1. Application of sulfur and carbon isotopes to oil–source rock correlation:
A case study from the Tazhong area, Tarim Basin, China
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 China
b
Key Lab of Exploration Technologies for Oil and Gas Resources of Ministry of Education, Yangtze University, Wuhan 430100, PR China
c
Liverpool University, Department of Earth and Ocean Sciences, Liverpool University, Liverpool, Merseyside L69 3GP, UK
d
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
Article history:
Received 17 December 2014
Received in revised form 11 March 2015
Accepted 17 March 2015
Available online 30 March 2015
Keywords:
Sulfur isotopes
Individual sulfur compounds
Carbon isotopes
Biomarker
Oil–rock correlation
Tarim Basin
a b s t r a c t
Up until now, it has been assumed that oil in the Palaeozoic reservoirs of the Tazhong Uplift was derived
from Upper Ordovician source rocks. Oils recently produced from the Middle and Lower Cambrian in
wells ZS1 and ZS5 provide clues concerning the source rocks of the oils in the Tazhong Uplift, Tarim
Basin, China. For this study, molecular composition, bulk and individual n-alkane d13
C and individual
alkyl-dibenzothiophene d34
S values were determined for the potential source rocks and for oils from
Cambrian and Ordovician reservoirs to determine the sources of the oils and to address whether d13
C
and d34
S values can be used effectively for oil–source rock correlation purposes. The ZS1 and ZS5
Cambrian oils, and six other oils from Ordovician reservoirs, were not significantly altered by TSR. The
ZS1 oils and most of the other oils, have a ‘‘V’’ shape in the distribution of C27–C29 steranes, bulk and
individual n-alkane d13
C values predominantly between À31‰ to À35‰ VPDB, and bulk and individual
alkyldibenzothiophene d34
S values between 15‰ to 23‰ VCDT. These characteristics are similar to those
for some Cambrian source rocks with kerogen d13
C values between À34.1‰ and À35.3‰ and d34
S values
between 10.4‰ and 21.6‰. The oil produced from the Lower Ordovician in well YM2 has similar features
to 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 not
from the Upper Ordovician. Conversely, the d13
C and d34
S values of ZS1C Cambrian oils have been shown
to shift to more positive values due to thermochemical sulfate reduction (TSR). Thus, d13
C and d34
S values
can be used as effective tools to demonstrate oil–source rock correlation, but only because there has been
little or no TSR in this part of the section.
Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction
There is uncertainty about which source rocks generated the oils
produced from Paleozoic strata in the Tarim Basin, China (Cai et al.,
2009a,b; Li et al., 2010 and references therein). Potential source
rocks for the oils include the Cambrian to Lower Ordovician and
the 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 the
Tazhong area are proposed to have been charged during three
periods, including remigration from previously charged oil pools
as a result of subsequent tectonic activity (Zhao et al., 2008; Cai
et 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 the
C27–C29 steranes. These characteristics have been matched pre-
viously with those of the Upper Ordovician source rocks (Zhang
et al., 2000; Li et al., 2010). The YM2-O1 oil shows much lighter
individual n-alkanes d13
C values than the presumed typical
Cambrian derived oils (TD2-Æ and TZ62-S), both of which are not
associated with H2S (Xiao et al., 2005). YM2-O1 oil was thus consid-
ered to be an end member oil from Upper Ordovician source rocks (Li
et al., 2010). Most Paleozoic oils in the Tarim Basin have biomarkers
and individual n-alkanes d13
C values that are much closer to those
http://dx.doi.org/10.1016/j.orggeochem.2015.03.012
0146-6380/Ó 2015 Elsevier Ltd. All rights reserved.
⇑ Corresponding author at: Key Lab of Exploration Technologies for Oil and Gas
Resources 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).
Organic Geochemistry 83-84 (2015) 140–152
Contents lists available at ScienceDirect
Organic Geochemistry
journal homepage: www.elsevier.com/locate/orggeochem

2. reportedfor theYM2-O1 oilsample thanto theTD2-Æ and TZ62-Soils
and it has been speculated that they may have been derived from the
an Upper Ordovician source rock mixed with small amounts of oils
derived from the Cambrian and Lower Ordovician source rocks (Li
et al., 2010, 2015; Yu et al., 2011, 2012; Tian et al., 2012). The
Lower Cambrian oil sample from well ZS1C (a lateral well of the
ZS1 well) has no detectable steranes or terpanes due to high matur-
ity, but has individual n-alkane d13
C values close to the TZ62 S and
TD2 Cambrian oils and was considered to have been derived from
Cambrian source rocks (Li et al., 2015).
However, these conclusions are contradicted by the following
published observations: (1) There are no reports of Upper
Ordovician source rocks having d13
C values as light as the oils
(Zhang et al., 2006; Yu et al., 2011). Indeed it is notable that the
C15+ saturates are reported to have lighter d13
C values in oils that
were derived from Precambrian and Cambrian source rocks than
oils derived from Ordovician source rocks based on 22 global oil
samples (Andrusevich et al., 1998). (2) The spatially and strati-
graphically limited distribution of the Upper Ordovician source
rock in the Tarim Basin does not support the discovered petroleum
resources (Cai et al., 2009a; Li et al., 2010; Yu et al., 2011). (3) The
d13
C value of an oil may be altered by TSR (Cai et al., 2001, 2003,
2008, 2009b). Petroleum is shifted to heavier d13
C values with
increasing TSR as a result of the preferential thermal cleavage
and oxidation of 12
C bonds (Krouse et al., 1988; Sassen, 1988;
Rooney, 1995; Manzano et al., 1997; Cai et al., 2003, 2004, 2013).
Thus, d13
C values can be only used for direct correlation of the
source rocks and oils where the oils are not TSR altered.
Sulfur isotopes have been used for the correlation of source
rocks 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 under
semi-closed to closed conditions. This feature, along with high H2S
solubility and rapid sulfur isotope homogenization, are believed to
result in small differences (up to 2‰) in d34
S values between
mature kerogens and their generated oils in the case studies and
experimental simulation (Cai et al., 2009a and references therein).
The Cambrian derived oils (including the TD2-Æ and TZ62-S oils)
have d34
S values ranging from +11.9 to +20.5‰, which are close
to values reported for Cambrian source rocks that range from
10.4–19.4‰ (Cai et al., 2009a,b). However, d34
S values of the oils
presumed to have been derived from the Upper Ordovician, with-
out associated H2S, have not been reported. It is not clear if these
oils, including the YM2-O1, have d34
S values that match with the
Upper Ordovician source rocks.
New clues may be supplied by oils recently produced from the
Cambrian from wells ZS1 and ZS5. The ZS1 oils (Æ2a) have the ‘‘V’’
shaped distribution of C27–C29 regular steranes and d13
C values
that match well with most of the Ordovician oils in the Tazhong
area (Cai, 2013; Yang, 2014; Li et al., 2015). As it is unlikely for this
Middle Cambrian oil to have been generated from an Upper
Ordovician source, the ZS1, and by inference most of the
Ordovician oils, are proposed to have been derived from a
Cambrian source (Cai, 2013; Yang, 2014). In contrast, Li et al.
(2015) consider the similarity in the distribution of C27–C29 regular
steranes and d13
C values between the ZS1 oils and the YM2-O1 oil
as evidence for the ZS1 oils to be derived from the Upper
Ordovician source rock. If the YM2-O1 oil is not an end member
oil 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, aromatics
and individual n-alkanes d13
C, and bulk and dibenzothiophenes
(DBTs) d34
S values of four oils produced from the Cambrian, twelve
oils from the Ordovician and one oil from the Silurian, along with
eight 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 d13
C values and biological
marker characteristics; (2) whether the YM2-O1 oil was derived
from the Cambrian source rocks; and (3) from what source rocks
the Lower Ordovician oils were derived.
2. Geological setting
The Tarim Basin has an area of 560,000 km2
and the maximum
accumulative thickness of $16,000 m for Ediacaran–Paleozoic
marine deposits and Mesozoic–Cenozoic terrigenous deposits
(Cai et al., 2009a) (Fig. 1). The Tazhong Uplift is an inherited
paleo-structural high in the central Tarim Basin and an important
area of oil and gas exploration and development with an explo-
ration area of $ 2.2 Â 104
km2
(Fig. 1). The tectonic history began
with oceanic spreading during the Cambrian–Early Ordovician. A
NW trending, basement involved fault system developed during
the Caledonian Orogeny at the end of Early Ordovician, resulting
in the formation of the tectonic framework and a ramp-edge type
carbonate platform. NW-SE directed tilting occurred during the
Late Ordovician resulting in erosion of the eastern area (Jia,
1997; Ren et al., 2011). The continuous compressive stress resulted
in NNE strike slip faults at the end of the Devonian (Wu et al.,
2009). From the end of the Caledonian to the early Hercynian (late
Devonian), the area underwent a period of uplift and tectonic
adjustment. As a result, the Devonian, Silurian and even
Ordovician strata were eroded and formed the most important
unconformity in the study area. During the Indosinian–
Himalayan Orogeny (J3 – N), the Paleozoic structures did not
change although there were overall fluctuations in the area (Jia,
1997).
The general stratigraphic column of the Tazhong Uplift (Fig. 2) is
described in Cai et al. (2001, 2009a,b) and Li et al. (2010). Briefly, the
Cambrian strata consist of the Lower Cambrian Xiaoerbulake
and Wusonggeer formations, the Middle Cambrian Shayilike and
Awatage formations and the Upper Cambrian Qiulitage Fm.
(Fig. 2). The Lower Cambrian section is composed of platform and
platform-margin dolomites with intercalated dark mudstones.
The Middle Cambrian is a supratidal, anhydrite bearing dolomite
and anhydrite dominated section. The Ordovician is predominantly
composed of open platform facies dolomite, packstone and bioclas-
tic limestone with reef and shoal facies grainstone in the Upper
Ordovician. The Silurian to Carboniferous sequence is composed
of marine sandstone and mudstone with intercalated micrite and
bioclastic limestone in the Carboniferous. The Permian consists of
transitional terrigenous-marine facies, gray or brown sandstones
and mudstones with intercalated volcanic rocks. The Mesozoic
and Cenozoic are mainly composed of terrigenous sandstones and
mudstones.
The Cambrian and Lower Ordovician source rocks include the
Cambrian abyssal to bathyal facies mudstone and shale, the
Cambrian evaporated lagoon facies, anhydritic dolomite and
argillaceous dolomite, and the Middle and Lower Ordovician
Heituao Formation under compensated basin facies mudstone
and shale. The source rocks are widespread in the basin [Fig. 10a
and b of Li et al. (2010)], and contain 1.0–3.0% TOC, predominantly
of type I to II1 kerogen, with vitrinite reflectance equivalences
(VRE) ranging from 1.5–2.3%, as determined from solid bitumen
reflectance measurements (VRE = 0.618Rb + 0.40) (Cai et al.,
2009a; Li et al., 2010 and references therein). Peak oil generation
from the Cambrian source rocks occurred during the late
Caledonian–early Hercynian period (S1 – D3), and from the
Ordovician Heituao Formation during the late Hercynian (C2 – P1)
(Zhao et al., 2008).
The Middle and Upper Ordovician (O2-3) source rocks include the
Saergan and Lianglitage formations. The Saergan Formation (O2-3s),
C. Cai et al. / Organic Geochemistry 83-84 (2015) 140–152 141

3. a marginal shelf to basin facies of black mudstones and shales, has
TOC generally greater than 1.0% and occurs in the Keping Uplift and
Awati 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. The
Lianglitage 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 and
TOC (Cai et al., 2009a). The O2-3 source rocks have VRE mainly from
0.81–1.3% and reached peak oil generation during the late
Yanshan–Himalayan period (K2 – N) (Zhao et al., 2008).
Petroleum is produced from the Carboniferous, Silurian, Lower
Ordovician (O1y) and Middle and Upper Ordovician (O2yj and O3l)
and recently from Cambrian reservoirs. Condensates are produced
from 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 well
ZS1) and ZS5 (Fig. 3A and B). Faults cross cutting the Cambrian
and Ordovician, and their associated fractures, are considered to
be the dominant conduits for upward hydrocarbon migration from
the 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 Lower
Cambrian source rocks including one mudstone at a depth of
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

4. 5790 m from the XH1 well in the Tabei Uplift. Seven mudstone,
shale and muddy dolomite samples from YJK, XEBLK and SGTBLK
outcrops in the Keping area in the west of the Tabei Uplift and from
KLTG 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 d13
C and d34
S values, bulk rock
extracted organic matter (EOM) and saturated and aromatic hydro-
carbon d13
C 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, d13
C and d34
S
analyses were not biodegraded and are not associated with signifi-
cant amounts of H2S (i.e. H2S concentration for all samples
is < 0.25%). Three other oils with slightly higher concentrations of
associated H2S were analyzed for comparison to assess the impact
of TSR on these geochemical parameters. Samples analyzed include
one oil produced from the Silurian, eight oils from the Upper
Ordovician (O3l), four oils from the Lower Ordovician (O1y) and four
oils from the Cambrian (Tables 1 and 2). One sour Cambrian oil
from well ZS1C and two sour Ordovician oils from wells TZ83
and ZG511, have associated H2S concentrations of 11%, 2.6% and
2.4%, respectively. All other Cambrian and Ordovician oils have
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

5. no significant H2S. The oils, except YM2-O1 oil, were collected from
different areas of Tazhong Uplift. YM2-O1 oil was collected from
the Lower Ordovician in the Tabei Uplift.
3.2. Biological marker analyses
About 80 g source rock samples were powdered for 3 min using
a grinding mill and then Soxhlet extracted using dichloromethane
(DCM) for 72 h. The extracts and whole oils were separated into
saturates, aromatics, resins (NSO) and asphaltenes by column chro-
matography using n-pentane, DCM and methanol as chromato-
graphic solvents. The saturated and aromatic fractions were
analyzed using a Hewlett Packard 6890GC-5973MSD mass spec-
trometer. The gas chromatograph (GC) was fitted with a HP-5MS
capillary 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
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 from
the 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 legend
in A).
144 C. Cai et al. / Organic Geochemistry 83-84 (2015) 140–152

6. 1 min. The temperature was then increased from 50–310 °C at a
rate of 3 °C/min, and then held at 310 °C for 18 min. Helium was
used 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, saturated
and aromatic fractions were determined following procedures
similar to those described by Sofer (1980). Carbon dioxide was
Table 1
Biomarker parameters of the Cambrian and Ordovician source rocks and oils.
Sample
No.
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
Source
Rocks
TD2*
Cambrian 1.06 0.27 28.4 31.7 39.9 0.44 0.48 0.46 0.82 0.23 0.54
XEBLK Cambrian 0.67 0.18 29.0 26.4 33 0.47 0.44 0.49 0.52 0.26 0.49
TZ12*
U. Ordo. 1.30 0.05 32.1 24.3 43.6 0.59 1.63 0.38 0.67 0.42 0.52
Z11*
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 0.50
TZ243 U. Ordo. 0.82 0 27.9 15.9 43.2 0.59 0.97 0.92 0.71 0.35 0.53
TZ44 U. Ordo. 0.86 0.18 21.8 26.5 42 0.35 0.66 0.82 0.66 0.23 0.49
ZG45 L. Ordo. 0.70 0 32.2 20.3 37.5 0.73 2.02 0.40 0.47 0.41 0.50
YM2 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 2
d13
C and d34
S values of source rocks and oils.
Well location Depth (m) Strata Lithology TOC (%) S (%) Den d13
C d34
S
Bulk oil/kerogen Saturates Aromatics Bulk oil/kerogen
Source rocks LN46 6160 O3l Marlstone 0.08 – À28.9Â
À30.7Â
À15.3*
TZ12 4669 O3l Bl. shale 0.68 – À26.2Â
À32.5Â
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 À34.7 À31.5 À31.8 –
TC1 5714.55 Æ Dolomite 0.80 – – – 10.4*
TC1 6421 Æ3 Dolomite 0.09 – À27.9Â
À28.5Â
–
TC1 7124 Æ1 Dolomite 0.12 – À29.7Â
À30.4Â
–
XH1 Core Æ1 Bl. mudstone 6.10 À34.2 À31.1 À29.1 –
KN1 4886 Æ1 Marlstone 0.14 – À28.8Â
À30.2Â
–
KN1 5503 Æ1 Marlstone 2.04 – À28.4Â
À31.1Â
17.8*
He4 5350.7 Æ1 Bl. dolomite – – À28.9 À28.0 –
XEBLK(KP) Outcrop Æ1 Bl. dolomite – – À29.5 À28.0 –
KLTG(Y2–15) Outcrop Æ1 Bl. shale – – – – 21.6
KLTG(Y2–34) Outcrop Æ1 Bl. shale – – – – 21.3
XEBLK(XD-Y) Outcrop Æ1 Bl. shale – À34.1 À32.6 À31.8 20.8
XEBLK(XK2) Outcrop Æ1 Bl. shlae 0.77 À35.3 À32.2 À31.4 18.0
XEBLK(Y4) Outcrop Æ Bl. shale 3.31 – – – 13.8*
TD2 4770.5 Æ Bl.mudstone 1.70 À26.8**
– – 19.4*
YJK Outcrop Æ1 Bl. shale 0.39 À28.5 – – 14.0
Oils TZ44 4822.00 O3l 0.28 0.795 À31.8 À31.8 À31.5 14.4
TZ623-H2 4694–5793 O3l 0.14 0.807 À31.9 À31.8 À31.2 –
TZ821–1 5218–5252 O3l 0.08 0.797 À31.4 À31.5 À30.0 –
ZG14–1 6133–6298 O3l 0.19 0.794 À31.5 À31.4 À29.7 13.2
ZG511 4824–5022 O3l 0.808 À31.1 À31.2 À30.0 19.9
ZG45 5637–5650 O1y 0.19 0.814 À31.4 À31.1 À29.9 12.9
TZ83 5666–5681 O1y 0.36 0.82 À30.3 À29.8 18.5*
TZ201C 5430–5779 O1y 0.28 0.805 À31.4 À31.2 À30.2 21.2
ZS1 6426–6496 Æ2 – – À33.5 À33.7 À31.1 18.1
ZS1 6439–6458 Æ2 – 0.789 À33.0 À33.1 À30.8 23.3
ZS1C 6806–6955 Æ1wx – – À29.9 À30.4 À30.0 –
ZS5 6562–6671 Æ1wx – – À34.7 À35.2 À32.1 25.9
TZ62 4053 S – 0.93 À28.8#
À29.4#
17.2*
TD2 4801 Æ – 1.02 À28.5 À29.3 À28.1 19.6*
YM2 5494–5953 O1y – – À33.5#
À34.0#
– 17.3
ZG54 4113–4193 S – – 14.2
Note: –: not available; Den and Bl are density and black in short, respectively.
Â
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

7. 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 allowed
to cool slowly (1 °C/min) to room temperature in order to ensure
reduction of any nitrous oxides. The resultant CO2 was separated
cryogenically and carbon isotope ratios were measured using a
VG SIRA 12 mass spectrometer. All data were corrected for 17
O
effects (Craig, 1957) and reported in conventional delta (d) nota-
tion in per mil (‰) relative to VPDB. Accuracy and reproducibility
of carbon isotopic data were assessed by replicate analysis of the
international standard NBS 22. The mean of eight replicates
(À29.60‰) was nearly identical with a precision of ± 0.042‰ and
within the experimental error reported by Gonfiantini et al. (1995).
3.4. Compound specific stable carbon isotope analyses
For compound specific d13
C analyses, a method similar to Li
et 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 a
60 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 holding
times of 1 min and 30 min, respectively. Helium was used as the
carrier gas at a flow rate of 1 ml/min with the injector operating
at constant flow.
The d13
C values were calculated by the integration of the
masses 44, 45 and 46 ion current counts of the CO2 peaks produced
by the combustion (copper oxide reaction furnace at 850 °C) of
hydrocarbons separated by GC. A CO2 reference gas (calibrated
relative to the PeeDee Belemnite (‰, PDB) with a known d13
C value
was pulsed into the mass spectrometer and the isotopic com-
position of samples was reported in the d notation relative to the
reference gas. The average values of at least two runs for each sam-
ple are reported and only results with a standard deviation of less
than 0.3‰ were used.
3.5. Bulk kerogen and oil sulfur isotope analysis
The methods for separation of kerogen and analysis of sulfur
isotopes of kerogen and oil were reported by Cai et al. (2009a).
Pyrite was removed from the kerogen by adding a mixture of hot
6N HCl and CrCl2 to the ground dry kerogen under a nitrogen flow
with the gas flow carrying the H2S to a trap where it was recovered
as Ag2S. Excess acids and acid soluble salts were removed from the
residual kerogen by water washing. About 2 h later the residual
kerogen was collected and reground to expose new pyrite surfaces
and the whole procedure was repeated once more. After the two
treatments, the residual kerogen was further analyzed using X-
ray diffraction (XRD) to determine whether pyrite was below the
detection limits (60.5% depending on conditions). If not, more
treatments 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 to
give total residual kerogen sulfur. Dissolved iron was measured
at pH < 2, using atomic absorption spectrometer, to determine
the 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 contamination
pyrite sulfur/total sulfur < 0.08, the produced BaSO4 were analyzed
for d34
S to guarantee low errors (depending on the differences in
d34
S 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 separated
by a GC (Clarus 580 Perkin Elmer, MA, USA) coupled to a
Neptune plus multi-collector inductively coupled plasma mass
spectrometer (MC-ICPMS, Thermo Scientific, Bremen, Germany)
located at the Hebrew University, Jerusalem. The system employed
similar conditions to those described in details in Amrani et al.
(2009, 2012) and Said-Ahmad and Amrani (2013). Duplicates for
some of the oils have been measured and the standard deviation
between the two duplicates was usually better than 1‰.
4. Results
4.1. Biomarkers
Organic matter extracted from the Cambrian source rock in the
XEBLK outcrop has maturity related parameters C29 aaa sterane
20S/(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 reported
values from the Cambrian in TD2 well and from the Upper
Ordovician in Z11 well (Cai et al., 2009a) and are lower than, or
close to, the respective equilibrium values (Peters et al., 2005), sug-
gesting that the values are not commensurate with the vitrinite
reflectance 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 related
parameters similar to previously reported ratios of the Cambrian
source 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 the
Upper Ordovician (Zhang et al., 2000; Li et al., 2010; Cai et al.,
2009a,b). However, this sample shows a ‘‘V’’ shaped distribution
of C27–C29 20R steranes, i.e., C27 > C28 < C29, or lowest percentage
of C28 aaa 20R among C27–C29 steranes (Fig. 4). Other source rocks
reported from the Cambrian show C27 6 C28 < C29 (Cai et al., 2009a;
Li et al., 2010, 2015). That is, the saturates extracted from
Cambrian source rock from XEBLK (XK2) outcrop show C27–C29
sterane distribution different from other Cambrian source rocks
but similar to the Upper Ordovician (Table 1).
With the exception of the TZ44 oil (Table 1; Fig. 4), oils from the
ZS1, 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 of
2.02 from ZG45 oil). The ZS1 oil shows a gammacerane/C30 hopane
ratio 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 and
individual n-alkanes
4.2.1. Carbon isotopes from source rocks
Source rocks from the Lower Cambrian in XH1 well, XEBLK and
SGTBLK outcrops have d13
C values ranging from À31.1‰ to
À32.2‰ for saturated hydrocarbons, À29.1‰ to À31.8‰ for aro-
matic hydrocarbons, and À33.7‰ to À35.3‰ (n = 5) for kerogens
(Fig. 5; Table 2). The values are significantly lighter than those of
Cambrian source rocks at wells TC1, He4 and KN1 that yield satu-
rated hydrocarbon d13
C values ranging from À27.9‰ to À29.7‰
(Cai et al., 2002, 2009a; Zhang et al., 2006), and from the Upper
Ordovician in LN46 and TZ12 wells that yield saturated hydrocar-
bon d13
C values ranging from À26.2‰ to À29.0‰ (Zhang et al.,
2006).
146 C. Cai et al. / Organic Geochemistry 83-84 (2015) 140–152

8. 4.2.2. Carbon isotopes of crude oils
Eight crude oils from the Ordovician have saturated hydrocar-
bon d13
C values ranging from À30.3‰ to À31.8‰, with an average
of À31.3‰, and aromatic hydrocarbon d13
C values ranging from
À29.8‰ to À31.6‰, with an average of À30.3‰. Two ZS1 middle
Cambrian oils from different depths show lighter d13
C values of
À33.1‰ and À33.7‰ for saturated hydrocarbons, À31.1‰ and
À30.8‰ for the aromatic fractions. These values are close to those
measured for extracts of the Lower Cambrian source rock in XH1
well, XEBLK and SGTBLK outcrops from À31.1‰ to À32.6‰ and
from À29.1‰ to À31.8‰ (n = 4), respectively. However, the
saturated hydrocarbon fractions are significantly lighter than those
from TC1, He4 and KN1 Cambrian source rocks (À27.9‰ to
À29.4‰) and than those from LN46 and TZ12 Upper Ordovician
source rocks (À28.9‰ and À26.2‰, respectively) (Fig. 5).
Compared to the ZS1 Cambrian oils, ZS5 Lower Cambrian oil show
relatively light d13
C values with bulk oil of À34.7‰, saturated
hydrocarbons of À35.2‰ and aromatic hydrocarbons of À32.1‰.
In contrast, the ZS1C Cambrian oil has d13
C values of about
À30.0‰ 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.
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

9. The ZS1 oils from the Cambrian have individual C14–C28 n-alka-
nes d13
C values from À33.8‰ to À36.5‰ (Fig. 6), which are close to
the YM2 oil, and much lighter than the TD2-Æ oil. Interestingly, the
ZS1C oil has C14–C31 n-alkane d13
C values from À30.2‰ to À28.8‰
(Li et al., 2015), similar to the TD2-Æ oil. All other oils analyzed
have the C14–C31 n-alkanes d13
C values predominantly from
À33.0‰ to À35.0‰, ranging between the ZS1 oils and the TD2-Æ
oil (Fig. 5).
4.3. Sulfur isotopes of bulk oil and individual sulfur compound
Source rock kerogens from the Cambrian from KLTG, YJK and
XEBLK outcrops were measured to have d34
S values ranging from
14.0‰ to 21.6‰ with an average of 19.1‰ (n = 5) (Table 2). The
values are close to those of the Cambrian kerogens from TD2,
KN1 and TC1 wells and XEBLK (Y-4) outcrop that yield d34
S values
ranging from 10.4‰ to 19.4‰ (n = 4, Cai et al., 2009a).
Bulk oils have d34
S values from 12.9‰ to 21.2‰ (n = 6) in the
Ordovician within the Tazhong area. ZS1 and ZS5 Cambrian oils
have d34
S values from 18.1‰ to 25.9‰, and YM2-O1 oil is 17.3‰
(Table 2). These values are close to those reported previously, from
11.9‰ to 19.9‰, for the Ordovician oils (n = 12; Cai et al., 2009b).
d34
S of individual alkylated dibenzothiophene compounds were
analyzed for d34
S values and show a narrow range from 17.1‰ to
21.2‰ for all six compounds from four analyzed oils, among which
DBT has the highest d34
S values (Fig. 7). These values are close to
those reported by Li et al. (2015) for the ZS1 oil at 6439–6458 m
(15–21‰) and the YM2-O1 oil (16–20‰). All individual compounds
have d34
S values close to their bulk oils and within the range of the
Cambrian kerogens (Fig. 7).
5. Discussion
5.1. Oil–source rock correlation based on geology, biomarkers and d13
C
values
The ZS1 oils are produced from reservoirs of the middle-lower
Awatage Fm. (_2a) within the Middle Cambrian anhydrite and
anhydrite-bearing dolomite (Fig. 3A). Bedded anhydrite occurs in
both the overlying and below the reservoirs although they are
thicker 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, have
been found in the study area (Fig. 3A), suggesting that the oils
are unlikely to have migrated downward from the Upper
Ordovician to the ZS1 and ZS5 pools in the Cambrian section.
Thus, the oils are considered to be typical of oils derived from a
Cambrian source rock. Both the ZS1 oils and XEBLK Cambrian
source 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 d13
C values of saturated and aro-
matic hydrocarbons (Table 1, Figs. 3 and 4). These features support
a Cambrian source for the ZS1 oils.
The YM2-O1 oil sample has a biomarker composition, and
individual n-alkane d13
C values that are similar to the characteris-
tics of ZS1 oils (Table 1 and Fig. 6), suggesting that they may have
been derived from the same source. Upper Ordovician source rocks
show 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 of
C27 6 C28 < C29, typical of the Cambrian–Lower Ordovician genetic
affinity (Xiao et al., 2005; Li et al., 2010, 2012; Liu et al., 2015).
Thus, it can be concluded that there are two facies of Cambrian
source rocks that have different biomarker distribution and d13
C
values. Their spatial and stratigraphic distribution remains to be
further 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 with
light d13
C values, similar to the YM2-O1 oil, have been reported
-33
-32
-31
-30
-29
-28
-27
-36 -34 -32 -30 -28 -26
δ13Car(‰)
δ13Csat (‰)
O3l oil
O1y oil
Є oil
O3l source rock
Є source rock
ZS1
TD2
ZS5
ZS1C
Fig. 5. Cross plot showing d13
C values of saturates and aromatics of extractable
organic matter and oils.
-39
-37
-35
-33
-31
-29
-27
C14 C15 C16 C17 C18 C19 C20 C21 C22 C23 C24 C25 C26 C27 C28 C29 C30 C31
δ13C(‰)
TZ6,O l source rock(Yu et al.,2012) TZ821-1,O l
TZ623-H2,O l ZG511,O l
ZG14-1,O y ZG45,O y
TZ201C,O y YM2,O y(Li et al.,2010)
ZS1,Є,6439-6458m ZS1,Є,6426-6497m
TD2,Є(Li et al.,2010) ZS1C,Є(Li et al.,2015)
Fig. 6. Distribution of individual n-alkane d13
C values showing similarly light values of ZS1 oils from the Cambrian to YM2-O1 oil and significantly lighter than those of TD2
and ZS1C Cambrian oils and a Upper Ordovician source rock from well TZ6 (See legend for data sources).
148 C. Cai et al. / Organic Geochemistry 83-84 (2015) 140–152

10. (Li et al., 2010; Yu et al., 2011). The reported Upper Ordovician
source rocks have saturated hydrocarbon d13
C values ranging from
À26.2‰ to À29.0‰ in the LN46 and TZ12 wells (Zhang et al., 2006),
and individual n-C13–n-C34 compounds from À29.4‰ to À30.8‰
from TZ6 well (Yu et al., 2012). These values are slightly heavier
than, or close to, those of the Cambrian from TC1, KN1, He4 and
TD2 wells with the n-C13–n-C29 d13
C values from À29.3‰ to
À32.6‰ in TD2 well (Table 2; Yu et al., 2012).
Interestingly, kerogens, and saturate and aromatic compounds
with the lightest d13
C values in the basin are found in the
Cambrian section in HX1 well, XEBLK and SGTBLK outcrops
(Table 2). These samples have kerogen d13
C values ranging
from À34.1‰ to À35.3‰, saturated hydrocarbons ranging from
À31.1‰ to À32.6‰ and aromatic hydrocarbons ranging from
À29.1‰ to À31.8‰. These values are close to, or slightly heavier
than, those of the ZS1 and YM2-O1 oils (À33.0‰ to À33.5‰,
À33.1‰ to À34.0‰ and À30.8‰ to À31.1‰ for bulk oils, saturated
and aromatic fractions, respectively). It is not fully clear why the
kerogens show lighter d13
C values than their derived saturates
and aromatics (Table 2). This may have resulted from two possible
causes. One possibility is that a portion of the extractable organic
matter (EOM) was generated from source rocks with lighter d13
C
values and migrated into the samples from the HX1 well, XEBLK
and SGTBLK outcrops. We do not believe it likely that all samples
were contaminated to the same degree. The second possible expla-
nation, that the difference results from heterogeneous primary bio-
mass and the selective preservation of 13
C enriched lipids from
prokaryotes, was suggested by Close et al. (2011) and Liu et al.
(2015). This could explain reports for other Cambrian source rock
samples (Li et al., 2015) and for Proterozoic sedimentary organics
(Logan et al., 1995, 1997) of kerogen being carbon isotopically
lighter than the associated lipids or EOM.
The ZS1C oil shows much heavier d13
C values in bulk oil, satu-
rated hydrocarbons and n-alkanes than the YM2-O1 oil and no
detectable 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 Lower
Cambrian is expected to have similarly heavy d13
C values. This is
in contradiction to the data (Table 2). In fact, the ZS1C oil is
associated with an H2S concentration of 11% and has alkylben-
zothiophenes and alkyldibenzothiophenes d34
S values from 35–
43‰ (Li et al., 2015). This oil must have been heavily altered by
TSR, and its d13
C values must, therefore, have been shifted signifi-
cantly. The present differences in d13
C value between individual
compound 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 d13
C value
has been reported from other TSR affected cases (Sassen, 1988;
Rooney, 1995) and is likely a result of preferential oxidation of
12
C bond of the individual n-alkanes during TSR. No evidence indi-
cates that this oil was derived from the source rock with d13
C val-
ues similar to the TZ62-S or TD2-Æ oil. In contrast, this oil is most
likely to have been derived from source rocks with d13
C values
similar to the ZS1 or YM2-O1 oil based on the assumption that
there are not more than two facies of the Cambrian source rocks.
Other oils from the Ordovician (Table 2) have d13
C values heav-
ier than those of ZS1, ZS5 and YM2-O1 oils (À33.0‰ to À34.7‰ for
bulk oils, À33.1‰ to À35.2‰ for the saturate fraction, À30.8‰ to
À32.1‰ for the aromatic fraction, and mainly À34.6‰ to
À36.5‰ for individual n-alkanes). These d13
C values are uniformly
lighter than those of the TD2-Æ and TZ62-S oils (about À28.6‰ for
bulk oils, À29.4‰ and À28.1‰ for saturated and aromatic frac-
tions, and À29.2‰ to À30.5‰ for individual n-alkanes) (Figs. 5
and 6). These oils may have been derived from either the mixing
of the two different organic facies in source rocks from the
Cambrian or the mixing of the isotopically lighter Cambrian source
rock 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 d34
S values
The criteria for using sulfur isotopes in the correlation of oils
and the source rocks in the Tarim Basin are: (1) Unaltered oils have
d34
S values that correlate with their parent source rock kerogens in
a rapidly buried basin (Thode, 1981; Orr, 1986; Cai et al., 2009a,b).
The Tarim Basin is such a basin as indicated by the Cambrian
source rocks being rapidly buried to > 4000 m as a result of subsi-
dence during the Cambrian to Ordovician (see Fig. 3A–D of Cai
et al., 2009a). (2) The oils analyzed (excluding TZ83, ZS1C and
ZG511 oil) are not biodegraded and have associated H2S concentra-
tions of < 0.25%, indicating no significant alteration by secondary
processes such as thermochemical or bacterial sulfate reduction
(Cai et al., 2009b). Specifically, the ZS1 Cambrian oils have
350 ppm associated H2S and H2S is below detection in the YM2-
O1 oil. This, along with the lack of biodegradation of the oils based
on whole oil GC data, indicates that there has been no significant
alteration, if any, of the d34
S values of these two oils.
The YM2-O1 oil has a bulk d34
S of 17.3‰, i.e., close to those
of the oils typically derived from the Cambrian (TZ62-S oil:
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 d34
S values with those of Cambrian and Upper Ordovician kerogens. Individual alkyl-dibenzothiophenes d34
S values
of YM2, ZS1, ZS1C and ZG19 wells are from Li et al. (2015), kerogen d34
S 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

11. 17.2‰, TD2-Æ: 19.6‰, ZS1: 18.1‰ and 23.3‰). The values are close
to 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 from
10.4‰ to 19.4‰ with an average of 15.4‰ (n = 4) as reported pre-
viously (Cai et al., 2009a). These values significantly higher than
the Middle and Upper Ordovician kerogen that from 3.8‰ to
6.8‰ with an average of 5.5‰ (n = 3) except for an abnormal value
of À15.3‰, and the two Lower Ordovician kerogens that yield d34
S
of 6.7‰ and 8.7‰ (Cai et al., 2009a). These characteristics may well
indicate that the YM2-O1 oil was derived from a Cambrian source
and not from an Upper Ordovician source as unaltered oils can
be enriched in 34
S up to 2‰ relative to their parent kerogen as
shown in field case studies (Thode, 1981; Orr, 1986) and closed
system dry and hydrous pyrolysis of immature kerogen (Idiz
et al., 1990; Amrani et al., 2005).
Other oils analyzed in this study (excluding TZ83-O1 oil) have
d34
S values ranging from 12.9‰ to 21.2‰ (Table 2), i.e., within
the range of the Cambrian kerogens but different than Upper
Ordovician source rocks (Fig. 7). This line of evidence supports a
Cambrian source for these oils as indicated by the discussion based
on the biomarkers and d13
C values in Section 5.1.
The TZ83-O1 oil is associated with 2.6% H2S and contains
alkylthiolanes derived from back reactions of pre-existing oil com-
pounds with TSR-derived H2S (Cai et al., 2009b); hence, its d34
S val-
ues may be influenced by TSR. However, alkylthiolanes are minor
components (Fig. 6a in Cai et al., 2009b) and the aromatic fraction
is dominated by alkyldibenzothiophenes (R-DBTs). As thermally
less stable thiols, thiolanes and benzothiophenes preferentially
incorporate TSR-H2S compared to thermally more stable sulfur
compounds (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 d34
S values is minor. The TZ83-O1 oil has a
bulk d34
S value of 18.5‰, which is close to other oils analyzed. d34
S
of individual compound were measured to determine the influence
of TSR alteration and if the bulk d34
S value still reflects the parent
kerogen. Unaltered oils have relatively homogeneous d34
S values
among different sulfur species (Thode et al., 1958; Thode, 1981).
That is, an unaltered oil is expected to have alkylbenzothiophenes
(BTs) d34
S values similar to DBTs. This has been verified by the
study of individual sulfur compounds (Amrani et al., 2012). Even
under conditions of low degrees of TSR, BTs rapidly adopt the
d34
S value of sulfate participating in TSR while DBTs d34
S values
remain essentially pristine. Consequently, at low extents of TSR,
there is a large difference in d34
S between BTs and DBTs (Amrani
et al., 2012). As TSR advances further, isotopically heavy TSR-H2S
is reported to be incorporated into DBTs, leading to DBTs d34
S 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 d34
S values of 19.7‰ to
20.4‰ (Li et al., 2015) and 18.9‰ to 20.0‰, respectively, indicating
that the oil has only a minor contribution, if any, of BTs sulfur
derived from TSR-H2S incorporation and that DBTs may have no
significant sulfur from TSR-H2S. Other analyzed oils, except for
the ZS1C oil, (Fig. 7) are not altered and altered to much less
extents than the TZ83-O1 oil as indicated by the associated H2S
concentration. Thus, we conclude that their DBTs d34
S values have
not been changed by TSR and that they reflect primary signals
inherited from their parent kerogen.
YM2-O1 oil has DBTs d34
S values close to those of DBTs and bulk
oils from ZS1, ZG511, ZG12 and ZG54 wells, predominantly from
15‰ to 20‰ (Fig. 7) and close to the Cambrian kerogens.
Interestingly, among the oils analyzed by Li et al. (2015), except
for the oils from ZG19 and ZG21, the other oils from the
Ordovician reservoirs show similar DBTs d34
S values to the results
presented here. This indicates that all oils may have been
predominantly derived from Cambrian source rocks. The ZG19
and ZG21 wells are located in the western part of the Tazhong
Uplift, from which the produced oils have DBTs d34
S values as
low as 10‰, and thus are considered to have been derived from dif-
ferent source rocks to the other oils, probably from an Upper and
Middle Ordovician Saergan Fm. source rock. This proposal is par-
tially supported by the following two lines of evidence: (1) the
gases in the west show much lower dryness coefficient (C1/C1-6)
and methane and ethane d13
C 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 ZG21
are located near the Awati area where the main Upper and Middle
Ordovician basin facies source rocks are considered to occur
(Fig. 10d of Li et al., 2010) and may have significant amounts of
petroleum contributed from the Upper and Middle Ordovician
source rocks. No similar source rocks have been reported in the
east and it is less likely for the Upper and Middle Ordovician
Saergan Fm. source rocks to have contributed significant amounts
of 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 ZG21
oils in the west, may have been derived from the Cambrian source
rocks. Most of the oils (including YM2-O1 oil) produced from
Cambrian–Ordovician reservoirs are believed to have been gener-
ated from Cambrian source rocks with similar organic d34
S, differ-
ences in d13
C value and some different biomarker composition seen
in the TD2-Æ and TZ62-S oils, suggesting multiple source facies.
Lateral heterogeneity in d13
Corg values has been shown from shelf
to basin environment during the Early Cambrian in the Yangtze plat-
form (South China) (Jiang et al., 2012). Similarly, d13
Csaturates values
of lower Cambrian platform dolomites in wells He4 and TC1 range
from À27.9‰ to À29.7‰ (Cai et al., 2009a and references therein)
and are significantly heavier than those of contemporary mudstones
and shales in XH1 well, XEBLK and SGTBLK outcrops from À31.1‰ to
À32.6‰ (Fig. 5). However, the distribution of the source rocks with
light d13
Corg in the Tarim Basin is not clear. The possible occurrence
and distribution of two different organic facies in the Cambrian
remain to be investigated.
The Cambrian source rocks that generated TZ62-S oil are con-
sidered as dolomites and have been shown to have significantly
lower TOC than shale and mudstone (Xiao et al., 2005; Cai et al.,
2009a). Thus, limited amounts of oil have been produced from
the 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 with
the Upper Ordovician as proposed by Li et al. (2010, 2015).
Present day oils produced from the Ordovician in the western
Tazhong area, such as wells ZG19 and ZG21, may have been
derived from the Cambrian with the light d13
C values and mixed
with the Upper and Middle Ordovician Saergan Fm. derived oil in
different proportions.
6. Conclusions
Based on geological and geochemical evidence, the ZS1 and ZS5
Cambrian oils were generated from a Cambrian source rock. These
oils show no significant alteration by TSR and have low C28 aaa
20R among C27–C29 steranes, low gammacerane/C30 hopane and
light d13
C values, which are considered to be typical characteristics
of an oil (YM2 oil) and proposed to be derived from the Upper
Ordovician source rocks (Zhang et al., 2000; Li et al., 2010). The
ZS1 and ZS5 Cambrian oils have bulk d13
C and d34
S values, and
individual n-alkanes d13
C and individual dibenzothiophene com-
pound d34
S values close to other oils from Ordovician reservoirs.
150 C. Cai et al. / Organic Geochemistry 83-84 (2015) 140–152

12. Bulk oil and individual DBTs d34
S values have here been shown to
be an effective tool to determine the source rock for oils that have
not 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 Cambrian
source rocks analyzed. We believe that these findings indicate that
most of the oils produced from Cambrian and Ordovician reservoirs
in the Tarim Basin are probably derived from Cambrian source
rocks and not from Upper Ordovician source rocks as previously
reported (Li et al., 2015). This proposal fully explains why the total
petroleum reserve identified in the basin is much higher than can
be predicted from a potential Upper Ordovician source rock.
Acknowledgments
This work is financially supported by China National Funds for
Distinguished Young Scientists (41125009) and Special Major
Project on Petroleum Study (2011ZX05008-003). Dr. Simon
Bottrell, 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, in
the online version, at http://dx.doi.org/10.1016/j.orggeochem.
2015.03.012.
Associate Editor—Cliff Walters
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