# Sulfate Sources of Thermal Sulfate Reduction (TSR) in the Permian Changxing and Triassic Feixianguan Formations, Northeastern Sichuan Basin, China

Authors: Pingping Li; Huayao Zou; Fang Hao; Xinya Yu
Journal: Geofluids
 (2019)

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## Abstract

Thermal sulfate reduction (TSR) occurred throughout the Permian Changxing (P2c) and Triassic Feixianguan (T1f) dolostone reservoirs in the western and eastern parts of the Kaijiang-Liangping (K-L) trough in the northeastern part of the Sichuan Basin. To determine the sulfate sources of this TSR, fourteen solid bitumen samples and eight anhydrite samples were collected from the northeastern part of the Sichuan Basin. These samples were analyzed to determine their sulfur isotopes. In addition, untreated, HNO3-treated, and CrCl2-treated solid bitumen samples were analyzed to determine their sulfur isotopes in order to obtain reliable δ34S data for the TSR solid bitumen. The results show that the HNO3 method is more effective at removing pyrite from solid bitumen than the method using CrCl2 thrice because the HNO3-treated solid bitumen has lower sulfur contents and higher δ34S. The δ34S of the T1f solid bitumen samples from the Puguang gas field (in the eastern part of the K-L trough, 12.0-24.0‰) is significantly lower than that of the samples from the Yuanba gas field (in the western part of the K-L trough, 24.1-34.2‰). The δ34S of the T1f1–2 anhydrite is 18.1-26.6‰, which is lower than that of the T1f3–4 anhydrite samples (29.9-39.6‰). The TSR sulfates from the Puguang gas field were most likely from the coeval T1f1–2 evaporating seawater and were enriched during the reflux-seepage dolomitization process. The TSR sulfates from the Yuanba gas field were primarily caused by the evaporation of seawater during the T1f4. First, the evaporating seawater would flow vertically into the P2c reservoirs in the adjacent area, and then, it would flow laterally into the P2c reservoirs in the Yuanba gas field. Considering the fact that the sulfate sources of TSR and the δ34S values of the TSR sulfates are different in the Puguang and Yuanba gas fields, the δ34S of TSR solid bitumen cannot be simply used to show the extent of TSR.


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## Body

## 1. Introduction

Thermal sulfate reduction (TSR) is the reaction of sulfate with a hydrocarbon (1) [1, 2]. As toxic and corrosive H2S can be produced during TSR, the mechanism and products of TSR have been extensively studied [1–4]. The sulfur in the H2S produced by TSR is derived from dissolved sulfate [1, 3, 5]. Different sulfate sources and whether the sulfates are sufficient or not can affect the H2S concentration [3, 6] and sulfur isotopic composition of H2S. Thus, determining the sulfate source is a key problem to predict the H2S concentration and distribution.
(1)Hydrocarbons+SO42−→AlteredHydrocarbons+SolidBitumen+CO2+H2SThe sulfur in the sulfate is mainly transferred into sulfur-rich solid bitumen, H2S, and some solid sulfides (such as pyrite) during TSR [7, 8]. The sulfur isotope of sulfur-rich solid bitumen and H2S is similar to the sulfur isotope of sulfate [9–11]. Thus, the sulfur isotopes of the sulfur-rich solid bitumen and the H2S can be used to determine the sulfate source. The sulfur isotopes of the sulfur-rich solid bitumen can be obtained relatively easily compared to those of the toxic H2S. As a result, we can use the sulfur isotopes of the sulfur-rich solid bitumen to determine the sulfate source.Pyrite is a byproduct of TSR [1], and it usually resides within the solid bitumen. Thus, we can obtain accurate sulfur isotopes for the solid bitumen only after the pyrite has been removed from the solid bitumen. There are two main methods that can be used to remove pyrite from solid bitumen. One method is to use CrCl2 to reduce the pyrite to H2S (2) [12]. Cai et al. [13, 14] successfully removed pyrite from kerogen and solid bitumen by this method. The second method is to use dilute HNO3 to oxidize the pyrite to H2SO4 (3) [15, 16]. The CrCl2 method is more complicated than the HNO3 method, but the HNO3 method is time consuming. However, there is no research on the use of both methods simultaneously to remove pyrite from solid bitumen, nor has an experiment been conducted to compare the two methods and to determine which method is more effective.
(2)FeS2+2CrCl2+4HCl→FeCl2+2CrCl3+2H2S(3)FeS2+8HNO3→FeNO33+2H2O+2H2SO4+5NOThe relatively high concentration of H2S (5-20%) in the Permian Changxing (P2c) and Triassic Feixianguan (T1f) Formations in the northeastern part of the Sichuan Basin has been concluded to be from TSR [4, 6, 17–19], whereas sulfur-rich solid bitumen has been found in other gas reservoirs [6, 14, 20, 21]. The H2S concentrations of the gas reservoirs in the eastern part of the Kaijiang-Liangping (K-L) trough (see detailed information in the background section) are higher than those of the gas reservoirs in the western part of the K-L trough [6]. Sulfate from the evaporative platform in T1f has been suggested as the sulfate source of the TSR [4, 18]. However, whether the sulfate was derived from the early (the first and second members of the T1f) or late T1f (the third and fourth members of the T1f) is unknown. In addition, there is no such evaporative platform in the P2c in the western part of the K-L trough, so the sulfate source of the TSR in this area is also unknown.The purpose of this paper is to determine which method is more effective at removing pyrite from solid bitumen by comparing the CrCl2 and HNO3 methods, to obtain reliable sulfur isotopic data for the solid bitumen, and to analyze the sulfate sources of TSR in the eastern and western parts of the K-L trough in the northeastern part of the Sichuan Basin based on the systematic correlation of the sulfur isotopes of the solid bitumen and anhydrite.


## 2. Geologic Setting

The Sichuan Basin is a rhombic basin in southwestern China (Figure1). The general evolutionary history of the Sichuan Basin can be divided into two main stages [22]. Before the Late Indosinian movement, the Sichuan Basin was primarily characterized by subsidence and uplift, and thick marine carbonates and shales were deposited at this stage. Since the Late Indosinian movement, large-scale lateral compression has been occurring, which has resulted in two obvious episodes of uplift, i.e., the Yanshan and Himalayan movements [22]. In addition, terrestrial fluvial-lacustrine deposits were formed.Figure 1
Main gas fields and sedimentary facies of the upper part of the Changxing Formation (P2c) in the northeastern part of the Sichuan Basin (modified from [23, 24]). YB = Yuanba; LG = Longgang; HB = Heba; PG = Puguang; TSP = Tieshanpo; LJZ = Luojiazhai; DKH = Dukouhe; JZP = Jinzhuping; JST = Jiaoshutang; K-L = Kaijiang-Liangping; CK = Chengkou.The Permian strata primarily consist of marine carbonates and shales, and from bottom to top, it can be divided into the Liangshan (P1l), Qixia (P1q), Maokou (P1m), Longtan (P2l)/Wujiaping (P2w), and Changxing (P2c) Formations [22]. The Early Triassic can be divided into the Fexianguan (T1f) and Jialingjiang (T1j) Formations, and the T1f can be furtherly divided into four members (T1f1, T1f2, T1f3, and T1f4) from bottom to top. During the P2c, the northwest-southeast trending K-L trough developed in the northeastern part of the Sichuan Basin (Figure 2). An open platform and a relatively isolated open platform developed in the western and eastern parts of the K-L trough, respectively [23]. During the early T1f, a restricted evaporative platform (characterized by anhydrite layers) developed in the eastern part of the K-L trough, whereas the western part of the K-L trough was still an open platform and contained no anhydrite layers. Until the end of the T1f, the evaporative platform remained restricted and widespread anhydrite layers continued to develop in the northeastern part of the Sichuan Basin (Figure 3). The T1j and Middle Triassic Leikoupo (T2l) Formations developed on restricted evaporative platforms and consist of limestone and widespread anhydrite [22].Figure 2
Key wells and sedimentary facies in the upper part of the second member of the Feixianguan Formation (T1f2) in the Sichuan Basin (modified from [24]).Figure 3
The lithologic variations in the Changxing (P2c) and Feixianguan (T1f) Formations in the Sichuan Basin from southwest to northeast. The data for wells Z4, GC2, LG1, and JZ2 are from Du et al. [24]. The figures to the left of each lithologic column are the burial depths (in m). T1f1–2 = the first and second members of T1f; T1f3–4 = the third and fourth members of T1f. The well locations are shown in Figure 2.Many gas fields, such as the Puguang, Dukouhe, Tieshanpo, Yuanba, and Longgang, are located in the P2c and T1f reefs and shoals in the eastern and western parts of the K-L trough [24, 25]. The natural gas in these gas fields was primarily produced by oil-cracking and was primarily derived from the Upper Permian P2l/P2w source rocks [4, 18, 21].


## 3. Sampling and Analytical Methods

Fourteen solid bitumen-bearing core samples (5 samples from the T1f in the Puguang gas field and 9 samples from the P2c in the Yuanba gas field) were collected from the northeast part of the Sichuan Basin, and eight anhydrite samples (7 samples from the T1f and 1 sample from the T1j) were collected for this study. The solid bitumen was primarily present as fill in the pores, while the anhydrite primarily occurred as nodules and layers (Figure 4).Figure 4
Photographs showing anhydrite (Anh) in an outcrop sample (a) and in cores (b–h), and black solid bitumen in thin section (i). (a) Outcrop JST, T1f4, anhydrite nodules in dolostone. (b) Well HB1, 4847.6 m, T1f4, anhydrite layers in mudstone. (c) Well HB1, 4501 m, T1j2 (second member of the Jialingjiang Formation), anhydrite nodules in dolostone. (d) Well MB6, 4015.9 m, T1f1, anhydrite nodules in limestone. (e) Well XHC1, 5468.2 m, T1f1, anhydrite nodules in dolostone. (f) Well DW2, 4750.7 m, T1f3, anhydrite nodules in dolostone. (g) Well MB1, 4302.3 m, T1f4, anhydrite layers in mudstone. (h) Well MB3, 3873 m, T1f4, large anhydrite nodules in dolostone. (i) Well YB271, 6327.4 m, P2c, black solid bitumen in the pores.
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)The anhydrite nodules and layers were crushed and powdered to less than 100 mesh using an agate mortar and pestle. The solid bitumen-bearing core samples were crushed in a rock crusher to less than 100 mesh, and then, this powder was treated with dichloromethane (DCM) to remove any soluble organic matter and elemental sulfur. The liquid and residue were filtered and flushed with DCM. The residue was dried and treated with hot 6 N HCl to remove carbonate minerals. This solution was heated for 2 hours at 70°C while using a magnetic stirrer. Next, the solid bitumen residue was filtered and rinsed with deionized water, and then, it was dried at 70°C. The solid bitumen residue was mainly between 100 and 300 mg, so the pyrite content cannot be evaluated precisely using XRD (X-ray diffraction). However, pyrite grains can be easily found using the SEM (Scanning Electron Microscopy) and EDS (Energy Dispersive Spectrometer) analysis (Figure5). As a result, it is necessary to remove pyrite to get accurate sulfur isotope of the solid bitumen. So the solid bitumen residue was treated with HNO3 and CrCl2 to remove the pyrite, respectively.Figure 5
SEM (Scanning Electron Microscopy) and EDS (Energy Dispersive Spectrometer) photographs showing pyrites in the untreated SB2 (a, c) and SB13 (b, d) solid bitumen samples.
(a)
(b)
(c)
(d)The HNO3 method requires the solid bitumen residue to react with 1 N HNO3 in a glass centrifuge tube at room temperature for two weeks. The centrifuge tubes were sonicated for 30 minutes every day. After reacting with 1 N HNO3 for two weeks, deionized water was added to the tubes, they were centrifuged again, and the liquid was poured off. This procedure was repeated twice more. Finally, the solid bitumen was dried at 70°C for 12 hours.The CrCl2 method used was similar to the method reported by Cai et al. [13, 14]. First, 12 N HCl and deionized water were added to solid CrCl3·6H2O to prepare a green CrCl3 solution. Then, the green CrCl3 solution was allowed to slowly flow past a flask column filled with granular zinc to convert it to a dark blue CrCl2 solution, which was stored in a ground glass stoppered bottle to prevent oxidation. Next, 6 N HCl and the CrCl2 solution were added to the solid bitumen residue in a glass tube, and then, the tube was covered with a plastic cap with a syringe needle, which allowed the generated H2S to flow out. The tube was heated at 80°C for 3 hours, then deionized water was added, the tube was centrifuged twice, the liquid was poured off, and the solid bitumen residue was reground. Finally, this procedure was repeated twice, and the solid bitumen was dried.The sulfur isotopes of the anhydrite powder, the untreated solid bitumen residue, and the HNO3-treated and CrCl2-treated solid bitumen were analyzed at the California Institute of Technology. Samples were combusted in a Costech elemental combustion system at 1000°C, and then, the isotopic ratios were determined using a Delta Plus XL mass spectrometer calibrated using International Atomic Energy Agency (IAEA) standards. The results are reported as δ34S relative to the Vienna Canyon Diablo Troilite (VCDT) standard. Average precision of analyses based on replicate analyses of standards was ±0.2‰.


## 4. Results

### 4.1. Sulfur Content and Isotope Ratios of the Solid Bitumen Samples

The sulfur contents and sulfur isotopes (δ34S) of the untreated solid bitumen, the HNO3-treated solid bitumen, and the CrCl2-treated solid bitumen are listed in Table 1. As the weight of the untreated solid bitumen is small, two samples (SB10 and SB12) were not treated with CrCl2.Table 1
Sulfur content and isotope composition of solid bitumen from the Yuanba and Puguang gas fields in the Sichuan Basin.
Gas field
Well
Sample no.
Strata
Depth (m)
Untreated SB
CrCl2-treated SB
HNO3-treated SB
S%
δ34S‰
S%
δ34S‰
S%
δ34S‰
Puguang
MB4
SB1
T1f2
3814.5
12.41
17.6
10.47
17.5
11.04
17.9
Puguang
MB4
SB2
T1f2
3836.3
6.18
15.4
5.76
16.6
4.63
17.8
Puguang
PG12
SB3
T1f2
6007.5
8.17
16.9
7.52
18.1
7.58
17.9
Puguang
PG2
SB4
T1f1
5059.0
11.53
20.1
10.57
19.0
10.54
18.5
Puguang
DW102
SB5
T1f1
4819.7
5.68
13.3
5.04
14.1
3.51
17.7
Yuanba
YB29
SB7
P2c
6642.4
12.4
27.7
11.6
27.9
12.1
28.5
Yuanba
YB205
SB8
P2c
6461.4
9.37
34.2
7.92
34.3
7.89
34.2
Yuanba
YB27
SB10
P2c
6295.3
11.53
25.9
/
/
11.08
26.5
Yuanba
YB27
SB11
P2c
6301.1
10.17
31.1
8.87
29.8
8.89
29.4
Yuanba
YB273
SB12
P2c
6826.2
10.34
25.1
/
/
10.29
25.6
Yuanba
YB271
SB13
P2c
6320.5
9.87
14.4
7.55
16.8
4.52
24.1
Yuanba
YB28
SB15
P2c
6807.3
11.75
30.5
11.6
29.8
11.1
30.3
Yuanba
YB224
SB16
P2c
6627.9
10.34
25.2
13.4
25.4
12.77
25.9
Yuanba
YB224
SB17
P2c
6641.2
14.57
24.7
14.28
24.9
13.66
26.5
Note: P2c = Changxing Formation (Fm); T1f1 = the first member of Feixianguan Fm; T1f2 = the second member of Feixianguan Fm.The sulfur contents of the untreated solid bitumen, the HNO3-treated solid bitumen, and the CrCl2-treated solid bitumen were different (Figure 6). Except for one sample (SB16), the sulfur contents of the HNO3-treated and CrCl2-treated solid bitumen samples were lower than those of the untreated solid bitumen samples. In addition, the sulfur contents of the HNO3-treated and CrCl2-treated solid bitumen samples were similar, but the sulfur contents of three of the HNO3-treated solid bitumen samples (SB2, SB5, and SB13) were lower than those of the CrCl2-treated solid bitumen samples (Figure 6).Figure 6
Variation in the sulfur contents of the untreated, CrCl2-treated, and HNO3-treated solid bitumen (SB) samples.Similarly, theδ34S values of the untreated solid bitumen samples, the HNO3-treated solid bitumen samples, and the CrCl2-treated solid bitumen samples were different (Figure 7). Except for two samples (SB4 and SB11), the δ34S values of the untreated solid bitumen samples were lower than those of the HNO3- and CrCl2-treated solid bitumen samples. In addition, the δ34S values of the HNO3- and CrCl2-treated solid bitumen samples were similar. However, in the case of samples SB2, SB5, SB13, and SB17, the δ34S values of the HNO3-treated solid bitumen samples were higher than those of the CrCl2-treated solid bitumen samples (Figure 7), while the sulfur contents of the HNO3-treated solid bitumen samples were lower than those of the CrCl2-treated solid bitumen samples (Figure 6). The δ34S difference between these four HNO3-treated solid bitumen samples and corresponding untreated solid bitumen was 3.2‰, 4.4‰, 9.7‰, and 1.8‰, respectively, and the δ34S difference between these four CrCl2-treated solid bitumen samples and corresponding untreated solid bitumen were 1.2‰, 3.6‰, 7.3‰, and 1.6‰, respectively.Figure 7
Variations in the sulfur isotopes of the untreated, CrCl2-treated, and HNO3-treated solid bitumen (SB) samples.Overall, there is no obvious difference in the sulfur contents of the solid bitumen samples from the Puguang and Yuan gas fields, and the sulfur contents ranged from 4.0% to 15.0% (Figure6), which are roughly similar to the sulfur content ranges reported for solid bitumen from the Puguang gas field (11.75-11.99%; [4]) and from the Yuanba gas field (6.64-17.99%; [6]). However, the δ34S of solid bitumen from the Puguang gas field (generally lower than 20.0‰) is significantly lower than that of solid bitumen from the Yuanba gas field (generally higher than 24.0‰).


### 4.2. Sulfur Isotope Ratios of the Anhydrite Samples

Theδ34S of the eight anhydrite samples analyzed and the δ34S reported for other anhydrites are listed in Table 2.Table 2
Sulfur isotope compositions of anhydrite from the northeastern Sichuan Basin.
Area/gas field
Well
Strata
Depth (m)
Sample description
δ34S/‰
XHC
XHC1
T1f1
5468.2
Anhydrite layer
26.2
PG
MB6
T1f1
4015.9
Anhydrite nodule
26.6
PG
DW2
T1f3
4750.7
Anhydrite nodule
29.9
HB
HB1
T1f4
4847.6
Anhydrite layer
34.3
PG
MB1
T1f4
4302.3
Anhydrite nodule
34.2
PG
MB3
T1f4
3873.0
Anhydrite nodule
34.4
JST
JST
T1f4
/
Anhydrite nodule
34.0
HB
HB1
T1j2
4501.0
Anhydrite layer
31.7
DKH∗
Du3
T1f1–3
/
Anhydrite layer
26.5
DKH∗
Du3
T1f1–3
/
Anhydrite nodule
30.0
DKH∗
D4
T1f1–3
/
Anhydrite nodule
30.7
TSP∗
Po1
T1f1–3
/
Anhydrite nodule
30.1
TSP∗
Po1
T1f1–3
/
Anhydrite nodule
30.6
HB#
HB102
T1f3
5176.8
Anhydrite vein
34.3
PG#
MB6
T1f1
4015.7
Anhydrite nodule
26.48
HB#
HB1
T1f4
4847.3
Anhydrite layer
35.88
PG#
MB2
T1f4
4144.0
Anhydrite layer
39.33
JZP†
Z2
T1f
3416.8
Anhydrite
19.71
LJZ†
Lj2
T1f1–3
2897.2
Anhydrite
22.59
DKH†
Du3
T1f1–3
4290.0
Anhydrite
18.12
DKH†
Du5
T1f1–3
4740.3
Anhydrite
24.34
DKH†
Du5
T1f1–3
4742.3
Anhydrite
22.8
DKH†
Du5
T1f1–3
4753.4
Anhydrite
22.83
TSP†
Po1
T1f1–3
3302.7
Anhydrite
19.46
TSP†
Po3
T1f1–3
3464.7
Anhydrite
18.92
QLX†
QL52
T1f1–3
3536.0
Anhydrite
24.64
QLX†
QL52
T1f1–3
3490.4
Anhydrite
23.57
JZP†
JZ1
T1f1–3
2825.8
Anhydrite
19.35
JZP†
JZ1
T1f1–3
2877.2
Anhydrite
22.07
JZP†
JZ1
T1f1–3
4766.0
Anhydrite
22.13
JZP†
ZJ1
T1f1–3
3941.9
Anhydrite
23.74
JZP†
Z1
T1f1–3
3350.5
Anhydrite
18.09
JZP†
Z1
T1f1–3
5648.9
Anhydrite
25.4
PG††
MB3
T1f4
3872.0-3880.2
Anhydrite vein
39.64
PG††
MB3
T1f4
3872.0-3880.2
Anhydrite vein
38.56
PG††
CY83
T1j4
3552.0-3554.0
Anhydrite layer
29.72
∗from Wang et al. [26], #from Zhang [39], †from Zhu et al. [19], ††from Zhu et al. [27].For the eight anhydrites analyzed in this study, theδ34S of the T1f1 anhydrite (26.2–26.6‰) was lower than that of the T1f3–T1f4 anhydrites (29.9-34.4‰) and the T1j anhydrite (31.7‰). The δ34S of the T1f1 anhydrite is similar to the δ34S range reported for the T1f1–T1f3 anhydrites (18.12-30.7‰; [19, 26]), and the δ34S of the T1f4 anhydrite is close to that reported for the T1f4 anhydrite (35.88-39.64‰, [27]) and the T1j anhydrite (24.7-32.5‰, [28]; 29.72‰, [27]). This suggests that the δ34S values of the anhydrite samples are reliable. The δ34S of the T1f1–T1f3 anhydrites (18.12-30.7‰) reported by Wang et al. [26] and Zhu et al. [19] is mainly for the T1f1–T1f2 because the anhydrite was mainly located within the T1f1–T1f2 in the Dukouhe, Tieshanpo, and Jinzhuping gas fields (Figure 3).Overall, theδ34S of the T1f1–T1f2 anhydrites (18.12-26.6‰) was lower than that of the T1f3–T1f4 anhydrites (29.9-39.6‰), which is consistent with the conclusion that the δ34S of seawater sulfate increased during the Early Triassic [29].


## 4.1. Sulfur Content and Isotope Ratios of the Solid Bitumen Samples

The sulfur contents and sulfur isotopes (δ34S) of the untreated solid bitumen, the HNO3-treated solid bitumen, and the CrCl2-treated solid bitumen are listed in Table 1. As the weight of the untreated solid bitumen is small, two samples (SB10 and SB12) were not treated with CrCl2.Table 1
Sulfur content and isotope composition of solid bitumen from the Yuanba and Puguang gas fields in the Sichuan Basin.
Gas field
Well
Sample no.
Strata
Depth (m)
Untreated SB
CrCl2-treated SB
HNO3-treated SB
S%
δ34S‰
S%
δ34S‰
S%
δ34S‰
Puguang
MB4
SB1
T1f2
3814.5
12.41
17.6
10.47
17.5
11.04
17.9
Puguang
MB4
SB2
T1f2
3836.3
6.18
15.4
5.76
16.6
4.63
17.8
Puguang
PG12
SB3
T1f2
6007.5
8.17
16.9
7.52
18.1
7.58
17.9
Puguang
PG2
SB4
T1f1
5059.0
11.53
20.1
10.57
19.0
10.54
18.5
Puguang
DW102
SB5
T1f1
4819.7
5.68
13.3
5.04
14.1
3.51
17.7
Yuanba
YB29
SB7
P2c
6642.4
12.4
27.7
11.6
27.9
12.1
28.5
Yuanba
YB205
SB8
P2c
6461.4
9.37
34.2
7.92
34.3
7.89
34.2
Yuanba
YB27
SB10
P2c
6295.3
11.53
25.9
/
/
11.08
26.5
Yuanba
YB27
SB11
P2c
6301.1
10.17
31.1
8.87
29.8
8.89
29.4
Yuanba
YB273
SB12
P2c
6826.2
10.34
25.1
/
/
10.29
25.6
Yuanba
YB271
SB13
P2c
6320.5
9.87
14.4
7.55
16.8
4.52
24.1
Yuanba
YB28
SB15
P2c
6807.3
11.75
30.5
11.6
29.8
11.1
30.3
Yuanba
YB224
SB16
P2c
6627.9
10.34
25.2
13.4
25.4
12.77
25.9
Yuanba
YB224
SB17
P2c
6641.2
14.57
24.7
14.28
24.9
13.66
26.5
Note: P2c = Changxing Formation (Fm); T1f1 = the first member of Feixianguan Fm; T1f2 = the second member of Feixianguan Fm.The sulfur contents of the untreated solid bitumen, the HNO3-treated solid bitumen, and the CrCl2-treated solid bitumen were different (Figure 6). Except for one sample (SB16), the sulfur contents of the HNO3-treated and CrCl2-treated solid bitumen samples were lower than those of the untreated solid bitumen samples. In addition, the sulfur contents of the HNO3-treated and CrCl2-treated solid bitumen samples were similar, but the sulfur contents of three of the HNO3-treated solid bitumen samples (SB2, SB5, and SB13) were lower than those of the CrCl2-treated solid bitumen samples (Figure 6).Figure 6
Variation in the sulfur contents of the untreated, CrCl2-treated, and HNO3-treated solid bitumen (SB) samples.Similarly, theδ34S values of the untreated solid bitumen samples, the HNO3-treated solid bitumen samples, and the CrCl2-treated solid bitumen samples were different (Figure 7). Except for two samples (SB4 and SB11), the δ34S values of the untreated solid bitumen samples were lower than those of the HNO3- and CrCl2-treated solid bitumen samples. In addition, the δ34S values of the HNO3- and CrCl2-treated solid bitumen samples were similar. However, in the case of samples SB2, SB5, SB13, and SB17, the δ34S values of the HNO3-treated solid bitumen samples were higher than those of the CrCl2-treated solid bitumen samples (Figure 7), while the sulfur contents of the HNO3-treated solid bitumen samples were lower than those of the CrCl2-treated solid bitumen samples (Figure 6). The δ34S difference between these four HNO3-treated solid bitumen samples and corresponding untreated solid bitumen was 3.2‰, 4.4‰, 9.7‰, and 1.8‰, respectively, and the δ34S difference between these four CrCl2-treated solid bitumen samples and corresponding untreated solid bitumen were 1.2‰, 3.6‰, 7.3‰, and 1.6‰, respectively.Figure 7
Variations in the sulfur isotopes of the untreated, CrCl2-treated, and HNO3-treated solid bitumen (SB) samples.Overall, there is no obvious difference in the sulfur contents of the solid bitumen samples from the Puguang and Yuan gas fields, and the sulfur contents ranged from 4.0% to 15.0% (Figure6), which are roughly similar to the sulfur content ranges reported for solid bitumen from the Puguang gas field (11.75-11.99%; [4]) and from the Yuanba gas field (6.64-17.99%; [6]). However, the δ34S of solid bitumen from the Puguang gas field (generally lower than 20.0‰) is significantly lower than that of solid bitumen from the Yuanba gas field (generally higher than 24.0‰).


## 4.2. Sulfur Isotope Ratios of the Anhydrite Samples

Theδ34S of the eight anhydrite samples analyzed and the δ34S reported for other anhydrites are listed in Table 2.Table 2
Sulfur isotope compositions of anhydrite from the northeastern Sichuan Basin.
Area/gas field
Well
Strata
Depth (m)
Sample description
δ34S/‰
XHC
XHC1
T1f1
5468.2
Anhydrite layer
26.2
PG
MB6
T1f1
4015.9
Anhydrite nodule
26.6
PG
DW2
T1f3
4750.7
Anhydrite nodule
29.9
HB
HB1
T1f4
4847.6
Anhydrite layer
34.3
PG
MB1
T1f4
4302.3
Anhydrite nodule
34.2
PG
MB3
T1f4
3873.0
Anhydrite nodule
34.4
JST
JST
T1f4
/
Anhydrite nodule
34.0
HB
HB1
T1j2
4501.0
Anhydrite layer
31.7
DKH∗
Du3
T1f1–3
/
Anhydrite layer
26.5
DKH∗
Du3
T1f1–3
/
Anhydrite nodule
30.0
DKH∗
D4
T1f1–3
/
Anhydrite nodule
30.7
TSP∗
Po1
T1f1–3
/
Anhydrite nodule
30.1
TSP∗
Po1
T1f1–3
/
Anhydrite nodule
30.6
HB#
HB102
T1f3
5176.8
Anhydrite vein
34.3
PG#
MB6
T1f1
4015.7
Anhydrite nodule
26.48
HB#
HB1
T1f4
4847.3
Anhydrite layer
35.88
PG#
MB2
T1f4
4144.0
Anhydrite layer
39.33
JZP†
Z2
T1f
3416.8
Anhydrite
19.71
LJZ†
Lj2
T1f1–3
2897.2
Anhydrite
22.59
DKH†
Du3
T1f1–3
4290.0
Anhydrite
18.12
DKH†
Du5
T1f1–3
4740.3
Anhydrite
24.34
DKH†
Du5
T1f1–3
4742.3
Anhydrite
22.8
DKH†
Du5
T1f1–3
4753.4
Anhydrite
22.83
TSP†
Po1
T1f1–3
3302.7
Anhydrite
19.46
TSP†
Po3
T1f1–3
3464.7
Anhydrite
18.92
QLX†
QL52
T1f1–3
3536.0
Anhydrite
24.64
QLX†
QL52
T1f1–3
3490.4
Anhydrite
23.57
JZP†
JZ1
T1f1–3
2825.8
Anhydrite
19.35
JZP†
JZ1
T1f1–3
2877.2
Anhydrite
22.07
JZP†
JZ1
T1f1–3
4766.0
Anhydrite
22.13
JZP†
ZJ1
T1f1–3
3941.9
Anhydrite
23.74
JZP†
Z1
T1f1–3
3350.5
Anhydrite
18.09
JZP†
Z1
T1f1–3
5648.9
Anhydrite
25.4
PG††
MB3
T1f4
3872.0-3880.2
Anhydrite vein
39.64
PG††
MB3
T1f4
3872.0-3880.2
Anhydrite vein
38.56
PG††
CY83
T1j4
3552.0-3554.0
Anhydrite layer
29.72
∗from Wang et al. [26], #from Zhang [39], †from Zhu et al. [19], ††from Zhu et al. [27].For the eight anhydrites analyzed in this study, theδ34S of the T1f1 anhydrite (26.2–26.6‰) was lower than that of the T1f3–T1f4 anhydrites (29.9-34.4‰) and the T1j anhydrite (31.7‰). The δ34S of the T1f1 anhydrite is similar to the δ34S range reported for the T1f1–T1f3 anhydrites (18.12-30.7‰; [19, 26]), and the δ34S of the T1f4 anhydrite is close to that reported for the T1f4 anhydrite (35.88-39.64‰, [27]) and the T1j anhydrite (24.7-32.5‰, [28]; 29.72‰, [27]). This suggests that the δ34S values of the anhydrite samples are reliable. The δ34S of the T1f1–T1f3 anhydrites (18.12-30.7‰) reported by Wang et al. [26] and Zhu et al. [19] is mainly for the T1f1–T1f2 because the anhydrite was mainly located within the T1f1–T1f2 in the Dukouhe, Tieshanpo, and Jinzhuping gas fields (Figure 3).Overall, theδ34S of the T1f1–T1f2 anhydrites (18.12-26.6‰) was lower than that of the T1f3–T1f4 anhydrites (29.9-39.6‰), which is consistent with the conclusion that the δ34S of seawater sulfate increased during the Early Triassic [29].


## 5. Discussion

### 5.1. Which Method Is More Effective at Removing Pyrite from Solid Bitumen?

Pyrite can be a byproduct of TSR [1, 8]. Pyrite grains were observed in the T1f reservoir cores and have been suggested to be formed by TSR because their δ34S is 18.7-20.16‰ [19], which differs from that of the pyrite from the source rock layers. Except for one sample (SB16), the sulfur contents of the HNO3-treated and CrCl2-treated solid bitumen samples are lower than those of the untreated solid bitumen samples (Figure 6), which support the conclusion that some of the sulfur was removed by the HNO3 and CrCl2 methods. The untreated solid bitumen samples were extracted by DCM so that the organic matter and elemental sulfur were removed from the solid bitumen. As a result, the decrease in the sulfur contents of the HNO3-treated and CrCl2-treated solid bitumen samples was most likely caused by the removal of some of the pyrite. In addition, the special smell of released H2S was smelled when the solid bitumen was treated with CrCl2, and small pyrite grains were observed in the untreated solid bitumen powder with SEM (Figure 5). XRD had been used to determine the pyrite content in kerogen and solid bitumen [13, 14], and an improved method to determine the pyrite content by measuring the dissolved iron at pH<2 using an atomic absorption spectrometer was developed by Cai et al. [30]. However, XRD and the dissolved iron analysis were not performed in this study because the obtained untreated solid bitumen samples weighed less than 100–300 mg each, so the pyrite content could not be determined. However, it is reasonable that the decrease in the sulfur contents of the HNO3- and CrCl2-treated solid bitumen samples was caused by removing some of the pyrite.Approximately 88.9-99.7% of pyrite was removed when the kerogen was treated twice with CrCl2 [12]. Cai et al. [13, 14, 30] used a similar method and successfully removed the pyrite from kerogen and solid bitumen samples, and sulfur isotopes were analyzed only when pyrite sulfur/total sulfur values were less than 0.08 [30]. However, no studies have been conducted to compare the HNO3 and CrCl2 methods. In this study, the sulfur contents and sulfur isotopes of most of the solid bitumen samples treated using the HNO3 and CrCl2 methods were quite similar (Figures 6 and 7), which demonstrate that pyrite was removed by both methods. However, some of the HNO3-treated solid bitumen samples (SB2, SB5, SB13, and SB17) had significantly lower sulfur contents and significantly higher δ34S than the CrCl2-treated solid bitumen samples (Figure 6), which suggests that the HNO3 method is more effective at removing pyrite from solid bitumen than the method using CrCl2 thrice. In particular, the δ34S of the CrCl2-treated sample SB13 (16.8‰) is still less than 20.0‰, but it is 2.4‰ higher than that of untreated sample SB13. The δ34S of HNO3-treated sample SB13 is 24.1‰, which is similar to the δ34S of the other HNO3-treated solid bitumen samples from the Yuanba gas field (greater than 24.0‰) (Figure 7). Therefore, the δ34S of HNO3-treated sample SB13 should be closer to that of solid bitumen without pyrite. In addition, the difference in sulfur content (ΔS) and the difference in the sulfur isotopes (Δδ34S) of the HNO3- and CrCl2-treated solid bitumen samples are negatively correlated (R2=0.83; Figure 8), indicating that the δ34S of the HNO3-treated solid bitumen samples increases more with decreasing sulfur content than the δ34S of the CrCl2-treated samples does. This also supports that the HNO3 method is more effective at removing pyrite from solid bitumen than the method using CrCl2 thrice.Figure 8
Variation in differences in the sulfur contents (ΔS) and the differences in the sulfur isotopes (Δδ34S) of the HNO3-treated and CrCl2-treated solid bitumen samples.As a result, theδ34S of the solid bitumen discussed in the next section is the δ34S of the HNO3-treated solid bitumen samples. Though we cannot be absolutely sure that all of the pyrite was completely removed from the solid bitumen by the HNO3 method, we believe that the δ34S of the HNO3-treated solid bitumen samples is quite similar to the true δ34S of solid bitumen without pyrite. Thus, the δ34S of solid bitumen samples can be used to investigate the origin of the sulfur.


### 5.2. Sulfate Sources of TSR

The solid bitumens from the T1f and P2c reservoirs in the northeastern part of the Sichuan Basin are insoluble pyrobitumens, which were formed in the advanced stages of thermal maturity [4, 21]. Insoluble solid bitumen can form due to the thermal chemical alteration (TCA) or thermochemical sulfate reduction (TSR) of migrated petroleum [31]. The S/C ratio of TSR bitumen is generally greater than 0.03 [31], while its δ34S is close to the δ34S of sulfate [11, 32, 33]. The S/C ratio of TCA bitumen is generally less than 0.03 [31], while its δ34S is similar to the δ34S of the source kerogen [20]. The S/C ratios of the T1f and P2c solid bitumen from the Puguang and Yuanba gas fields are greater than 0.03 [4, 6]. In addition, the δ34S of this solid bitumen is 12.0-35.0‰ (Figure 9), which is significantly higher than that of the kerogen from the Permian source rocks from well HB1 (−26.7‰; [30]), but it is close to the δ34S of the T1f anhydrite (18.0-35.0‰; Figure 9). The S/C ratios and the δ34S values of the solid bitumen both support its TSR origin.Figure 9
Correlation between the sulfur isotopes of the solid bitumen and anhydrite samples in the northeastern part of the Sichuan Basin.The mechanism by which the sulfur in the sulfate is transferred into the solid bitumen is not yet clearly understood. However, when the TSR proceeds gradually, the saturate/aromatic ratio of the TSR-altered oil decreases, while the sulfur and oxygen contents of the solid bitumen increase [32, 34], which suggests that the sulfur was transferred from the sulfate into the hydrocarbon. In addition, the TSR-altered oils are rich in sulfur compounds, such as thiophenes, benzothiophens, and dibenzothiophenes [32, 33]. The isotopic fractionation of sulfur that occurs during the TSR process is minimal, so the δ34S of the TSR-altered oil and solid bitumen is close to that of the TSR-involved sulfate [9–11, 28, 32, 33]. TSR can occur in both gas zone and gas-water transition zone [17], but the sulfur fractionations were different in these two zones [14]. In the gas-water transition zone, the gas saturation was low so that some dissolved sulfate remains unreacted, and the TSR solid bitumen would be enriched in δ34S and H2S would be depleted in δ34S. In the gas zone, the limited dissolved sulfate would be reduced completely, so that the δ34S of TSR solid bitumen would be close to the δ34S of sulfate. However, the solid bitumen samples from the Yuanba and Puguang gas fields were both collected from present gas zones, so we considered that the sulfur fractionation during the TSR was insignificant.As shown in Figure9, the δ34S of the solid bitumen from the Puguang gas field in the eastern part of the K-L trough is generally less than 24.0‰ (except for one sample with 31.2‰), whereas the δ34S of the solid bitumen from the Yuanba gas field in the western part of the K-L trough is generally greater than 24.0‰ (24.1-34.2‰). Thus, assuming minimal isotopic fractionation of sulfur during TSR, it is impossible for the sulfur in the solid bitumen from the Yuanba gas field to have come from the T1f1 and T1f2 sulfates because the δ34S of these sulfates is lower than that of the solid bitumen. However, the δ34S of the solid bitumen is similar to that of the T1f3 and T1f4 anhydrites. In addition, there is no anhydrite in T1f1 and T1f2 in the Yuanba gas field, and the anhydrite is only present in T1f4 (Figure 3). As a result, the sulfate of the TSR was most likely from T1f4 in the Yuanba gas field. The primary source of the sulfate of the TSR in the Puguang gas field was the T1f1 and T1f2 anhydrite layers, which have slightly higher δ34S than the solid bitumen, but part of the sulfate was from the T1f3 and T1f4 anhydrite layers because the δ34S of some of the solid bitumen is higher than that of the T1f1 and T1f2 anhydrites (Figure 9). Based on the correlation between the δ34S values of the solid bitumen and anhydrite samples, it can be concluded that the sulfate source of the TSR is different in the western and eastern parts of the K-L trough.The H2S-rich natural gases of the T1f and P2c are mainly located in the dolostone reservoirs in the northeastern part of the Sichuan Basin, whereas no H2S was found in limestone reservoirs [6]. Thus, we suggest that the TSR sulfates (including anhydrites and dissolved sulfates) were enriched during the dolomitization process. Geological observations and numerical simulations have demonstrated that anhydrite can be a byproduct of the dolomitization process [35, 36], which indicates that sulfates can be enriched during the dolomitization process. According to the calculations by Li et al. [6], if all of the SO42− in the formation water were converted to H2S during TSR, the generated H2S concentration would be 8.8-23.3%, which is similar to the range observed in the Puguang (5.09–19.22%, [4]) and Yuanba (1.20-12.16%, [6]) gas fields. In addition, the anhydrite produced in the dolomitization process can also supply additional sulfates. TSR calcites have been observed to replace anhydrite in the T1f dolostone reservoir in the northeastern part of the Sichuan Basin [19]. Therefore, the sulfates enriched during the dolomitization process can supply sufficient sulfur to produce the observed H2S concentration in the northeastern part of the Sichuan Basin.The sulfates of the TSR in the Puguang gas field in the eastern part of the K-L trough were most probably enriched during the reflux-seepage dolomitization process [37–39]. The coevally evaporating seawater of the T1f flowed laterally into the porous T1f oolitic limestone and the P2c reef limestone, causing broad dolomitization, during which the TSR sulfates (anhydrites and dissolved sulfates) were enriched. The down migration model of dolomitization fluid from T1f and P2c had been supported by the evidence that the P2c dolostones have 87Sr/86Sr ratios close to the ratios of T1f seawater but heavier than P2c seawater [38]. This is similar to the Permian Khuff Formation, offshore Dubai [40]. Finally, the sulfur from the sulfates was transferred into the solid bitumen and the H2S. The SO42− concentration of the formation water from the Puguang gas field (H2S concentration of 10-20%) is less than 1.0 g/L, which is far lower than that of the formation water from the Jiannan gas field (H2S concentration less than 5%), indicating that the dissolved sulfates in the formation water were consumed during the TSR process in the Puguang gas field [6].As was previously discussed, the sulfates of the TSR in the Yuanba gas field were most likely from the evaporating seawater in T1f4. However, the micritic limestone of T1f1 and T1f2 can act as a barrier to prevent evaporating seawater of T1f4 from flowing vertically into the P2c reservoirs (Figure 3). However, there are porous dolostone reservoirs in T1f1 and T1f2 in the Longgang gas field (Figure 3), which is also in the western part of the K-L trough and adjacent to the Yuanba gas field, so we suggest that the evaporating seawater may have initially flowed vertically from T1f4 into T1f1 and T1f2, then it flowed into the P2c reservoir layers in the Longgang gas field, and finally, it flowed laterally into the P2c reservoirs in the Yuanba gas field, which caused the dolomitization and enrichment of the sulfates of the TSR.Cai et al. [30] proposed that the δ34S and S/C ratio of the TSR solid bitumen samples can be used to show the TSR extent based on systematic analysis of TSR and non-TSR solid bitumen samples in the northeastern part of the Sichuan Basin. However, the δ34S values of the P2c solid bitumen in the western part of the K-L trough, such as those from the Yuanba and Longgang gas fields, are not included in that paper. As was previously discussed, the δ34S of the solid bitumen from the Yuanba gas field is significantly higher than that from the Puguang gas field. However, the S/C of the solid bitumen from the Yuanba gas field (less than 0.06) is less than the S/C of the solid bitumen from the Puguang gas field (greater than 0.06). Also, the H2S concentration of the Yuanba gas field (5-10%) is lower than that of the Puguang gas field (10-20%), and the corresponding GSI (gas sourcing index), which was used to reflect the extent of TSR [41], is lower in the Yuanba gas field (less than 0.1) than in the Puguang gas field (0.1-0.2). Thus, we conclude that the extent of TSR was greater in the Puguang gas field than in the Yuanba gas field. As a result, we suggest that the δ34S of TSR solid bitumen can be used as an indicator of the TSR extent only when the sulfate source of the TSR is similar (δ34S of sulfate source is similar).


## 5.1. Which Method Is More Effective at Removing Pyrite from Solid Bitumen?

Pyrite can be a byproduct of TSR [1, 8]. Pyrite grains were observed in the T1f reservoir cores and have been suggested to be formed by TSR because their δ34S is 18.7-20.16‰ [19], which differs from that of the pyrite from the source rock layers. Except for one sample (SB16), the sulfur contents of the HNO3-treated and CrCl2-treated solid bitumen samples are lower than those of the untreated solid bitumen samples (Figure 6), which support the conclusion that some of the sulfur was removed by the HNO3 and CrCl2 methods. The untreated solid bitumen samples were extracted by DCM so that the organic matter and elemental sulfur were removed from the solid bitumen. As a result, the decrease in the sulfur contents of the HNO3-treated and CrCl2-treated solid bitumen samples was most likely caused by the removal of some of the pyrite. In addition, the special smell of released H2S was smelled when the solid bitumen was treated with CrCl2, and small pyrite grains were observed in the untreated solid bitumen powder with SEM (Figure 5). XRD had been used to determine the pyrite content in kerogen and solid bitumen [13, 14], and an improved method to determine the pyrite content by measuring the dissolved iron at pH<2 using an atomic absorption spectrometer was developed by Cai et al. [30]. However, XRD and the dissolved iron analysis were not performed in this study because the obtained untreated solid bitumen samples weighed less than 100–300 mg each, so the pyrite content could not be determined. However, it is reasonable that the decrease in the sulfur contents of the HNO3- and CrCl2-treated solid bitumen samples was caused by removing some of the pyrite.Approximately 88.9-99.7% of pyrite was removed when the kerogen was treated twice with CrCl2 [12]. Cai et al. [13, 14, 30] used a similar method and successfully removed the pyrite from kerogen and solid bitumen samples, and sulfur isotopes were analyzed only when pyrite sulfur/total sulfur values were less than 0.08 [30]. However, no studies have been conducted to compare the HNO3 and CrCl2 methods. In this study, the sulfur contents and sulfur isotopes of most of the solid bitumen samples treated using the HNO3 and CrCl2 methods were quite similar (Figures 6 and 7), which demonstrate that pyrite was removed by both methods. However, some of the HNO3-treated solid bitumen samples (SB2, SB5, SB13, and SB17) had significantly lower sulfur contents and significantly higher δ34S than the CrCl2-treated solid bitumen samples (Figure 6), which suggests that the HNO3 method is more effective at removing pyrite from solid bitumen than the method using CrCl2 thrice. In particular, the δ34S of the CrCl2-treated sample SB13 (16.8‰) is still less than 20.0‰, but it is 2.4‰ higher than that of untreated sample SB13. The δ34S of HNO3-treated sample SB13 is 24.1‰, which is similar to the δ34S of the other HNO3-treated solid bitumen samples from the Yuanba gas field (greater than 24.0‰) (Figure 7). Therefore, the δ34S of HNO3-treated sample SB13 should be closer to that of solid bitumen without pyrite. In addition, the difference in sulfur content (ΔS) and the difference in the sulfur isotopes (Δδ34S) of the HNO3- and CrCl2-treated solid bitumen samples are negatively correlated (R2=0.83; Figure 8), indicating that the δ34S of the HNO3-treated solid bitumen samples increases more with decreasing sulfur content than the δ34S of the CrCl2-treated samples does. This also supports that the HNO3 method is more effective at removing pyrite from solid bitumen than the method using CrCl2 thrice.Figure 8
Variation in differences in the sulfur contents (ΔS) and the differences in the sulfur isotopes (Δδ34S) of the HNO3-treated and CrCl2-treated solid bitumen samples.As a result, theδ34S of the solid bitumen discussed in the next section is the δ34S of the HNO3-treated solid bitumen samples. Though we cannot be absolutely sure that all of the pyrite was completely removed from the solid bitumen by the HNO3 method, we believe that the δ34S of the HNO3-treated solid bitumen samples is quite similar to the true δ34S of solid bitumen without pyrite. Thus, the δ34S of solid bitumen samples can be used to investigate the origin of the sulfur.


## 5.2. Sulfate Sources of TSR

The solid bitumens from the T1f and P2c reservoirs in the northeastern part of the Sichuan Basin are insoluble pyrobitumens, which were formed in the advanced stages of thermal maturity [4, 21]. Insoluble solid bitumen can form due to the thermal chemical alteration (TCA) or thermochemical sulfate reduction (TSR) of migrated petroleum [31]. The S/C ratio of TSR bitumen is generally greater than 0.03 [31], while its δ34S is close to the δ34S of sulfate [11, 32, 33]. The S/C ratio of TCA bitumen is generally less than 0.03 [31], while its δ34S is similar to the δ34S of the source kerogen [20]. The S/C ratios of the T1f and P2c solid bitumen from the Puguang and Yuanba gas fields are greater than 0.03 [4, 6]. In addition, the δ34S of this solid bitumen is 12.0-35.0‰ (Figure 9), which is significantly higher than that of the kerogen from the Permian source rocks from well HB1 (−26.7‰; [30]), but it is close to the δ34S of the T1f anhydrite (18.0-35.0‰; Figure 9). The S/C ratios and the δ34S values of the solid bitumen both support its TSR origin.Figure 9
Correlation between the sulfur isotopes of the solid bitumen and anhydrite samples in the northeastern part of the Sichuan Basin.The mechanism by which the sulfur in the sulfate is transferred into the solid bitumen is not yet clearly understood. However, when the TSR proceeds gradually, the saturate/aromatic ratio of the TSR-altered oil decreases, while the sulfur and oxygen contents of the solid bitumen increase [32, 34], which suggests that the sulfur was transferred from the sulfate into the hydrocarbon. In addition, the TSR-altered oils are rich in sulfur compounds, such as thiophenes, benzothiophens, and dibenzothiophenes [32, 33]. The isotopic fractionation of sulfur that occurs during the TSR process is minimal, so the δ34S of the TSR-altered oil and solid bitumen is close to that of the TSR-involved sulfate [9–11, 28, 32, 33]. TSR can occur in both gas zone and gas-water transition zone [17], but the sulfur fractionations were different in these two zones [14]. In the gas-water transition zone, the gas saturation was low so that some dissolved sulfate remains unreacted, and the TSR solid bitumen would be enriched in δ34S and H2S would be depleted in δ34S. In the gas zone, the limited dissolved sulfate would be reduced completely, so that the δ34S of TSR solid bitumen would be close to the δ34S of sulfate. However, the solid bitumen samples from the Yuanba and Puguang gas fields were both collected from present gas zones, so we considered that the sulfur fractionation during the TSR was insignificant.As shown in Figure9, the δ34S of the solid bitumen from the Puguang gas field in the eastern part of the K-L trough is generally less than 24.0‰ (except for one sample with 31.2‰), whereas the δ34S of the solid bitumen from the Yuanba gas field in the western part of the K-L trough is generally greater than 24.0‰ (24.1-34.2‰). Thus, assuming minimal isotopic fractionation of sulfur during TSR, it is impossible for the sulfur in the solid bitumen from the Yuanba gas field to have come from the T1f1 and T1f2 sulfates because the δ34S of these sulfates is lower than that of the solid bitumen. However, the δ34S of the solid bitumen is similar to that of the T1f3 and T1f4 anhydrites. In addition, there is no anhydrite in T1f1 and T1f2 in the Yuanba gas field, and the anhydrite is only present in T1f4 (Figure 3). As a result, the sulfate of the TSR was most likely from T1f4 in the Yuanba gas field. The primary source of the sulfate of the TSR in the Puguang gas field was the T1f1 and T1f2 anhydrite layers, which have slightly higher δ34S than the solid bitumen, but part of the sulfate was from the T1f3 and T1f4 anhydrite layers because the δ34S of some of the solid bitumen is higher than that of the T1f1 and T1f2 anhydrites (Figure 9). Based on the correlation between the δ34S values of the solid bitumen and anhydrite samples, it can be concluded that the sulfate source of the TSR is different in the western and eastern parts of the K-L trough.The H2S-rich natural gases of the T1f and P2c are mainly located in the dolostone reservoirs in the northeastern part of the Sichuan Basin, whereas no H2S was found in limestone reservoirs [6]. Thus, we suggest that the TSR sulfates (including anhydrites and dissolved sulfates) were enriched during the dolomitization process. Geological observations and numerical simulations have demonstrated that anhydrite can be a byproduct of the dolomitization process [35, 36], which indicates that sulfates can be enriched during the dolomitization process. According to the calculations by Li et al. [6], if all of the SO42− in the formation water were converted to H2S during TSR, the generated H2S concentration would be 8.8-23.3%, which is similar to the range observed in the Puguang (5.09–19.22%, [4]) and Yuanba (1.20-12.16%, [6]) gas fields. In addition, the anhydrite produced in the dolomitization process can also supply additional sulfates. TSR calcites have been observed to replace anhydrite in the T1f dolostone reservoir in the northeastern part of the Sichuan Basin [19]. Therefore, the sulfates enriched during the dolomitization process can supply sufficient sulfur to produce the observed H2S concentration in the northeastern part of the Sichuan Basin.The sulfates of the TSR in the Puguang gas field in the eastern part of the K-L trough were most probably enriched during the reflux-seepage dolomitization process [37–39]. The coevally evaporating seawater of the T1f flowed laterally into the porous T1f oolitic limestone and the P2c reef limestone, causing broad dolomitization, during which the TSR sulfates (anhydrites and dissolved sulfates) were enriched. The down migration model of dolomitization fluid from T1f and P2c had been supported by the evidence that the P2c dolostones have 87Sr/86Sr ratios close to the ratios of T1f seawater but heavier than P2c seawater [38]. This is similar to the Permian Khuff Formation, offshore Dubai [40]. Finally, the sulfur from the sulfates was transferred into the solid bitumen and the H2S. The SO42− concentration of the formation water from the Puguang gas field (H2S concentration of 10-20%) is less than 1.0 g/L, which is far lower than that of the formation water from the Jiannan gas field (H2S concentration less than 5%), indicating that the dissolved sulfates in the formation water were consumed during the TSR process in the Puguang gas field [6].As was previously discussed, the sulfates of the TSR in the Yuanba gas field were most likely from the evaporating seawater in T1f4. However, the micritic limestone of T1f1 and T1f2 can act as a barrier to prevent evaporating seawater of T1f4 from flowing vertically into the P2c reservoirs (Figure 3). However, there are porous dolostone reservoirs in T1f1 and T1f2 in the Longgang gas field (Figure 3), which is also in the western part of the K-L trough and adjacent to the Yuanba gas field, so we suggest that the evaporating seawater may have initially flowed vertically from T1f4 into T1f1 and T1f2, then it flowed into the P2c reservoir layers in the Longgang gas field, and finally, it flowed laterally into the P2c reservoirs in the Yuanba gas field, which caused the dolomitization and enrichment of the sulfates of the TSR.Cai et al. [30] proposed that the δ34S and S/C ratio of the TSR solid bitumen samples can be used to show the TSR extent based on systematic analysis of TSR and non-TSR solid bitumen samples in the northeastern part of the Sichuan Basin. However, the δ34S values of the P2c solid bitumen in the western part of the K-L trough, such as those from the Yuanba and Longgang gas fields, are not included in that paper. As was previously discussed, the δ34S of the solid bitumen from the Yuanba gas field is significantly higher than that from the Puguang gas field. However, the S/C of the solid bitumen from the Yuanba gas field (less than 0.06) is less than the S/C of the solid bitumen from the Puguang gas field (greater than 0.06). Also, the H2S concentration of the Yuanba gas field (5-10%) is lower than that of the Puguang gas field (10-20%), and the corresponding GSI (gas sourcing index), which was used to reflect the extent of TSR [41], is lower in the Yuanba gas field (less than 0.1) than in the Puguang gas field (0.1-0.2). Thus, we conclude that the extent of TSR was greater in the Puguang gas field than in the Yuanba gas field. As a result, we suggest that the δ34S of TSR solid bitumen can be used as an indicator of the TSR extent only when the sulfate source of the TSR is similar (δ34S of sulfate source is similar).


## 6. Conclusions

The following conclusions can be drawn based on our study:(1)
Both the HNO3 method and the CrCl2 method successfully remove pyrite from solid bitumen, but the HNO3 method is more effective than the method using CrCl2 thrice
(2)
Theδ34S of the T1f solid bitumen from the Puguang gas field (12.0-24.0‰) is lower than the δ34S of the P2c solid bitumen from the Yuanba gas field (24.1-34.2‰), and the sulfates of the TSR of the Puguang gas field were mainly formed by evaporating seawater in T1f1–2, whereas the sulfates of the TSR in the Yuanba gas field were mainly formed by evaporating seawater in T1f4
(3)
When theδ34S of the sulfates of the TSR is similar, the δ34S of the TSR solid bitumen can be used to show the extent of TSR



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*Source: 5898901-2019-05-07.xml*