Applied Geochemistry 20 (2005) 1427–1444
AppliedGeochemistry
www.elsevier.com/locate/apgeochem
Origin of sulfur rich oils and H2S in Tertiary lacustrinesections of the Jinxian Sag, Bohai Bay Basin, China
Chunfang Cai a,*, Richard H. Worden b,*, George A. Wolff b, Simon Bottrell c,Donglian Wang d, Xin Li d
a Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences,
P.O. Box 9825, Beijing 100029, PR Chinab Department of Earth and Ocean Sciences, University of Liverpool, 4 Brownlow Street, Liverpool L69 3GP, UK
c School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UKd Huabei Petroleum Bureau, PetroChina,Renqiu, PR China
Received 5 November 2004; accepted 10 March 2005
Available online 22 June 2005
Editorial handling by B.R.T. Simoneit
Abstract
Very high S oils (up to 14.7%) with H2S contents of up to 92% in the associated gas have been found in the Tertiary
in the Jinxian Sag, Bohai Bay Basin, PR China. Several oil samples were analyzed for C and S stable isotopes and bio-
markers to try to understand the origin of these unusual oil samples.
The high S oils occur in relatively shallow reservoirs in the northern part of the Jinxian Sag in anhydrite-rich reservoirs,
and are characteristic of oils derived from S-rich source rocks deposited in an enclosed and productive stratified hyper-
saline water body. In contrast, low S oils (as low as 0.03%) in the southern part of the Jinxian Sag occur in Tertiary lacus-
trine reservoirs withminimal anhydrite. These southern oils were probably derived from less S-rich source rocks deposited
under a relatively open and freshwater to brackish lake environment that had larger amounts of higher plant inputs.
The extremely high S oil samples (>10%) underwent biodegradation of normal alkanes resulting in a degree of con-
centration of S in the residual petroleum, although isoprenoid alkanes remain showing that biodegradation was not
extreme. Interestingly, the high S oils occur in H2S-rich reservoirs (H2S up to 92% by volume) where the H2S was
derived from bacterial SO4 reduction, most likely in the source rock prior to migration. Three oils in the Jinxian
Sag have d34S values from +0.3& to +16.2& and the oil with the highest S content shows the lightest d34S value. This
d34S value for that oil is close to the d34S value for H2S (�0&). It is possible that H2S was incorporated into function-
alized compounds within the residual petroleum during biodegradation at depth in the reservoir thus accounting for the
very high concentrations of S in petroleum.
� 2005 Elsevier Ltd. All rights reserved.
0883-2927/$ - see front matter � 2005 Elsevier Ltd. All rights reserv
doi:10.1016/j.apgeochem.2005.03.005
* Corresponding authors.
E-mail addresses: [email protected], [email protected].
ac.cn (C. Cai), [email protected] (R.H. Worden).
1. Introduction
The Jinxian Sag is located in the south of the Jizhong
Depression, namely in the western Bohai Bay Basin,
which is a faulted basin in East China (Fig. 1). The
Jinxian Sag has its axis lying NE–SW with an area of
ed.
Boundary of south and north
N
Fig. 1. Map showing geological and tectonic features and the location of the major petroleum exploration wells in the Jinxian Sag,
Bohai Bay Basin.
1428 C. Cai et al. / Applied Geochemistry 20 (2005) 1427–1444
1700 km2 and is a Mesozoic and Cenozoic fault-
bounded sag. The basement is composed of a wide range
of rock types and stratigraphic ages: Middle and Upper
Proterozoic and Lower Palaeozoic carbonates, Upper
Palaeozoic marine and non-marine limestones, coal
measures and siliciclastic sediments and Mesozoic non-
marine clastics and volcanoclastics.
An oil discovery on the Zhaolz structure in the north-
ern Jinxian Sag has reserves of 23.3 · 106 tons of oil with
an area covering 42 km2 and an H2S reserve of
2.17 · 106 m3 over an area of 52 km2 (Qi et al., 1998).
Sulfur-rich oils and kerogens are of considerable
interest in petroleum geochemistry (e.g. Orr, 1986; Sin-
ninghe Damste et al., 1990) because of the economic
implications (high refining costs), as well as implications
for interpretation of the environments of source rock
deposition. High S oils are usually associated with
hypersaline marine/lacustrine evaporites and carbon-
ates, and tend to be from low maturity source rocks
(Fu et al., 1986; Orr, 1986).
In China, high S oils (defined as >1% S, Waldo et al.,
1991) have been reported in the Jianghan Basin (Fu
et al., 1986; Sheng et al., 1986) and the Zhuanhua
Depression (Huang and Pearson, 1999). Sulfur-rich
petroleum also occurs in the Jinxian Sag (Fig. 1). Oils
with unusually high S concentrations of up to 14.7%
have been found in the Jinxian Sag. Whilst the source
rocks of the Jinxian Sag have been discussed briefly
(Bao and Li, 2001), the oils themselves have not been
characterised. This paper describes the petroleum
geochemistry and stable isotope data of ultra-high S
petroleum from the Jinxian Sag. The results suggest that
the high S oils were generated from marginally mature,
S-rich source rocks but that secondary processes, includ-
ing biodegradation and late diagenetic assimilation of S
into oil, are required to explain anomalous S concentra-
tions of up to 14.7%.
Thus, the general issue being addressed in this paper
is why some oil samples in the Jinxian Sag have such
high S concentrations. The specific issues being ad-
C. Cai et al. / Applied Geochemistry 20 (2005) 1427–1444 1429
dressed are: (1) why some oils in the Jinxian Sag have
such extreme S concentrations and yet others fall more
into the normal range, (2) whether the S in the oil is a
result of the sedimentary depositional environment,
early diagenetic changes, biodegradation or addition of
S to petroleum during burial diagenesis; and (3) the rela-
tionships of the S in the oil to other S-bearing minerals
and compounds in the basin.
2. Geological setting
2.1. Sedimentation and burial history
There are distinct differences in both the styles of sed-
imentation and burial histories of the northern and
southern parts of the Jinxian Sag. The boundary be-
tween the northern and southern parts of the basin lies
close to the Zhaoxian anticline structure and Nanboshe
(Fig. 1, Lei et al., 1999).
The late Palaeocene–early Eocene lake, responsible
for the deposition of the main source rocks in the north
Jinxian Sag, may have been relatively deep and anoxic,
as indicated by the presence of laminated mudstones,
shales and dark massive mudstones in the basin center
(Table 1). Evaporites, carbonates and thin sandstones
deposited under shallow lacustrine conditions occur at
the basin margins in the north. Close to the boundary
Table 1
Synthetic stratigraphy for Jixian Sag
System General lithology and dep
Quaternary Alluvial brown, yellow san
Neogene Alluvial yellow, brown pe
brown-yellow silty mudsto
(Nm), Guantao Fm (Ng),
Paleogene
Oligocene Dongying Fm (Ed) Ed-Es1 Alluvial brown, purple–re
brown argillaceous siltston
Eocene Shahejie Fm (Es)
Upper Eocene (Es1) Alluvial (see above)
Middle Eocene (Es2-3) Lacustrine brown, purple,
mudstone interbedded wit
grained Sandstone (Reserv
Lower Eocene
and Upper Paleocene (Es4–Ek1)
Lacustrine grey–brown, gr
Gypsiferous mudstone, lim
middle-fine grained sandst
bottom of Es4 (source roc
Paleocene Kongdian Fm (Ek)
Upper Paleocene (Ek1) See above
Middle Paleocene (Ek2) Lacustrine brown, dark gr
fine-grained sandstone, ar
Lower Paleocene (Ek3) Alluvial-lacustrine brown–
siltstone unconformably o
faults in the east, subsurface fan sandstones up to
600 m thickness have been found. These sandstones are
texturally and compositionally immature and contain
pebbles and mud. In direct contrast, in the south of the
Jinxian Sag, bedded evaporites and carbonates are not
found. However, anhydrite-bearing mudstones are found
in both the south and north of the Sag, being thicker in
the north (with discrete beds varying from �3 mm to
10 cm). These anhydrite-bearing mudstones are typically
interlayered with dark, organic-rich mudstones. Overall,
the north of the sag has more evaporitic lacustrine source
rocks than the south of the sag (Guo et al., 1997).
Burial and thermal history modelling of the Jinxian
Sag (Fig. 2) using Thermodel software (Hu and Zhang,
1998) and based upon R0 data, indicated more rapid sed-
imentation in the north during the deposition of the
Upper Palaeocene–Lower Eocene (Es4–Ek1) than in
the south, in contrast to the Middle Eocene (Es2-3) per-
iod. At the present time, the Es4–Ek1 source rocks are
buried more deeply in the south than in the north. The
Es4–Ek1 reservoirs in the north Jinxian were heated
more slowly than in the south.
2.2. Source rock richness and type
In the north, Es4–Ek1 source rocks have H/C ratios
from 0.6 to 1.6. There is a range of distinct kerogen
types, ranging from type I to oxidized type III (Fig. 3).
ositional setting Thickness (m)
dy mud, sand and pebble 400
bbly sandstone, siltstone interlayers with
ne, mudstone, including Minghuazhen
unconformably over Eocene.
200–1400
d mudstone interbedded with grey,
e, fine-grained sandstone
0–1600
dark purple mudstone and sandy
h grey mudstone, siltstone and middle-fine
oir)
0–1222
ey, black mudstone, laminated black shale,
estone, dolomite nipping anhydrite beds and
one. Up to 100 m sandstone occurs at the
k and reservoir)
0–1200
ey, black mudstone interbedded with
gillaceous sandstone (source and reservoir)
0–2500
red pebble-bearing, argillaceous sandstone,
ver Mesozoic carbonates
0–900
Well Zhaoxin2 in the north
Ek -Es1 4 Ed Ng Nm Qs s s3 2 1
30 Co
50 Co
70 Co
90 Co
110 Co
R =0.45%o
R =0.60%o
0
1600
48 36 24 12 0 (my)
Depth(m)
Depth(m)
800
2400
3200
Ek Ek -E Ed Nm Ng Q2 1 4 3 2 1s s s s
(b)
(a)
Well 42 in the south
Fig. 2. Diagrams showing burial histories in well Zhaoxin2 in
the north (a) and in well 42 in the south (b) Jinxian Sag based
on vitrinite reflectance (R0) data. Note: Ek represents Palaeo-
cene Kongdian Fm, Es is Eocene Shahejie Fm, and Ed is
Oligocene Dongying Fm.
(North)
(South)
Fig. 3. Plot of H/C–O/C atomic ratio for sedimentary organic
matter in source rocks from the Jinxian Sag (modified after
Guo et al., 1997).
0.25 0
Ro
1500
2000
2500
3000
1000
Depth (m) TOC (%) EOM (%)
Fig. 4. Synthetic profile of bulk parameters for sour
1430 C. Cai et al. / Applied Geochemistry 20 (2005) 1427–1444
The Es4–Ek1 anoxic evaporitic lacustrine source rocks
in the north are primarily type I and mixed with type
III. In contrast, in the southern part of the sag, Es4–
Ek1 and Lower–Middle Palaeocene (Ek2-3) source
rocks are of sapropelic-humic type II2 and type III,
respectively (Fig. 3).
2.2.1. Hypersaline lacustrine organic facies
In the northern part of the sag, the Es4–Ek1 anhy-
drite-rich mudstone is currently buried to depths be-
tween 1400 and 3200 m. Migrating petroleum has been
found in horizontal and vertical micro-fractures in the
.5 0.75 420 440 460 480 0.6 0.8 1.0 1. 2
, % Tmax , C OEP O
Source rock Oils
ce rocks and oils from the north Jinxian Sag.
C. Cai et al. / Applied Geochemistry 20 (2005) 1427–1444 1431
dark mudstone at depths of 2508 m in well Zhaoxin1
and 2459 m in well Zhaoxin2 (He et al., 1995). The mud-
stone at 2459 m has a vitrinite reflectance value (R0) of
�0.45% and a Tmax of �430 �C (Fig. 4) whilst the
threshold depth of petroleum generation is considered
to be approximately 2800 m in this area (Huabei, 1988;
Bao and Li, 2001). The Es4–Ek1 source rock
(>2450 m) has TOC values from 0.4% to 1.2% with an
average of 0.84% (n = 16). The Ek2-3 source rock has
a mean TOC of 0.61% (n = 27; He et al., 1995).
Although these source rocks have relatively low to
moderate TOC values, their extractable organic matter
(EOM) contents are relatively high, being up to 0.47%
(Fig. 4). The EOM values are similar to Eocene hypersa-
line lacustrine source rock in the Jianghan Basin (Peng
et al., 1998). Saturate fractions in the EOM range from
12% to 46%, aromatic fractions from 3% to 53%, resins
from 21% to 74% and asphaltenes from 2% to 25%, with
average values of 24%, 23%, 39% and 14% (n = 21),
respectively (He et al., 1995; Guo et al., 1997). Hydro-
carbon compounds account for about 50% of the total
extractable organic matter on average.
2.2.2. Freshwater lacustrine organic facies
In the southern part of the Jinxian Sag, Es2-3, Es4–
Ek1 and Ek2-3 source rocks have no anhydrite or are
less anhydrite-rich than the northern source rocks and
have mean TOC values of 0.47% (n = 77), 0.63%
(n = 107) and 0.63% (n = 100), respectively (Guo et al.,
1997).
3. Analytical methods
3.1. Sulfur species analysis
Mudstone source rock samples were analyzed for to-
tal sulfur, pyrite S, elemental S and organically bound S
(He et al., 1995). The total S was weighed as BaSO4 and
the HCl-soluble SO4 bound sulfur (SSO4) was assessed by
ICP; elemental S was calculated from CuS generated
through contact with hot Cu; the pyrite-bound S was
calculated from the Fe content of the pyrite, isolated
by H2SO4 + HF treatment (the Fe–S atomic ratio of
pyrite was considered to equal 1:2). Organically bound
S was calculated from the difference between total S
and the sum of SSO4, pyrite S and elemental S.
3.2. Isotopic composition: d34S and d13C
Finely ground samples (2–6 g) were treated with hot
10% HCl in an inert atmosphere to dissolve the acid-sol-
uble SO4 minerals. After filtration, BaCl2 was added to
the solution and aqueous SO4 was precipitated as
BaSO4. The precipitate was added to a mixture of CrCl2,
concentrated HCl and excess CuCl2 in an inert atmo-
sphere, which led to precipitation of CuS. Oils were
combusted in a Parr bomb apparatus to oxidize organi-
cally bound S to SO4. Dissolved SO4 was precipitated as
BaSO4 and weighed.
The BaSO4 and CuS were converted to SO2 by com-
bustion and analyzed for S isotope compositions (Cai
et al., 2003). The data were reported relative to the
V-CDT standard.
Stable C isotopic compositions of the whole oils, sat-
urates, aromatics, resins and asphaltenes were deter-
mined following procedures similar to those described
by Sofer (1980). Carbon dioxide was prepared by com-
busting (850 �C, 2 h) aliquots (0.5–1 mg) of petroleum
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�1) to
room temperature in order to ensure reduction of any
nitrous oxides. The resultant CO2 was separated cryo-
genically and C isotope ratios were measured using a
VG SIRA 12 mass spectrometer. All data were corrected
for 17O effects (Craig, 1957) and reported in conven-
tional delta (d) notation in per mil (&) relative to V-
PDB. Accuracy and reproducibility of C isotopic data
were assessed by replicate analysis of the international
standard NBS 22. The mean of 8 replicates (�29.60&)
was identical within experimental error to the value re-
ported by Gonfiantini et al. (1995) and gave a precision
(sn-1) of ±0.042&.
3.3. Biological markers
Oils and source rock extracts were separated into sat-
urates, aromatics, resins (NSO) and asphaltenes by col-
umn chromatography using n-pentane, dichloromethane
(DCM) and methanol as developing solvents. The 3 oils
with the highest, middle and lowest contents of saturates
were selected for GC and GC-MS analyses. Whole oils
were treated with a silicalite to remove n-alkanes (Han-
son et al., 2001).
Gas chromatography (GC) was performed on a Hew-
lett-Packard 5890 series II instrument equipped with a
Gerstel temperature-programmed cold injection system
and a fused silica capillary column (60 m · 0.25 mm
i.d., film thickness 0.25 lm; DB-5, J&W). Helium was
used as the carrier gas, and the GC oven was pro-
grammed from 30 �C (after being held for 1 min) to
360 �C at a rate of 3 �C/min and was held isothermally
for 50 min.
All saturate and aromatic fractions and whole oils
(after removal of n-alkanes) were analysed by gas chro-
matograph-mass spectrometry (GC-MS) using a Trace
2000 Series gas chromatograph fitted with a split/split-
less injector (320 �C), and a fused silica column
(60 m · 0.25 mm i.d.; DB5, 0.1 lm film thickness,
J&W), with He as the carrier gas (ca. 1.6 mL min�1).
Typically, the oven temperature was programmed from
1432 C. Cai et al. / Applied Geochemistry 20 (2005) 1427–1444
60 �C to 170 �C at 6 �C min�1 after 1 min, and then up
to 315 �C at 2.5 �C min�1 where it was held for
10 min. The column was fed directly into the EI source
of a Thermoquest Finnigan TSQ7000 mass spectrome-
ter. Typical GC-MS operating conditions were: ionisa-
tion potential 70 eV; source temperature 215 �C; trap
current 300 lA. The instrument was operated in Selected
Ion Monitoring mode (SIM; m/z 125, 127, 177, 191, 205,
217, 218, 231, 245, 253, 259) and cycled every 1 s (50–
600 D). Data were processed using Xcalibur software.
4. Results
4.1. Bulk properties of petroleum samples
Petroleum samples have medium to high densities
(from 0.83 to 1.07 g/cm3, at 20 �C) and low to very high
Dep
th (
m)
Sulfur (%)
North
South
0 5 10 15
Fig. 5. Variation of S contents of Jinxian oils with reservoir
depths.
R=0.
75
2
(a) (b)
Sul
fur(
%)
Density (g/cm3)
Fig. 6. Relationships (a) between S contents and densities, and (b)
S contents (from 0.03% to 14.7%). Most oils from the
northern part of the Jinxian Sag have S contents >1%,
and are considered to be high S oils (Fig. 5; Waldo
et al., 1991). In well 2, the fluid contains elevated H2S
concentrations (up to 92%; Yan et al., 1982). There is
an inverse correlation between S content and oil density
(Fig. 6(a)) as reported for many petroleum samples
(Tissot and Welte, 1984; Baskin and Peters, 1992). These
characteristics follow a general geographical trend with
the northern oil samples having the highest densities
(g/cm3) and the highest S contents.
The oil samples with highest S contents (>5%) occur in
the shallower parts of the section (<2400 m; Fig. 5). These
high S oils tend to have high proportions of resins and
asphaltenes, and commensurately lower proportions of
saturates and aromatics. Oil-S contents show a positive
relationship with resin and asphaltene contents
(R2 = 0.51, n = 33, Fig. 6(b)). This suggests that the S in
the oil residesmainly in the resin and asphaltene fractions.
Thiophenic S in the aromatic fractions is a relatively
minor component of the oils; the identifiable compounds
are benzothiophenes and dibenzothiophenes.
The chemical composition data show that saturates
are as low as 12% in well 7, in the S-rich oils of the
northern Jinxian Sag, but account for up to 46% in
the less S-rich oil from well 39 in the middle Jinxian
Sag (Table 2).
4.2. Saturated biomarkers
In the northern part of the Jinxian Sag, the n-alkanes
in the majority of the 24 Es4–Ek1 source rock extract
samples are dominated by the C18 or C16 homologues
(Fig. 7 and Table 2; He et al., 1995; Guo et al., 1997).
In a few samples, C17, C20 or C22 were the major homo-
logues. However, phytane is the dominant alkane (Fig.
7) and the Pr/Ph ratio ranges from 0.17 to 0.57. The
odd/even preference (OEP) ranges from 0.51 to 0.84
R=0.51
2
Resin + asphaltene (%)
between S and resin plus asphaltene contents for Jinxian oils.
Table
2d1
3C
values
andpercentages
offractions,
sulfurcontents
andd3
4Sforcrudeoilsandextractable
organic
matter
Oil/source
Location
Code
Well
Depth
(m)
Fm
Sulfur
(%)
d34S
(&,CDT)
GC
parameters
Percentages
d13C
(&,PDB)
Pr/Ph
Ph/nC18
OEP
Peak
Sat.
Ar.
Resin
Asp.
Sat.
Ar.
Resin
Asp.
Woil/EOM
aKerogen
Oil
North
Oil2
41–3
1722
Es4
10.86
16.2
0.50
–b
––
23.7
36.9
19.6
19.8
�27.7
�26.2
�26.6
�26.5
�26.7
–
Oil3
72232
Es4
14.69
0.3
0.48
––
–12.0
56.1
12.4
19.5
�26.1
�24.9
�24.7
�25.3
�25.1
–
Oil5
39
2794
Es4
3.49
10.9
0.55
1.69
0.76
C14
46.2
28.5
8.8
16.5
�27.6
�25.3
�25.8
�25.9
�26.2
–
South
Oil6
60c
2031
Es2-3
2.15
–0.46
1.80
0.85
C24
––
––
�28.0
�26.4
�26.4
�26.5
�27.0
–
Oil7
61c
1952
Es2-3
––
––
–C24
––
––
�28.3
�27.2
�27.2
�26.6
�27.6
–
Oil8
29c
4152
Ek2-3
0.03
–0.61
1.10
0.97
C17
––
––
––
––
�27.5
–
Oil9
46c
4100
Ek2-3
0.25
–0.72
0.94
0.93
C22
––
––
––
––
�27.3
–
Potential
source
rock
North
Sour1
Zhaoxin2
2392
Es4
––
––
––
30.9
3.7
18.6
46.8
�26.3
�24.0
�23.4
�24.7
––
Sour2
Zhaoxin2
2984
Ek1
––
0.32
1.23
0.65
C16
31.9
31.2
24.7
12.2
––
––
––
South
Sour3
105c
2578
Es4–Ek1
––
––
––
––
––
�27.0
�26.1
�26.1
�25.6
�26.2
�24.6
Sour4
52c
3042
Ek2-3
––
––
––
––
––
�27.0
�25.4
�25.2
�24.8
�25.4
�24.8
Sour5
29c
3916
Ek2-3
––
1.12
1.45
1.01
C19
––
––
––
––
––
Sour6
66c
3928
Ek2-3
––
0.92
0.96
1.02
C25
––
––
––
––
––
aWoil/EOM
represents
whole
oilorextractable
organic
matter.
bnomeasurementornotavailable
formeasurement.
cdata
from
Guoet
al.(1997).
C. Cai et al. / Applied Geochemistry 20 (2005) 1427–1444 1433
(He et al., 1995; Guo et al., 1997), i.e., even C-numbered
n-alkanes dominate.
The biomarkers from a calcareous mudstone at a
depth of 2984m (TOC = 1.1%, R0 = 0.68%; Fig. 8
and Table 3) are characterized by a high gammace-
rane/C30 17a, 21b hopane ratio and C20 and C21 pregn-
anes, and a minor contribution from b-carotane (not
shown). Steranes are present in higher concentrations
than hopanes (Table 3). The maturity parameters
C29aaa sterane 20 S/(S + R) and C32ab hopane 22
S/(S + R) ratios are 0.51 and 0.56, respectively, sug-
gesting the organic matter is thermally mature. The
feature is similar to that from the mudstone at
3167 m in the same well (Bao and Li, 2001). Compared
with the sample at 2392 m in the same well
(TOC = 0.5%, R0 �0.4%), the mudstone at the depth
of 2984 m shows relatively high organic matter matu-
rity parameter values, gammacerane/C30 17a, 21b ho-
pane ratio and abundant C20 and C21 pregnanes.
Sulfur-containing compounds, including alkyldibenzo-
thiophenes and alkylbenzothiophenes were detected in
the aromatic fraction in both mudstone samples from
2392 and 3167 m (not shown).
In 5 samples from Ek2-3, Es4 and Es2-3 strata in
south Jinxian, n-alkanes are dominated by the C25,
C26, C29 or C19 homologues and the OEP is close to 1
(Table 2). The Pr/Ph ratios of the Ek2-3 source rocks
vary from 0.92 to 1.12 (n = 5); there is a virtual absence
of pregnanes and low amounts of gammacerane (Guo
et al., 1997). However, for the shallower Es4–Ek1 and
Es2-3 source rocks, Pr/Ph ratios are lower, ranging from
0.37 to 0.66 (n = 3), the OEP varies from 0.63 to 1.09
and the pregnanes and gammacerane abundances are
between Ek2-3 source rock in the south and Ek1–Es4
in the north.
Both ultrahigh S oil samples (from wells 41–3 and
7) in the northern part of the Jinxian Sag are proba-
bly moderately biodegraded because the n-alkanes
have been removed, although the isoprenoid alkanes
are generally intact (Fig. 9). Oil from well 39 has a
S content of 3.5%, is unbiodegraded (Fig. 9(a)) and
has n-alkanes dominated by nC14 The Pr/Ph ratios
of the oils from wells 41–3, 7 and 39 range from
0.49 to 0.55 (Table 2).
All 3 oils show relatively abundant gammacerane
and C35 homohopanes (Fig. 8) and there is no indica-
tion of biodegradation of the sterane or triterpane com-
pounds. The two high S oils have relatively high
amounts of C27 abb steranes and pregnanes, and low
oleanane/C30 ab hopane ratios compared with the oil
in well 39 (Table 3).
Pregnanes and gammacerane occur in greatest abun-
dance (Table 3; Guo et al., 1997) in oil samples from the
north of the Jinxian Sag (wells 41–3, 7, 37 and 57), while
those from the south (wells 61 and 29) have low abun-
dances of pregnanes and gammaceranes.
Increasing retention time
Rel
ativ
e R
espo
nse
Fig. 7. Gas chromatogram of Es4–Ek1 calcareous mudstone at 2984 m from well Zhaoxin2 (taken from He et al., 1995).
1434 C. Cai et al. / Applied Geochemistry 20 (2005) 1427–1444
4.3. d13C and d34S values
The two source rock extract samples from the south-
ern Jinxian Sag have relatively lighter d13C values in the
saturates, aromatics, resins and asphaltenes than the ex-
tract from a source rock in the northern part of the Jinx-
ian Sag (well Zhaoxin2; Table 2). d13C values for whole
oils range from �27.6& to �25.1& (Table 2). The two
oils from the north show significantly heavier d13C val-
ues in the saturate, aromatic, resin and asphaltene frac-
tions, compared to the oils and source rock from the
south (Fig. 10). Three of the 5 oils have aromatic hydro-
carbon fractions rich in d13C compared to the asphaltene
fractions.
Positive correlations between the S contents and the
d13C values of the saturates (Table 2) and between S
contents and whole oil d13C values are shown in Fig.
11. Additionally, there is a relationship between S con-
tent and the difference in d13C between saturates and
asphaltenes (d13Csat.�d13Casp.) (Fig. 11).
4.4. Sulfur species and their d34S values
Sulfur minerals and compounds detected in the Jinx-
ian Sag include anhydrite, gypsum, elemental S, pyrite,
organically bound S in oils and mudstone as well as
H2S gas, dissolved H2S and SO2�4 in oilfield formation
waters.
X-ray diffraction analyses of two mudstone samples
in well Zhaoxin2 and well 105 revealed that anhydrite
and minor amounts of gypsum are the only detectable
inorganic S species (Table 4). Chemical analyses indicate
that pyrite and elemental S account for up to 2.2 and
3.2 wt.%, respectively (Table 5).
Anhydrite and gypsum beds are found in the north
and middle parts of the Jinxian Sag but were not found
in the southern part. The very high S oils and H2S gas
occur exclusively within the area with bedded anhydrite
and gypsum occurrences.
H2S gas was produced mainly from Es4–Ek1 dolo-
mite or argillaceous dolomite and partially from sand-
stone and argillaceous anhydrite in north Jinxian. The
H2S pools in the Lower Dolomite group are shown to
have a broader area than in the Upper Dolomite group
(Fig. 12) and both groups are capped by thick-bedded
anhydrite. Natural gas produced from the Lower Dolo-
mite group in Well 2 has a H2S concentration of 92%,
CO2 = 3.6%, CH4 + C2H6 = 0.3%, and C3H8 = 0.1% be-
tween 2603 m and 2618 m. The temperature for the H2S-
bearing reservoir in well 2 is �90 �C (Fig. 2).
In north Jinxian, the Es4–Ek1 oilfield waters contain
2096–9475 mg/L dissolved H2S whilst no significant dis-
solved H2S was detected in south Jinxian wells. In the
northern Jinxian Sag, the Es4–Ek1 oilfield waters also
have higher aqueous SO4 concentrations than Es2-3
and Ek2 oilfield waters. The average aqueous SO4 con-
centrations are 357.6 mg/L for Es2-3 reservoirs,
1614 mg/L for Es4–Ek1 reservoirs, and 1088 mg/L for
Ek2 reservoirs (He et al., 1995).
Organically bound S in mudstone rock samples
ranges from trace to 0.73 wt.%. In these mudstones,
there is no correlation between organic S and pyrite S
contents or between TOC and pyrite S contents (Table
5).
Pyrite d34S values of well Zhaoxin2 (northern Jinx-
ian) and well 105 (middle Jinxian) are �2.8& and
�12.5& respectively (Table 4 and Fig. 13), whilst H2S
gas has d34S values close to 0& (Huan et al., 1992; He
m/z=191
Well 39 oil
Zhaoxin2 2984m
ca lcar eous mudsto ne
Zhaoxin2 2392m anhydr ite- bearing mu dstone
Well 41 -3 oi l
Well 7 oi l
Well 39 oi l
Rel
ativ
e In
tens
ityR
elat
ive
Inte
nsity
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
(j)
mor
etan
e
Ole
anan
e
29T
sC
29ho
pane
C30
hopa
ne
gam
mac
eran
e
homohopanes
Fig. 8. GC-MS fragmentograms (m/z = 191, 217) for oils and extractable organic matter from well Zhaoxin2.
C. Cai et al. / Applied Geochemistry 20 (2005) 1427–1444 1435
Table
3
Biomarker
parametersfortheoilsandextractable
organic
matter
Oil/
source
rock
Steranes
Triterpanes
Percentage
C29S/
R+S
C29bb
/
bb+aa
Ster./
hop.
C30hop.
ab/ba
C29-30-norhop./
C3017ahop.
Gamm./
C3017ahop.
Olean./
C3017ahop.
C32S/
S+R
C33S/
S+R
C34S/
S+R
C35S/
S+R
Ts/Tm
C27
C28
C29
Oil2
29
24
41
0.53
0.56
1.85
12.56
0.43
0.77
0.10
0.58
0.58
0.62
0.55
0.12
Oil3
38
30
33
0.49
0.56
1.33
–0.45
0.77
00.49
0.62
0.62
0.46
0.03
Oil5
26
22
52
0.55
0.57
1.34
11.17
0.59
0.85
0.47
0.63
0.59
0.58
0.59
0.28
Oil8
35
27
38
0.49
0.48
––
–0.07
–0.57
––
––
Sour1
19
26
55
0.38
0.40
1.37
1.80
0.60
0.43
0.16
0.30
0.27
0.28
0.24
0.30
Sour2
––
–0.51
0.56
––
–0.39
–0.56
––
––
Sour5
30
19
51
0.39
0.39
––
–0.08
–0.56
––
––
Sour6
35
24
40
0.44
0.42
––
–0.11
–0.55
––
––
C27,C28andC29bb
steranepercertages
are
calculatedfrom
m/z=218;Ster./hop.isallsteranes/allhopanes;C29-30-norhop./C3017ahop.representsC2917a,
21b-30-norhapane/C30
17a,
21b-hopane;
Ster.,hop.,Gamm.andOlean.are
steranes,hopanes
andoleananes,respectively.
1436 C. Cai et al. / Applied Geochemistry 20 (2005) 1427–1444
et al., 1995). These values are much lighter than those of
coexisting anhydrite across the northern and southern
parts of the Jinxian Sag (d34S �+34& to +40&). Ele-
mental S from well Zhaoxin1 in the northern part of
the Jinxian Sag has a d34S value of +6& (Table 4).
Three oils from the northern Jinxian Sag analyzed for
S isotopes have d34S values from +0.3& to +16.2& (Ta-
ble 2 and Fig. 13). The values are quite different (much
lower) than those of anhydrite in the basin, but higher
than those of pyrite. The oil sample with the highest S
content (14.7%) has the lowest d34S value of +0.3&,
which is close to that of H2S gas (Huan et al., 1992;
He et al., 1995).
5. Discussion
5.1. Source rock depositional environment
Biomarker parameters in the oils and mudstone
bitumen extracts from the Es4–Ek1 strata in the north,
namely low Pr/Ph, high gammacerane/C30-17ab-hopaneratios, abundant pregnanes and the presence of b-caro-tane, are consistent with organic matter deposition
from a stratified hypersaline geochemically reduced
water body (Fu et al., 1986; Sinninghe Damste et al.,
1995). Relatively high percentages of C27 steranes
(>30%) in the oils indicate a significant contribution
to the organic matter from algae. This corroborates
the H/C-O/C atomic ratio data (Fig. 3), which indicates
that the source rock contained Type I kerogen. Simi-
larly, the high sterane/hopane ratios (>1.3) of the oils
and source rocks indicate a dominantly algal contribu-
tion and are typical of a eutrophic lacustrine environ-
ment of deposition (Kuo, 1994). Thus, it can be
concluded that the Es4–Ek1 sediments in the northern
Jinxian Sag were deposited under a relatively arid cli-
mate in a hydrologically closed, hypersaline eutrophic
lake, similar to the Green River Formation in the
Washakie Basin (Horsfield et al., 1994). Such a deposi-
tional environment would also be consistent with the
relatively high d13C values of the sedimentary organic
matter (inferred from the isotopically heavy oils),
which indicate high algal growth rates (e.g. Peters
et al., 1996; Schidlowski et al., 1994). However, the
presence of oleanane in the oils and sediment, suggests
an additional contribution from allochthonous terres-
trial organic matter to the northern Jinxian Sag lacus-
trine sediments (Bao and Li, 2001).
In the southern Jinxian Sag, organic matter in the
Ek2 strata was mainly derived from terrestrial plants
as indicated by the alkane peak at C25 to C29 and gam-
macerane and pregnanes being less abundant than in the
north (Table 3; Guo et al., 1997). Sedimentary organic
matter d13C values are relatively low (Table 2). It can
thus be inferred that the Ek2 sediments were deposited
Increasing retention time
Ph
Ph
Pr
Rel
ativ
e In
tens
ity
(a)
(b)
Fig. 9. Gas chromatogram of whole oils produced from wells (a) 39 and (b) 41–3 showing biodegradation of n-alkanes in well 41–3 oil
but no degradation in well 39 oil.
C. Cai et al. / Applied Geochemistry 20 (2005) 1427–1444 1437
in a relatively hydrologically open, brackish to freshwa-
ter lake.
5.2. Oil-source correlation
The 3 oils in wells 41–3, 7 and 39 and the extract from
a depth of 2984 m mudstone in well Zhaoxin2 from the
northern part of the Jinxian Sag show similarly low Pr/
Ph ratios, high gammacerane/C30 17aa hopane ratios
(Tables 2 and 3) and abundant pregnanes (Fig. 8), sug-
gesting that the oils were derived from source rocks that
are similar to the Es4–Ek1 hypersaline lacustrine facies.
The biomarker maturity parameters, namely the C29 aaasterane 20 S/(S + R) and C31 ab hopane 22S/(S + R) ra-
tios, are close to the equilibrium values of 0.52–0.55 and
0.57–0.62, respectively (Peters and Moldowan, 1993),
indicating that they were derived from mature source
rocks. The same conclusion can be arrived at from the
C30 ba (H)-norhopane/C30 ab-hopane ratio, which is
<0.1 (Table 3; Peters and Moldowan, 1993).
When biomarker maturity parameters (Table 3) and
R0 values (�0.4%) are considered, it is apparent that
the organic matter at 2392 m in well Zhaoxin2 is too
immature (shallow) to have sourced the oils. Note that
the depth of 2392 m for the extract is similar to the depth
at which the oil samples were recovered. Moreover, it is
unlikely that the oils have experienced post-generational
maturation, since the present burial depth of the reser-
voirs (<2400 m) is relatively shallow. Thus, the oils must
have been derived from more mature source rocks at
depths >2800 m (Fig. 4) such as from the calcareous
mudstones at 2984 and at 3167 m in well Zhaoxin2
(Fig. 8 and Table 3; Bao and Li, 2001).
The oil from well 39 differs slightly from the two oils
(from wells 7 and 37) reservoired further north in that it
has a relatively higher ratio of oleanane/C30 ab-hopane,but lower C27 abb/C27-29 abb sterane ratios (Table 3),
which may indicate a relatively larger contribution from
higher land plants and slightly different depositional
conditions (e.g. Peters and Moldowan, 1993).
The S-lean oils in the Ek2-3 reservoirs in wells 29 and
46 in the south of the Jinxian Sag (Table 2), which have
no significant gammacerane and pregnanes, are consid-
ered to have been derived from the Ek2-3 freshwater
lacustrine source rock (Table 3; Guo et al., 1997).
5.3. Origin of gas phase H2S
In the Jinxian Sag, pyrite in source rocks has d34Svalues of �2.8 to �12.5&. H2S gas in reservoirs has
d34 S values close to 0& (Huan et al., 1992; He et al.,
1995). These values are significantly lighter than most
Asp
Resin
Ar
Whole oil
Sat
No rt h
So uth
(a)
we ll 7
we ll 41-3
we ll 39
we ll 60we ll 61
-28 -26 -24
-28 -26 -24
Asp
Resin
Ar
Sat
Kerogen
(b)
EOM
Zhaoxin2 Es -k4 1
we ll 105 Es -k4 1
we ll 52 Ek2-3
δ13C (%)
Fig. 10. Stable C isotope type-curves of (a) oils and (b)
extractable organic matter from potential source rocks. Note:
Sat, Ar and Asp represent saturate, aromatic and asphaltene
fraction, respectively; EOM is extractable organic matter.
0 10 20
-1.5
-1.0
-0.5-28
-26
-24
-28
-27
-26
Saturates
Wholeoil
Sat.-Asp.
R =0.662
R =0.692
R =0.882
δ13
C (
%)
Sulfur (%)
Fig. 11. Relationships between S contents and whole oil,
saturates, and saturates minus asphaltene d13C values
(d13Csat.�d13Casp.).
1438 C. Cai et al. / Applied Geochemistry 20 (2005) 1427–1444
of the petroleum and elemental S samples (+6.1& to
+16.2&). All these S species are isotopically much lighter
than Eocene anhydrite (ranging from +34.5& to
+39.7& with a mean of +37.8&). The H2S with d34Svalues heavier than the pyrite suggests that the H2S
was generated later than the reduced S in pyrite (e.g.
Raiswell et al., 1993; Tuttle and Goldhaber, 1993).
The Eocene anhydrite d34S values are significantly
higher than worldwide Eocene evaporitic marine SO4
values (+20& to +22&; Claypool et al., 1980). The rea-
son for the difference is not clear although 3 possible
explanations are presented here. The first arises from
the similarity of the Eocene in the Jinxian Sag and Tri-
assic Jialingjiang Formation anhydrite d34S values (Cai
et al., 2003) suggesting large scale basinal and cross-for-
mational flow of dissolved Triassic anhydrite may have
occurred resulting in reprecipitation in the Eocene sedi-
ments. The second possibility is that the heavy d34S val-
ues of anhydrite resulted from Rayleigh fractionation
during bacterial SO4 reduction of seawater in a closed
system. The third explanation is that the Eocene anhy-
drite, being lacustrine in origin, bears no relation to
marine systems since it is not subject to overturn and
homogenization of oceans but is subject to localized in-
puts (possibly from weathered Triassic outcrops). The
Eocene anhydrite is certainly isotopically anomalous in
comparison to contemporary marine anhydrite and the
difference in depositional environment probably ex-
plains the difference (option 3).
In north Jinxian, H2S in the reservoirs coexists with
S-enriched oils and has concentrations up to 92% by vol-
ume. However, the H2S with high concentrations is not
the result of thermochemical SO4 reduction (TSR) since
the difference in d34S values between the H2S (close to
0&) and parent sulfates (about +35&) is too large for
natural TSR (e.g. Machel et al., 1995; Worden and
Smalley, 1996; Cai et al., 2001). Rather the gas phase
H2S was probably derived from bacterial sulfate reduc-
tion as suggested by Yan et al. (1982), Huan et al.
(1992) and He et al. (1995). However, Wade et al.
(1989) suggested that even moderate concentrations of
H2S (>3%) might inhibit the activity of SO4 reducing
bacteria. Thus, it is likely that the site of the H2S gener-
ation was different to the site of H2S accumulation.
Bacterial SO4 reduction (BSR) is considered to take
place in anoxic marine and lacustrine environments,
resulting in the eventual transformation of aqueous
SO4 into H2S, polysulfides and elemental S. BSR ini-
tially results in reduced forms of inorganic sulfur
(H2S) that may, in some circumstances, be incorporated
into sedimentary organic matter and pyrite (e.g. Sinnin-
Table
4
Sulfurisotopecompositionofsulphurspecies
Well
Form
ation
Depth
(m)
Rock/m
ineral
XRD
resulta
d34S(&
,CDT)
Pyrite
Ele.sulfur
Anhy./gyp.
Zhaoxin2
Es4
2392
Anhydriticmudstone
Anhydrite,quartz,
dolomite,
kaolinite,
muscovite,
illite,chlorite
�2.8
�ffi
pb
+39.7
Well105
Es4
2573
Anhydriticlimestone
Anhydrite,quartz,
calcite,
gypsum,illite,kaolinite
�12.5
�ffi
p+37.5
Zhaoxin1
Ek1
2725
Anhydriticmudstone
–c
–+6.1
d–
Zhaoxin1
Es4
2218
Anhydrite
––
–+34.5
d
Zhaoxin2
Es4
2780
Anhydrite
––
–+39.3
d
aIn
order
ofamountfrom
highto
low.
bRepresents
sampleswithelem
entalsulfurbutnotanalyzedford3
4S.
cNotanalyzed.
dData
from
Heet
al.(1995);othersfrom
thisstudy.
C. Cai et al. / Applied Geochemistry 20 (2005) 1427–1444 1439
ghe Damste et al., 1990). BSR can result in S isotope
fractionation of up to �46& under pure culture experi-
ments (Kaplan and Rittenberg, 1964). In this case it
seems that BSR, in the source rock during burial, re-
sulted in elevated H2S. This suggests that at least some
of the bacterially reduced S was not incorporated into
organic matter. The freedom of the H2S to escape the
source rock prior to formation of pyrite suggests that
the source rocks had negligible quantities of available
Fe.
The precise site of BSR is not clear but it may have
taken place in the anhydrite-rich source rock and the
generated H2S may have migrated to the reservoirs
along with the evolved petroleum phase. H2S accumula-
tion in the reservoirs indicates that there was no avail-
able reactive Fe in the reservoir and that the rate of
H2S generation (by BSR) was higher than the rate of
H2S loss by any other diagenetic process.
5.4. Origin of sulfur-enriched oils
Sulfur-enriched oils are generally considered to be de-
rived from type I-S or type II-S kerogens with high S/C
atomic ratios (>0.04) (e.g. Orr, 1986; Peters et al., 1996).
However, the oils with S contents >10% may have S de-
rived from secondary processes (Thompson, 1981).
There are 4 main processes, or effects, that must be con-
sidered in attempting to explain the ultra-high S oils of
the Jinxian Sag. These are the roles of: (1) high S kero-
gen, (2) maturity effects since early generated oils tend to
have the highest S content, (3) biodegradation since the
residual S increases as hydrocarbons are selectively
metabolized, (4) diagenetic addition of S to oil during
burial.
5.4.1. Source rock influence
The low Pr/Ph ratio, the high gammacerane/C30-
17ab-hopane ratio, the occurrence of pregnanes and
b-carotane in the source rock extracts and petroleum
samples from the northern part of the Jinxian Sag,
discussed in Section 5.1, are all consistent with primary
organic matter that was deposited from stratified, hyper-
saline, geochemically reduced water typical of evaporitic
lacustrine environments. The great abundance of anhy-
drite with anomalous (i.e. non-marine) S isotope values
in the source rocks, and the Upper Palaeocene and Lower
Eocene sections in general (Es4–Ek1), also show that the
environment was rich in SO4. The predominance of car-
bonate and evaporitic sediments in the northern Jinxian
(Section 2.1) suggests that few Fe-rich minerals (e.g. clay
minerals) were co-deposited with the sulfate and carbon-
ate minerals. It is thus very likely that the initial kerogen
in the northern Jinxian was S-rich. Localised early dia-
genetic bacterial SO4 reduction (as opposed to burial-
related BSR, see earlier) would have led to reduced forms
of S being available for reaction either with the organic
Table 5
Sulfur contents in different species and TOC analyses for sediments
Well Depth (m) Fm. Rock Sulfur (%) TOC (%)
Total Sulfate Pyrite Ele. S Org S
Zhaoxin1 2510 Es4 Anhy. M. 8.19 6.07 1.58 0.1 0.44 0.62
Zhaoxin1 2725 Ek1 Anhy. M. 5.56 1.71 0.2 3.25 0.4 0.81
Zhaoxin1 2727 Ek1 Anhy. M. 4.29 3.07 0.27 – – 1.14
Zhaoxin2 2650 Es4 Anhy. M. 7.36 2.14 0.32 – – 1.05
Zhaoxin2 2652 Es4 Anhy. M. 8.19 4.78 0.22 – – 0.88
Zhaoxin2 3106 Ek1 Anhy. M. 6.06 4.46 0.84 0.03 0.73 1.17
Zhaoxin1 2508 Es4 M. 3.02 0.44 2.21 0.1 0.27 0.62
Zhaoxin1 2523 Es4 M. 1.69 0.6 1.08 DT 0.01 0.2
Zhaoxin1 2685 Ek1 M. 2.51 0.63 1.88 DT 0 0.28
Zhaoxin1 2792 Ek1 M. 1.7 0.95 0.18 0.53 0.04 0.73
Zhaoxin2 2459 Es4 M. 2.25 1.84 0.08 – – 1.17
Zhaoxin2 2656 Es4 M. 3.25 1.78 0.29 0.75 0.43 0.84
Zhaoxin2 2715 Es4 M. 3.46 0.75 1.08 1.0 0.63 1.2
Zhaoxin2 2852 Es4 M. 2.92 0.68 1.22 0.61 0.41 0.72
Zhaoxin2 2974 Ek1 M. 2.87 1.49 0.9 0.27 0.21 0.42
Zhaoxin2 3131 Ek1 M. 3.03 0.75 1.91 0.07 0.3 0.6
Zhaoxin2 3177 Ek1 M. 3.28 0.52 2.15 0.16 0.45 0.6
108 1845 Es4 M. 2.16 0.12 2.02 DT 0.02 0.85
Zhaoxin2 2328 Es4 Argi. Lime. 3.52 3.21 0.3 DT 0.01 0.26
Anhy. M. represents anhydritic mudstone in short; M.: mudstone; Argi. Lime.: argillaceous limestone; Ele. S: Elemental sulfur; Org. S:
Organically bound sulfur; –: No measurement; DT: below detection limit.
1440 C. Cai et al. / Applied Geochemistry 20 (2005) 1427–1444
matter or any available forms of Fe. The nature of the
mineralogy of the Es4–Ek1 sediments (Fe mineral poor)
led to S rich kerogen.
In contrast, in the southern Jinxian Sag, organic mat-
ter was derived from terrestrial plants as indicated by the
alkane peak at C25–C29 and gammacerane and pregn-
anes being less abundant than in the north (Section
5.1; Table 3; Guo et al., 1997). The source rocks in the
south were deposited in a much less evaporated lacus-
trine environment than in the northern part of the basin.
Although anhydrite-bearing mudstones are present in
the south, they are less abundant and thinner than in
the north (Section 2.1).
Thus petroleum from the northern Jinxian Sag is
likely to have a higher S concentration than petroleum
from the southern Jinxian Sag. It is not surprising that
the petroleum from the northern Jinxian Sag is relatively
S-rich although concentrations of up to 14% remain
anomalous despite the sedimentary environment and or-
ganic facies and still require explanation.
5.4.2. Maturity influence
Marginally mature petroleum source rocks generate
petroleum with a higher S concentration than equivalent
fully mature source rocks. In many basins, the S content
of petroleum decreases with increasing depth of burial of
the source rock due to this effect. However, source rock
maturity seems not to have been important in this case
since there is no correlation between the S content of
the petroleum and the maturity (as indicated by molec-
ular maturity parameters; Table 3).
5.4.3. Biodegradation
Significantly, the petroleum samples with >5% S only
occur at depths of less than 2400 m (Fig. 5) with palae-
otemperatures <80 �C. The environment is suitable for
an aerobic biodegradation of hydrocarbons and anaero-
bic SO4 reduction of byproducts of the biodegradation
organic acids and anions (e.g. Jobson et al., 1979). Re-
cent work has shown that degradation of hydrocarbons
in subsurface oil reservoirs is dominated by anaerobic
bacteria (e.g. Larter et al., 2003). For example, it was
proposed that hydrocarbons are directly degraded by
SO4 reducing bacteria (e.g. Bechtel et al., 1996), or cou-
pled to organic acids and anions (e.g. Cai et al., 2002). In
the northern Jinxian Sag, the saturate contents of petro-
leums range from 12% to 46%. Among the oils, the two
high S oils (wells 41–3 and 7) show evidence of biodeg-
radation of normal alkanes whilst isoprenoids, steranes,
triterpanes seem to have remained intact (Figs. 8 and 9).
The oil from well 7 has the lowest content of saturates
and the highest S (Table 2). The oil from well 39 has
the highest content of saturates, and shows no evidence
of biodegradation (Figs. 8 and 9). If the oil from well 7
were the result of biodegradation of the oil from well 39
with S of 3.5%, i.e., saturate totals decreased from 46%
to 12%, then about 75% of the saturates have been
degraded. Consequently, the oil may concentrate S by
LD
UDUD
Dep
th(m
)
(a) (b)
(c)
Fig. 12. H2S distribution in Kongdian Fm (Upper Palaeocene) (a) Upper Dolomite and (b) Lower Dolomite groups superposed over
isopachs of the top of the Upper Dolomite and Lower Dolomite groups, respectively; (c) cross-section showing H2S gas reservoirs
encountered in wells.
Sam
ple
Elemental sulfur
δ 34S (%)
Fig. 13. Distribution of d34S values of anhydrite, pyrite, H2S
and elemental S and petroleum from the Jinxian Sag.
C. Cai et al. / Applied Geochemistry 20 (2005) 1427–1444 1441
up to about 1.6 times and the resulting oil may have a S
content of about 5.6%. This value is still significantly
lower than the S content of the oil from well 7 (14.7%).
Biodegradation typically results in residual saturates
with relatively high d13C values (as seen in Fig. 10(a))
and thus the whole oil has increased d13C values. How-
ever, negligible significant change in d34S values would
be expected, since S-containing compounds are thought
to be resistant to biodegradation (e.g. Manowitz et al.,
1990). If organic S bonds were ruptured during biodeg-
radation, normal kinetic isotope effects favouring faster
reaction of the lighter 32S would increase the d34S value
of unreacted material rather than the decrease observed
in the Jinxian Sag. Thus biodegradation, and the conse-
quent partial removal of the saturates fractions, alone
cannot account for the observed broad range of d34Svalues of the oils (+0.3& to +16.2&) and the low d34Svalue in the most heavily degraded oil in the Jinxian
Sag.
1442 C. Cai et al. / Applied Geochemistry 20 (2005) 1427–1444
5.4.4. Sulfur incorporation into oils?
The 3 oils in the Jinxian Sag have d34S values ranging
from +0.3& to +16.2&. The oil from well 7 has an ex-
tremely high S content and is also the most depleted in
d34S, with a d34S value close to co-existing BSR-derived
H2S gas in the reservoir (Table 2). In contrast, the lowest
S oil from well 39 has the highest d34S value. Manowitz
et al. (1990) proposed that relatively high S contents and
low d34S values in degraded oils from the Bolivar Coastal
Fields (Venezuela) were the result of SO4 reduction.
Thus, it cannot be discounted that S with low d34S val-
ues has been incorporated into the oil in the Jinxian Sag.
Labile compounds that may be generated by biodeg-
radation of petroleum, particularly functionalised com-
pounds, are considered to be suitable precursors to
facilitate the incorporation of reduced S generated by
BSR into oil (Tuttle and Goldhaber, 1993; Rowland
et al., 1993). Petroleum that increasingly incorporates
BSR-derived S can be expected to have an elevated S con-
tent and d34S value that approaches BSR-derived S d34Svalues. This possibility is supported by the fact that the
oil from well 7, with the highest S content, has the lowest
d34S value, and this is close to coexisting BSR-derived
H2S gas in reservoirs. Additionally, S incorporation into
labile hydrocarbons is expected to result in a decrease in
the saturates fraction, and an increase in aromatic, resin
and asphaltene fractions (Table 2). Since saturates have
the lowest C isotope values of all the 4 fractions, the
other 3 fractions are expected to show lower d13C values
than before S incorporation into biodegraded saturates.
Sulfur incorporation typically generates asphaltene and
resin compounds as evidenced by the positive relation-
ship between S and resin + asphaltene contents (Fig.
6(b)). Thus, a relatively significant change in d13C value
may occur in the asphaltene and/or resin, compared with
the aromatic fraction. The negative relationship between
S content and (d13Csat.–d13Casp.; Fig. 11) indicates that
increasing S incorporation has resulted in asphaltene
d13Casp. closer to saturates, and thus asphaltenes with
lighter d13C values than aromatics.
In brief, the above data indicate that part of the
asphaltene and resin compounds in the high S oils may
have been derived from a secondary process in the reser-
voir. The most likely process is the incorporation of
BSR-derived reduced S (most likely to be H2S) into bio-
degraded labile hydrocarbons.
Orr (1977) suggested that it is unlikely for reduced S
to be incorporated into oils under conditions of low H2S
concentrations and low temperatures (<80 �C), which
are the conditions favourable for SO4-reducing bacteria
to grow. However, this may not be the case for the Jinx-
ian Sag. The coexistence of high H2S concentrations
with labile hydrocarbons, where the H2S was the result
of BSR outside of the reservoir, may have been suitable
for the incorporation of reduced S generated by BSR
(Tuttle and Goldhaber, 1993).
6. Conclusions
(1) Petroleum from the northern Jinxian Sag in the
Bohai Basin has anomalous S concentrations
ranging from 3.49% to 14.69%. In contrast, the
southern Jinxian Sag has petroleum with 0.03%
to 2.15% S.
(2) Es4–Ek1 source rock extracts and oils have low Pr/
Ph, high gammacerane/C30 hopane ratios, abun-
dant C35 homohopanes and high sterane/hopane
with thepresence of pregnanes andb-carotene. Thisbiomarker distribution is consistent with organic
matter being deposited under a relatively closed
and stratified hypersaline eutrophic water body.
In contrast, Ek2-3 sedimentary organic matter
was deposited under a relatively open freshwater
environment dominated by higher plants.
(3) High S oils in the Es4–Ek1 and Es2-3 reservoirs in
the northern Jinxian Sag were derived from Es4–
Ek1 lacustrine calcareous mudstones although
the S concentrations of petroleum from the north-
ern Jinxian Sag are much greater than is typical of
most type I evaporitic source rocks.
(4) Bacterial SO4 reduction (BSR) during burial dia-
genesis was the likely origin of the H2S gas in
the reservoir. The highest S concentration is found
in oil with low d34S values that are similar to the
d34S values of the co-existing H2S. The S-rich oil
has d34S values significantly heavier than those
of pyrite and much lighter than those of co-exist-
ing anhydrite.
(5) Biodegradation of the saturates fraction led to
increased S in the residual petroleum although it
is unlikely to have been capable of producing
the highest S concentrations found in the northern
Jinxian Sag.
(6) Sulfur, in the form of BSR-derived H2S, may have
been assimilated into the petroleum in the north-
ern Jinxian Sag during burial diagenesis. The sec-
ondary addition of S into the petroleum was
possibly contemporary with mild biodegradation
of the saturates fraction.
Acknowledgements
The research was financially supported by the Natu-
ral Sciences Foundation of China (grant No. 40173023),
China National Major Basic Development Program 973
(2003CB214605), FANEDD, SRF for ROCS (SEM)
and the UK Royal Society, UK. Dr. Anu Thompson
helped with GC-MS analyses. GCMS facilities were pro-
vided by HEFCE grant No. JR98LIWO. Comments by
Henry Halpern from Saudi Aramco, helped to improve
the manuscript.
C. Cai et al. / Applied Geochemistry 20 (2005) 1427–1444 1443
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