Distribution and significance of novel low molecular weight sterenes in an immature evaporitic sediment from the Jinxian Sag, North China

Organic Geochemistry 40 (2009) 902–911

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Distribution and significance of novel low molecular weight sterenes in an immature evaporitic sediment from the Jinxian Sag, North China Hong Lu a,*, Tengshui Chen a, Kliti Grice b, Paul Greenwood c, Ping’an Peng a,*, Guoying Sheng a a

State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, Guangdong 510640, China WA Organic and Isotope Geochemistry Centre, Institute for Geoscience Research, Curtin University of Technology, Perth, Australia c John De Laeter Centre of Excellence in Mass Spectrometry and WA Biogeochemistry Centre, University of Western Australia, Crawley 6009, WA, Australia b

a r t i c l e

i n f o

Article history: Received 29 December 2008 Received in revised form 2 April 2009 Accepted 29 April 2009 Available online 5 May 2009

a b s t r a c t An unusual series of C22–C27 monounsaturated sterenes and C24–C30 tetracyclic terpanes (17,21-secohopanes) were detected in relatively high concentrations in an immature evaporitic marl sediment of the Jinxian Sag, Bohai Bay Basin, North China. The site of unsaturation in these novel sterenes is assigned tentatively to the D ring on the basis of mass spectral interpretation, which also distinguishes them from reported unsaturated sterenes. Other hydrocarbon biomarker or stable isotope characteristics are indicative of microbial (e.g. methyl hopanes), phytoplankton or higher plant (depleted d13C values of isoprenoids and hopanes) inputs and an anoxic carbonate depositional environment (hexacyclic hopanes; tetracyclic terpanes). The hydrocarbon composition showed no obvious biodegradation and the relatively high concentration of unsaturated terpenoids (e.g. gammacerene) and low values of other established maturity parameters (Ts/Tm = 0.23; Ro = 0.44%; Tmax = 417 °C), are consistent with sediments of low maturity. The novel, low molecular weight sterenes and the tetracyclic terpanes may be early diagenetic products of microbial sources in a carbonate environment. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Sterane and terpane hydrocarbons are common constituents of sedimentary organic matter and petroleum. Ubiquitous in sediments and oil and containing a broad range of molecular weight (MW) and isomeric forms, these important biomarkers are representative of source and diagenetic complexity. As such, their detection can provide important information about the origin, depositional setting, diagenetic formation, or subsequent alteration of the hydrocarbon composition of oil or petroleum source rocks (Peters et al., 2005). Sterols that are precursors to sedimentary steranes (e.g. D5 and 7 D ) normally occur within a narrow C27–C29 range, although 4methyl analogues and C30 dinosterols also occur. Correspondingly, the most common steroid products of sedimentary processes are the C27–C29 regular steranes and diasteranes. C30 or low MW steranes (e.g. C21, C22, C26) have occasionally been detected and are thought to reflect selective, less common environments of deposition (Wingert and Pomerantz, 1986; Moldowan et al., 1991; Suzuki et al., 1993; Schouten et al., 1994; Requejo et al., 1997; Holba et al., 1998; Bao and Li, 2001).

* Corresponding authors. Tel.: +86 20 85290191; fax: +86 20 85290706. E-mail addresses: [email protected] (H. Lu), [email protected] (P. Peng). 0146-6380/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2009.04.015

The diagenetic pathway leading to the fully saturated steranes includes several steps, including oxidation of sterols to sterones, dehydration to sterenes, isomerization and reduction. Thus, sterenes are key intermediates in the conversion of oxygenated steroids to their more commonly detected saturated and aromatic counterparts (e.g. Mackenzie et al., 1982; Brassell et al., 1984; de Leeuw and Baas, 1986; Peakman and Maxwell, 1988; de Leeuw et al., 1989, 1993). A broad range of unsaturated C27–C29 sterenes, including D4, D5, D8(14) 5a-and D14 5a-sterenes, D7-5a-sterenes, D2-sterenes, D3,5-and DN,22-steradienes, have been detected on occasion in immature sediments (Shi et al., 1988; de Leeuw et al., 1989; Peakman et al., 1989, 1992; van Duin et al.,1996; Amo et al., 2007). Here, we report the occurrence of a novel series of C22–C27 sterenes in an immature evaporitic marl sediment from the Jinxian Sag, Bohai Bay Basin, North China. Interestingly, the same samples also include C24–C30 tetracyclic terpanes (17,21-secohopanes). These are thought to derive from the same microbial lipid precursors as the more common higher MW hopanes (Aquino Neto et al., 1983). A comprehensive review of the hydrocarbon composition of Jinxian Sag sediments revealed the presence of many established aliphatic biomarkers. These were scrutinised for molecular or stable isotopic evidence of potential biological precursors or depositional dependence of the unusual LMW sterenes and tetracyclic terpanes in the Jinxian sediments.

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aromatic and polar (NSO) fractions using silica column chromatography with n-hexane (60 ml), benzene (40 ml) and EtOH (40 ml), respectively. The saturate fraction was further separated into straight chain and branched/cyclic hydrocarbon fractions using 5A molecular sieving.

2. Samples and analysis 2.1. Samples and location Drill core samples were collected from 17 different depths (2366, 2454, 2494, 2459, 2543, 2625, 2708, 2792, 2859, 2899, 2937, 2968, 2940, 3009, 3087, 3211 and 3232 m) spanning the E2k1–E2s4 Formations of the Zhaoxin 2 well in the northern region of the Jinxian Sag, Bohai Bay Basin (Fig. 1). Rocks from this formation are described as having been deposited in a hypersaline lacustrine environment (Bao and Li, 2001; Cai et al., 2005; Zhang et al., 2005). The Jinxian Sag is one of several faulted Eocene depressions in the south part of Bohai Bay Basin (Fig. 1), characterized by carbonate and evaporitic deposits. H2S production and leakage was evident during the drilling of the Zhao-2 well. The sulfur content for some of the crude oils in the region is as high as ca. 15 wt%. Elemental sulfur can be seen on the surface of sediments and in Soxhlet extracts. The Eocene sediments in the Jinxian Sag are subdivided into the Kongdian Formation (E2k), the Oligocene Shahejie Formation (E2s4–E3s1) and the Dongying Formation (E3d;Fig. 1). The carbonate and evaporitic deposits of E2k1 and E2s4 are of Upper Paleocene age and Lower Eocene age, respectively. The structural geology, stratigraphy, and petroleum geology of the Jinxian Sag have been described in detail by Bao and Li (2001), Cai et al. (2005) and Zhang et al. (2005).

2.3. Instrumental analysis Gas chromatography–mass spectrometry (GC–MS) was carried out with an Agilent 5890 gas chromatograph interfaced to an Agilent 5973 mass spectrometer operated at 70 eV, a range of m/z 50– 600 and a cycle time of 1.8 s. The chromatograph was equipped with a DB-5 (film thickness 0.25 lm) fused silica column (60 m  0.25 mm). He was used as carrier gas. Samples were eluted using a temperature programme of 40 °C (1 min) to 315 °C (held 30 min) at 3 °C/min. Selected ion (m/z 218, 191, 329) analysis and multiple reaction monitoring (MRM) were conducted with an Agilent 5890 Series II gas chromatograph interfaced to an Micromass Autospec (UltimaQ) double focussing mass spectrometer. The MRM ion transitions separately monitored included M+ ? m/z 191 and M+ ? m/ z 329 transitions specific to the C25–C30 tetracyclic terpanes (M+: 344, 358, 372,. . .,414) and the M+. ? m/z 218 transitions specific to the C22–C27 sterenes (M+: 300, 314, 328,. . .,370). A 60 m  0.25 mm  0.25 lm DB-5 column was used with He carrier gas at a constant flow of 1.5 ml/min. Cool on-column injection was used with a GC programme of 40 °C (1 min) to 325 °C (30 min) at 3 °C/min. The MS source was run using an electron energy of 70 eV, a filament current of 200 lA and source temperature of 250 °C, The Autospec analyzer was run using an accelerating potential of 8 kV, with an electron multiplier set at 200 V for full scan (FS) and selected ion recording (SIR) analysis and 350 V for MRM

2.2. Sample preparation The samples were crushed and Soxhlet extracted with CH2Cl2/ MeOH (93:7 v/v, 72 h). Sulfur was removed by the addition of activated Cu filings. Asphaltenes were precipitated in n-hexane and isolated by filtration. The maltenes were separated into saturate,

system Formation section

China Beijing

Dongying Formation

38

Bohai Bay Basin

14 5

Jinxian Sag North

1

37

3

4

2

Nanboshe 41-3 Zhaoxian 105 57 39

Ed2

10

Zhaoxin 1 Zhao1x Zhaoxin 2

5

Ed1

Es1 E O G E N E

Shahejie Formation

Es2 Es3 Es4

N

Ek1 South

29 61

Kongdian Formation

35

Ek2 Boundary of south and north

Ek3 0

4

8 km

Fig. 1. Map showing geological setting, stratigraphic section and location of Zhaoxin 2 well and Jinxian Sag in the Bohai Bay Basin, North China.

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C30H G

a) TIC saturate

C29H

nC20

nC15

nC25

35.0

40.0

45.0

50.0 55.0 Ph

60.0

65.0

70.0

75.0

80.0

85.0

90.0

C30H

b) TIC

G

Branched/cyclic C29H pregnane

Pr

G* Tm

C21

Nor-Pr

C22

HH33 HH34

BNH Ts

C21 10 C22

c) m/z 218

7

34 2 1 5

9

8

6

C27-C29 steranes range

C30H G

d) m/z 191

C29H G* Tricyclic Terpane C20 C21 40

C23 C24

Tm

C24Te d b c e BNH g T a f Ts C25

60

80

100

Ret time (min) Fig. 2. GC–MS analysis of evaporitic marl sample (2459 m, E2k; Zhaoxin 2 well, Jinxian Sag, Bohai Bay Basin, North China) showing (a) total ion chromatogram (TIC) of the saturate fraction; (b) TIC of the branched/cyclic fraction; (c) m/z 218; and (d) m/z 191 of the branched cyclic/fraction. Pr = pristane; Nor-Pr = norpristane; Ph = phytane; BNH = bisnorhopane; C29H = C29 hopane; G = gammacerane; G* = gammacerene (tentative); C24TeT = C24 tetracyclic terpane; HH33 = C33 hexacyclic hopane; HH34 = C34 hexacyclic hopane; 1–10, = unspecified C22–C27 sterenes; (a–g) = C25–C30 tetracyclic terpanes.

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analysis. Mass resolution was set at 1000 for FS and (de facto) 600 for SIR and MRM. Component assignments were made on the basis of elution order and MS correlation with MS libraries and literature reports, as well as comparison with the well established profiles of several oil fractions previously characterised in our laboratories. Compound-specific carbon isotope analysis (GC-irMS) was performed with an IsoPrime isotope ratio mass spectrometer interfaced to an Agilent 6890 gas chromatograph. The GC oven was programmed from 50 °C to 310 °C at 3 °C/min with initial and final hold times of 1 and 20 min, respectively. Upon elution, individual compounds were quantitatively converted to CO2 and H2O in the combustion reactor (CuO, 850 °C); water was cryogenically removed with liquid N2 and the isotopic composition of the CO2 was then measured. Isotopic compositions were calculated by integration of the m/z 44, 45, and 46 ion currents of the CO2 peak. The 13 12 C/ C composition is reported relative to a reference CO2 gas of known 13C/12C content. Concentrated straight chain and branched/cyclic fractions were separately analysed using GC–irMS. Average values and standard deviations are reported from at least three analyses.

3. Results and discussion The total ion chromatogram of the saturate fraction of the 2459 m (E2k) Jinxian sediment is shown in Fig. 2a. The n-alkanes ranged from n-C14 to n-C28, maximizing at n-C18, and dominated by the LMW components. Low concentrations of >C22 n-alkanes show no obvious even/odd preference. Phytane is similar in abundance to n-C18 while pristane, in contrast, is much lower in abundance than n-C17. The relatively high concentration of phytane is consistent with an anoxic depositional environment (Peters et al, 2005). The total ion chromatogram of the branched/cyclic fraction of the same sample is shown in Fig. 2b. The fraction is dominated by the regular isoprenoids nor-pristane (nor-Pr), pristane (Pr), and phytane (Ph) and higher MW steroids and hopanoids. 3.1. Steroid distributions The major steroid products were C21 pregnane and C22 homopregnane (Fig. 2b and c). The m/z 218 mass chromatogram also shows a number of relatively abundant C22–C27 monounsaturated sterenes (1–10; Fig. 2c) representing two or more homologous

Peak 1

Peak 3

Peak 5

Peak 7

Peak 9

Peak 10

Fig. 3. Selective mass spectra of sterenes in branched/cyclic fraction of an evaporitic marl sample (2459 m, E2k) from Zhaoxin 2 well in the Jinxian Sag, North China. Peaks numbered with reference to Fig. 2c.

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series. Specific stereochemical assignment was not possible in the absence of correlation with authentic standards. Much lower concentrations of the C27–C29 regular and diasteranes were detected. The mass spectra of selected monounsaturated sterenes are shown in Fig. 3. A prominent M+ (300, 314, 328,. . .370) allowed MW determination. The consistent occurrence of a base peak at m/z 218 reflects a fully saturated A–C ring fragment and the prominent m/z 149 fragment (m/z > 151) suggests a possible 5a(H),14b(H) structure (Peters et al., 2005, p. 223). In contrast to these mass spectra, diasterenes (D13(17)) as well as D14(14–15) and D15(15–16) sterenes have a base peak at m/z 257 attributed to an unsaturated D-ring substituent (Peakman et al., 1989, 1992). The spectra of the present PC22 monounsaturated sterene series show a prominent high MW monounsaturated fragment series with m/z 243 (peak 3; Fig. 3), 257 (peak 5), 271 (peak 7),. . .313, every 57 difference locating the double bond between C-17 and C-20. The m/z 243 and m/z 257 fragments of the C22 and C23 sterenes, respectively, exclude any of the D-ring alkyl substituent, pointing to Dring unsaturation. The sedimentary formation of sterenes reported to date has been attributed to catagenetic side chain cleavage of higher MW steranes (Wingert and Pomerantz, 1986; Requejo et al., 1997). However, formation of the sterenes reported here via a thermally related process is not supported by the low thermal maturity of the Jinxian Sag sediments, implied from some low values of traditional maturity parameters (e.g. Ts/Tm = 0.23; C29Ts/C29H = 0.06)

Table 1 Stable carbon isotopic composition of hydrocarbons in straight chain and branched/ cyclic fractions of 2459 m (E2k) sediments of Zhaoxin 2 well. Compound

d13C (‰)

Dev.

Compound C21 pregnane C22 pregnane C22 pregnane C22 pregnene C23 sterene C23 sterene C25 sterene C26 sterene C27 sterene C26 secohopane C27 secohopane C27 secohopane Tm Bisnorhopane C29 hopane C30 hopane Gammacerene Gammacerane

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

29.6 28.9 28.8 28.7 29.0 29.3 30.2 30.5 30.3 31.1 30.7 30.8 30.8

0.3 0.2 0.2 0.4 0.4 0.4 0.4 0.5 0.4 0.5 0.4 0.5 0.5

Nor-Pr Pr Ph

32.5 31.0 32.4

0.5 0.3 0.2

d13C (‰) 28.1 28.7 28.5 29.8 31.4 30.7 29.7 31.9 30.1 29.8 28.6 28.9 25.3 26.0 25.5 26.3 26.2 29.3

Dev. 0.5 0.5 0.3 0.3 0.3 0.4 0.5 0.5 0.3 0.4 0.5 0.3 0.4 0.1 0.1 0.1 0.7 0.4

and the co-occurrence of high amounts of (tentatively assigned) gammacerene – or a closely related hopene. Furthermore, the low values of vitrinite reflectance Ro (0.44%) and Tmax (417 °C) for the 2459 m sediment of the Zhaoxin 2 well are consistent with low maturity.

Fig. 4. Mass spectra of selected LMW steranes (C20–C26) and C27 sterene in the branched/cyclic fraction of the evaporitic marl sediment from Zhaoxin 2 well and crude oil samples from the Jinxian Sag, North China.

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H. Lu et al. / Organic Geochemistry 40 (2009) 902–911

C30H

350000

G

m/ z 191

300000 250000 200000

C29H

150000 100000 50000 0 88.00

88.50

89.00

89.50

90.00

90.50

91.00

91.50

92.00

92.50

93.00

93.50

94.00

94.50

350000 300000

426→ → 191

250000 200000

3β-Me

2αMe 150000 100000 50000 0 88.00

88.50

89.00

89.50

90.00

90.50

91.00

91.50

92.00

92.50

93.00

93.50

94.00

94.50

2αMe 160000 140000

426→ 205

120000 100000

3β-Me

80000 60000 40000 20000 0 88.00

88.50

89.00

89.50

90.00

90.50

91.00

91.50

92.00

92.50

93.00

93.50

94.00

94.50

Rention time (Mins) Fig. 5. (a) m/z 191 SIR; (b) m/z 426 ? 191 MRM and (c) m/z 426 ? 205 MRM data highlighting C31 2a and 3b methylhopanes in the branched/cyclic fraction of an evaporitic marl sample (2459 m, E2k) from Zhaoxin 2 well.

The C29 sterane ratios 20S/(20S+20R) and abb/(aaa + abb) in this sample were 0.39 and 0.66, respectively, and the 22S/ (22S + 22R) values of C31 and C32 hopanes were 0.72 and 0.62, respectively, which are not typical of low maturity. Thermally stable hopanes were most abundant, but ba hopanes were also detected in low concentration, indicating incomplete isomerisation. The absence of other unsaturated hopanoid products, as well as hopanes displaying the original stereochemistry of their bacteriohopanepolyol precursors, suggests some level of thermal alteration beyond diagenesis. However, recent hypersaline environments are often characterized by high amounts of relatively uncommon sterols, such as D7 sterols (ten Haven et al., 1986). The diagenetic pathway of such sterols, might rapidly lead to formation of 20R and 20S 5a(H),l4b(H),l7b(H)-steranes, provid-

ing relatively immature samples with a mature sterane appearance. Extended 17(H),21(H)-hopanes and hop 17(21)-enes, present in ancient hypersaline environments, can also be altered by non-thermal processes, such as diagenetic isomerisation at C-22 (ten Haven et al., 1986). Therefore, the somewhat anomalous biomarker maturity indicators are consistent with an immature evaluation. The sterenes in the Jinxian Sag exstracts may be early, microbially mediated, diagenetic products. In addition to the detection of methyl and other hopane microbial biomarkers, the high relative abundance of C15–C19 n-alkanes (Fig. 2a) may be attributed to extensive heterotrophy/bacterial reworking and the generally low concentrations of PC25 n-alkanes may be due to low level biodegradation. The depleted d13C values of the isoprenoids

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C2-DBT

C3-DBT

Aromatic fraction Zhaonxin 2 well 2459m, E2k C1-DBT

TIC C4-DBT

C3-BT

C1-C5

Benzo-bisbenzothiophene

benzothienobenzothiophene C2-BT

DBT C4-BT

Fig. 6. TIC of aromatic hydrocarbon fraction of 2459 m (E2k) evaporitic marl sample. DBT = dibenzothiophene.

(cf. n-alkanes; Section 3.3) are consistent with a heterotrophic source (Grice et al., 2005). While microbial metabolites are normally fully saturated, several occurrences of unsaturated products are known. Arthrobacter simplex, for example, has the capacity to produce unsaturated pregnanes from cholesterol sources (Arima et al., 1969; Mahato et al., 1988). The LMW sterenes (e.g. C22 pregnene, Table 1, Fig. 3), present in high concentration in the Jinxian sediment, may be products of similar metabolic processing. Microbial transformation may also account for the strong predominance of terpenoids over steroids (Fig. 2a and b). The low MW C22–C26 sterenes were only detected in the 2459 m sample, but C27 sterenes can be seen in other depth sediments (e.g. 2494, 2899, 3009, 3211 m). A relatively broad distribution of C21–C29 steranes (Fig. 4) in the Jinxian Sag sediments (2494, 2543, 2899, 2940, 2968, 3009, 3087, 3211 m), previously detected in several regional oils (Pan et al., 1991), partially coin-

cident with the C22–C27 range of the novel sterene series in the 2459 m sample, suggests a close source or diagenetic relationship. 3.2. Hopanoid distributions The hopanoid distribution in the m/z 191 mass chromatogram (Fig. 2d) is dominated by 17a, 21b-norhopane (C29H), 17a, 21bhopane (C30H) and gammacerane (G). A peak tentatively assigned as gammacerene (G*, Fig. 2d) was also detected in relatively high abundance and the presence of both 2a- and 3bmethylhopanes in lower concentrations was confirmed from MRM analysis (Fig. 5). The methyl hopanes derive from cyanobacterial or microaerophilic proteobacteria – particularly type 1 methanotrophs (Summons and Jahnke, 1992; Summons et al., 1999).

Fig. 7. Mass spectra of selected C24–C30 tetracyclic terpanes detected in branched/cyclic fraction of the 2459 m (E2k) evaporitic marl sample. Peak assignments (c, e, f and g) relate to Fig. 2d peak assignments.

H. Lu et al. / Organic Geochemistry 40 (2009) 902–911

Carbon number 16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

-24

n-alkanes

Tm

-25

‰ 13C,

-27

C30H

pregnane C21-

-28

C22-

C27-secoH C27-secoH

C22-

-29

C25-sterene C22-sterene

-30 Pr

Ga

C23-sterene

-32 -33

C26-secoH C27-sterene

C23-sterene

-31

C26-sterene NPr

1982). However, the C24–C30 tetracyclic terpanes in the sediments from the Zhaoxin 2 well, as with the C22–C27 sterenes, are inconsistent with a thermal mechanism.

C29H BNH

-26

909

Ph

Fig. 8. Stable carbon isotopic compositions of hydrocarbons in straight chain and branched/cyclic fractions of 2459 m (E2k) evaporitic marl sample.

A high concentration of gammacerane and low Pr/Ph values (0.29–0.61) are consistent with a hypersaline depositional environment (ten Haven et al., 1986; Sinninghe Damsté et al., 1995). The sample suite also showed consistently high abundances of C33–C34 hexacyclic hopanes (HH33, HH34; Fig. 2a, b and d), and alkyl dibenzothiophenes (DBTs) and alkyl benzothienobenzothiophenes (BTBTs Fig. 6), both attributed to anoxic carbonate anhydrite palaeoenvironments (Connan and Dessort, 1987; Hughes et al., 1995). Low concentrations of C20–C24 tricyclic and C24–C30 tetracylcic (a–g; Fig. 2d) terpanes also were evident. These tetracyclic terpanes, or 17,21-secohopanes, have identical spectra (Fig. 7a) to authentic compounds reported by Aquino Neto et al. (1983). While not as common as hopanes, these biomarkers have been detected in a broad range of sedimentary materials and crude oils from carbonate sources (e.g. Aquino Neto et al., 1983). They typically range from C24 to C27, but alkyl side chain extension to C35 is possible (Aquino Neto et al., 1983). The C24 tetracyclic terpane is typically the major component of the series and its high relative abundance in petroleum normally indicates carbonate/evaporite depositional environments (Connan and Dessort, 1987), although a terrigenous source might also be possible (Grice et al., 2001). The tetracyclic terpanes are believed to form via selective scission of the vulnerable C-17 to C-21 bond (see Appendix) of the regular C30 aabb hopanes (Trendel et al., 1982; Aquino Neto et al., 1983). This may occur during early diagenesis or thermal maturation. The ring opening may be microbially mediated during the early stages of diagenesis, with subsequent geochemical reduction to a saturated form (Aquino Neto et al., 1983; Bisseret and Rohmer, 1990, 1993; Bisseret et al., 1990). The capacity of the bacterium Arthrobacter simplex to introduce unsaturation into the hopane structure was demonstrated by the generation of hop-17(21)-ene, 17(21)-epoxyhopane, hop17(21)-en-21-one and 17,21-secohopane-17,21-dione products on its incubation with tritium labelled hopane and bacteriohopane substrates (Tritz et al., 1999). The latter might be precursors of the 17,21-secohopane, in which case the overall mechanism would account for the opening of the D17(21) double bond (Tritz et al., 1999). The hydrocarbon composition of the Jinxian sediments shows no evidence of biodegradation, suggesting that any microbially mediated transformation of hopanoid precursors to tetracyclic terpanes must occur at a very early stage of diagenesis. Alternatively, the 17,21-secohopanes may derive from the thermocatalytic degradation of hopane precursors (Trendel et al.,

3.3. Stable carbon isotopic composition of biomarkers The stable carbon isotopic composition of selected straight chain and branched/cyclic hydrocarbon products from a concentrated branched/cyclic fraction from the 2459 m Jinxian sediment, are shown in Fig. 8 and Table 1. While the full sample suite was analysed, this was the only sample from which reliable d13C data were obtained for the sterenes. The low LMW sterenes (C23–C27), 17,21-secohopanes (C26–C27) and C16–C28 n28.1‰ to alkanes have similar d13C values in the range 31.4‰. The slightly increased depletion of PC22 n-alkanes suggests that these hydrocarbons may be preferentially mineralised. Microbial utilisation of C27–C29 cholesteroids might similarly contribute to the sterenes and 17,21-secohopanes, which 25.3‰ to have similar d13C values to the n-alkanes. The 26.3‰ d13C range for the regular hopanes (Tm, C29 hopane, C30 hopane, and bisnorphopane) is consistent with a heterotrophic bacterial source (Schoell et al., 1992). Sulfur bacteria may also be significant, given the high concentrations of benzothiophenes (Fig. 6) in the aromatic fraction of the 2459 m sediment. The distinct d13C depletion (2–3‰) of Pr ( 31.0‰) and Ph ( 32.4‰) relative to the LMW sterenes and even more so to the regular hopanes is consistent with a phytoplankton chlorophyll a origin (Brooks et al., 1969; Powell and McKirdy, 1973; Didyk et al., 1978). The >1‰ enrichment of Pr relative to Ph may be due to a contribution from a higher plant sources (Goosens et al., 1984). 4. Conclusions Series of previously unknown LMW sterenes reflecting an unsaturated D ring were detected in an upper zone sediment from the Zhaoxin 2 well, Jinxian Sag, Bohai Bay Basin, North China, partly proving the classical diagenetic theory of sterol transformation to sterenes and then steranes. Co-occuring C24–C30 tetracyclic terpanes (17,21-secohopanes) reflected similar d13C values to the LMW sterenes. A high sulfur content – inferred from high benzothiophene concentrations, low Pr/ Ph values, and high concentration of gammacerane and hexacyclic hopanes in the Jinxian sediments are consistent with a hypersaline/anoxic, sulfate-rich carbonate depositional environment. Low values of vitrinite reflectance Ro (0.44%) and Tmax (417 °C), together with a low ratio of Ts/Tm and presence of gammacerene, suggest a low maturity that does not support a thermal mechanism for the origin of the 17,21-secohopanes. The co-occurrence and isotopic signatures of other hydrocarbon biomarkers (e.g. Tm, C29 hopane, C30 hopane and bisnorphopane) indicative of microbial inputs, suggests that the LMW sterenes and tetracyclic terpanes originate from microbially mediated diagenetic processes in anoxic carbonate settings. Acknowledgements G. Chidlow and S. Wang are thanked for technical support. The research was supported by State 973 Project (2006CB701404), CAS Project (KZCX2-YW-114-1), NSFC Project (40873032) and the Earmarked Fund of State Key Laboratory of Organic Geochemistry (SKLOG2008A01). This is contribution No. IS-1078 from GIGCAS. W. Meredith and J.A. Curiale provided helpful comments that significantly improved the manuscript.

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29 28 21

22

19 20 12 11

18 1

13

10

A

5

3 4

26

23

?

27

16

257 243

15

14

8

B

? ?

25

D

C

9

2

17

24

232

7

6

H

H

5α (H),14 (H)-sterene

5α(H)-sterene

12 11 1 2

10

3

4 23

26 13 14

25

5

17 28

χ 21

6

8 7

22

11

31 16

9

1 29

15

22

2

10

23

3

4 20

hopane Appendix Structure of sterenes and tetracyclic terpanes. Associate Editor—C. C. Walters References Amo, M., Suzuki, N., Shinoda, T., Ratnayake, N.P., Takahashi, K., 2007. Diagenesis and distribution of sterenes in Late Miocene to Pliocene marine siliceous rocks from Horonobe (Hokkaido, Japan). Organic Geochemistry 38, 1132–1145. Aquino Neto, F.R., Trendel, J.-M., Restle, A., Connan, J., Albrecht, P., 1983. Occurrence and formation of tricyclic and tetracyclic terpanes in sediments and petroleum. In: Bjorøy, M. et al. (Eds.), Advances in Organic Geochemistry 1981. Wiley, Chichester, pp. 659–667. Arima, K., Nagasawa, M., Bae, M., Tamura, G., 1969. Microbial transformation of sterols. Part I. Decomposition of cholesterol by microorganisms. Agricultural Biology and Chemistry 33, 1636–1643. Bao, J., Li, M., 2001. Unprecedented occurrence of novel C26–C28 21-norcholestanes and related triaromatic series in evaporitic lacustrine sediments. Organic Geochemistry 32, 1031–1036. Bisseret, P., Rohmer, M., 1990. Bromine, N-bromosuccinimide and sulphur induced isomerizations in the hopane series. Tetrahedron Letters 31, 7445–7448. Bisseret, P., Rohmer, M., 1993. Dimethyldioxirane oxidation of isomeric triterpenes of the hopane series. Tetrahedron Letters 34, 1131–1132. Bisseret, P., Armspach, D., Neunlist, S., Rohmer, M., 1990. Oxidation of the triterpenic hopane skeleton by peracids. Tetrahedron Letters 31, 6523–6526. Brassell, S.C., McEvoy, J., Hoffmann, C.F., Lamb, N.A., Peakman, T.M., Maxwell, J.R., 1984. Isomerisation, rearrangement and aromatisation of steroids in distinguishing early stages of diagenesis. Organic Geochemistry 6, 11–23. Brooks, J.D., Gould, K., Smith, J., 1969. Isoprenoid hydrocarbons in coal and petroleum. Nature 222, 257–259. Cai, C.F., Worden, R.H., Wolff, G.A., Bottrell, S., Wang, D.L., Li, X., 2005. Origin of sulfur rich oils and H2S in tertiary lacustrine sections of the Jinxian Sag, Bohai Bay Basin, China. Applied Geochemistry 20, 1427–1444. Connan, J., Dessort, D., 1987. Novel family of hexacyclic hopanoid alkanes (C32–C35) occurring in sediments and oils from anoxic paleoenvironments. Organic Geochemistry 11, 103–113.

5

13 14

9 8

27

24

19

329 12

32

30

18

6

28

26

25 20

19

18 15

27

17 16

24

7 191

21

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