Organic Geochemistry 83-84 (2015) 140–152

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Organic Geochemistry

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Application of sulfur and carbon isotopes to oil–source rock correlation:A case study from the Tazhong area, Tarim Basin, China

http://dx.doi.org/10.1016/j.orggeochem.2015.03.0120146-6380/� 2015 Elsevier Ltd. All rights reserved.

⇑ Corresponding author at: Key Lab of Exploration Technologies for Oil and GasResources of Ministry of Education, Yangtze University, Wuhan 430100, PR China.Tel.: +86 10 82998127; fax: +86 10 62010846.

E-mail address: cai_cf@mail.iggcas.ac.cn (C. Cai).

Chunfang Cai a,b,⇑, Chunming Zhang b, Richard H. Worden c, Tiankai Wang a, Hongxia Li a, Lei Jiang a,Shaoying Huang d, Baoshou Zhang d

a Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, PR Chinab Key Lab of Exploration Technologies for Oil and Gas Resources of Ministry of Education, Yangtze University, Wuhan 430100, PR Chinac Liverpool University, Department of Earth and Ocean Sciences, Liverpool University, Liverpool, Merseyside L69 3GP, UKd Research Institute of Petroleum Exploration and Development, Tarim Oilfield Company, PetroChina, Korla, Xinjiang 841000, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 17 December 2014Received in revised form 11 March 2015Accepted 17 March 2015Available online 30 March 2015

Keywords:Sulfur isotopesIndividual sulfur compoundsCarbon isotopesBiomarkerOil–rock correlationTarim Basin

Up until now, it has been assumed that oil in the Palaeozoic reservoirs of the Tazhong Uplift was derivedfrom Upper Ordovician source rocks. Oils recently produced from the Middle and Lower Cambrian inwells ZS1 and ZS5 provide clues concerning the source rocks of the oils in the Tazhong Uplift, TarimBasin, China. For this study, molecular composition, bulk and individual n-alkane d13C and individualalkyl-dibenzothiophene d34S values were determined for the potential source rocks and for oils fromCambrian and Ordovician reservoirs to determine the sources of the oils and to address whether d13Cand d34S values can be used effectively for oil–source rock correlation purposes. The ZS1 and ZS5Cambrian oils, and six other oils from Ordovician reservoirs, were not significantly altered by TSR. TheZS1 oils and most of the other oils, have a ‘‘V’’ shape in the distribution of C27–C29 steranes, bulk andindividual n-alkane d13C values predominantly between �31‰ to �35‰ VPDB, and bulk and individualalkyldibenzothiophene d34S values between 15‰ to 23‰ VCDT. These characteristics are similar to thosefor some Cambrian source rocks with kerogen d13C values between �34.1‰ and �35.3‰ and d34S valuesbetween 10.4‰ and 21.6‰. The oil produced from the Lower Ordovician in well YM2 has similar featuresto the ZS1 Cambrian oils. These new lines of evidence indicate that most of the oils in the Tazhong Uplift,contrary to previous interpretations, were probably derived from the Cambrian source rocks, and notfrom the Upper Ordovician. Conversely, the d13C and d34S values of ZS1C Cambrian oils have been shownto shift to more positive values due to thermochemical sulfate reduction (TSR). Thus, d13C and d34S valuescan be used as effective tools to demonstrate oil–source rock correlation, but only because there has beenlittle or no TSR in this part of the section.

� 2015 Elsevier Ltd. All rights reserved.

1. Introduction

There is uncertainty about which source rocks generated the oilsproduced from Paleozoic strata in the Tarim Basin, China (Cai et al.,2009a,b; Li et al., 2010 and references therein). Potential sourcerocks for the oils include the Cambrian to Lower Ordovician andthe Upper Ordovician and have been shown to have significant dif-ferences in maturity, biomarkers and carbon and sulfur isotopes(Zhang et al., 2000; Cai et al., 2009a; Li et al., 2010). The oils in theTazhong area are proposed to have been charged during three

periods, including remigration from previously charged oil poolsas a result of subsequent tectonic activity (Zhao et al., 2008; Caiet al., 2009b and references therein).

An oil produced from the Lower Ordovician section in well YM2(YM2-O1) is not associated with H2S and possesses a low gam-macerane/C30 hopane ratio and very low C28 steranes among theC27–C29 steranes. These characteristics have been matched pre-viously with those of the Upper Ordovician source rocks (Zhanget al., 2000; Li et al., 2010). The YM2-O1 oil shows much lighterindividual n-alkanes d13C values than the presumed typicalCambrian derived oils (TD2-Æ and TZ62-S), both of which are notassociated with H2S (Xiao et al., 2005). YM2-O1 oil was thus consid-ered to be an end member oil from Upper Ordovician source rocks (Liet al., 2010). Most Paleozoic oils in the Tarim Basin have biomarkersand individual n-alkanes d13C values that are much closer to those

C. Cai et al. / Organic Geochemistry 83-84 (2015) 140–152 141

reported for the YM2-O1 oil sample than to the TD2-Æand TZ62-S oilsand it has been speculated that they may have been derived from thean Upper Ordovician source rock mixed with small amounts of oilsderived from the Cambrian and Lower Ordovician source rocks (Liet al., 2010, 2015; Yu et al., 2011, 2012; Tian et al., 2012). TheLower Cambrian oil sample from well ZS1C (a lateral well of theZS1 well) has no detectable steranes or terpanes due to high matur-ity, but has individual n-alkane d13C values close to the TZ62 S andTD2 Cambrian oils and was considered to have been derived fromCambrian source rocks (Li et al., 2015).

However, these conclusions are contradicted by the followingpublished observations: (1) There are no reports of UpperOrdovician source rocks having d13C values as light as the oils(Zhang et al., 2006; Yu et al., 2011). Indeed it is notable that theC15+ saturates are reported to have lighter d13C values in oils thatwere derived from Precambrian and Cambrian source rocks thanoils derived from Ordovician source rocks based on 22 global oilsamples (Andrusevich et al., 1998). (2) The spatially and strati-graphically limited distribution of the Upper Ordovician sourcerock in the Tarim Basin does not support the discovered petroleumresources (Cai et al., 2009a; Li et al., 2010; Yu et al., 2011). (3) Thed13C value of an oil may be altered by TSR (Cai et al., 2001, 2003,2008, 2009b). Petroleum is shifted to heavier d13C values withincreasing TSR as a result of the preferential thermal cleavageand oxidation of 12C bonds (Krouse et al., 1988; Sassen, 1988;Rooney, 1995; Manzano et al., 1997; Cai et al., 2003, 2004, 2013).Thus, d13C values can be only used for direct correlation of thesource rocks and oils where the oils are not TSR altered.

Sulfur isotopes have been used for the correlation of sourcerocks and oils not altered by TSR in a rapidly buried basin(Thode, 1981; Orr, 1986; Cai et al., 2009a,b). In such a basin, hydro-carbons are generated rapidly and peak oil is likely to occur undersemi-closed to closed conditions. This feature, along with high H2Ssolubility and rapid sulfur isotope homogenization, are believed toresult in small differences (up to 2‰) in d34S values betweenmature kerogens and their generated oils in the case studies andexperimental simulation (Cai et al., 2009a and references therein).The Cambrian derived oils (including the TD2-Æ and TZ62-S oils)have d34S values ranging from +11.9 to +20.5‰, which are closeto values reported for Cambrian source rocks that range from10.4–19.4‰ (Cai et al., 2009a,b). However, d34S values of the oilspresumed to have been derived from the Upper Ordovician, with-out associated H2S, have not been reported. It is not clear if theseoils, including the YM2-O1, have d34S values that match with theUpper Ordovician source rocks.

New clues may be supplied by oils recently produced from theCambrian from wells ZS1 and ZS5. The ZS1 oils (Æ2a) have the ‘‘V’’shaped distribution of C27–C29 regular steranes and d13C valuesthat match well with most of the Ordovician oils in the Tazhongarea (Cai, 2013; Yang, 2014; Li et al., 2015). As it is unlikely for thisMiddle Cambrian oil to have been generated from an UpperOrdovician source, the ZS1, and by inference most of theOrdovician oils, are proposed to have been derived from aCambrian source (Cai, 2013; Yang, 2014). In contrast, Li et al.(2015) consider the similarity in the distribution of C27–C29 regularsteranes and d13C values between the ZS1 oils and the YM2-O1 oilas evidence for the ZS1 oils to be derived from the UpperOrdovician source rock. If the YM2-O1 oil is not an end memberoil typical of generation from the Upper Ordovician, the model pre-dictions of Li et al. (2015) are based on an incorrect premise.

To resolve this conflict, biomarkers, bulk, saturates, aromaticsand individual n-alkanes d13C, and bulk and dibenzothiophenes(DBTs) d34S values of four oils produced from the Cambrian, twelveoils from the Ordovician and one oil from the Silurian, along witheight Cambrian source rocks, were analyzed. The specific objec-tives of this study are to determine: (1) if two facies of source rocks

exist in the Cambrian with different d13C values and biologicalmarker characteristics; (2) whether the YM2-O1 oil was derivedfrom the Cambrian source rocks; and (3) from what source rocksthe Lower Ordovician oils were derived.

2. Geological setting

The Tarim Basin has an area of 560,000 km2 and the maximumaccumulative thickness of �16,000 m for Ediacaran–Paleozoicmarine deposits and Mesozoic–Cenozoic terrigenous deposits(Cai et al., 2009a) (Fig. 1). The Tazhong Uplift is an inheritedpaleo-structural high in the central Tarim Basin and an importantarea of oil and gas exploration and development with an explo-ration area of � 2.2 � 104 km2 (Fig. 1). The tectonic history beganwith oceanic spreading during the Cambrian–Early Ordovician. ANW trending, basement involved fault system developed duringthe Caledonian Orogeny at the end of Early Ordovician, resultingin the formation of the tectonic framework and a ramp-edge typecarbonate platform. NW-SE directed tilting occurred during theLate Ordovician resulting in erosion of the eastern area (Jia,1997; Ren et al., 2011). The continuous compressive stress resultedin NNE strike slip faults at the end of the Devonian (Wu et al.,2009). From the end of the Caledonian to the early Hercynian (lateDevonian), the area underwent a period of uplift and tectonicadjustment. As a result, the Devonian, Silurian and evenOrdovician strata were eroded and formed the most importantunconformity in the study area. During the Indosinian–Himalayan Orogeny (J3 – N), the Paleozoic structures did notchange although there were overall fluctuations in the area (Jia,1997).

The general stratigraphic column of the Tazhong Uplift (Fig. 2) isdescribed in Cai et al. (2001, 2009a,b) and Li et al. (2010). Briefly, theCambrian strata consist of the Lower Cambrian Xiaoerbulakeand Wusonggeer formations, the Middle Cambrian Shayilike andAwatage formations and the Upper Cambrian Qiulitage Fm.(Fig. 2). The Lower Cambrian section is composed of platform andplatform-margin dolomites with intercalated dark mudstones.The Middle Cambrian is a supratidal, anhydrite bearing dolomiteand anhydrite dominated section. The Ordovician is predominantlycomposed of open platform facies dolomite, packstone and bioclas-tic limestone with reef and shoal facies grainstone in the UpperOrdovician. The Silurian to Carboniferous sequence is composedof marine sandstone and mudstone with intercalated micrite andbioclastic limestone in the Carboniferous. The Permian consists oftransitional terrigenous-marine facies, gray or brown sandstonesand mudstones with intercalated volcanic rocks. The Mesozoicand Cenozoic are mainly composed of terrigenous sandstones andmudstones.

The Cambrian and Lower Ordovician source rocks include theCambrian abyssal to bathyal facies mudstone and shale, theCambrian evaporated lagoon facies, anhydritic dolomite andargillaceous dolomite, and the Middle and Lower OrdovicianHeituao Formation under compensated basin facies mudstoneand shale. The source rocks are widespread in the basin [Fig. 10aand b of Li et al. (2010)], and contain 1.0–3.0% TOC, predominantlyof type I to II1 kerogen, with vitrinite reflectance equivalences(VRE) ranging from 1.5–2.3%, as determined from solid bitumenreflectance measurements (VRE = 0.618Rb + 0.40) (Cai et al.,2009a; Li et al., 2010 and references therein). Peak oil generationfrom the Cambrian source rocks occurred during the lateCaledonian–early Hercynian period (S1 – D3), and from theOrdovician Heituao Formation during the late Hercynian (C2 – P1)(Zhao et al., 2008).

The Middle and Upper Ordovician (O2-3) source rocks include theSaergan and Lianglitage formations. The Saergan Formation (O2-3s),

Fig. 1. Map showing geological structures of the Tazhong Uplift and locations of sampled wells and cross sections AB and CD in Fig. 3.

142 C. Cai et al. / Organic Geochemistry 83-84 (2015) 140–152

a marginal shelf to basin facies of black mudstones and shales, hasTOC generally greater than 1.0% and occurs in the Keping Uplift andAwati Depression. The Lianglitage Formation (O3l), a marginal plat-form to slope facies of lime-mud mound marlstones and argilla-ceous limestones, is found in the Tazhong and Tabei Uplifts. TheLianglitage Formation (O3l) source rock has TOC values gener-ally < 0.8%, contains type I and type II2 to III kerogen and has signifi-cant stratigraphic (vertical) and lateral variability in thickness andTOC (Cai et al., 2009a). The O2-3 source rocks have VRE mainly from0.81–1.3% and reached peak oil generation during the lateYanshan–Himalayan period (K2 – N) (Zhao et al., 2008).

Petroleum is produced from the Carboniferous, Silurian, LowerOrdovician (O1y) and Middle and Upper Ordovician (O2yj and O3l)and recently from Cambrian reservoirs. Condensates are producedfrom the Middle Cambrian Awatage Fm. (Æ2a) and gases were

discovered in the Xiaoerbulake Fm. (Æ1x) in the well ZS1.Condensate oils and gases are produced from the Wusonggeer–Xiaoerbulake (Æ1wx) Formation in wells ZS1C (a sidetrack off wellZS1) and ZS5 (Fig. 3A and B). Faults cross cutting the Cambrianand Ordovician, and their associated fractures, are considered tobe the dominant conduits for upward hydrocarbon migration fromthe Cambrian to the Ordovician (Cai et al., 2001; Lü et al., 2004).

3. Sampling and analytical methods

3.1. Sampling

Eight samples have been collected from potential LowerCambrian source rocks including one mudstone at a depth of

Fig. 2. General stratigraphic column of the Tazhong Uplift (modified from Li et al., 2010).

C. Cai et al. / Organic Geochemistry 83-84 (2015) 140–152 143

5790 m from the XH1 well in the Tabei Uplift. Seven mudstone,shale and muddy dolomite samples from YJK, XEBLK and SGTBLKoutcrops in the Keping area in the west of the Tabei Uplift and fromKLTG outcrop in the NE Tarim basin also were collected (Fig. 1A).These samples represent different organic facies and were ana-lyzed for TOC, bulk kerogen d13C and d34S values, bulk rockextracted organic matter (EOM) and saturated and aromatic hydro-carbon d13C values. Saturated hydrocarbons from one of the sam-ples from the XEBLK outcrop was analyzed using GC–MS.

In order to eliminate the masking effects of secondary alter-ation, the oil samples selected for the biomarker, d13C and d34S

analyses were not biodegraded and are not associated with signifi-cant amounts of H2S (i.e. H2S concentration for all samplesis < 0.25%). Three other oils with slightly higher concentrations ofassociated H2S were analyzed for comparison to assess the impactof TSR on these geochemical parameters. Samples analyzed includeone oil produced from the Silurian, eight oils from the UpperOrdovician (O3l), four oils from the Lower Ordovician (O1y) and fouroils from the Cambrian (Tables 1 and 2). One sour Cambrian oilfrom well ZS1C and two sour Ordovician oils from wells TZ83and ZG511, have associated H2S concentrations of 11%, 2.6% and2.4%, respectively. All other Cambrian and Ordovician oils have

Fig. 3. (A) Cross section AB showing location of ZS1 well. Note that no stratigraphic juxtapositions caused by faulting indicating that the oil may not have been derived fromthe Upper Ordovician. (B) Cross section CD showing distribution of wells ZS1 and ZS5 with oil distribution in the Middle Cambrian and gas in the Lower Cambrian (see legendin A).

144 C. Cai et al. / Organic Geochemistry 83-84 (2015) 140–152

no significant H2S. The oils, except YM2-O1 oil, were collected fromdifferent areas of Tazhong Uplift. YM2-O1 oil was collected fromthe Lower Ordovician in the Tabei Uplift.

3.2. Biological marker analyses

About 80 g source rock samples were powdered for 3 min usinga grinding mill and then Soxhlet extracted using dichloromethane

(DCM) for 72 h. The extracts and whole oils were separated intosaturates, aromatics, resins (NSO) and asphaltenes by column chro-matography using n-pentane, DCM and methanol as chromato-graphic solvents. The saturated and aromatic fractions wereanalyzed using a Hewlett Packard 6890GC-5973MSD mass spec-trometer. The gas chromatograph (GC) was fitted with a HP-5MScapillary column (30 m � 0.25 mm � 0.25 lm). The injection tem-perature was 300 �C and the oven was initially held at 50 �C for

Table 1Biomarker parameters of the Cambrian and Ordovician source rocks and oils.

SampleNo.

Strata Pr/Ph**

Gm/C30H

C27

20R%C28

20R%C29

20R%Ts/(Ts + Tm)

C24Te/C26TT

C23TT/C30H

C21TT/C23TT

C29Ts/(C29Ts + C29H)

C29 aaaS/S + R

SourceRocks

TD2* Cambrian 1.06 0.27 28.4 31.7 39.9 0.44 0.48 0.46 0.82 0.23 0.54XEBLK Cambrian 0.67 0.18 29.0 26.4 33 0.47 0.44 0.49 0.52 0.26 0.49TZ12* U. Ordo. 1.30 0.05 32.1 24.3 43.6 0.59 1.63 0.38 0.67 0.42 0.52Z11* U. Ordo. 1.28 0.06 24.3 20.9 54.8 0.22 6.23 0.05 1.08 0.13 0.44

Oils ZS1-L Cambrian 0.92 0.16 36.8 18.1 45.2 0.63 0.72 0.79 0.55 0.37 0.50TZ243 U. Ordo. 0.82 0 27.9 15.9 43.2 0.59 0.97 0.92 0.71 0.35 0.53TZ44 U. Ordo. 0.86 0.18 21.8 26.5 42 0.35 0.66 0.82 0.66 0.23 0.49ZG45 L. Ordo. 0.70 0 32.2 20.3 37.5 0.73 2.02 0.40 0.47 0.41 0.50YM2 L. Ordo. 0.70 0.12 27.5 17 44.8 0.34 0.96 0.56 0.50 0.20 0.51

Note: Gm: gammacerane, C30H: C30 hopane.* From Cai et al. (2009a).

** Calculated from m/z = 85.

Table 2d13C and d34S values of source rocks and oils.

Well location Depth (m) Strata Lithology TOC (%) S (%) Den d13C d34S

Bulk oil/kerogen Saturates Aromatics Bulk oil/kerogen

Source rocks LN46 6160 O3l Marlstone 0.08 – �28.9� �30.7� �15.3*

TZ12 4669 O3l Bl. shale 0.68 – �26.2� �32.5� 6.8*

TZ72 5061.5 O3l Marlstone 0.42 – – – 3.81*

He3 4042 O3l Micrite 0.93 – – – 5.80*

YG-08 Outcrop O2-3s Shale 4.40 6.83*

SGTBLK Outcrop Æ1 Bl. shale 1.06 �34.7 �31.5 �31.8 –TC1 5714.55 Æ Dolomite 0.80 – – – 10.4*

TC1 6421 Æ3 Dolomite 0.09 – �27.9� �28.5� –TC1 7124 Æ1 Dolomite 0.12 – �29.7� �30.4� –XH1 Core Æ1 Bl. mudstone 6.10 �34.2 �31.1 �29.1 –KN1 4886 Æ1 Marlstone 0.14 – �28.8� �30.2� –KN1 5503 Æ1 Marlstone 2.04 – �28.4� �31.1� 17.8*

He4 5350.7 Æ1 Bl. dolomite – – �28.9 �28.0 –XEBLK(KP) Outcrop Æ1 Bl. dolomite – – �29.5 �28.0 –KLTG(Y2–15) Outcrop Æ1 Bl. shale – – – – 21.6KLTG(Y2–34) Outcrop Æ1 Bl. shale – – – – 21.3XEBLK(XD-Y) Outcrop Æ1 Bl. shale – �34.1 �32.6 �31.8 20.8XEBLK(XK2) Outcrop Æ1 Bl. shlae 0.77 �35.3 �32.2 �31.4 18.0XEBLK(Y4) Outcrop Æ Bl. shale 3.31 – – – 13.8*

TD2 4770.5 Æ Bl.mudstone 1.70 �26.8** – – 19.4*

YJK Outcrop Æ1 Bl. shale 0.39 �28.5 – – 14.0

Oils TZ44 4822.00 O3l 0.28 0.795 �31.8 �31.8 �31.5 14.4TZ623-H2 4694–5793 O3l 0.14 0.807 �31.9 �31.8 �31.2 –TZ821–1 5218–5252 O3l 0.08 0.797 �31.4 �31.5 �30.0 –ZG14–1 6133–6298 O3l 0.19 0.794 �31.5 �31.4 �29.7 13.2ZG511 4824–5022 O3l 0.808 �31.1 �31.2 �30.0 19.9ZG45 5637–5650 O1y 0.19 0.814 �31.4 �31.1 �29.9 12.9TZ83 5666–5681 O1y 0.36 0.82 �30.3 �29.8 18.5*

TZ201C 5430–5779 O1y 0.28 0.805 �31.4 �31.2 �30.2 21.2ZS1 6426–6496 Æ2 – – �33.5 �33.7 �31.1 18.1ZS1 6439–6458 Æ2 – 0.789 �33.0 �33.1 �30.8 23.3ZS1C 6806–6955 Æ1wx – – �29.9 �30.4 �30.0 –ZS5 6562–6671 Æ1wx – – �34.7 �35.2 �32.1 25.9TZ62 4053 S – 0.93 �28.8# �29.4# 17.2*

TD2 4801 Æ – 1.02 �28.5 �29.3 �28.1 19.6*

YM2 5494–5953 O1y – – �33.5# �34.0# – 17.3ZG54 4113–4193 S – – 14.2

Note: –: not available; Den and Bl are density and black in short, respectively.� From Zhang et al. (2006).* From Cai et al. (2009a).

** From Zhang et al. (2004).# From Jia et al. (2013).

C. Cai et al. / Organic Geochemistry 83-84 (2015) 140–152 145

1 min. The temperature was then increased from 50–310 �C at arate of 3 �C/min, and then held at 310 �C for 18 min. Helium wasused as the carrier gas (1.0 ml/min). Operating conditions were:ion source, 230 �C; emission current, 34.6 lA; quadrupole tem-perature, 150 �C and electron energy, 70 eV.

3.3. Whole oils and fractions stable carbon isotope analyses

Stable carbon isotopic compositions of the whole oils, saturatedand aromatic fractions were determined following proceduressimilar to those described by Sofer (1980). Carbon dioxide was

146 C. Cai et al. / Organic Geochemistry 83-84 (2015) 140–152

prepared by combusting (850 �C, 2 h) aliquots (0.5–1 mg) of petro-leum samples in clean, evacuated quartz tubes containing Cu(II)O,Ag and Cu metals. Following combustion the samples were allowedto cool slowly (1 �C/min) to room temperature in order to ensurereduction of any nitrous oxides. The resultant CO2 was separatedcryogenically and carbon isotope ratios were measured using aVG SIRA 12 mass spectrometer. All data were corrected for 17Oeffects (Craig, 1957) and reported in conventional delta (d) nota-tion in per mil (‰) relative to VPDB. Accuracy and reproducibilityof carbon isotopic data were assessed by replicate analysis of theinternational standard NBS 22. The mean of eight replicates(�29.60‰) was nearly identical with a precision of ± 0.042‰ andwithin the experimental error reported by Gonfiantini et al. (1995).

3.4. Compound specific stable carbon isotope analyses

For compound specific d13C analyses, a method similar to Liet al. (2010) was used. Normal alkanes were isolated from the satu-rated hydrocarbon fractions of the oils with 5 Å molecular sieves.Analyses were carried out on a Micromass IsoPrime mass spec-trometer attached to a HP 6890 GC. Separate was made using a60 m � 0.25 mm i.d. capillary column coated with 0.25 lm 5%phenylmethylsilicone stationary phase. The GC oven was pro-grammed from 50–310 �C at 3 �C/min with initial and final holdingtimes of 1 min and 30 min, respectively. Helium was used as thecarrier gas at a flow rate of 1 ml/min with the injector operatingat constant flow.

The d13C values were calculated by the integration of themasses 44, 45 and 46 ion current counts of the CO2 peaks producedby the combustion (copper oxide reaction furnace at 850 �C) ofhydrocarbons separated by GC. A CO2 reference gas (calibratedrelative to the PeeDee Belemnite (‰, PDB) with a known d13C valuewas pulsed into the mass spectrometer and the isotopic com-position of samples was reported in the d notation relative to thereference gas. The average values of at least two runs for each sam-ple are reported and only results with a standard deviation of lessthan 0.3‰ were used.

3.5. Bulk kerogen and oil sulfur isotope analysis

The methods for separation of kerogen and analysis of sulfurisotopes of kerogen and oil were reported by Cai et al. (2009a).Pyrite was removed from the kerogen by adding a mixture of hot6N HCl and CrCl2 to the ground dry kerogen under a nitrogen flowwith the gas flow carrying the H2S to a trap where it was recoveredas Ag2S. Excess acids and acid soluble salts were removed from theresidual kerogen by water washing. About 2 h later the residualkerogen was collected and reground to expose new pyrite surfacesand the whole procedure was repeated once more. After the twotreatments, the residual kerogen was further analyzed using X-ray diffraction (XRD) to determine whether pyrite was below thedetection limits (60.5% depending on conditions). If not, moretreatments were employed.

A known weight (between 350 mg and 900 mg) of kerogen iso-late or 1–4 g oil, was combusted in a Parr bomb apparatus at�25 atm oxygen to oxidize organically bound sulfides to sulfate.Dissolved sulfate was then precipitated as BaSO4 and weighed togive total residual kerogen sulfur. Dissolved iron was measuredat pH < 2, using atomic absorption spectrometer, to determinethe maximum residual pyrite content in the kerogen after the chro-mium reduction (assuming that all Fe occurs as pyrite in the kero-gen). Only when residual kerogen samples contain contaminationpyrite sulfur/total sulfur < 0.08, the produced BaSO4 were analyzedfor d34S to guarantee low errors (depending on the differences ind34S value between kerogen and the associated pyrite).

3.6. Individual dibenzothiophene sulfur isotope analysis

To determine the sulfur isotope ratios of individual diben-zothiophenes, the aromatic fractions of the oils were separatedby a GC (Clarus 580 Perkin Elmer, MA, USA) coupled to aNeptune plus multi-collector inductively coupled plasma massspectrometer (MC-ICPMS, Thermo Scientific, Bremen, Germany)located at the Hebrew University, Jerusalem. The system employedsimilar conditions to those described in details in Amrani et al.(2009, 2012) and Said-Ahmad and Amrani (2013). Duplicates forsome of the oils have been measured and the standard deviationbetween the two duplicates was usually better than 1‰.

4. Results

4.1. Biomarkers

Organic matter extracted from the Cambrian source rock in theXEBLK outcrop has maturity related parameters C29 aaa sterane20S/(20S + 20R) ratio of 0.49, C29Ts/(C29Ts + C29H) of 0.26 and Ts/(Ts + Tm) of 0.47 (Table 1). These values are similar to the reportedvalues from the Cambrian in TD2 well and from the UpperOrdovician in Z11 well (Cai et al., 2009a) and are lower than, orclose to, the respective equilibrium values (Peters et al., 2005), sug-gesting that the values are not commensurate with the vitrinitereflectance equivalent (Ro) values (1.5–2.3%), overmature charac-teristics for the Cambrian source rocks (Wang et al., 2003).

The XEBLK sample shows some biological precursor relatedparameters similar to previously reported ratios of the Cambriansource rocks, i.e., higher C23/C21 tricyclic terpane (> 1) and gam-macerane/C30 17a, 21b-hopane (0.18) ratios, and lower Pr/Ph(close to 1.0) and C24Te/C26TT ratios (< 1.0) than those from theUpper Ordovician (Zhang et al., 2000; Li et al., 2010; Cai et al.,2009a,b). However, this sample shows a ‘‘V’’ shaped distributionof C27–C29 20R steranes, i.e., C27 > C28 < C29, or lowest percentageof C28 aaa 20R among C27–C29 steranes (Fig. 4). Other source rocksreported from the Cambrian show C27 6 C28 < C29 (Cai et al., 2009a;Li et al., 2010, 2015). That is, the saturates extracted fromCambrian source rock from XEBLK (XK2) outcrop show C27–C29

sterane distribution different from other Cambrian source rocksbut similar to the Upper Ordovician (Table 1).

With the exception of the TZ44 oil (Table 1; Fig. 4), oils from theZS1, TZ243, ZG45 and YM2 wells have the same distribution pat-tern of C27 > C28 < C29. All oils possess C21/C23 tricyclic terpane < 1,Pr/Ph < 1.0 and C24Te/C26TT < 1.0 (except for an abnormal value of2.02 from ZG45 oil). The ZS1 oil shows a gammacerane/C30 hopaneratio similar to YM2 and TZ44 oils but higher than the other oils(Table 1).

4.2. Carbon isotopes of bulk oil/kerogen, EOM and oil fractions andindividual n-alkanes

4.2.1. Carbon isotopes from source rocksSource rocks from the Lower Cambrian in XH1 well, XEBLK and

SGTBLK outcrops have d13C values ranging from �31.1‰ to�32.2‰ for saturated hydrocarbons, �29.1‰ to �31.8‰ for aro-matic hydrocarbons, and �33.7‰ to �35.3‰ (n = 5) for kerogens(Fig. 5; Table 2). The values are significantly lighter than those ofCambrian source rocks at wells TC1, He4 and KN1 that yield satu-rated hydrocarbon d13C values ranging from �27.9‰ to �29.7‰

(Cai et al., 2002, 2009a; Zhang et al., 2006), and from the UpperOrdovician in LN46 and TZ12 wells that yield saturated hydrocar-bon d13C values ranging from �26.2‰ to �29.0‰ (Zhang et al.,2006).

Fig. 4. Partial GC–MS chromatograms (m/z = 217) for three oils and extractable organic matter from the Cambrian source rock (XEBLK) (XK2, TOC = 0.77%).

C. Cai et al. / Organic Geochemistry 83-84 (2015) 140–152 147

4.2.2. Carbon isotopes of crude oilsEight crude oils from the Ordovician have saturated hydrocar-

bon d13C values ranging from �30.3‰ to �31.8‰, with an averageof �31.3‰, and aromatic hydrocarbon d13C values ranging from�29.8‰ to �31.6‰, with an average of �30.3‰. Two ZS1 middleCambrian oils from different depths show lighter d13C values of�33.1‰ and �33.7‰ for saturated hydrocarbons, �31.1‰ and�30.8‰ for the aromatic fractions. These values are close to thosemeasured for extracts of the Lower Cambrian source rock in XH1well, XEBLK and SGTBLK outcrops from �31.1‰ to �32.6‰ andfrom �29.1‰ to �31.8‰ (n = 4), respectively. However, the

saturated hydrocarbon fractions are significantly lighter than thosefrom TC1, He4 and KN1 Cambrian source rocks (�27.9‰ to�29.4‰) and than those from LN46 and TZ12 Upper Ordoviciansource rocks (�28.9‰ and �26.2‰, respectively) (Fig. 5).Compared to the ZS1 Cambrian oils, ZS5 Lower Cambrian oil showrelatively light d13C values with bulk oil of �34.7‰, saturatedhydrocarbons of �35.2‰ and aromatic hydrocarbons of �32.1‰.In contrast, the ZS1C Cambrian oil has d13C values of about�30.0‰ for bulk oil and saturated and aromatic hydrocarbons(Table 2), being much heavier than ZS1 and ZS5 Cambrian oils,but close to the Ordovician oils.

-33

-32

-31

-30

-29

-28

-27

-36 -34 -32 -30 -28 -26

δ13 C

ar(‰

)

δ13Csat (‰)

O3l oil

O1y oil

Є oil

O3l source rock

Є source rock

ZS1

TD2

ZS5

ZS1C

Fig. 5. Cross plot showing d13C values of saturates and aromatics of extractableorganic matter and oils.

148 C. Cai et al. / Organic Geochemistry 83-84 (2015) 140–152

The ZS1 oils from the Cambrian have individual C14–C28 n-alka-nes d13C values from �33.8‰ to �36.5‰ (Fig. 6), which are close tothe YM2 oil, and much lighter than the TD2-Æ oil. Interestingly, theZS1C oil has C14–C31 n-alkane d13C values from �30.2‰ to �28.8‰

(Li et al., 2015), similar to the TD2-Æ oil. All other oils analyzedhave the C14–C31 n-alkanes d13C values predominantly from�33.0‰ to �35.0‰, ranging between the ZS1 oils and the TD2-Æoil (Fig. 5).

4.3. Sulfur isotopes of bulk oil and individual sulfur compound

Source rock kerogens from the Cambrian from KLTG, YJK andXEBLK outcrops were measured to have d34S values ranging from14.0‰ to 21.6‰ with an average of 19.1‰ (n = 5) (Table 2). Thevalues are close to those of the Cambrian kerogens from TD2,KN1 and TC1 wells and XEBLK (Y-4) outcrop that yield d34S valuesranging from 10.4‰ to 19.4‰ (n = 4, Cai et al., 2009a).

Bulk oils have d34S values from 12.9‰ to 21.2‰ (n = 6) in theOrdovician within the Tazhong area. ZS1 and ZS5 Cambrian oilshave d34S values from 18.1‰ to 25.9‰, and YM2-O1 oil is 17.3‰

(Table 2). These values are close to those reported previously, from11.9‰ to 19.9‰, for the Ordovician oils (n = 12; Cai et al., 2009b).

d34S of individual alkylated dibenzothiophene compounds wereanalyzed for d34S values and show a narrow range from 17.1‰ to21.2‰ for all six compounds from four analyzed oils, among whichDBT has the highest d34S values (Fig. 7). These values are close tothose reported by Li et al. (2015) for the ZS1 oil at 6439–6458 m

-39

-37

-35

-33

-31

-29

-27

C14 C15 C16 C17 C18 C19 C20 C21 C2

δ13C

(‰)

TZ6,O l source rock(Yu et al.,2012)TZ623-H2,O lZG14-1,O yTZ201C,O yZS1,Є,6439-6458mTD2,Є(Li et al.,2010)

Fig. 6. Distribution of individual n-alkane d13C values showing similarly light values of Zand ZS1C Cambrian oils and a Upper Ordovician source rock from well TZ6 (See legend

(15–21‰) and the YM2-O1 oil (16–20‰). All individual compoundshave d34S values close to their bulk oils and within the range of theCambrian kerogens (Fig. 7).

5. Discussion

5.1. Oil–source rock correlation based on geology, biomarkers and d13Cvalues

The ZS1 oils are produced from reservoirs of the middle-lowerAwatage Fm. (_2a) within the Middle Cambrian anhydrite andanhydrite-bearing dolomite (Fig. 3A). Bedded anhydrite occurs inboth the overlying and below the reservoirs although they arethicker below the reservoir interval [Fig. 2(a) of Li et al. (2015)].The ZS5 oil is produced from the Lower Cambrian below the anhy-drite beds (Fig. 3B). No major faults, leading to stratigraphic juxta-positions between the Upper Ordovician and the Cambrian, havebeen found in the study area (Fig. 3A), suggesting that the oilsare unlikely to have migrated downward from the UpperOrdovician to the ZS1 and ZS5 pools in the Cambrian section.Thus, the oils are considered to be typical of oils derived from aCambrian source rock. Both the ZS1 oils and XEBLK Cambriansource rock extracts show similar biomarker distribution features;i.e., a ‘‘V’’ shaped distribution of C27–C29 aaa 20R steranes, C24

tetracyclic terpane (C24Te) less than C26 tricyclic terpane (C26TT)and similar C21 triterpanes/C23 triterpanes (C21TT/C23TT) ratios,C29Ts/(C29Ts + C29H) ratios and d13C values of saturated and aro-matic hydrocarbons (Table 1, Figs. 3 and 4). These features supporta Cambrian source for the ZS1 oils.

The YM2-O1 oil sample has a biomarker composition, andindividual n-alkane d13C values that are similar to the characteris-tics of ZS1 oils (Table 1 and Fig. 6), suggesting that they may havebeen derived from the same source. Upper Ordovician source rocksshow the lowest C28 sterane in the C27–C29 sterane distribution(Zhang et al., 2000; Li et al., 2010; Yu et al., 2011), which is differ-ent from the TD2-Æ and TZ62-S oils with sterane distribution ofC27 6 C28 < C29, typical of the Cambrian–Lower Ordovician geneticaffinity (Xiao et al., 2005; Li et al., 2010, 2012; Liu et al., 2015).Thus, it can be concluded that there are two facies of Cambriansource rocks that have different biomarker distribution and d13Cvalues. Their spatial and stratigraphic distribution remains to befurther investigated.

The Upper Ordovician source rock may have a similar biomar-ker composition to that of the Cambrian, especially in the dis-tribution of C27 to C29 steranes. However, no source rocks withlight d13C values, similar to the YM2-O1 oil, have been reported

2 C23 C24 C25 C26 C27 C28 C29 C30 C31

TZ821-1,O lZG511,O lZG45,O yYM2,O y(Li et al.,2010)ZS1,Є,6426-6497mZS1C,Є(Li et al.,2015)

S1 oils from the Cambrian to YM2-O1 oil and significantly lighter than those of TD2for data sources).

0

5

10

15

20

25

30

35

40

δ34

(‰)

TZ83,O y

ZG511,O l

ZG12,O y

ZG54,S

Є kerogen

O kerogen

YM2,O y

ZS1,Є2

ZS1C,Є1

ZG19,O +

Є

Fig. 7. Comparison of bulk oils, individual S compounds d34S values with those of Cambrian and Upper Ordovician kerogens. Individual alkyl-dibenzothiophenes d34S valuesof YM2, ZS1, ZS1C and ZG19 wells are from Li et al. (2015), kerogen d34S values are from Cai et al. (2009a) and this study and all others from this study.

C. Cai et al. / Organic Geochemistry 83-84 (2015) 140–152 149

(Li et al., 2010; Yu et al., 2011). The reported Upper Ordoviciansource rocks have saturated hydrocarbon d13C values ranging from�26.2‰ to�29.0‰ in the LN46 and TZ12 wells (Zhang et al., 2006),and individual n-C13–n-C34 compounds from �29.4‰ to �30.8‰

from TZ6 well (Yu et al., 2012). These values are slightly heavierthan, or close to, those of the Cambrian from TC1, KN1, He4 andTD2 wells with the n-C13–n-C29 d13C values from �29.3‰ to�32.6‰ in TD2 well (Table 2; Yu et al., 2012).

Interestingly, kerogens, and saturate and aromatic compoundswith the lightest d13C values in the basin are found in theCambrian section in HX1 well, XEBLK and SGTBLK outcrops(Table 2). These samples have kerogen d13C values rangingfrom �34.1‰ to �35.3‰, saturated hydrocarbons ranging from�31.1‰ to �32.6‰ and aromatic hydrocarbons ranging from�29.1‰ to �31.8‰. These values are close to, or slightly heavierthan, those of the ZS1 and YM2-O1 oils (�33.0‰ to �33.5‰,�33.1‰ to �34.0‰ and �30.8‰ to �31.1‰ for bulk oils, saturatedand aromatic fractions, respectively). It is not fully clear why thekerogens show lighter d13C values than their derived saturatesand aromatics (Table 2). This may have resulted from two possiblecauses. One possibility is that a portion of the extractable organicmatter (EOM) was generated from source rocks with lighter d13Cvalues and migrated into the samples from the HX1 well, XEBLKand SGTBLK outcrops. We do not believe it likely that all sampleswere contaminated to the same degree. The second possible expla-nation, that the difference results from heterogeneous primary bio-mass and the selective preservation of 13C enriched lipids fromprokaryotes, was suggested by Close et al. (2011) and Liu et al.(2015). This could explain reports for other Cambrian source rocksamples (Li et al., 2015) and for Proterozoic sedimentary organics(Logan et al., 1995, 1997) of kerogen being carbon isotopicallylighter than the associated lipids or EOM.

The ZS1C oil shows much heavier d13C values in bulk oil, satu-rated hydrocarbons and n-alkanes than the YM2-O1 oil and nodetectable steranes and terpenes (not shown). This oil is consid-ered to have been generated from the Lower Cambrian source rock(Li et al., 2015). If so, the ZS5 oil produced from the LowerCambrian is expected to have similarly heavy d13C values. This isin contradiction to the data (Table 2). In fact, the ZS1C oil isassociated with an H2S concentration of 11% and has alkylben-zothiophenes and alkyldibenzothiophenes d34S values from 35–43‰ (Li et al., 2015). This oil must have been heavily altered byTSR, and its d13C values must, therefore, have been shifted signifi-cantly. The present differences in d13C value between individualcompound from n-C14 to n-C28 of ZS1C oil and ZS1 oil at depths

of 6426–6497 m are up to 5‰. A similarly large shift in d13C valuehas been reported from other TSR affected cases (Sassen, 1988;Rooney, 1995) and is likely a result of preferential oxidation of12C bond of the individual n-alkanes during TSR. No evidence indi-cates that this oil was derived from the source rock with d13C val-ues similar to the TZ62-S or TD2-Æ oil. In contrast, this oil is mostlikely to have been derived from source rocks with d13C valuessimilar to the ZS1 or YM2-O1 oil based on the assumption thatthere are not more than two facies of the Cambrian source rocks.

Other oils from the Ordovician (Table 2) have d13C values heav-ier than those of ZS1, ZS5 and YM2-O1 oils (�33.0‰ to �34.7‰ forbulk oils, �33.1‰ to �35.2‰ for the saturate fraction, �30.8‰ to�32.1‰ for the aromatic fraction, and mainly �34.6‰ to�36.5‰ for individual n-alkanes). These d13C values are uniformlylighter than those of the TD2-Æ and TZ62-S oils (about �28.6‰ forbulk oils, �29.4‰ and �28.1‰ for saturated and aromatic frac-tions, and �29.2‰ to �30.5‰ for individual n-alkanes) (Figs. 5and 6). These oils may have been derived from either the mixingof the two different organic facies in source rocks from theCambrian or the mixing of the isotopically lighter Cambrian sourcerock with the isotopically heavier Upper Ordovician source rocks.Further work is needed to resolve which of these possible explana-tions is most plausible.

5.2. Oil–source rock correlation based on bulk d34S values

The criteria for using sulfur isotopes in the correlation of oilsand the source rocks in the Tarim Basin are: (1) Unaltered oils haved34S values that correlate with their parent source rock kerogens ina rapidly buried basin (Thode, 1981; Orr, 1986; Cai et al., 2009a,b).The Tarim Basin is such a basin as indicated by the Cambriansource rocks being rapidly buried to > 4000 m as a result of subsi-dence during the Cambrian to Ordovician (see Fig. 3A–D of Caiet al., 2009a). (2) The oils analyzed (excluding TZ83, ZS1C andZG511 oil) are not biodegraded and have associated H2S concentra-tions of < 0.25%, indicating no significant alteration by secondaryprocesses such as thermochemical or bacterial sulfate reduction(Cai et al., 2009b). Specifically, the ZS1 Cambrian oils have350 ppm associated H2S and H2S is below detection in the YM2-O1 oil. This, along with the lack of biodegradation of the oils basedon whole oil GC data, indicates that there has been no significantalteration, if any, of the d34S values of these two oils.

The YM2-O1 oil has a bulk d34S of 17.3‰, i.e., close to thoseof the oils typically derived from the Cambrian (TZ62-S oil:

150 C. Cai et al. / Organic Geochemistry 83-84 (2015) 140–152

17.2‰, TD2-Æ: 19.6‰, ZS1: 18.1‰ and 23.3‰). The values are closeto those of the Cambrian kerogen ranging from 14.0‰ to 21.6‰

(n = 5) with an average of 18.9‰ (this study, Table 2), and from10.4‰ to 19.4‰ with an average of 15.4‰ (n = 4) as reported pre-viously (Cai et al., 2009a). These values significantly higher thanthe Middle and Upper Ordovician kerogen that from 3.8‰ to6.8‰ with an average of 5.5‰ (n = 3) except for an abnormal valueof �15.3‰, and the two Lower Ordovician kerogens that yield d34Sof 6.7‰ and 8.7‰ (Cai et al., 2009a). These characteristics may wellindicate that the YM2-O1 oil was derived from a Cambrian sourceand not from an Upper Ordovician source as unaltered oils canbe enriched in 34S up to 2‰ relative to their parent kerogen asshown in field case studies (Thode, 1981; Orr, 1986) and closedsystem dry and hydrous pyrolysis of immature kerogen (Idizet al., 1990; Amrani et al., 2005).

Other oils analyzed in this study (excluding TZ83-O1 oil) haved34S values ranging from 12.9‰ to 21.2‰ (Table 2), i.e., withinthe range of the Cambrian kerogens but different than UpperOrdovician source rocks (Fig. 7). This line of evidence supports aCambrian source for these oils as indicated by the discussion basedon the biomarkers and d13C values in Section 5.1.

The TZ83-O1 oil is associated with 2.6% H2S and containsalkylthiolanes derived from back reactions of pre-existing oil com-pounds with TSR-derived H2S (Cai et al., 2009b); hence, its d34S val-ues may be influenced by TSR. However, alkylthiolanes are minorcomponents (Fig. 6a in Cai et al., 2009b) and the aromatic fractionis dominated by alkyldibenzothiophenes (R-DBTs). As thermallyless stable thiols, thiolanes and benzothiophenes preferentiallyincorporate TSR-H2S compared to thermally more stable sulfurcompounds (Orr, 1974; Cai et al., 2003, 2009b; Amrani et al.,2012), the low concentrations of alkylthiolanes suggests that influ-ence of TSR on its bulk d34S values is minor. The TZ83-O1 oil has abulk d34S value of 18.5‰, which is close to other oils analyzed. d34Sof individual compound were measured to determine the influenceof TSR alteration and if the bulk d34S value still reflects the parentkerogen. Unaltered oils have relatively homogeneous d34S valuesamong different sulfur species (Thode et al., 1958; Thode, 1981).That is, an unaltered oil is expected to have alkylbenzothiophenes(BTs) d34S values similar to DBTs. This has been verified by thestudy of individual sulfur compounds (Amrani et al., 2012). Evenunder conditions of low degrees of TSR, BTs rapidly adopt thed34S value of sulfate participating in TSR while DBTs d34S valuesremain essentially pristine. Consequently, at low extents of TSR,there is a large difference in d34S between BTs and DBTs (Amraniet al., 2012). As TSR advances further, isotopically heavy TSR-H2Sis reported to be incorporated into DBTs, leading to DBTs d34S val-ues that are close to BTs and also close to those of the initial (par-ent) sulfate mineral (Amrani et al., 2012).

The TZ83-O1 oil yields BTs and DBT d34S values of 19.7‰ to20.4‰ (Li et al., 2015) and 18.9‰ to 20.0‰, respectively, indicatingthat the oil has only a minor contribution, if any, of BTs sulfurderived from TSR-H2S incorporation and that DBTs may have nosignificant sulfur from TSR-H2S. Other analyzed oils, except forthe ZS1C oil, (Fig. 7) are not altered and altered to much lessextents than the TZ83-O1 oil as indicated by the associated H2Sconcentration. Thus, we conclude that their DBTs d34S values havenot been changed by TSR and that they reflect primary signalsinherited from their parent kerogen.

YM2-O1 oil has DBTs d34S values close to those of DBTs and bulkoils from ZS1, ZG511, ZG12 and ZG54 wells, predominantly from15‰ to 20‰ (Fig. 7) and close to the Cambrian kerogens.Interestingly, among the oils analyzed by Li et al. (2015), exceptfor the oils from ZG19 and ZG21, the other oils from theOrdovician reservoirs show similar DBTs d34S values to the resultspresented here. This indicates that all oils may have been

predominantly derived from Cambrian source rocks. The ZG19and ZG21 wells are located in the western part of the TazhongUplift, from which the produced oils have DBTs d34S values aslow as 10‰, and thus are considered to have been derived from dif-ferent source rocks to the other oils, probably from an Upper andMiddle Ordovician Saergan Fm. source rock. This proposal is par-tially supported by the following two lines of evidence: (1) thegases in the west show much lower dryness coefficient (C1/C1-6)and methane and ethane d13C values (Wang et al., 2014; Li et al.,2015) and may be derived from source rocks with lower maturity,which are different from those in the east; (2) wells ZG19 and ZG21are located near the Awati area where the main Upper and MiddleOrdovician basin facies source rocks are considered to occur(Fig. 10d of Li et al., 2010) and may have significant amounts ofpetroleum contributed from the Upper and Middle Ordoviciansource rocks. No similar source rocks have been reported in theeast and it is less likely for the Upper and Middle OrdovicianSaergan Fm. source rocks to have contributed significant amountsof oils to reservoirs in the east. Further work is needed to deter-mine the source of the oils in the western Tazhong area.

From the above discussion, all these oils, except ZG19 and ZG21oils in the west, may have been derived from the Cambrian sourcerocks. Most of the oils (including YM2-O1 oil) produced fromCambrian–Ordovician reservoirs are believed to have been gener-ated from Cambrian source rocks with similar organic d34S, differ-ences in d13C value and some different biomarker composition seenin the TD2-Æ and TZ62-S oils, suggesting multiple source facies.

Lateral heterogeneity in d13Corg values has been shown from shelfto basin environment during the Early Cambrian in the Yangtze plat-form (South China) (Jiang et al., 2012). Similarly, d13Csaturates valuesof lower Cambrian platform dolomites in wells He4 and TC1 rangefrom �27.9‰ to �29.7‰ (Cai et al., 2009a and references therein)and are significantly heavier than those of contemporary mudstonesand shales in XH1 well, XEBLK and SGTBLK outcrops from�31.1‰ to�32.6‰ (Fig. 5). However, the distribution of the source rocks withlight d13Corg in the Tarim Basin is not clear. The possible occurrenceand distribution of two different organic facies in the Cambrianremain to be investigated.

The Cambrian source rocks that generated TZ62-S oil are con-sidered as dolomites and have been shown to have significantlylower TOC than shale and mudstone (Xiao et al., 2005; Cai et al.,2009a). Thus, limited amounts of oil have been produced fromthe wells TD2 and TZ62. It is possible that no significant con-tribution of petroleum from this facies of source rock has occurred,arguing against widespread mixing of an oil from this facies withthe Upper Ordovician as proposed by Li et al. (2010, 2015).Present day oils produced from the Ordovician in the westernTazhong area, such as wells ZG19 and ZG21, may have beenderived from the Cambrian with the light d13C values and mixedwith the Upper and Middle Ordovician Saergan Fm. derived oil indifferent proportions.

6. Conclusions

Based on geological and geochemical evidence, the ZS1 and ZS5Cambrian oils were generated from a Cambrian source rock. Theseoils show no significant alteration by TSR and have low C28 aaa20R among C27–C29 steranes, low gammacerane/C30 hopane andlight d13C values, which are considered to be typical characteristicsof an oil (YM2 oil) and proposed to be derived from the UpperOrdovician source rocks (Zhang et al., 2000; Li et al., 2010). TheZS1 and ZS5 Cambrian oils have bulk d13C and d34S values, andindividual n-alkanes d13C and individual dibenzothiophene com-pound d34S values close to other oils from Ordovician reservoirs.

C. Cai et al. / Organic Geochemistry 83-84 (2015) 140–152 151

Bulk oil and individual DBTs d34S values have here been shown tobe an effective tool to determine the source rock for oils that havenot been altered by secondary processes such as biodegradation,TSR and BSR in a rapidly buried basin such as in the Tarim Basin.

These measurements correlate well to some of the Cambriansource rocks analyzed. We believe that these findings indicate thatmost of the oils produced from Cambrian and Ordovician reservoirsin the Tarim Basin are probably derived from Cambrian sourcerocks and not from Upper Ordovician source rocks as previouslyreported (Li et al., 2015). This proposal fully explains why the totalpetroleum reserve identified in the basin is much higher than canbe predicted from a potential Upper Ordovician source rock.

Acknowledgments

This work is financially supported by China National Funds forDistinguished Young Scientists (41125009) and Special MajorProject on Petroleum Study (2011ZX05008-003). Dr. SimonBottrell, an anonymous reviewer and the Associate Editor, Dr.Clifford C. Walters, are sincerely acknowledged for helpful com-ments on an earlier version of this manuscript.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.orggeochem.2015.03.012.

Associate Editor—Cliff Walters

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