# The Effect of Water Chemistry on Thermochemical Sulfate Reduction: A Case Study from the Ordovician in the Tazhong Area, Northwest China

Authors: Hongxia Li; Chunfang Cai; Lianqi Jia; Chenlu Xu; Ke Zhang
Journal: Geofluids
 (2017)

---

## Abstract

Formation water chemistry, sulfate sulfur isotopes, and associated H2S contents and sulfur isotopes were measured from the Ordovician in Tazhong area, Tarim Basin. The aim is to elucidate the effects of geochemical composition of formation water on thermochemical sulfate reduction (TSR) and potential usage of SO4/Cl ratios as a new proxy for TSR extents in areas, where H2S and thiaadamantanes (TAs) data are not available. The formation water has SO4/Cl ratios from 0.0002 to 0.016, significantly lower than 0.04 to 0.05 from 3 to 7 times evapoconcentrated seawater. Thus, the low values are explained to result from TSR. Furthermore, the SO4/Cl ratios show negative correlation relationships to TAs and H2S concentrations, indicating that TSR occurred in a relatively closed system and SO4/Cl ratio can be used to indicate TSR extents in this area. Extensive TSR in the Cambrian in the Tazhong area, represented by low SO4/Cl ratios and high H2S and TAs concentrations, is accompanied by formation water with high TDS and Mg concentrations, indicating the effects of water chemistry on TSR under a realistic geological background. In contrast, the low TSR extent in the Ordovician may have resulted from limited TSR reaction duration and total contribution of aqueous SO42-.


---

## Body

## 1. Introduction

Thermochemical sulfate reduction (TSR), a process whereby aqueous sulfate and petroleum compounds react at temperatures higher than 120°C (CnH2n+2 + SO42-→CO2 + H2S + altered petroleum), is considered to result in elevated H2S concentrations in many carbonate reservoirs [1–8]. Significant advance has occurred on mechanisms of TSR. A great number of organic sulfides such as thiols and thiolanes [7, 9, 10] and 1- to 3-cage thiadiamondoids with 1 to 4 sulfur atoms were detected from TSR areas [11–14]. The presence of these organic sulfur compounds, especially labile sulfur compounds such as 1-pentanethiol or diethyl disulfide, has been experimentally showed to significantly increase the rate of TSR [15]. However, hydrocarbons cannot directly react with solid sulfate in temperature from 180°C to 350°C in the laboratory [16]. Reactions between solid sulfate and gaseous hydrocarbon are quite slow even under temperatures of several hundred degrees Celsius (Kiyosu et al., 1990). Water is the solvent for chemical species and provides the aqueous matrix for all chemical reactions. Theoretical calculations by Ma et al. [17] showed that bisulfate ions (HSO4-) and/or magnesium sulfate contact ion-pairs (MgSO4 CIP) are most likely reactive sulfur species involved in TSR. Experiments indicated that the concentrations of MgSO4 CIP are related to temperatures and SO4/Mg ratios in the solutions [18, 19]. Consequently, water chemistry and geologic environment can strongly influence the TSR process [8, 17]. However, the effects of water chemistry on TSR are limited to theoretical and experimental studies. More researches in the real geological setting should be done.H2S, common in the Ordovician carbonate reservoir in the Tazhong area, is generated by TSR [20, 21]. H2S concentration from the Ordovician in the Tazhong area is less than 10%, which is lower than that in the Khuff formation in Abu Dhabi (up to 50% [6]), the Nisku Formation in western Canada (up to 31% [22]), and the Feixianguan Formation in the northeastern Sichuan basin (up to 17% [23, 24]). The low concentrations of H2S in the Tazhong area are considered to result from TSR process which is limited by the burial temperature [25, 26]. Besides, TSR process can be limited by water chemistry [8]. The Tazhong area is chosen as a research target and compared with the northeastern Sichuan Basin because relatively abundant information about TSR and formation water was published by previous studies. The work presented here seeks to address the following research questions: (1) what is the effect of water chemistry on TSR process and/or extent? (2) Why TSR extent in the Ordovician Yingshan Formation in the Tazhong area is low? A better understanding about the TSR mechanism will be provided through this work.


## 2. Geological Setting

### 2.1. Structural Units and Stratigraphy

Tazhong area is located in the center of Tazhong Uplift, Tarim Basin, northwest China. It is surrounded by the Manjiaer Sag, South Depression, Bachu Uplift, and Tadong Uplift (Figure1(a)). It can be divided into number 1 Fault-slope Zone, North Slope, number 10 Structural Belt, Central Faulted Horst Belt, South Slope, and East Burial Hill Zone (Figure 1(b)). Tazhong Uplift is one of the major petroleum production areas in the Tarim Basin. Oils and natural gases have been found in the Cambrian-Ordovician carbonate reservoirs and the Silurian-Carboniferous clastic reservoirs [27].Figure 1
(a) Map of the Tarim Basin showing tectonic units and location of Tazhong Uplift; (b) map of the study area showing geological tectonics and location of wells from where formation waters were collected.The general stratigraphic columns of the Tazhong area were described previously [20, 25, 28, 29]. Briefly, the Cambrian strata are composed of tidal, platform, and platform-marginal carbonates. The Ordovician strata include the Upper Ordovician Sangtanmu (O3s) and Lianglitage (O3l) Formations and the Lower and Middle Ordovician Yingshan (O1-2y) and Penglaiba (O1p) Formations (Figure 2). The Lower Ordovician is predominantly composed of thick, platform facies dolomite in the lower part and limestone in the upper part. The Upper Ordovician is represented by reef and shoal facies packstone and bioclastic limestone and slope facies limestone and marlstone [9] The Silurian to the Carboniferous sequence consists of marine sandstones and mudstones. The Permian strata are composed of lacustrine sediments and volcanic rocks. The Mesozoic and the Cenozoic are nonmarine sandstones and mudstones [20, 30, 31].Figure 2
Stratigraphic column for the Tazhong Uplift.Anhydritic dolomites and anhydrite were observed in supratidal facies of the Middle Cambrian. Bedded anhydrites of 44 m~98 m thick are present in the eastern ZS1 and ZS5 wells [21]. No anhydritic carbonate was observed in the Ordovician, which makes the geological background of TSR in the Tazhong area differentiate from the northeast Sichuan Basin.


### 2.2. Burial and Thermal History

TZ12 is located at the central part of number 10 Structural Belt (Figure1(b)). Based on the burial history that rebuilt on well TZ12 (Figure 3(a)), the Lower Ordovician reached temperature of 120°C at the late Cretaceous and then reached to the maximum depth of 5000 m and temperature of 150°C at present day, whereas the Triassic Feixianguan Formation has reached temperature of 150°C since the end of the Triassic and reached temperature of 200°C at the middle Jurassic (Figure 3(b)). TSR occurred in the limestone reservoirs of the Yingshan Formation above a temperature of 120°C [20, 29].Figure 3
(a) Burial and paleotemperature history of the Ordovician Yingshan Formation (marked with grey color) based on the well TZ12 in the Tazhong area (modified from Chen et al., 2010). (b) Burial and paleotemperature history of the Triassic Feixianguan Formation (marked with grey color) based on the well LJ2 from the Northeast Sichuan Basin (modified from [23]).
(a)
(b)


## 2.1. Structural Units and Stratigraphy

Tazhong area is located in the center of Tazhong Uplift, Tarim Basin, northwest China. It is surrounded by the Manjiaer Sag, South Depression, Bachu Uplift, and Tadong Uplift (Figure1(a)). It can be divided into number 1 Fault-slope Zone, North Slope, number 10 Structural Belt, Central Faulted Horst Belt, South Slope, and East Burial Hill Zone (Figure 1(b)). Tazhong Uplift is one of the major petroleum production areas in the Tarim Basin. Oils and natural gases have been found in the Cambrian-Ordovician carbonate reservoirs and the Silurian-Carboniferous clastic reservoirs [27].Figure 1
(a) Map of the Tarim Basin showing tectonic units and location of Tazhong Uplift; (b) map of the study area showing geological tectonics and location of wells from where formation waters were collected.The general stratigraphic columns of the Tazhong area were described previously [20, 25, 28, 29]. Briefly, the Cambrian strata are composed of tidal, platform, and platform-marginal carbonates. The Ordovician strata include the Upper Ordovician Sangtanmu (O3s) and Lianglitage (O3l) Formations and the Lower and Middle Ordovician Yingshan (O1-2y) and Penglaiba (O1p) Formations (Figure 2). The Lower Ordovician is predominantly composed of thick, platform facies dolomite in the lower part and limestone in the upper part. The Upper Ordovician is represented by reef and shoal facies packstone and bioclastic limestone and slope facies limestone and marlstone [9] The Silurian to the Carboniferous sequence consists of marine sandstones and mudstones. The Permian strata are composed of lacustrine sediments and volcanic rocks. The Mesozoic and the Cenozoic are nonmarine sandstones and mudstones [20, 30, 31].Figure 2
Stratigraphic column for the Tazhong Uplift.Anhydritic dolomites and anhydrite were observed in supratidal facies of the Middle Cambrian. Bedded anhydrites of 44 m~98 m thick are present in the eastern ZS1 and ZS5 wells [21]. No anhydritic carbonate was observed in the Ordovician, which makes the geological background of TSR in the Tazhong area differentiate from the northeast Sichuan Basin.


## 2.2. Burial and Thermal History

TZ12 is located at the central part of number 10 Structural Belt (Figure1(b)). Based on the burial history that rebuilt on well TZ12 (Figure 3(a)), the Lower Ordovician reached temperature of 120°C at the late Cretaceous and then reached to the maximum depth of 5000 m and temperature of 150°C at present day, whereas the Triassic Feixianguan Formation has reached temperature of 150°C since the end of the Triassic and reached temperature of 200°C at the middle Jurassic (Figure 3(b)). TSR occurred in the limestone reservoirs of the Yingshan Formation above a temperature of 120°C [20, 29].Figure 3
(a) Burial and paleotemperature history of the Ordovician Yingshan Formation (marked with grey color) based on the well TZ12 in the Tazhong area (modified from Chen et al., 2010). (b) Burial and paleotemperature history of the Triassic Feixianguan Formation (marked with grey color) based on the well LJ2 from the Northeast Sichuan Basin (modified from [23]).
(a)
(b)


## 3. Sample Collection and Analysis

A total of 17 water samples were collected from wells in the Tazhong area. These samples are used for analysis of water chemistry and S isotopic compositions ofSO42-. 7 H2S samples were collected and analyzed for S isotope. TAs concentrations in oils are obtained from previous studies.pH was measured using an electrode method within 2 hours after sampling in the field. TDS were measured by the gravimetric method according to Clescerl et al. [32]. After filtration with a 0.45 ml filter, 0.5 ml samples of the brines were dried at 180°C until a constant weight was reached. The anions were measured by ion chromatography following appropriate dilution (5000 times for Cl and 1000 times for Br and SO4) with a Dionex ICS900 instrument with an AS19 ion-exchange column. The analytical precisions were better than 0.8% for Cl and 4.3% for SO42-. The major cations in the diluted solutions (5000 times for all cations) were analyzed with a Varian Vista-Pro inductively coupled plasma-optical emission spectrometer (ICP-OES) with an analytical precision better than 5%.DissolvedSO42- was quantitatively precipitated as BaSO4 by reacting with excess BaCl2. This reaction was performed at a pH between 3 and 4 with HCl to prevent precipitation of BaCO3. The precipitation of BaSO4 was then filtered using a Buchner funnel and washed with distilled water. And then precipitation of BaSO4 was dried and used for sulfur isotopic analysis on a Thermo Finnigan Delta S mass spectrometer. H2S was precipitated immediately in the field by the quantitative reaction with excess zinc acetate, Zn(CH3COO)2, to form ZnS at a pH in the range of 10-11 (the pH was adjusted with NaOH). The solution with ZnS was put aside overnight and then filtered with a 0.45 μm filter on site. In the laboratory, ZnS was transformed to Ag2S by adding HCl and passing the evolved H2S under an inert atmosphere through AgNO3 solution at a pH of 4. Ag2S were used for sulfur isotopic analysis on a Thermo Finnigan Delta S mass spectrometer calibrated by a series of International Atomic Energy Agency standards. Results are presented as δ34S relative to the Vienna Canyon Diablo Troilite (VCDT) standard. The reproducibility for δ34S measurement is ±0.3‰.


## 4. Results

### 4.1. Water Chemistry

Chemical compositions of formation water are shown in Table1. The Yingshan Formation (O1-2y) formation waters have Na+ concentrations ranging from 20140 mg/L to 64000 mg/L and Cl− concentrations ranging from 50861 mg/L to 126000 mg/L, characterized by Na/Cl molar ratios of 0.54~0.89 with an average of 0.74. The range of SO42- concentrations is from 31 mg/L to 891 mg/L. The SO4/Cl ratios (expressed in weight units) range from 0.0002 to 0.016 with a mean value of 0.005 (SO4/Cl ratio of seawater is 0.144). The range of Mg2+ concentrations of O1-2y formation waters is from 92 mg/L to 1070 mg/L with an average of 638 mg/L, and the Mg/Cl ratios lie between 0.002 and 0.014 with a mean value of 0.007 (Mg/Cl ratio of seawater is 0.067). Na/Cl molar ratios of formation water are close to that of seawater (0.86). SO4/Cl and Mg/Cl ratios of formation water are significantly depleted compared to seawater and vary largely compared to Na/Cl molar ratios.Table 1
Geochemical and sulfur isotopic composition of formation water, concentrations and sulfur isotopic composition of H2S in the gas, and concentrations of TAs in the oil in Tazhong area.
Well
Depth (m)
Strata
pH
H
C
O
3
- (mg/l)
Cl− (mg/l)
S
O
4
2
- (mg/l)
Ca2+ (mg/l)
Mg2+ (mg/l)
Na+ (mg/l)
TDS (g/L)
δ
34
S
S
O
4 (‰)
δ
34
S
H
2
S (‰)
H2S (%)
TAs (μg/g)
TZ44
4873
O
1
-
2
y
7.0
781
50861
654
7333
92
—
84
—
—
0.1
32.96#
TZ82
5460
O
1
-
2
y
6.7▯
778▯
62400▯
341▯
10500▯
374▯
—
107▯
24.7▯
—
0.2
—
TZ83-1
5638
O
1
-
2
y
6.4&
99&
97900&
156&
8741&
684&
51530&
164&
—
—
0.7
—
ZG103
6191
O
1
-
2
y
6.3
371
54800
321
3122
341
31970
96
22.7
11.9
0.4
—
ZG11
6398
O
1
-
2
y
6.8
527
55340
224
3240
234
—
92
26.8▯
16.3
0.6∗
46.63#
ZG2
5880
O
1
-
2
y
7.1▯
239▯
58000▯
353▯
9840▯
395▯
24320▯
129▯
25.7▯
1.3
—
ZG433C
6133
O
1
-
2
y
8.0&
484&
120000&
73&
13500&
1070&
62400&
204&
—
13.2
4.9∗
—
ZG44
5647
O
1
-
2
y
7.2&
617&
100200&
450&
9635&
770&
53510&
170&
—
—
—
—
ZG44C
5647
O
1
-
2
y
5.5&
3080&
109000&
571&
14000&
698&
56900&
188&
—
—
—
—
ZG44-H2C
6056
O
1
-
2
y
6.8&
369&
95990&
343&
11150&
768&
46340&
159&
24.2
15.5※
0.4∗
—
ZG46
5148
O
1
-
2
y
7.0▯
6358▯
97300▯
330▯
1580▯
1050▯
—
166▯
24.2▯
13.4
1.5∗
—
ZG461
5560
O
1
-
2
y
6.0
1317
56990
891
19800
810
20140
104
—
13.4
0.1
ZG5
6406
O
1
-
2
y
6.3&
—
84900&
617&
43110&
474&
138&
27.8
15.5
3.5∗
104.19#
ZG511-2
5238
O
1
-
2
y
6.5&
88&
112000&
34&
10630&
1003&
52770&
182&
—
—
—
—
ZG6
6053
O
1
-
2
y
6.4&
127&
126000&
31&
13770&
918&
59260&
206&
29.8
7.8∗
147.20#
ZG7
5794
O
1
-
2
y
7.8▯
871▯
102000▯
53▯
8890▯
532▯
—
112▯
27.8▯
—
4.4∗
100.75#
ZS1C
6520
∈
1
6.0
274
164000
182
41300
25500
248
11
∗
1936.6#
Note that※ is from Cai et al. [10], ∗ is from Cai et al. [21], # is Cai et al. [13], & is from Li and Cai [33], ▯ is from Zhang et al. [25], and “—” represents no data available.The Cambrian formation water hasSO42- concentration of 182 mg/L, Cl− concentration of 164000 mg/L, SO4/Cl ratio of 0.001 which is lower than most of the O1-2y formation water. The Cambrian formation water has Mg2+ concentration of 25500 mg/L and Mg/Cl ratios of 0.155 which is higher than that of the O1-2y formation water.


### 4.2. Sulfur Isotopic Composition

δ
34
S
H
2
S values in the O1-2y carbonate reservoir range from 11.9‰ to 16.3‰ with an average of 14.2‰ (Table 1). δ34S value of H2S from the Cambrian is 33‰ which is significantly higher than that from the O1-2y carbonate reservoir. δ34SSO4 values of the O1-2y formation water range from 22.7‰ to 29.8‰ with an average of 26‰ which is slightly heavier than that of coeval seawater (Figure 4). δ34S values of the Cambrian bedded anhydrite lie between 26.2‰ and 33.7‰, which is similar to the Cambrian seawater. δ34S values of H2S from the Ordovician are lighter than that of the Cambrian seawater, the Ordovician seawater, and the Cambrian bedded anhydrite (Figure 4). Sulfur isotope fractionation between SO4 and H2S in the Ordovician lies between 8.7‰ and 12.3‰ with an average of 10.6‰.Figure 4
Sulfur isotopic composition of water, H2S, and anhydrite. Sulfur isotopic of bedded anhydrite is from Cai et al. [10]. Sulfur isotopic of H2S in the Cambrian is from Cai et al. [21].


## 4.1. Water Chemistry

Chemical compositions of formation water are shown in Table1. The Yingshan Formation (O1-2y) formation waters have Na+ concentrations ranging from 20140 mg/L to 64000 mg/L and Cl− concentrations ranging from 50861 mg/L to 126000 mg/L, characterized by Na/Cl molar ratios of 0.54~0.89 with an average of 0.74. The range of SO42- concentrations is from 31 mg/L to 891 mg/L. The SO4/Cl ratios (expressed in weight units) range from 0.0002 to 0.016 with a mean value of 0.005 (SO4/Cl ratio of seawater is 0.144). The range of Mg2+ concentrations of O1-2y formation waters is from 92 mg/L to 1070 mg/L with an average of 638 mg/L, and the Mg/Cl ratios lie between 0.002 and 0.014 with a mean value of 0.007 (Mg/Cl ratio of seawater is 0.067). Na/Cl molar ratios of formation water are close to that of seawater (0.86). SO4/Cl and Mg/Cl ratios of formation water are significantly depleted compared to seawater and vary largely compared to Na/Cl molar ratios.Table 1
Geochemical and sulfur isotopic composition of formation water, concentrations and sulfur isotopic composition of H2S in the gas, and concentrations of TAs in the oil in Tazhong area.
Well
Depth (m)
Strata
pH
H
C
O
3
- (mg/l)
Cl− (mg/l)
S
O
4
2
- (mg/l)
Ca2+ (mg/l)
Mg2+ (mg/l)
Na+ (mg/l)
TDS (g/L)
δ
34
S
S
O
4 (‰)
δ
34
S
H
2
S (‰)
H2S (%)
TAs (μg/g)
TZ44
4873
O
1
-
2
y
7.0
781
50861
654
7333
92
—
84
—
—
0.1
32.96#
TZ82
5460
O
1
-
2
y
6.7▯
778▯
62400▯
341▯
10500▯
374▯
—
107▯
24.7▯
—
0.2
—
TZ83-1
5638
O
1
-
2
y
6.4&
99&
97900&
156&
8741&
684&
51530&
164&
—
—
0.7
—
ZG103
6191
O
1
-
2
y
6.3
371
54800
321
3122
341
31970
96
22.7
11.9
0.4
—
ZG11
6398
O
1
-
2
y
6.8
527
55340
224
3240
234
—
92
26.8▯
16.3
0.6∗
46.63#
ZG2
5880
O
1
-
2
y
7.1▯
239▯
58000▯
353▯
9840▯
395▯
24320▯
129▯
25.7▯
1.3
—
ZG433C
6133
O
1
-
2
y
8.0&
484&
120000&
73&
13500&
1070&
62400&
204&
—
13.2
4.9∗
—
ZG44
5647
O
1
-
2
y
7.2&
617&
100200&
450&
9635&
770&
53510&
170&
—
—
—
—
ZG44C
5647
O
1
-
2
y
5.5&
3080&
109000&
571&
14000&
698&
56900&
188&
—
—
—
—
ZG44-H2C
6056
O
1
-
2
y
6.8&
369&
95990&
343&
11150&
768&
46340&
159&
24.2
15.5※
0.4∗
—
ZG46
5148
O
1
-
2
y
7.0▯
6358▯
97300▯
330▯
1580▯
1050▯
—
166▯
24.2▯
13.4
1.5∗
—
ZG461
5560
O
1
-
2
y
6.0
1317
56990
891
19800
810
20140
104
—
13.4
0.1
ZG5
6406
O
1
-
2
y
6.3&
—
84900&
617&
43110&
474&
138&
27.8
15.5
3.5∗
104.19#
ZG511-2
5238
O
1
-
2
y
6.5&
88&
112000&
34&
10630&
1003&
52770&
182&
—
—
—
—
ZG6
6053
O
1
-
2
y
6.4&
127&
126000&
31&
13770&
918&
59260&
206&
29.8
7.8∗
147.20#
ZG7
5794
O
1
-
2
y
7.8▯
871▯
102000▯
53▯
8890▯
532▯
—
112▯
27.8▯
—
4.4∗
100.75#
ZS1C
6520
∈
1
6.0
274
164000
182
41300
25500
248
11
∗
1936.6#
Note that※ is from Cai et al. [10], ∗ is from Cai et al. [21], # is Cai et al. [13], & is from Li and Cai [33], ▯ is from Zhang et al. [25], and “—” represents no data available.The Cambrian formation water hasSO42- concentration of 182 mg/L, Cl− concentration of 164000 mg/L, SO4/Cl ratio of 0.001 which is lower than most of the O1-2y formation water. The Cambrian formation water has Mg2+ concentration of 25500 mg/L and Mg/Cl ratios of 0.155 which is higher than that of the O1-2y formation water.


## 4.2. Sulfur Isotopic Composition

δ
34
S
H
2
S values in the O1-2y carbonate reservoir range from 11.9‰ to 16.3‰ with an average of 14.2‰ (Table 1). δ34S value of H2S from the Cambrian is 33‰ which is significantly higher than that from the O1-2y carbonate reservoir. δ34SSO4 values of the O1-2y formation water range from 22.7‰ to 29.8‰ with an average of 26‰ which is slightly heavier than that of coeval seawater (Figure 4). δ34S values of the Cambrian bedded anhydrite lie between 26.2‰ and 33.7‰, which is similar to the Cambrian seawater. δ34S values of H2S from the Ordovician are lighter than that of the Cambrian seawater, the Ordovician seawater, and the Cambrian bedded anhydrite (Figure 4). Sulfur isotope fractionation between SO4 and H2S in the Ordovician lies between 8.7‰ and 12.3‰ with an average of 10.6‰.Figure 4
Sulfur isotopic composition of water, H2S, and anhydrite. Sulfur isotopic of bedded anhydrite is from Cai et al. [10]. Sulfur isotopic of H2S in the Cambrian is from Cai et al. [21].


## 5. Discussion

### 5.1. Sulfur Isotope Composition and Fractionation

H2S from the Ordovician carbonate reservoir in this and previous studies have δ34S values from 12‰ to 16‰ which are 15‰~20‰ lighter than the counterpart in the Cambrian (33‰) [10, 21, 34]. The large differences indicate that the H2S in the Ordovician was probably generated from in situ TSR rather than TSR that happened in the Cambrian [13, 21]. A positive relationship exists between δ34SH2S values and δ34SSO4 values (Figure 5(a)). δ34SSO4 values tend to increase with a decrease of SO42- concentrations in the formation water (Figure 5(b)). This indicates that δ34S values of H2S are related to those of the remaining SO42-. There is a positive relationship between δ34SSO4-H2S and SO4/Cl ratios (Figure 5(c)), likely indicating that the more dissolved SO42- is converted to H2S, the smaller sulfur isotope fractionation occurs. This may imply that δ34SH2S values are controlled by both δ34SSO4 values and TSR extent.Figure 5
Relationship between (a)δ34SSO4 and δ34SH2S; (b) SO4/Cl and δ34SSO4; (c) SO4/Cl and δ34SSO4-H2S.
(a)
(b)
(c)Sulfur isotope fractionation is nearly negligible if complete conversion of the available sulfate during TSR [5] (Krouse, 2001). Sulfur isotope fractionation is observed when only part of sulfate was reduced during TSR [35]. H2S in the Cambrian carbonate reservoir has δ34SH2S values close to coeval bedded anhydrite (Figure 4), whereas H2S in the Ordovician carbonate reservoir has δ34SH2S values lighter than coeval seawater (22~26‰, [36]) and formation water (Figure 4). δ34SSO4-H2S differences of the Ordovician in the study area fall within a range of 8.7‰ to 12.3‰ with a mean value of 10.6‰ (Table 1). The differences of δ34S values between H2S and SO42- in the Ordovician carbonate reservoir are higher than those of the Khuff Formation (2‰ to 3‰; [6]). This is probably due to the different geologic settings between the Abu Dhabi and the Tazhong area. TSR in the Khuff Formation of Abu Dhabi happened in the gas intervals with faster sulfate reduction than supply of reactive sulfates from anhydrite dissolution; consequently, almost all dissolved SO42- are converted into H2S and thus H2S shows similar δ34S values to the parent anhydrite [6]. In contrast, TSR in the Tazhong area may have happened at the oil-water transition zone [13, 14, 21]. TSR around the oil-water transition zone may have not consumed all the dissolved SO42-; SO4342- may have been reduced preferentially as the result of kinetic isotopic fractionation; thus, significant δ34SSO4-H2S differences are observed. Similar cases were reported from gas-water transitions in local areas from the western Canadian Basin [5] and the northeastern Sichuan Basin (Cai et al., 2010). The relatively lower H2S concentration and higher δ34SSO4-H2S differences in the O1-2y Formation than those in the Cambrian may indicate that only part of the dissolved SO42- was reduced in situ in the Tazhong area, and TSR in the Ordovician is in the early stage. Zhang et al. [25] and Su et al. [26] also suggested that the overall TSR extent in the Ordovician of the Tazhong area is limited by the burial temperatures that reservoirs experienced. In other words, TSR extent is low and dissolved SO42- is excessive for in situ TSR in the Ordovician of the Tazhong area. SO4/Cl ratios, relating to the remaining dissolved SO42- amounts in formation water, probably can be used as a proxy of in situ TSR extent under some circumstances, where H2S and TAs concentrations are unavailable.


### 5.2. SO4/Cl Ratio: A Potential Proxy of TSR Extent

#### 5.2.1. Effects of Water Evolution on SO4/Cl Ratio

When initial seawater is evaporated and concentrated to 10 times, SO4/Cl ratio of seawater decreases from 0.144 to 0.04 as the result of the precipitation of sulfates [37, 38]. TDS of formation water in the study area lies between 84 g/L and 206 g/L with an average of 144 g/L, which is 3 to 7 folds of seawater (35 g/L). SO4/Cl ratios of 3 times and 7 times concentrated seawater are 0.05 and 0.04, respectively. Whereas the O1-2y formation water has SO4/Cl ratios from 0.0002 to 0.016, which are significantly lower than that which can be generated from the seawater evaporation. Assuming that the Cambrian to the Middle Ordovician seawater has a similar SO4/Cl ratio, formation waters from both the Cambrian and O1-2y evolved from evaporated seawater alone are expected to have SO4/Cl ratios higher than 0.04; thus, it is unlikely for the mixing of evaporated formation water between the Cambrian and the O1-2y Formation to have the low SO4/Cl ratios (<0.016).


#### 5.2.2. Consumption of AqueousSO42- by TSR

TSR is ubiquitous in the carbonate reservoirs the Tazhong area [13, 20, 21, 29, 39, 40]. TSR in the Cambrian is more extensive than that in the Ordovician, as higher H2S concentration, higher TAs concentration, and lower SO4/Cl ratio were observed (Table 1). TSR consumes dissolved SO42-, leading to lower SO4/Cl ratios in the formation water than original seawater. The depletion of SO42- in the O1-2y formation water was not compensated by anhydrite dissolution as no anhydrite or anhydritic carbonate rocks develop in the Ordovician strata (Figure 2).H2S, a direct product of TSR, can dissolve in formation water, precipitate as pyrite, and be incorporated into oils and solid bitumens producing alkylthiolanes, alkylthiols, and alkyl 2-thiaadamantanes [7, 13, 21]. Thiaadamantanes (TAs) concentrations in petroleum are considered to better reflect TSR extents because TAs is quite stable even under high temperature [13, 14, 41]. However, TAs concentrations are only measured in several wells. Negative relationships exist in SO4/Cl ratios versus H2S concentrations and SO4/Cl ratios versus TAs concentrations (Figures 6(a) and 6(b)). This indicates that SO42- was transformed to H2S by TSR and subsequently to incorporate into TAs in a relatively closed system. Thus, SO4/Cl ratio is expected to be a good proxy to reflect TSR extent if they are not significantly changed by water mixing or anhydrite dissolution.Figure 6
Relationship between (a) SO4/Cl ratios and H2S concentrations; (b) SO4/Cl ratios and TAs concentrations.
(a)
(b)


### 5.3. Influence of Water Chemistry on TSR Initiation

TSR is commonly observed in carbonate reservoirs with high-temperature, but it is difficult to repeat the TSR process in the laboratory under conditions resembling nature. DissolvedSO42-, with symmetrical molecular structure and spherical electronic distributions, have extremely low reactivity in the absence of catalysis [17]. TSR reactions that occur in natural environments are most likely to involve magnesium sulfate (MgSO4) rather than “free” dissolved sulfate ions (SO42-) or solvated sulfate ion-pairs. MgSO4 has been proved to be an effective catalysis for TSR in the laboratory [17, 42]. MgSO4 exists as a main magnesium-bearing specie in solutions with Mg2+ being dominant [43]. As temperature increases, MgSO4 solutions were separated into MgSO4-rich phase and MgSO4-poor phase due to the formation of the complex Mg2+-SO42- ion association in the fused silica capillary capsules, and the phase separation temperature decreases with increasing Mg/SO4 ratios [19]. This indicates that formation water with high Mg concentrations and high temperature is preferable to form MgSO4 and initiate TSR.Figures7(a), 7(b), and 7(c) show a negative relationship between Mg concentrations and SO4/Cl ratios, a positive relationship between Mg concentrations and H2S concentrations, and a positive relationship between Mg concentrations and TAs concentrations. This indicates that the TSR extent is high in formation water with high Mg concentrations, which prove the catalysis of MgSO4. TDS concentrations show a negative correlation with SO4/Cl ratios, a positive correlation with H2S concentrations, and a positive correlation with TAs concentrations (Figures 7(d), 7(e), and 7(f)). This also indicates the catalysis of MgSO4 as Mg concentration is in proportion to TDS in this study.Figure 7
Relationship between (a) SO4/Cl ratios and Mg concentrations; (b) H2S concentrations and Mg concentrations; (c) TAs concentrations and Mg concentrations; (d) SO4/Cl ratios and TDS; (e) H2S concentrations and TDS; (f) TAs concentrations and TDS.
(a)
(b)
(c)
(d)
(e)
(f)Mean Mg/Cl ratios in the formation water from the Yingshan Formation is 0.007, which is similar to that from the Feixianguan Formation (0.009 [44]). But average Mg/SO4 ratio in the formation water from the Yingshan Formation is 6.24, which is significantly higher than that from the Feixianguan Formation (0.035 [44]). The low Mg/SO4 ratios in the formation water from the Feixianguan Formation resulted from the high SO4 concentration contributed by anhydrite dissolution. The limitation on TSR initiation from low Mg/SO4 ratios in the Feixianguan Formation was probably compensated by the high temperature in the reservoir (Figure 3(b)). Similarly, the limitation on TSR initiation from low temperature in the Yingshan Formation was compensated by the high Mg/SO4 ratios in the formation water. Duration time of TSR in the Feixianguan Formation is much longer than that in the Yingshan Formation as temperature of the Feixianguan Formation has reached 200°C since the Middle Jurassic (Figure 3(b)). And reaction rate of TSR in the Feixianguan Formation is also faster than that in the Yingshan Formation. Moreover, contribution of total aqueous SO42- in the Feixianguan Formation is more than that in the Yingshan Formation as many anhydrites develop in the Feixianguan Formation. These are probably the main reasons for the lower H2S concentrations in the Tazhong area than that in the northeast Sichuan Basin.


## 5.1. Sulfur Isotope Composition and Fractionation

H2S from the Ordovician carbonate reservoir in this and previous studies have δ34S values from 12‰ to 16‰ which are 15‰~20‰ lighter than the counterpart in the Cambrian (33‰) [10, 21, 34]. The large differences indicate that the H2S in the Ordovician was probably generated from in situ TSR rather than TSR that happened in the Cambrian [13, 21]. A positive relationship exists between δ34SH2S values and δ34SSO4 values (Figure 5(a)). δ34SSO4 values tend to increase with a decrease of SO42- concentrations in the formation water (Figure 5(b)). This indicates that δ34S values of H2S are related to those of the remaining SO42-. There is a positive relationship between δ34SSO4-H2S and SO4/Cl ratios (Figure 5(c)), likely indicating that the more dissolved SO42- is converted to H2S, the smaller sulfur isotope fractionation occurs. This may imply that δ34SH2S values are controlled by both δ34SSO4 values and TSR extent.Figure 5
Relationship between (a)δ34SSO4 and δ34SH2S; (b) SO4/Cl and δ34SSO4; (c) SO4/Cl and δ34SSO4-H2S.
(a)
(b)
(c)Sulfur isotope fractionation is nearly negligible if complete conversion of the available sulfate during TSR [5] (Krouse, 2001). Sulfur isotope fractionation is observed when only part of sulfate was reduced during TSR [35]. H2S in the Cambrian carbonate reservoir has δ34SH2S values close to coeval bedded anhydrite (Figure 4), whereas H2S in the Ordovician carbonate reservoir has δ34SH2S values lighter than coeval seawater (22~26‰, [36]) and formation water (Figure 4). δ34SSO4-H2S differences of the Ordovician in the study area fall within a range of 8.7‰ to 12.3‰ with a mean value of 10.6‰ (Table 1). The differences of δ34S values between H2S and SO42- in the Ordovician carbonate reservoir are higher than those of the Khuff Formation (2‰ to 3‰; [6]). This is probably due to the different geologic settings between the Abu Dhabi and the Tazhong area. TSR in the Khuff Formation of Abu Dhabi happened in the gas intervals with faster sulfate reduction than supply of reactive sulfates from anhydrite dissolution; consequently, almost all dissolved SO42- are converted into H2S and thus H2S shows similar δ34S values to the parent anhydrite [6]. In contrast, TSR in the Tazhong area may have happened at the oil-water transition zone [13, 14, 21]. TSR around the oil-water transition zone may have not consumed all the dissolved SO42-; SO4342- may have been reduced preferentially as the result of kinetic isotopic fractionation; thus, significant δ34SSO4-H2S differences are observed. Similar cases were reported from gas-water transitions in local areas from the western Canadian Basin [5] and the northeastern Sichuan Basin (Cai et al., 2010). The relatively lower H2S concentration and higher δ34SSO4-H2S differences in the O1-2y Formation than those in the Cambrian may indicate that only part of the dissolved SO42- was reduced in situ in the Tazhong area, and TSR in the Ordovician is in the early stage. Zhang et al. [25] and Su et al. [26] also suggested that the overall TSR extent in the Ordovician of the Tazhong area is limited by the burial temperatures that reservoirs experienced. In other words, TSR extent is low and dissolved SO42- is excessive for in situ TSR in the Ordovician of the Tazhong area. SO4/Cl ratios, relating to the remaining dissolved SO42- amounts in formation water, probably can be used as a proxy of in situ TSR extent under some circumstances, where H2S and TAs concentrations are unavailable.


## 5.2. SO4/Cl Ratio: A Potential Proxy of TSR Extent

### 5.2.1. Effects of Water Evolution on SO4/Cl Ratio

When initial seawater is evaporated and concentrated to 10 times, SO4/Cl ratio of seawater decreases from 0.144 to 0.04 as the result of the precipitation of sulfates [37, 38]. TDS of formation water in the study area lies between 84 g/L and 206 g/L with an average of 144 g/L, which is 3 to 7 folds of seawater (35 g/L). SO4/Cl ratios of 3 times and 7 times concentrated seawater are 0.05 and 0.04, respectively. Whereas the O1-2y formation water has SO4/Cl ratios from 0.0002 to 0.016, which are significantly lower than that which can be generated from the seawater evaporation. Assuming that the Cambrian to the Middle Ordovician seawater has a similar SO4/Cl ratio, formation waters from both the Cambrian and O1-2y evolved from evaporated seawater alone are expected to have SO4/Cl ratios higher than 0.04; thus, it is unlikely for the mixing of evaporated formation water between the Cambrian and the O1-2y Formation to have the low SO4/Cl ratios (<0.016).


### 5.2.2. Consumption of AqueousSO42- by TSR

TSR is ubiquitous in the carbonate reservoirs the Tazhong area [13, 20, 21, 29, 39, 40]. TSR in the Cambrian is more extensive than that in the Ordovician, as higher H2S concentration, higher TAs concentration, and lower SO4/Cl ratio were observed (Table 1). TSR consumes dissolved SO42-, leading to lower SO4/Cl ratios in the formation water than original seawater. The depletion of SO42- in the O1-2y formation water was not compensated by anhydrite dissolution as no anhydrite or anhydritic carbonate rocks develop in the Ordovician strata (Figure 2).H2S, a direct product of TSR, can dissolve in formation water, precipitate as pyrite, and be incorporated into oils and solid bitumens producing alkylthiolanes, alkylthiols, and alkyl 2-thiaadamantanes [7, 13, 21]. Thiaadamantanes (TAs) concentrations in petroleum are considered to better reflect TSR extents because TAs is quite stable even under high temperature [13, 14, 41]. However, TAs concentrations are only measured in several wells. Negative relationships exist in SO4/Cl ratios versus H2S concentrations and SO4/Cl ratios versus TAs concentrations (Figures 6(a) and 6(b)). This indicates that SO42- was transformed to H2S by TSR and subsequently to incorporate into TAs in a relatively closed system. Thus, SO4/Cl ratio is expected to be a good proxy to reflect TSR extent if they are not significantly changed by water mixing or anhydrite dissolution.Figure 6
Relationship between (a) SO4/Cl ratios and H2S concentrations; (b) SO4/Cl ratios and TAs concentrations.
(a)
(b)


## 5.2.1. Effects of Water Evolution on SO4/Cl Ratio

When initial seawater is evaporated and concentrated to 10 times, SO4/Cl ratio of seawater decreases from 0.144 to 0.04 as the result of the precipitation of sulfates [37, 38]. TDS of formation water in the study area lies between 84 g/L and 206 g/L with an average of 144 g/L, which is 3 to 7 folds of seawater (35 g/L). SO4/Cl ratios of 3 times and 7 times concentrated seawater are 0.05 and 0.04, respectively. Whereas the O1-2y formation water has SO4/Cl ratios from 0.0002 to 0.016, which are significantly lower than that which can be generated from the seawater evaporation. Assuming that the Cambrian to the Middle Ordovician seawater has a similar SO4/Cl ratio, formation waters from both the Cambrian and O1-2y evolved from evaporated seawater alone are expected to have SO4/Cl ratios higher than 0.04; thus, it is unlikely for the mixing of evaporated formation water between the Cambrian and the O1-2y Formation to have the low SO4/Cl ratios (<0.016).


## 5.2.2. Consumption of AqueousSO42- by TSR

TSR is ubiquitous in the carbonate reservoirs the Tazhong area [13, 20, 21, 29, 39, 40]. TSR in the Cambrian is more extensive than that in the Ordovician, as higher H2S concentration, higher TAs concentration, and lower SO4/Cl ratio were observed (Table 1). TSR consumes dissolved SO42-, leading to lower SO4/Cl ratios in the formation water than original seawater. The depletion of SO42- in the O1-2y formation water was not compensated by anhydrite dissolution as no anhydrite or anhydritic carbonate rocks develop in the Ordovician strata (Figure 2).H2S, a direct product of TSR, can dissolve in formation water, precipitate as pyrite, and be incorporated into oils and solid bitumens producing alkylthiolanes, alkylthiols, and alkyl 2-thiaadamantanes [7, 13, 21]. Thiaadamantanes (TAs) concentrations in petroleum are considered to better reflect TSR extents because TAs is quite stable even under high temperature [13, 14, 41]. However, TAs concentrations are only measured in several wells. Negative relationships exist in SO4/Cl ratios versus H2S concentrations and SO4/Cl ratios versus TAs concentrations (Figures 6(a) and 6(b)). This indicates that SO42- was transformed to H2S by TSR and subsequently to incorporate into TAs in a relatively closed system. Thus, SO4/Cl ratio is expected to be a good proxy to reflect TSR extent if they are not significantly changed by water mixing or anhydrite dissolution.Figure 6
Relationship between (a) SO4/Cl ratios and H2S concentrations; (b) SO4/Cl ratios and TAs concentrations.
(a)
(b)


## 5.3. Influence of Water Chemistry on TSR Initiation

TSR is commonly observed in carbonate reservoirs with high-temperature, but it is difficult to repeat the TSR process in the laboratory under conditions resembling nature. DissolvedSO42-, with symmetrical molecular structure and spherical electronic distributions, have extremely low reactivity in the absence of catalysis [17]. TSR reactions that occur in natural environments are most likely to involve magnesium sulfate (MgSO4) rather than “free” dissolved sulfate ions (SO42-) or solvated sulfate ion-pairs. MgSO4 has been proved to be an effective catalysis for TSR in the laboratory [17, 42]. MgSO4 exists as a main magnesium-bearing specie in solutions with Mg2+ being dominant [43]. As temperature increases, MgSO4 solutions were separated into MgSO4-rich phase and MgSO4-poor phase due to the formation of the complex Mg2+-SO42- ion association in the fused silica capillary capsules, and the phase separation temperature decreases with increasing Mg/SO4 ratios [19]. This indicates that formation water with high Mg concentrations and high temperature is preferable to form MgSO4 and initiate TSR.Figures7(a), 7(b), and 7(c) show a negative relationship between Mg concentrations and SO4/Cl ratios, a positive relationship between Mg concentrations and H2S concentrations, and a positive relationship between Mg concentrations and TAs concentrations. This indicates that the TSR extent is high in formation water with high Mg concentrations, which prove the catalysis of MgSO4. TDS concentrations show a negative correlation with SO4/Cl ratios, a positive correlation with H2S concentrations, and a positive correlation with TAs concentrations (Figures 7(d), 7(e), and 7(f)). This also indicates the catalysis of MgSO4 as Mg concentration is in proportion to TDS in this study.Figure 7
Relationship between (a) SO4/Cl ratios and Mg concentrations; (b) H2S concentrations and Mg concentrations; (c) TAs concentrations and Mg concentrations; (d) SO4/Cl ratios and TDS; (e) H2S concentrations and TDS; (f) TAs concentrations and TDS.
(a)
(b)
(c)
(d)
(e)
(f)Mean Mg/Cl ratios in the formation water from the Yingshan Formation is 0.007, which is similar to that from the Feixianguan Formation (0.009 [44]). But average Mg/SO4 ratio in the formation water from the Yingshan Formation is 6.24, which is significantly higher than that from the Feixianguan Formation (0.035 [44]). The low Mg/SO4 ratios in the formation water from the Feixianguan Formation resulted from the high SO4 concentration contributed by anhydrite dissolution. The limitation on TSR initiation from low Mg/SO4 ratios in the Feixianguan Formation was probably compensated by the high temperature in the reservoir (Figure 3(b)). Similarly, the limitation on TSR initiation from low temperature in the Yingshan Formation was compensated by the high Mg/SO4 ratios in the formation water. Duration time of TSR in the Feixianguan Formation is much longer than that in the Yingshan Formation as temperature of the Feixianguan Formation has reached 200°C since the Middle Jurassic (Figure 3(b)). And reaction rate of TSR in the Feixianguan Formation is also faster than that in the Yingshan Formation. Moreover, contribution of total aqueous SO42- in the Feixianguan Formation is more than that in the Yingshan Formation as many anhydrites develop in the Feixianguan Formation. These are probably the main reasons for the lower H2S concentrations in the Tazhong area than that in the northeast Sichuan Basin.


## 6. Conclusions

Formation water is the solvent for sulfates, and water chemistry has a great influence on TSR which explain the low TSR extent in the Tazhong area.(1) MgSO4 contact ion pair in formation water is catalyst for TSR. High Mg/SO4 ratios and high temperatures are preferable to form MgSO4 contact ion pair in solutions and thus will increase TSR extent. High Mg/SO4 ratios of the O1-2y formation water compensated the low temperature which would limit the initiation of TSR in the Tazhong area.(2) The lower TSR extent in the Tazhong area is limited by the shorter reaction time and less total aqueousSO42- contribution in the reservoir.



---

*Source: 6351382-2017-10-15.xml*