4- + 1,6)-DMDBT ratio with maturation is not attributable to the decrease in the absolute concentration of 1,4-DMDBT, but to the more rapid increase of 4,6-DMDBT. Both of the 4,6-DMDBT and 1,4-DMDBT + 1.6-DMDBT isomers attain the highest concentration at a depth of 3750 m, followed by a decrease. Then the increase of the 4,6-/(1,4- + 1,6)DMDBT ratio is attributable to the more rapid concentration reduction of 1,4-DMDBT. 4. Conclusions

3.5. Distribution of dimethyldibenzothiophenes as maturity indicators Based on the same thermodynamic stability principles as used to explain the utility of the 4-/1-methyldibenzothiophene ratio (4-/1-MDBT ratio: MDR) as a maturity indicator (Radke et al., 1986; Radke, 1988), two DMDBT based parameters, 4,6-/1,4DMDBT and 2,4-/1,4-DMDBT, have been proposed as effective maturity indicators (Chakhmakhchev et al., 1997). The lithology and organic facies seemed to have no significant influence on the distributions of the DMDBTs with increasing maturity for moderate to high maturity oils and sediment extracts. The depth trend of the 4,6-/(1,4- + 1,6)-DMDBT ratio in lacustrine shales with Type II and III kerogen from well SG1 is shown in Fig. 11a. The 4,6-/(1,4- + 1,6)-DMDBT ratio shows no regular trend of change with depth of burial down to about 3000 m. Therefore, the 4,6-/(1,4- + 1,6)-DMDBT ratio is not an ideal indicator to assess the thermal maturity of immature and low maturity source rocks. For maturity indicators based on PAHs, such as the methylphenanthrene related MPI-1 (defined by Radke and Welte (1981)), the steric strain driven rearrangement, where a methyl group is shifted

A strong correlation between the absolute concentration of 3,30 dimethylbiphenyl (3,30 -DMBP) and the summed concentrations of 4,6-dimethyldibenzothiophene (4,6-DMDBT); 2,6-DMDBT and 2,8-DMDBT in a set of source rock extracts suggests their possible original relationship in sedimentary organic matter or oils. The sulfur radical mechanism and thermodynamic stability show that the 4,6-DMDBT and 3,6-DMDBT are favored to form from 3,30 -DMBP and 3,40 -DMBP, respectively. This was supported by the most abundant isomers in the m/z 212 mass chromatograms of most of the samples in our study. The relative concentration of 4,6DMDBT among the three isomers above shows no regular changing trend with increasing maturity at low thermal stress due to its greater steric hindrance toward S atom insertion. However, it is the predominant isomer within the main phase of oil generation due to its relative high thermodynamic stability. Thus, the maturity indicator 4,6-/(1,4- + 1,6)-DMDBT shows a progressive change with depth within the oil generation window, but no regular trend at low maturation levels. The distributions of the DMDBT isomers in sediment extracts depends on the maturity stage, differences in

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thermodynamic stability, the abundance of the corresponding DMBP isomers, as well as steric effects. Acknowledgements The work was funded by the National Natural Science Foundation of China (Grant No. 40972089) and the State Key Laboratory of Petroleum Resources and Prospecting (PRPDX-200801). The authors are grateful for the assistance of Shengbao Shi and Lei Zhu in the GC–MS analysis. Hui Wang of the Liaohe oil field provided much help with sample collection. The author Meijun Li is highly indebted to Dr. Geoffrey S. Ellis for assistance during his stay as a visiting scholar at the U.S. Geological Survey. The authors would like to thank Dr. Leszek Marynowski, Dr. Michael Erdmann and four anonymous reviewers for their comments and constructive suggestions which significantly improved the quality of this paper. Associate Editor – Michael Erdmann References Alexander, R., Kagi, R.I., Rowland, S.J., Sheppard, P.N., Chirila, T.V., 1985. The effects of thermal maturity on distributions of dimethylnaphthalenes and trimethylnaphthalenes in some Ancient sediments and petroleums. Geochimica et Cosmochimica Acta 49, 385–395. Alexander, R., Cumbers, K.M., Kagi, R.I., 1986. Alkylbiphenyls in ancient sediments and petroleums. In: Leythaeuser, D., Rullkötter, J. (Eds.), Advances in Organic Geochemistry 1985. Pergamon Press, Oxford, pp. 841–845, Organic Geochemistry, vol.10. Alexander, R., Ngo, S.H., Kagi, R.I., 1991. Identification and analysis of trimethylbiphenyls in petroleum. Journal of Chromatography A 538, 424–430. Asif, M., Alexander, R., Fazelat, T., Pierce, K., 2009. Geosynthesis of dibenzothiophene and alkyldibenzothiophenes in crude oils and sediments by carbon catalysis. Organic Geochemistry 40, 895–901. Bao, J., Wang, T., Chen, F., 1996. Relative abundance of alkyl dibenzothiophenes in the source rocks and their geochemical significances. Journal of China University of Petroleum 20, 19–23 (in Chinese). Bastow, T.P., Alexander, R., Fisher, S.J., Singh, R.K., van Aarssen, B.G.K., Kagi, R.I., 2000. Geosynthesis of organic compounds. Part V—methylation of alkylnaphthalenes. Organic Geochemistry 31, 523–534. Budzinski, H., Garrigues, P., Radke, M., Connan, J., Rayez, J.C., Rayez, M.T., 1993. Use of molecular modeling as a tool to evaluate thermodynamic stability of alkylated polycyclic aromatic hydrocarbons. Energy & Fuels 7, 505–511. Burnham, A.K., Sweeney, J.J., 1989. A chemical kinetic model of vitrinite maturation and reflectance. Geochimica et Cosmochimica Acta 53, 2649–2657. Chakhmakhchev, A., Suzuki, M., Takayama, K., 1997. Distribution of alkylated dibenzothiophenes in petroleum as a tool for maturity assessments. Organic Geochemistry 26, 483–490. Cumbers, K.M., Alexander, R., Kagi, R.I., 1987. Methylbiphenyl, ethylbiphenyl and dimethylbiphenyl isomer distributions in some sediments and crude oils. Geochimica et Cosmochimica Acta 51, 3105–3111. Douglas, A.G., Mair, B.J., 1965. Sulfur: Role in genesis of petroleum. Science 147, 99– 501. Espitalié, J., 1986. Use of Tmax as a maturation index for different types of organic matter. Comparison with vitrinite reflectance. In: Burrus, J., (Ed.), Thermal Modeling in Sedimentary Basins. Institut Français du Pétrole Exploration Research Conference, pp. 475–496. Ge, T., Chen, Y., 1993. Liaohe Oilfield. Petroleum Geology of China 3. Petroleum Industry Press, Beijing (in Chinese). Gransch, J.A., Posthuma, J., 1974. On the origin of sulphur in crudes. Advances in Organic Geochemistry 1974, 727–739. Hanson, W.E., 1960. Origin of petroleum. In: Gruse, W.A., Stevens, D.R. (Eds.), Chemical Technology of Petroleum. McGraw-Hill, New York, Chapter 5, p. 247. Hu, L., Fuhrmann, A., Poelchau, H.S., Horsfield, B., Zhang, Z., Wu, T., Chen, Y., Li, J., 2005. Numerical simulation of petroleum generation and migration in the Qingshui sag, western depression of the Liaohe basin, northeast China. American Association of Petroleum Geologists Bulletin 89, 1629–1649. Huang, H., Bowler, B.F.J., Zhang, Z., Oldenburg, T.B.P., Larter, S.R., 2003. Influence of biodegradation on carbazole and benzocarbazole distributions in oil columns from the Liaohe basin, NE China. Organic Geochemistry 34, 951–969. Hughes, W.B., Holba, A.G., Dzou, L.I.P., 1995. The ratios of dibenzothiophene to phenanthrene and pristane to phytane as indicators of depositional environment and lithology of petroleum source rocks. Geochimica et Cosmochimica Acta 59, 3581–3598. Kruge, M.A., 2000. Determination of thermal maturity and organic matter type by principal components and analysis of the distributions of polycyclic aromatic compounds. International Journal of Coal Geology 43, 27–51.

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