Origin of sulfur rich oils and H S in Tertiary lacustrine ...sourcedb.igg.cas.cn/cn/zjrck/200907/W020101123534997067373.pdf · The Es4–Ek1 anoxic evaporitic lacustrine source rocks

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.

mailto:[email protected]




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


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.


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


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).


(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).


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.


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.


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.


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.).


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.


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.


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.


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.


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.


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