Geofluids
Volume 2019 (2019), Article ID 8608596, 23 pages
https://doi.org/10.1155/2019/8608596

Research Article

Carbon and Hydrogen Isotopic Reversals in Highly Mature Coal-Derived Gases: A Case Study of Paleozoic Gases in the Southern Ordos Basin, China

Dan Liu1,2

¹Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
²Institute of Earth Science, Chinese Academy of Sciences, Beijing 100029, China

Correspondence should be addressed to Dan Liu; liudan@mail.iggcas.ac.cn

Received 12 February 2019; Revised 13 May 2019; Accepted 11 June 2019; Published 11 July 2019

Academic Editor: Francesco Italiano

Copyright © 2019 Dan Liu. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The compositional carbon isotopic series δ¹³C-CH₄<δ¹³C-C₂H₆<δ¹³C-C₃H₈<δ¹³C-C₄H₁₀ is common in thermogenic gases. With the exploration of deeper strata, however, isotopic reversals (δ¹³C-CH₄>δ¹³C-C₂H₆>δ¹³C-C₃H₈) in overmature unconventional shale gases and conventional (coal-derived) gases have been identified. Paleozoic gases in the southern Ordos Basin, China, with partial or complete isotopic reversals, were studied as examples of isotopic fractionation in overmature coal-derived gases. Isotopic compositions of gases of different maturities from the Ordos Basin and shale gases from around the world were compared. Results indicate that carbon isotopic series are related to maturity. Complete isotopic reversal occurs mostly in regions with vitrinite reflectance . Where , almost all gases display partial isotopic reversal, with δ¹³C-CH₄>δ¹³C-C₂H₆ or δ¹³C-C₂H₆>δ¹³C-C₃H₈. Carbon isotopic reversal in coal-derived gases is not caused by abiotic origin, the mixing of gases from different types of source rock, abiotic polymerization, wet gas cracking, and other mechanisms that contribute to reversal in shale gases. Based on the unique structure of coaly source rock and the geology of the Ordos Basin, closed-system aromatization-polycondensation reactions are considered the most likely cause of carbon isotopic reversal. During the reactions, isotopically light gases are generated by recombination of previously formed hydrocarbons and residual kerogen-coal. Hydrogen isotopic reversal in the southern Ordos Basin might also be caused by aromatization-polycondensation reactions.

1. Introduction

The carbon isotopic composition of natural gas can be used to determine its origin, source, and maturity [1, 2]. In thermogenic gas reactions, ¹²C–¹²C bonds usually break before ¹²C–¹³C bonds, resulting in kinetic isotopic fractionation [2] and causing natural gas to follow two evolutionary trends, with carbon isotopes becoming heavier as the component carbon number increases, forming a positive isotopic series with δ¹³C-CH₄<δ¹³C-C₂H₆<δ¹³C-C₃H₈<δ¹³C-C₄H₁₀, and with each component C becoming enriched in ¹³C as gas maturity increases [1].

As exploration of Paleozoic gas in the Ordos Basin has expanded into the southern part of the basin, more gases have been found to have carbon and hydrogen isotopic reversals. Most previous studies have attributed the partial carbon isotopic reversal (δ¹³C-CH₄<δ¹³C-C₂H₆>δ¹³C-C₃H₈) of Ordos gases to the mixing of gases generated from different source rocks [3], (Yang et al., 2012), [4–7]. Recent studies have reported complete carbon isotopic reversals (δ¹³C-CH₄>δ¹³C-C₂H₆>δ¹³C-C₃H₈) in Ordos gases [8–12], with all suggesting that such reversals are caused by high temperatures (>200°C), with isotopic reversal in shale gas having the same mechanism. However, there are problems with this explanation as follows: (1) shale gas is an oil-type gas generated from type I and type II kerogen, whereas Ordos gas is mainly coal derived and generated from type III kerogen, with different gas-generation reactions occurring in these sources; (2) complete carbon isotopic reversal is universal in overmature shale gases [13–15], but not all overmature coal-derived gas has complete carbon isotopic reversals, as in gas from the southern Ordos Basin [16]; and (3) gases in the southern Ordos Basin display unique characteristics that differ from those of overmature shale gas, such as the evolutionary trend of hydrogen isotopes.

Here, the geochemical characteristics of southern Ordos gas and shale gas are compared, with isotopic fractionation of overmature coal-derived gas being considered separately from shale gas. Geochemical data for gases from other fields in the Ordos Basin with different maturities are considered to elucidate isotopic evolution in the Ordos Basin. The cause of hydrogen isotopic reversal in the southern Ordos Basin is also discussed.

2. Geological Setting

The Ordos Basin is the second largest sedimentary basin in China, with Paleozoic strata occurring over an area of >250,000 km² [16]. This basin is a petroliferous cratonic basin with stable subsidence, depression displacement, visible torsion, and multiple cycles of deposition [8]. Several giant gas fields, each containing >100 billion cubic metres (bcm) of proven gas reserves, have been discovered in the Paleozoic strata of the basin, with most gas being in the upper Paleozoic reservoirs such as the Sulige, Shenmu, Daniudi, and Yan’an gas fields. Upper Paleozoic and lower Paleozoic-Ordovician carbonate rocks provide the reservoir in the Jingbian field (Figure 1). Exploration of the Jingbian field has increased recently, with many wells being drilled in the overmature southern Jingbian area, targeting both lower and upper Paleozoic gas systems [16] and providing an unprecedented opportunity to improve our understanding of the geochemistry of overmature gas systems.

(a)

(b)

(c)

Figure 1: (a) Maturity map of the upper Paleozoic strata of the Ordos Basin, with sample locations labelled by well name (after [16]); (b) stratigraphic column of the Paleozoic strata of the Ordos Basin (after [16]); (c) cross section for Ordos Basin (after [16]).

2.1. Source Rocks

The predominant source rock of Paleozoic gases is Carboniferous-Permian (C-P) coal and mudstone. Ordovician marine carbonates contribute only minor amounts of gas owing to their low organic content. Previous studies indicate that gases of the Sulige, Daniudi, Shenmu, Wushenqi, Yulin, Zizhou, and Mizhi fields were all produced from upper Paleozoic coaly source rocks [4, 17–20]. The origin of Jingbian gas, from the lower Paleozoic-Ordovician Majiagou formation, is still unresolved, although the prevailing view is that it was produced mainly from upper Paleozoic coal measures, migrated downward, and mixed with oil-type gas from lower Paleozoic-Ordovician source rocks [11, 21].

2.2. Reservoirs

The two main reservoirs in the Paleozoic strata of the Ordos Basin are the upper C-P paralic and continental clastic reservoir and lower Paleozoic (Ordovician) marine carbonate reservoir. Most natural gas from the Sulige, Daniudi, Shenmu, Wushenqi, Yulin, Zizhou, and Mizhi fields is produced from the upper Paleozoic reservoirs, with tight sandstones of the Permian Shanxi and Xiashihezi formations being the major reservoirs. Gas of the Jingbian field is produced from both the upper and lower Paleozoic reservoirs, with the major lower Paleozoic reservoir being the fifth member (O₁m₅) of the Majiagou formation.

2.3. Caprocks

Caprocks of the lower Paleozoic reservoirs are Carboniferous bauxitic mudstones that are <10 m thick, coal measures that are 15–40 m thick, and argillaceous dolomite and marlstone [21]. The regional seal of the upper Paleozoic reservoirs consists of stable, laterally distributed, Shangshihezi and Shiqianfeng formation lacustrine mudstones [16].

2.4. Thermal History and Gas Charge Model

During the Late Triassic-Early Jurassic (T₃-J₁), at the threshold of hydrocarbon generation, the gas first accumulated within C-P source rocks. During the Late Jurassic-Early Cretaceous (J-K₁), >1000 m of J-K₁ sediment was deposited, with a maximum depth of >4000 m and a maximum palaeogeothermal temperature of 160–200°C. Most C-P gases were generated during this period and migrated to nearby upper Paleozoic C-P reservoirs between source rocks. Driven by buoyancy and abnormal pressure, the gases also migrated directly upward or downward into the lower Paleozoic reservoirs over distances of <200 m [16, 21, 22]. From the end of the Early Cretaceous, thick strata were eroded and gas generation terminated, with the temperature reducing to the current 99.6–113.5°C [23].

3. Sampling and Analysis

3.1. Data Analyses

Geochemical characteristics of 199 gas samples from Paleozoic reservoirs in the Ordos Basin gas fields, including the Jingbian, Yan’an, Sulige, Shenmu, and Daniudi fields (Figure 1), are presented in Table 1. The data pertain to 94 samples from previous studies [9, 16], 41 from newly collected samples in this study, and 64 recorded in the geochemical database of the Exploration and Development Research Institute, Changqing Oil Company (EDRICOC), Xi’an, China. Eight of the 27 samples of Feng et al. [9] were analysed for rare-gas isotopes (Table 1). Data for the gas samples were obtained using similar techniques [11, 16] to ensure consistency. The maturity of the upper Paleozoic source rocks in the Ordos Basin increases from north to south, and the gases come from similar source rocks, so a comparison of isotopic compositions of gases in these fields may provide information on the evolution of coal-derived gases based on geological setting. Lower Paleozoic South Jingbian gases are included in the discussion of the origin of isotopic reversal. Shale gas from the Sichuan Basin and other basins with different maturities is included to provide information on differences between coal-derived and oil-type gases (data for shale gas were obtained from [13–16, 24–31]). Shale gases also display isotopic reversal in high maturity conditions, providing insight into the origin of unusual geochemical characteristics of overmature coal-derived gases.

Table 1: Composition and carbon isotopes of Paleozoic gas in Ordos Basin.

Field
Well
Strata
Depth
Main component (%)
δ¹³C (‰), VPDB
δD (‰), VSMOW
Rare gas
Ro-Dai
Reference

CH₄
C₂H₆
C₃H₈
iC₄H₁₀
nC₄H₁₀
iC₅H₁₂
nC₅H₁₂
C₅+
CO₂
N₂
CH₄
C₂H₆
C₃H₈
C₄H₁₀
CO₂
CH₄
C₂H₆
C₃H₈
³He/⁴He
⁴⁰Ar/³⁶Ar
%

Yan’an
Sh2
P₂h

96.68
0.73
0.09
0.02
0.06

1.41
1.07
-29.2
-30.7
-31.9

-12.9
-168
-190

2.34
The main component data and carbon and hydrogen isotope data are from Feng et al., [9]; the rare gas data are from this study.

Sh217
P₂h

96.30
0.62
0.05

2.27
0.76
-27.6
-34.9

-0.2
-170
-183

3.04

Sh6
P₁s

96.32
0.76
0.07

1.97
0.86
-28.1
-30.5
-30.4

-9.1
-168
-187

2.80

Sh36
P₁s

93.90
0.43
0.02

4.93
0.72
-29.2
-35.4

-3.6
-166
-182

2.34

Sh38
P₁s

95.91
0.42
0.03

3.11
0.53
-28.2
-36.1

-1.1
-167
-185

2.75

Y127
P₁s

93.45
0.43
0.03

5.72
0.37
-29.3
-33.7
-30.7

-0.2
-168
-184

2.30

Sh210
P₁s

93.39
0.43
0.03

5.86
0.30
-29.7
-34.9
-34.5

-0.1
-168
-187

2.15

Sh212
P₁s

93.24
0.41
0.02

5.63
0.69
-29.7
-35.1
-34.5

-0.7
-167
-184

2.15

Sh209
P₁s

89.90
0.42
0.02

9.08
0.57
-28.9
-34.7

-170
-190

2.46

Sh217-1
P₁s

94.45
0.30
0.02

4.79
0.43
-29.3
-34.0

2.4
-167
-178

2.30

Sh225
P₁s

93.87
0.42
0.03

5.01
0.67
-28.8
-34.1

-0.9
-163
-167

2.50

Sh231
P₁s

93.14
0.40
0.02

5.96
0.47
-29.4
-34.4
-34.0

0.2
-168
-197

2.26

Sh12
C₂b

95.31
0.53
0.04

3.51
0.59
-30.6
-37.2
-35.8

-0.6
-165
-183

1.86

Sh37
C₂b

96.60
0.42
0.03

2.74
0.22
-30.8
-37.1
-37.3

-2.1
-170
-173

1.80

Sh48
C₂b

94.89
0.52
0.04

4.29
0.25
-29.9
-36.5

1.7
-163
-186

2.09

Sh56
P₁s

95.96
0.48
0.03
0.00
0.00

3.00
0.47
-28.2
-35.0
-33.7

-166

2.75

Y201
P₁s

97.48
0.39
0.04
0.00
0.00

1.37
0.65
-28.9
-34.9
-32.3

-166

2.46

Sh28-1
P₁s

97.39
0.41
0.02
0.00
0.00

1.52
0.39
-29.1
-36.8

-165
-189

2.38

Y186
P₁s

95.98
0.42
0.02
0.00
0.00

2.97
0.40
-28.1
-36.4
-35.5

-165

2.80

Sh26
P₁s

97.15
0.37
0.02
0.00
0.00

2.14
0.24
-29.2
-35.5

-163

2.34

Sh25
P₁s

95.84
0.43
0.02
0.00
0.00

3.15
0.48
-28.6
-36.2

-166
-193

2.58

Y175
C₂b

96.49
0.62
0.05
0.00
0.00

2.16
0.59
-27.5
-33.4
-33.3

-164
-178

3.09

Sh229
P₁s

94.74
0.47
0.03
0.00
0.00

4.16
0.47
-27.8
-34.5
-33.5

-167

2.94

Sh228
P₁s

94.68
0.41
0.03
0.00
0.00

4.30
0.46
-28.5
-35.2
-34.7

-167
-186

2.62

Sh214
P₁s

94.86
0.43
0.03
0.00
0.00

4.14
0.43
-28.3
-35.4

-168

2.71

Sh207
P₁s

95.94
0.28
0.02
0.00
0.00

3.09
0.56
-27.9
-33.7

-167

2.89

Y161
P₁s

95.37
0.47
0.03
0.00
0.00

3.53
0.47
-28.2
-34.5

-167

2.75

Standard deviation
0.85
1.60
1.93

4.16
1.98
6.91

0.34

South Jingbian-upper Paleozoic
Tong25
P₂h₄
2647
90.81
2.68
0.60
0.12
0.12
0.06
0.03
0.00
0.78
4.86
-21.0
-21.1
-22.2

8.89
Database of EDRICOC

Shan281
P₂h
3522
79.55
0.97
0.16
0.02
0.02
0.01
0.01
0.02
1.30
18.06
-31.3
-35.5
-32.9

1.67

Shan285
C₂b
3150
94.85
1.25
0.14
0.02
0.02
0.00
0.00
0.01
3.37
0.26
-33.6
-33.9
-33.0

1.14

Shan381
P₂h₈
3344
92.37
1.12
0.15
0.02
0.02
0.02
0.01
0.01
1.06
5.20
-26.4
-27.4
-29.4

3.68

Su353
P₁s₁
3498
93.12
1.11
0.17
0.02
0.02
0.01
0.00
0.01
1.86
3.69
-24.1
-25.6
-28.7

5.39

Shan438
C₂b
3525
93.02
1.06
0.14
0.02
0.02
0.01
0.00
0.01
4.62
1.11
-30.3
-36.7
-36.3

1.95

Shan303
P₂h₈
3093
88.47
0.90
0.20
0.02
0.02
0.01
0.00
0.00
2.02
8.36
-27.7
-31.8
-29.3

3.01

Su222
P₁s₁
3795
93.01
0.95
0.17
0.01
0.02
0.01
0.00
0.01
1.42
4.41
-27.2
-28.6
-30.4

3.24

Shan380
P₂h₈
3306
90.58
0.94
0.13
0.01
0.01
0.01
0.00
0.01
1.13
7.18
-24.5
-28.3
-29.3

5.05

Shan437
P₂h₈
3453
80.74
0.76
0.20
0.01
0.01
0.00
0.00
0.00
2.25
16.02
-28.8
-31.8
-30.1

2.49

Shan135
P₂h₈
3378
95.80
0.99
0.13
0.01
0.03
0.01
0.00
0.00
1.15
1.88
-25.9
-29.0
-30.1

3.98

Shan292
P₁s₂
3363
86.13
0.77
0.22
0.01
0.01
0.00
0.00
0.00
0.99
11.86
-26.8
-31.2
-29.3

3.47

Su243
P₂h₈
4038
92.81
0.80
0.14
0.01
0.01
0.00
0.00
0.00
0.56
5.51
-26.2
-28.9
-30.6

3.82

Shan316
P₂h₈
3217
93.21
0.72
0.19
0.01
0.01
0.00
0.00
0.01
2.74
3.10
-27.9
-32.6
-29.6

2.91

Shan428
P₁s₁
3927
90.20
0.67
0.11
0.01
0.01
0.00
0.00
0.02
3.21
5.79
-28.1
-29.2
-29.3

2.82

Shan340
P₂h₈
3046
66.02
0.47
0.09
0.01
0.01
0.00
0.00
0.01
0.71
32.68
-26.9
-32.2
-29.7

3.39

Shan339
P₁s₂
3527
93.31
0.59
0.10
0.02
0.02
0.01
0.01
0.00
3.34
2.58
-29.1
-34.2

2.39

Su127
P₂h₈
3899
94.46
0.63
0.10
0.01
0.01
0.00
0.00
0.02
1.20
3.59
-29.0
-33.7
-34.1

2.42

Yi6
P₂h₈
2149
65.72
0.40
0.04
0.00
0.00
0.00
0.00
0.00
0.14
33.70
-32.3
-36.8

1.42

Shan341
P₁s₂
3188
93.77
0.44
0.09
0.01
0.02
0.01
0.01
0.00
3.09
2.55
-28.0
-35.7

2.84

Shan323
P₁s₁
3751
93.70
0.50
0.07
0.01
0.01
0.00
0.00
0.00
3.66
2.06

Shan441
P₂h₈
3266
95.13
0.47
0.05
0.00
0.00
0.00
0.00
0.01
2.66
1.68
-27.0
-31.5
-28.7

3.37

Shan429
P₁s₂
3146
93.52
0.46
0.04
0.00
0.00
0.00
0.00
0.00
2.72
3.25
-28.8
-34.2
-35.5

2.51

Shan383
P₂h₈
3160
89.97
0.41
0.05
0.00
0.00
0.00
0.00
0.02
4.26
5.31
-27.4
-32.0
-29.6

3.12

Shan383
P₁s₁
3214
92.71
0.41
0.05
0.00
0.00
0.00
0.00
0.00
3.15
3.67
-27.5
-32.2
-29.8

3.10

Shan441
P₁s₂
3389
92.78
0.33
0.03
0.00
0.00
0.00
0.00
0.00
3.67
3.16
-27.0
-31.5
-28.7

3.37

Shan429
P₁s₂
3232
93.42
0.36
0.02
0.00
0.00
0.00
0.00
0.00
3.34
2.86
-29.8
-35.7
-30.5

2.12

Yi8
P₁s₂
2164
91.82
0.23
0.02
0.00
0.00
0.00
0.00
0.00
0.42
7.51
-31.3
-31.4
-30.8

1.66

Shan371
P₂h₈
3921
93.33
0.22
0.02
0.00
0.00
0.00
0.00
0.00
4.61
1.82
-32.5
-34.3
-30.1

1.37

Shan340
C₂b
3186
77.56
0.17
0.00
0.00
0.00
0.00
0.00
0.00
1.21
21.05
-28.8
-32.0
-31.2

2.51

G69-9C3
P₂h₈

94.95
0.72
0.09
0
0.01
0
0
0
3.31
0.85
-27.6
-29.3
-30
—
-9.2
-157
-163
-178

3.04
This study

G69-9C4
P₂h₈

95.54
0.84
0.09
0.01
0.01
0
0
0
2.64
0.79
-27.2
-28.4
-29.6
—
-5.3
-158
-165
-167

3.24

Standard deviation
2.57
3.43
2.53

2.76
0.71
1.41
7.78

1.45

South Jingbian-lower Paleozoic
G69-9
O₁m₅

91.16
0.31
0.03
0

7.98
0.48
-32.1
-33.5
-29.4

-0.7
-157
-191
-156

1.46
This study

G71-13
O₁m₅

93.65
0.32
0.01
0
0
0
0
0
5.42
0.54
-32.5
-37.5
-32.2
—
-1.9
-159
-185
-94

1.36

JN57-9H1
O₁m₅

91.79
0.33
0.04
0
0
0
0
0
7.43
0.37
-31.8
—
—
—
—
-166

1.53

JN57-9H2
O₁m₅

91.64
0.32
0.04
0
0
0
0
0
7.61
0.36
-32
—
—
—
—
-166

1.48

JN57-9H3
O₁m₅

91.51
0.32
0.04
0
0
0
0
0
7.6
0.5
-32.9
-34.3
-27.5
—
10.9
-157
-178
-163

1.28

G19-15
O₁m₅

93.18
0.60
0.06
0.01
0.00

5.80
0.31
-29.1
-33.8
-30.6

-0.4
-169
-184
-186

2.38
Dai et al., [16]

G19-13
O₁m₅

93.32
0.70
0.07
0.01
0.01

5.55
0.31
-27.0
-32.7
-30.1
-27.7
-0.5
-167
-184
-189

3.35

Shan133
O₁m₅1,4
3370
93.22
0.49
0.05
0
0

5.88
0.3
-29.6
-35
-29.7
—
—
-168
-190
-183
—
—
2.19

Shan62
O₁m₅
3715
96.55
0.55
0.07
0.01
0.01

—
—
-32.7
-33.1
-30
—
—

4.10
727.00
1.32

Shan13
O₁m₅1-2,4
3540
94.2
0.56
0.07
0
0

4.82
0.27
-31
-32.5
-29.4
—
—
-167
-178
-181
—
—
1.74

Shan95
O₁m₅¹⁻²
3455
95.25
1.02
0.11
0.01
0.01

3
0.52
-27.4
-28.1
-27.1
-27.6
-2.8
-168
-176
-187
—
—
3.14

Shan52
O₁m₅¹
3450
93.08
0.5
0.07
0.01
0.01

6.18
0.12
-32.9
-33.8
-28.9
-23.9
—
-171
-181
-190
—
—
1.28

Shan30
O₁m₅¹⁻⁴
3594
95.23
0.43
0.05
0
0

2.81
1.44
-32.8
-33
-25
—
—

—
—
1.30

Shan89
O₁m₅¹⁻²
/
93.35
0.74
0.09
0.01
0.01

5.64
0.13
-32.5
-32.8
-28.8
-24.6
-2.3
-169
-173
-175
6.08
614.00
1.36

Yu70
O₁m₅
2762
97.81
1.07
0.40
0.07
0.10
0.01
0.01
0.01
0.44
/
-30.83
-33.6
-28.3

1.79
Database of EDRICOC

Shan339
O₁m₅
3606
93.51
0.72
0.13
0.02
0.02
0.01
0.01
0.01
3.43
2.12
-31.59
-37.3
-29.4

1.58

Shan430
O₁m₅
3959
55.54
0.27
0.05
0.00
0.01
0.00
0.00
0.01
31.54
12.58
-31.21
-32.7
-26.2

1.68

Shan323
O₁m₅
3895
91.14
0.40
0.06
0.01
0.01
0.01
0.00
0.00
5.93
2.43
-33.39
-35.9
-30.1

1.18

Shan438
O₁m₅
3486
94.92
0.41
0.03
0.00
0.00
0.00
0.00
0.00
3.99
0.63
-31.73
-37.8
-33.3

1.55

Shan265
O₁m₅
3450
96.09
0.38
0.06
0.00
0.01
0.00
0.00
0.00
4.26
0.25
-30.98
-37.3
-32.8

1.75

Shan430
O₁m₅
3994
79.63
0.29
0.04
0.00
0.00
0.00
0.00
0.01
6.25
13.78
-32.16
-33.8
-27.4

1.44

Shan434
O₁m₅
3558
92.74
0.35
0.03
0.00
0.00
0.00
0.00
0.01
4.07
2.77
-31.56
-35.8
-30.6

1.59

Shan441
O₁m₅
3486
85.03
0.21
0.04
0.02
0.03
0.02
0.01
0.00
14.65
0.00
-32.16
-36.8
-29.7

1.44

Shan373
O₁m₅
4003
92.36
0.30
0.03
0.00
0.00
0.00
0.00
0.00
5.93
1.37
-32.70
-33.6
-25.6

1.32

Shan322
O₁m₅
3965
85.01
0.25
0.03
0.01
0.01
0.00
0.00
0.00
2.07
12.63
-34.04
-37.9
-33.4

1.06

Su222
O₁m₅
4001
92.26
0.27
0.03
0.00
0.00
0.00
0.00
0.00
5.67
1.76
-32.68
-34.2
-30.0

1.32

Shan377
O₁m₅
3305
95.86
0.27
0.03
0.00
0.00
0.00
0.00
0.00
2.80
1.05
-32.92
-36.5

1.27

Su222
O₁m₅
3942
90.87
0.24
0.03
0.00
0.00
0.00
0.00
0.00
4.82
4.04
-31.80
-33.6
-29.6

1.53

Shan323
O₁m₅
3936
93.29
0.25
0.02
0.00
0.00
0.00
0.00
0.00
6.44
0.00
-34.41
-36.3
-31.3

1.00

Su127
O₁m₅
4072
84.28
0.19
0.01
0.00
0.00
0.00
0.00
0.00
7.41
8.11
-32.72
-35.7
-30.6

1.32

Su379
O₁m₅
3810
89.40
0.05
0.00
0.00
0.00
0.00
0.00
0.00
7.98
2.57
-36.48
-39.4

0.71

Yu70
O₁m₅
2762
97.81
1.07
0.40
0.07
0.10
0.01
0.01
0.01
0.44
/
-30.83
-33.6
-28.3

1.79

Shan339
O₁m₅
3606
93.51
0.72
0.13
0.02
0.02
0.01
0.01
0.01
3.43
2.12
-31.59
-37.3
-29.4

1.58

Shan430
O₁m₅
3959
55.54
0.27
0.05
0.00
0.01
0.00
0.00
0.01
31.54
12.58
-31.21
-32.7
-26.2

1.68

Shan323
O₁m₅
3895
91.14
0.40
0.06
0.01
0.01
0.01
0.00
0.00
5.93
2.43
-33.39
-35.9
-30.1

1.18

Shan438
O₁m₅
3486
94.92
0.41
0.03
0.00
0.00
0.00
0.00
0.00
3.99
0.63
-31.73
-37.8
-33.3

1.55

Shan265
O₁m₅
3450
96.09
0.38
0.06
0.00
0.01
0.00
0.00
0.00
4.26
0.25
-30.98
-37.3
-32.8

1.75

Shan430
O₁m₅
3994
79.63
0.29
0.04
0.00
0.00
0.00
0.00
0.01
6.25
13.78
-32.16
-33.8
-27.4

1.44

Shan434
O₁m₅
3558
92.74
0.35
0.03
0.00
0.00
0.00
0.00
0.01
4.07
2.77
-31.56
-35.8
-30.6

1.59

Shan441
O₁m₅
3486
85.03
0.21
0.04
0.02
0.03
0.02
0.01
0.00
14.65
0.00
-32.16
-36.8
-29.7

1.44

Shan373
O₁m₅
4003
92.36
0.30
0.03
0.00
0.00
0.00
0.00
0.00
5.93
1.37
-32.70
-33.6
-25.6

1.32

Shan322
O₁m₅
3965
85.01
0.25
0.03
0.01
0.01
0.00
0.00
0.00
2.07
12.63
-34.04
-37.9
-33.4

1.06

Su222
O₁m₅
4001
92.26
0.27
0.03
0.00
0.00
0.00
0.00
0.00
5.67
1.76
-32.68
-34.2
-30.0

1.32

Shan377
O₁m₅
3305
95.86
0.27
0.03
0.00
0.00
0.00
0.00
0.00
2.80
1.05
-32.92
-36.5

1.27

Su222
O₁m₅
3942
90.87
0.24
0.03
0.00
0.00
0.00
0.00
0.00
4.82
4.04
-31.80
-33.6
-29.6

1.53

Shan323
O₁m₅
3936
93.29
0.25
0.02
0.00
0.00
0.00
0.00
0.00
6.44
0.00
-34.41
-36.3
-31.3

1.00

Su127
O₁m₅
4072
84.28
0.19
0.01
0.00
0.00
0.00
0.00
0.00
7.41
8.11
-32.72
-35.7
-30.6

1.32

Su379
O₁m₅
3810
89.40
0.05
0.00
0.00
0.00
0.00
0.00
0.00
7.98
2.57
-36.48
-39.4

0.71

Standard deviation
1.72
2.21
2.19
1.98
4.76
4.85
5.89
29.14

0.48

Shenmu
Shuang10-13
P₂h
2990
91.55
4.66
0.95
0.22
0.19
0.10
0.05
0.23
1.62
0.41
-35.9
-23.5
-22.3
-22.2

-198
-157
-152

0.78
This study

Shuang10-3
P₁s₁
2878
91.33
4.95
1.03
0.19
0.20
0.09
0.04
0.19
1.56
0.38
-36.5
-24.8
-23.7
-23.0

-196
-161
-159

0.71

Shuang10-5
P₂h₈
2888
92.17
3.78
0.75
0.16
0.15
0.07
0.03
0.14
2.22
0.51
-36.1
-23.8
-22.8
-22.0

-195
-157
-156

0.76

Shuang10-9
P₂h₈
3105
91.62
4.18
0.85
0.19
0.17
0.08
0.04
0.13
2.28
0.44
-36.5
-23.5
-22.3
-21.7

-199
-157
-154

0.71

Shuang11-15C1
P₂h₈
3008
91.34
4.20
0.88
0.18
0.17
0.08
0.04
0.26
2.50
0.33
-36.4
-23.3
-22.3
-21.7

-200
-159
-156

0.72

Shuang11-15C3
P₂h₈
3206
91.69
3.81
0.76
0.16
0.15
0.08
0.04
0.25
2.74
0.29
-36
-23.2
-22.2
-21.5

-200
-158
-158

0.77

Shuang11-4C5
P₂h
2990
89.42
6.92
1.53
0.25
0.27
0.12
0.07
0.22
0.57
0.57
-36.8
-25.4
-24.2
-23.8

-198
-161
-155

0.68

Shuang19-12
P₂h₈
2996
92.17
3.09
0.60
0.14
0.11
0.06
0.03
0.16
3.36
0.26
-35.9
-23.2
-21.8
-21.1

-193
-156
-155

0.78

Shuang20
P₁t
2960
91.37
4.40
0.80
0.14
0.15
0.07
0.04
0.21
2.46
0.34
-36.2
-25.8
-24.2
-23.5

-193
-163
-160

0.75

Shuang21-11
P₂h₈
2983
91.61
4.10
0.75
0.14
0.13
0.06
0.03
0.14
2.66
0.36
-35.9
-24.1
-22.9
-21.0

-195
-159
-157

0.78

Shuang3
P₁s₂
2990
91.49
4.48
0.88
0.20
0.17
0.09
0.04
0.21
2.17
0.24
-36.7
-23.2
-22
-21.7

-199
-156
-152

0.69

Shuang5-19
P₂h₈
3146
92.12
4.00
0.79
0.16
0.15
0.07
0.04
0.16
2.12
0.36
-36.5
-23.9
-22.7
-22.2

-197
-158
-157

0.71

Shuang6-18
P₂h₈t
2853
91.77
4.00
0.83
0.17
0.17
0.08
0.04
0.16
2.46
0.29
-36.7
-23.7
-22.4
-21.8

-198
-157
-156

0.69

Shuang6-19
P₂h₈
2933
91.65
4.28
0.89
0.18
0.18
0.09
0.04
0.15
2.26
0.25
-37
-23.8
-22.4
-21.9

-201
-161
-159

0.65

Shuang7-11
P₂h₆
2872
93.09
4.09
0.86
0.17
0.18
0.09
0.05
0.25
0.53
0.63
-36.7
-24.7
-23.1
-22.5

-197
-161
-155

0.69

Shuang7-12
P₂h
3083
92.58
3.93
0.74
0.16
0.15
0.08
0.04
0.18
1.78
0.34
-36.1
-23.4
-22.2
-21.9

-199
-157
-155

0.76

Shuang8-12
P₂h
2927
94.43
2.63
0.41
0.09
0.10
0.06
0.03
0.10
1.97
0.17
-37.3
-24.7
-26.4
-25.6
-12.3
-185
-189
-178

0.62

Shuang8-17C3
P₂h
3026
92.08
4.18
0.86
0.19
0.18
0.09
0.05
0.26
1.82
0.26
-36.6
-25.6
-23.4
-22.4

-197
-163
-156

+
0.70

Shuang8-8
P₂h₈
2891
91.46
5.53
1.16
0.20
0.24
0.10
0.05
0.18
0.37
0.65
-37.1
-25.1
-24.3
-23.6

-198
-161
-158

0.64

Shuang9-11
P₂h₈
2891
91.44
4.42
0.88
0.21
0.18
0.09
0.04
0.24
2.22
0.26
-36
-23.1
-22
-21.7

-199
-156
-152

0.77

Shuang9-11H2
P₁t
5043
91.97
3.39
0.61
0.14
0.11
0.06
0.03
0.19
3.21
0.26
-35.6
-22.8
-22
-21.0

-198
-155
-157

0.82

Shuang9-12
P₂h₈
2899
92.26
3.57
0.68
0.15
0.13
0.07
0.03
0.16
2.63
0.31
-36.5
-23.2
-22.1
-21.1

-198
-157
-156

0.71

Shuang9-13H2
P₁s₂
4264
91.34
4.77
1.06
0.25
0.22
0.11
0.05
0.26
1.61
0.30
-36
-23.2
-22.1
-21.9

-198
-156
-154

0.77

Shuang8-17c1
P₁s+P₂h

90.92
4.94
1.16
0.24
0.26
0.13
0.07
0.37
0.52
1.37
-37.5
-22.8
-22.8
-22.3
-1.2
-192
-149
-146

0.60

Standard deviation
0.49
0.90
1.06
1.06
7.85
3.40
7.00
5.46

0.06

Sulige
Su53
P₁s, P₂h

86.05
8.36
2.17
0.37
0.44
0.00
0

1.13
0.72
-35.6
-25.3
-23.7
-23.9

-202
-165
-160

0.82
Dai et al., [16]

Su75
P₂h

92.47
3.92
0.66
0.11
0.11
0.00
0

1.30
1.10
-33.2
-23.8
-23.4
-22.4

-194
-163
-157

1.22

Su76
P₁s, P₂h

86.41
8.37
2.33
0.39
0.51
0.00
0

0.13
1.21
-35.1
-24.6
-24.4
-24.4

-203
-165
-161

0.89

Su53-78-46H
P₁s, P₂h

89.82
6.21
1.24
0.22
0.24
0.00
0

0.93
0.87
-33.9
-23.9
-23.0
-23.2

-198
-165
-156

1.09

Su75-64-5X
P₂h

89.45
6.36
1.26
0.22
0.24
0.00
0

0.13
0.93
-33.5
-24.0
-23.3
-22.8

-199
-167
-159

1.16

Su76-1-4
P₂h

90.38
6.03
1.18
0.21
0.22
0.00
0

0.82
0.71
-32.7
-23.6
-22.9
-23.0

-198
-168
-165

1.32

Zhao61
P₁s

88.98
6.83
1.53
0.31
0.37
0.00
0

0.55
0.85
-33.2
-23.5
-23.3
-23.2

-194
-159
-154

1.22

Mi37-13
P₁s

94.19
3.77
0.53
0.11
0.09
0.00
0

0.71
0.39
-33.0
-23.2
-22.4
-21.1

-182
-156
-145

1.26

Zhou35-28
P₁s₂

94.81
2.97
0.44
0.06
0.07
0.00
0

1.2
0.37
-32.5
-25.7
-23.6
-23.3

-181
-164
-157

1.36

Yu69
P₁s₂

94.93
2.85
0.4
0.06
0.06
0.00
0

1.27
0.35
-32.8
-26.3
-24.1
-21.7

-179
-162
-151

1.30

Mi38-13A
P₁s₂

94.53
3.04
0.45
0.08
0.07
0.00
0

1.34
0.37
-33.1
-25.0
-22.8
-22.0

-181
-155
-141

1.24

Zhou21-24
P₁s₂

94.22
3.12
0.48
0.08
0.07
0.00
0

1.58
0.32
-32.7
-25.1
-23.2
-22.2

-183
-163
-155

1.32

Yu45
P₁s₂

94.17
3.12
0.48
0.08
0.08
0.00
0

1.58
0.36
-33.2
-25.2
-23.1
-22.5

-183
-164
-155

1.22

Mi40-13
P₁s₂

94.45
2.99
0.45
0.07
0.07
0.00
0

1.52
0.32
-32.8
-25.3
-23.3
-22.4

-183
-167
-155

1.30

Zhou25-38
P₁s₂

94.67
2.87
0.42
0.06
0.07
0.00
0

1.4
0.38
-32.6
-25.7
-23.3
-22.9

-185
-165
-154

1.34

Su11-18-36
P₂h₈

90.16
5.5
1.15
0.21
0.21
0.00
0

1.47
0.94
-33.0
-23.3
-22.3
-22.9

-196
-165
-167

1.26

Su75-70-5x

90.7
5.19
1.02
0.18
0.18
0.00
0

1.48
0.93
-32.8
-23.6
-23.1
-22.7

-196
-166
-170

1.30

Su55

88.96
7.07
1.47
0.22
0.27
0.00
0

0.68
0.88
-35.1
-24.6
-24.1
-24.8

-202
-164
-173

0.89

Su76-15-18

85.63
8.18
2.56
0.47
0.64
0.00
0

0.41
1.29
-35.7
-25.3
-24.8
-24.8

-205
-164
-167

0.81

Sunan9-61

88.28
5.49
1.16
0.74
0.23
0.00
0

1.47
1.77
-32.3
-20.4
-17.7
-18.5

-190
-161
-155

1.41

Standard deviation
1.06
1.31
1.41
1.38

8.75
3.41
7.86

0.19

South Sulige
Su21
P₁s, P₂h

92.39
4.48
0.83
0.13
0.14
0.00
0

0.99
0.68
-33.4
-23.4
-23.8
-22.7

-194
-167
-163

1.18
Dai et al., [16]

Su95
P₂h

92.24
3.95
0.66
0.11
0.11
0.00
0

1.64
1.00
-32.5
-23.9
-24.0
-22.7

-193
-167
-160

1.36

Su139
P₁s, P₂h

93.16
3.05
0.51
0.07
0.07
0.00
0

1.31
1.45
-30.4
-24.2
-26.8
-23.7

-192
-178
-180

1.92

Su336
P₁s, P₂h

90.20
1.40
0.15
0.02
0.01
0.00
0

0.00
8.06
-28.7
-22.6
-25.1

-189
-169
-168

2.54

Su14-0-31
P₁s, P₂h

93.00
4.05
0.65
0.11
0.10
0.00
0

1.20
0.59
-32.0
-23.8
-24.7
-22.0

-196
-168
-172

1.48

Su48-2-86
P₁s

92.85
4.00
0.63
0.11
0.10
0.00
0

1.44
0.57
-31.7
-23.2
-24.3
-22.3

-190
-172
-170

1.55

Su48-14-76
P₁s, P₂h

92.73
3.48
0.65
0.13
0.11
0.00
0

1.47
1.14
-33.5
-22.8
-24.2
-22.2

-192
-172
-171

1.16

Su48-15-68
P₂h₈

92.79
3.28
0.61
0.11
0.12
0.00
0

1.70
1.07
-29.8
-23.4
-25.0
-22.6

-195
-170
-172

2.12

Su77-2-5
P₂h

89.90
5.53
1.24
0.24
0.27
0.00
0

1.46
0.70
-30.8
-22.7
-23.3
-22.9

-194
-168
-164

1.80

Su77-6-8
P₂h₈

89.90
5.80
1.24
0.22
0.24
0.00
0

0.60
0.79
-33.6
-23.9
-24.1
-23.5

-201
-165
-165

1.14

Su120-52-82
P₁s, P₂h

91.64
3.69
0.64
0.11
0.10
0.00
0

2.58
0.93
-31.1
-23.3
-25.6
-23.6

-192
-176
-179

1.71

Yu85
P₁s₂

93.83
2.83
0.39
0.06
0.06
0.00
0

1.22
1.48

Yu30
P₁s₂

94.1
3.14
0.48
0.07
0.08
0.00
0

1.62
0.38
-33.1
-23.0
-23.4
-21.7

-183
-161
-154

1.24

Su77-4-6
P₂h₈

90.95
5.1
1.14
0.21
0.24
0.00
0

0.21
0.93
-33.5
-23.7
-24.0
-23.3

-198
-161
-166

1.16

Su14-8-45
P₂h₈

92.97
3.93
0.74
0.13
0.13
0.00
0

1.1
0.77
-33.2
-24.3
-24.3
-23.0

-188
-170
-165

1.22

Su14-18-36
P₁s₁

93.08
3.92
0.73
0.13
0.14
0.00
0

1.13
0.66
-33.4
-24.0
-24.3
-22.8

-190
-170
-172

1.18

Su14-11-09
P₂h₈

92.52
3.78
0.75
0.16
0.17
0.00
0

1.18
1.1
-31.6
-24.0
-24.2
-22.6

-188
-167
-173

1.58

Su48-13-79C3

92.82
3.33
0.61
0.12
0.12
0.00
0

1.55
1.18
-30.2
-22.9
-23.4
-21.9

-187
-171
-168

1.99

Su120-42-84
P₂h₈

91.15
4.19
0.79
0.15
0.14
0.00
0

2.25
1.04
-31.9
-23.6
-24.7
-22.7

-190
-165
-173

1.50

Sunan3-45

79.77
4.53
0.96
0.23
0.17
0.00
0

0.04
13.93
-31.3
-22.1
-22.8
-20.7

-188
-166
-169

1.66

Standard deviation
1.46
0.61
0.91
0.74

4.23
4.31
6.20

0.39

Daniudi
D66-3
P₁h³

97.49
1.90
0.30
0.06
0.06

0.09
0.00
-35.8
-26.1
-24.8
-24.2

-181

0.80
Dai et al., [16]

DK19
P₁h³

96.46
1.85
0.27

0.08

0.67
0.59
-34.4
-25.7
-24.3
-22.0

1.00

D16
P₁x³

94.37
2.52
0.26
0.06
0.09

0.37
1.96
-34.7
-26.7
-26.7
-24.4

0.95

DK13
P₁x³

94.49
1.71
0.31

0.07

0.28
2.55
-34.7
-25.6
-24.2
-22.4

0.95

DK14
P₁x³

93.94
1.79
0.27

0.08

0.26
2.98
-35.0
-25.6
-24.2
-22.4

0.91

DK30
P₁x¹

85.73
2.83
0.50
0.08
0.10

0.10
1.83
-34.3
-25.9
-25.3
-25.4

-181

1.02

DP 14
P₁x¹

87.91
8.07
2.32
0.34
0.43

0.42
0.00
-37.4
-25.7
-25.3
-24.6

-201

0.61

D66-28
P₁x¹

88.87
7.94
1.86
0.28
0.30

0.42
0.00
-37.7
-25.2
-24.2
-23.4

-197

0.58

D1-1-154
P₁x¹

90.49
6.23
1.59
0.25
0.31

0.64
0.10
-37.1
-25.9
-26.5
-24.5

-197

0.64

DK29
P₁x¹

88.67
6.88
2.24
0.49
0.46

0.55
0.15
-34.7
-25.4
-23.8
-22.8

-189

0.95

D1-4-110
P₁s¹

91.51
4.86
1.13
0.18
0.19

1.89
0.00
-34.1
-24.8
-23.2
-22.3

-186

1.05

D10
P₁s²

81.15
4.01
0.85
0.27
0.32

1.12
9.71
-36.0
-24.0
-23.5
-23.3

0.77

D1-4-107
P₁s²

89.30
6.39
2.31
0.44
0.42

0.65
0.00
-33.3
-24.8
-23.0
-22.6

-179

1.20

D24
P₁s²

87.95
6.92
1.83
0.45
0.63

0.33
1.49
-36.7
-25.4
-25.0
-24.5

0.69

D12-1
P₁s²

89.91
6.85
1.78
0.30
0.24

0.65
0.00
-35.8
-24.5
-23.5
-22.7

-193

0.80

D47--47
C₃t²

90.89
5.80
1.48
0.25
0.28

0.73
0.00
-37.5
-26.1
-24.6
-24.2

-204

0.60

D35
C₃t²

90.31
5.70
1.26
0.19
0.20

1.98
0.00
-37.9
-25.2
-23.4
-21.9

-197

0.56

D35-22
C₃t²

90.41
5.60
1.22
0.18
0.19

2.11
0.00
-37.7
-25.0
-23.1
-22.6

-195

0.58

ES1

93.96
3.62
0.87
0.14
0.23

0.20
0.81
-33.5
-25.1
-24.6
-23.6
-11.6
-189
-168
-170

1.16
This study

ES4

93.71
3.57
0.86
0.15
0.22

0.19
1.08
-33.3
-24.5
-23.2
-22.9
-6.8
-186
-166
-172

1.20

Jin11

93.69
3.57
0.87
0.14
0.20

1.34
-33.8
-25
-24.5
-23.6

-187
-171
-179

1.10

ESP2

93.74
3.64
0.85
0.12
0.17

1.32
-33.2
-25.3
-24.9
-24.4

-190
-173
-183

1.22

J11P4H

93.87
3.71
0.92
0.13
0.19

0.03
1.04
-33.1
-25.1
-24.6
-23.6

-189
-170
-158

1.24

Jin26

93.79
3.67
0.90
0.20
0.01

0.09
1.13
-33.7
-25.6
-25.3
-23.8
-22.8
-190
-175
-162

1.12

DPS4

85.20
8.16
1.92
0.27
0.32

2.96
0.90
-36.7
-23.8
-23.1
-22.7
-7.2
-219
-161
-138

0.69

DK13

92.72
3.29
0.76
0.11
0.12

2.63
0.32
-35.2
-25.3
-24.6
-23.1
-4.7
-191
-170
-151

0.88

DK3

95.20
2.87
0.55
0.08
0.08

0.52
0.78
-35.6
-25.1
-22.7
-23.0
-12.4
-193
-166
-145

0.82

DK6

89.95
5.96
1.54
0.25
0.29

1.28
0.52
-36.8
-24.7
-24.1
-23.5
-9.2
-209
-169
-158

0.68

The Ro-Dai was calculated by the formula [32].

3.2. Geochemical and C–H Isotopic Analyses

Analyses of gas composition and C and H isotopic ratios were conducted at the Institute of Petroleum Exploration and Development (RIPED), Langfang, China. A Hewlett Packard HP7890A capillary gas chromatograph was used to determine hydrocarbon compositions with a PLOT Al₂O₃ column ().

Carbon isotopic compositions were determined by gas chromatography–combustion–isotope ratio mass spectrometry (GC–C–IRMS; Thermo Delta V Advantage). Each sample was analysed three times, yielding an analytical precision of ±0.3‰. Results are reported relative to Vienna Pee Dee Belemnite (VPDB). Hydrogen isotopes were analysed by GC–thermal conversion–IRMS (GC–TC–IRMS; Finnigan MAT 253), using a HP-PLOTQ GC column (). Analytical precision was ±3‰. Hydrogen isotopic compositions are reported relative to Vienna Standard Mean Ocean Water (VSMOW). The detailed analytical procedure is described in Feng et al. [9].

3.3. He Isotopic Composition

Seventeen gas samples were analysed at RIPED for He content and ³He/⁴He ratios using a sampling system attached to an IRMS. The natural gas cylinder was connected to the injection port of the instrument through a pressure relief valve, with the pipeline evacuated to ultrahigh vacuum. Sampling was controlled by an injection valve, and the gas was purified of hydrocarbons, N₂, O₂, CO₂, H₂S, and H₂, by a zirconium base furnace. He, Ne, Ar, Kr, and Xe were separated cryogenically and analysed by MS. Based on internationally recognized contents of inert gases in air, the relative standard deviations of He content and ³He/⁴He ratio were ±3.4% and ±4.5%, respectively.

4. Results

4.1. Natural Gas Composition

Southern Ordos Basin gases from the Yan’an and South Jingbian gas fields are of high dryness, with dryness coefficients (Table 1), and their cumulative contents of heavier hydrocarbon components (C2–5) are low (Figure 2). Ethane content is 0.05%–2.68% (mean 0.51%), and propane content is <0.60% for Paleozoic gases from both fields. Concentrations of nonhydrocarbon gases, including CO₂ and N₂, vary widely. CO₂ and N₂ concentrations in the Yan’an gas field are relatively low at 0.56%–31.54% (mean 4.38%) and 0.00%–32.68% (mean 5.23%), respectively, whereas the nonhydrocarbon content of South Jingbian gas is high, with its methane content being relatively low. The CO₂ content of lower Paleozoic gas from the Jingbian gas field is 0.44%–31.54% (mean 6.61%), whereas the N₂ content of upper Paleozoic gas is 0.26%–33.7% (mean 7.07%). The high CO₂ content of lower Paleozoic South Jingbian gas is probably due to its carbonate reservoir. Upper Paleozoic gases are contained in Carboniferous-Permian reservoirs, whereas lower Paleozoic gases are from Ordovician carbonate reservoirs, and it is likely that the high CO₂ content of the latter arose from the cracking of Ordovician carbonates. High temperatures are necessary for the thermal cracking of carbonate, whereas the existence of acidic water in the Ordovician reservoir [16] makes the generation of CO₂ possible at any temperature.

Figure 2: Relative proportions and dryness coefficients of gas components of natural gas in gas fields of the Ordos Basin.

Gases from the central and northern Ordos Basin (Sulige, Shenmu, and Daniudi fields) have compositions different to those of gases from the southern basin (South Jingbian and Yan’an fields; Figure 2). Gases from the central and northern fields are relatively wet, with (mean 0.941); their heavy hydrocarbon contents are higher (ethane mean 3.04%); and nonhydrocarbon contents lower (N₂ and CO₂ means 1.86% and 3.07%, respectively). These characteristics are consistent with the Ro distribution of source rocks in the Ordos Basin (Figure 1). Differences in hydrocarbon content indicate that gases from the southern Ordos Basin have been exposed to higher palaeogeothermal stress and were generated in a later evolutionary stage, whereas their nonhydrocarbon content could be explained by some secondary mechanism occurring in a late maturity stage.

4.2. Carbon Isotopic Compositions of Alkanes and CO₂

Gas δ¹³C-CH₄ values in the upper Paleozoic reservoir of the South Jingbian and Yan’an gas fields range from −21.02‰ to −33.62‰ (SD 2.57‰) and −27.5‰ to −30.8‰ (SD 0.85‰), respectively, with heavier isotopic compositions than those of upper Paleozoic coal-derived gases from other Ordos Basin gas fields such as the Sulige, Shenmu, and Daniudi fields (Table 1; Figure 3). In contrast, most upper Paleozoic gases from the southern Ordos Basin have unusually light δ¹³C-C₂H₆ values, with those of South Jingbian gas averaging −31.5‰ and Yan’an gas averaging −34.8‰, which are lighter than typical coal-derived gases (≥29‰). Equivalent vitrinite reflectance values of the sources of gases of the upper Paleozoic South Jingbian and Yan’an are in the range 1.14–5.39%Ro (SD 1.45%Ro) and 1.80–3.09%Ro (SD 0.34%Ro), respectively, with the reflectance values of most of these gases being in the range 2.0–3.5%Ro, based on the relationship proposed by Dai et al. [32] for gases generated from type III kerogen-coal (; 34.39%Ro), consistent with measured vitrinite reflectance of coal source rocks in the upper Paleozoic strata (Figure 1).

(a)

(b)

Figure 3: (a) Plot of δ¹³C-C₂H₆ versus δ¹³C-CH₄; (b) relationship between δ¹³C-CO₂ and CO₂ content of gases in different gas fields of the Ordos Basin (modified from [33]). Note: symbols for samples from different areas of the Ordos Basin are used in all subsequent figures.

Compared with upper Paleozoic South Jingbian gases, lower Paleozoic gases contain alkanes more depleted in ¹³C, with δ¹³C-CH₄ values of −27.00‰ to −36.48‰ (mean −32.05‰, SD 1.72‰) and δ¹³C-C₂H₆ values of −28.10‰ to −39.42‰ (mean −35.06‰, SD 2.21‰). The equivalent vitrinite reflectance of lower Paleozoic gas sources, calculated as proposed by Dai et al. [32] for coal-derived gas, is in the range 0.71–3.35%Ro (mean 1.50%Ro, SD 0.48%Ro). Calculated Ro values for lower Paleozoic source rocks of South Jingbian gas are lower than those of upper Paleozoic source rocks in the same region (Figure 1), indicating two possibilities: that the gas is not coal-derived and the calculation method is therefore inapplicable or that the gases were generated from other coal source rocks of lower maturity than the upper Paleozoic source rocks. Jurassic coaly source rock has lower maturity than C-P coaly source rock, although it produces mainly oil and there is no pathway for any gas generated to migrate to the lower Paleozoic reservoir, precluding the second possibility. Type I and type II kerogen occur in lower Paleozoic Ordovician source rocks [11, 34], so oil-type gases generated there could be mixed with lower Paleozoic gases, and the Ro–δ¹³C-CH₄ relationship is not applicable to mixed gases. It appears, therefore, that geochemical differences between lower and upper Paleozoic South Jingbian gases indicate that they are produced from different sources.

Thrasher and Fleet [35] reported that the δ¹³C values for large gas accumulations (over 15 vol.% of CO₂) are in the range −10‰ to 0‰, suggesting an inorganic origin. In contrast, gas accumulations that have less than 15 vol.% of CO₂ and low δ¹³C-CO₂ values (<10‰) are of organic origin. Dai et al. [33] reviewed the distribution of δ¹³C-CO₂ values in Chinese basins and proposed a model of CO₂ production in which gases of organic origin contain <15% CO₂ with δ¹³C-CO₂ values below −10‰, whereas gases of inorganic origin contain >60% CO₂ with δ¹³C-CO₂ values of −8‰ to −3‰. The δ¹³C-CO₂ values of 23 Ordos Basin gas samples range from −12.9‰ to 10.9‰, with 20 having values above −8% (Figure 3(b), Table 1), indicating an inorganic origin. Considering that a unit of thick limestone occurs in the Benxi (C₂b) and Taiyuan (P₁t) formations in the Ordos Basin, it is likely that CO₂ was produced by the cracking of inorganic carbonates. The C-P coaly source rock generated organic acid while approaching and reaching maturation [16], with the palaeotemperature reaching 160–200°C. These factors created a favourable environment for the generation of CO₂.

4.3. Hydrogen Isotopic Composition

The hydrogen isotopic composition of methane (δD-CH₄) of natural gas in the southern Ordos Basin is heavy compared with that of the other gas fields (Figure 4(a)). Yan’an gas δD-CH₄ values range from −163.0‰ to −170.0‰ (mean −166.6‰, SD 1.98‰). Lower Paleozoic South Jingbian gases have similar hydrogen isotopic compositions, with δD-CH₄ values of −157‰ to −171‰ (mean −165.33‰, SD 4.85‰). The heavy δD-CH₄ values of Yan’an and South Jingbian gases are consistent with their high maturities. The hydrogen isotopic compositions of ethane (δD-C₂H₆) of southern Ordos Basin gases are light compared with those of gases from all other Ordos Basin fields (Table 1). The δD-C₂H₆ values of Yan’an gases range from −167.0‰ to −197.0‰ (mean −184.3‰, SD 6.91‰), whereas those of South Jingbian gases range from −173.0‰ to −191.0‰ (mean −182.0‰, SD 5.89‰). Gases of the southern Ordos Basin thus display hydrogen isotopic reversal (δD-CH₄>δD-C₂H₆) (Figure 4(b)).

(a)

(b)

(c)

Figure 4: (a) Plot of δD-CH₄ versus δ¹³C-CH₄; (b) hydrogen isotopic series of alkanes in lower and upper Paleozoic gases of South Jingbian and Yan’an gas fields (all gases show hydrogen isotopic reversal with δD-CH₄>δD-C₂H₆); (c) hydrogen isotopic series of alkane gases in Sulige, Daniudi, and Shenmu gas fields (with positive hydrogen isotopic series).

4.4. Carbon Isotopic Reversal in Gases from the Southern Ordos Basin

Carbon isotopic reversal is a distinctive feature of Paleozoic natural gas of the southern Ordos Basin (Figure 5). The lower Paleozoic South Jingbian gases all display partial reversal (δ¹³C-CH₄>δ¹³C-C₂H₆<δ¹³C-C₃H₈), whereas upper Paleozoic gases display partial (δ¹³C-CH₄>δ¹³C-C₂H₆<δ¹³C-C₃H₈) or complete (δ¹³C-CH₄>δ¹³C-C₂H₆>δ¹³C-C₃H₈) reversals, and Yan’an gases display complete reversals. For all Ordos Basin upper Paleozoic gas fields, there is a strong relationship between maturity (indicated by Ro values) and carbon isotopic series (Figures 1(a) and 5; Table 2).

(a)

(b)

(c)

(d)

Figure 5: Carbon isotopic series of alkane gases in (a) Yan’an and upper Paleozoic South Jingbian gas fields; (b) lower Paleozoic South Jingbian field; (c) southern Sulige field; and (d) Daniudi, northern Sulige, and Shenmu gas fields.

Table 2: Relationship between maturity and carbon isotopic series in the Ordos Basin.

Gas field
Source rock maturity
Carbon isotopic series
Detailed carbon isotopic series

Yan’an/South Jingbian

Complete reversal
δ¹³C-CH₄>δ¹³C-C₂H₆>δ¹³C-C₃H₈

South Jingbian

Partial reversal
δ¹³C-CH₄>δ¹³C-C₂H₆<δ¹³C-C₃H₈

South Sulige

Partial reversal
δ¹³C-CH₄<δ¹³C-C₂H₆>δ¹³C-C₃H₈

North Sulige, Shenmu, and Daniudi

Positive series
δ¹³C-CH₄<δ¹³C-C₂H₆<δ¹³C-C₃H₈

On the basis of the new data and the results of previous studies, some preliminary conclusions can be drawn as follows. A large portion of the gases with complete carbon isotopic reversal appears in regions with , but not all gases from such regions display complete reversal (some have partial reversals); in regions with , almost all gases display partial reversals, some with δ¹³C-CH₄>δ¹³C-C₂H₆ and others with δ¹³C-C₂H₆>δ¹³C-C₃H₈; and most gases from regions with display positive carbon isotopic series. The correlation between carbon isotopic series in Paleozoic Ordos Basin gases and gas maturity indicates that isotopic reversal may be caused by elevated temperatures.

4.5. Carbon Isotopic Evolution in Gases of Different Maturity

As observed for shale gases of varying maturity, carbon isotopic compositions of methane and ethane indicate different evolutionary trends, with δ¹³C-CH₄ values increasing with decreasing wetness and δ¹³C-C₂H₆ values first increasing and then decreasing with decreasing wetness (Figure 6). The evolution of mainly coal-derived natural gases in the Paleozoic strata of the Ordos Basin can be divided into two zones in terms of δ¹³C-C₂H₆ versus wetness trends: a prerollover zone with wetness of >2.0% and a rollover zone with wetness of <2.0% (Figure 6(a)). Paleozoic gases from the South Jingbian and Yan’an fields are distributed mainly in the rollover zone, where δ¹³C-C₂H₆ values become more negative with decreasing wetness. Upper Paleozoic gases from the Sulige, Shenmu, and Daniudi fields are distributed in the prerollover zone, where δ¹³C values of ethane become heavier with decreasing wetness (Figure 6(a)). δ¹³C-C₂H₆ rollover occurs with wetness of <2.0% (this study), whereas for shale gases it occurs with wetness of <5.0% [15]. A possible cause of this difference is that at the same maturity level, shale gases that originated from types I and II kerogen contain more wet gases than do coal-derived gases that originated from type III kerogen, owing to the different structures of parent sources. In the case of shale gas, the retained petroleum could closely contact the kerogen and/or source rock mineralogy, so the cracking rate of the crude oil is much earlier than the conventional gas formation [36]. The δ¹³C-C₂H₆ content of Ordos gases increases with decreasing wetness (Figure 6(b)).

(a)

(b)

Figure 6: (a) δ¹³C-C₂H₆ versus wetness (modified from [15]); (b) δ¹³C-CH₄ versus wetness (modified from [15]). (Data sources for shale gas in (a) and (b) are as follows: [13, 24–27, 29, 30]; from [14–16, 28, 31]).

5. Discussion

5.1. Source of Natural Gas

Numerous models have been proposed to identify the origin of hydrocarbon gases. One of the most widely used models is the Bernard diagram, which combines (C₁/C₂+C₃) and δ¹³C-CH₄ values. Whiticar [37] modified the diagram after Bernard et al. [38] and Faber and Stahl [39] to identify primary gases and gases altered by secondary effects. Based on previous work, Dai et al. [40] developed a similar model by plotting (C₁/C₂+C₃) versus δ¹³C-CH₄ values for natural gases with different origins from various basins in China, with the model being especially applicable for gas identification in China. A modified Dai plot (Figure 7) of the data in Table 1 reveals that most gases of the Sulige, Shenmu, and Daniudi fields are derived from type III kerogen coaly source rocks, supporting results of previous studies [16]. However, the source of Paleozoic gases in the southern Ordos Basin is more complicated. As shown in Figure 7, data for most upper Paleozoic South Jingbian and Yan’an gases plot in the field of “coal-type gas,” whereas data for lower Paleozoic gases of South Jingbian plot in either the field of “cracking gas from oil and type II kerogen” or the field of “coal-type gas.” This pattern indicates that the upper Paleozoic gas of the southern Ordos Basin is mainly coal-type gas derived from coaly source rocks, whereas the lower Paleozoic gas of the southern Ordos Basin is a mixture of coal-type gas and oil-type gas.

Figure 7: Plot of C₁/C2–3 ratio versus δ¹³C-CH₄ (modified after [38–40]).

5.2. Origin of Carbon Isotopic Reversals

5.2.1. Possible Causes for the Carbon Isotopic Reversals

There are four possible causes of carbon isotopic reversal as follows.

(1) Inorganic Origin. Inorganic gases are formed by Fischer-Tropsch synthesis when mantle degassing releases CO₂ or CO, forming hydrocarbons by reduction with reversed carbon isotopic compositions [41]. Such gases are typically characterized by δ¹³C-CH₄>δ¹³C-C₂H₆>δ¹³C-C₃H₈>δ¹³C-C₄H₁₀, with δ¹³C-CH₄> − 25‰ [42]. Of the 107 Paleozoic gas samples from the southern Ordos Basin, 15 have complete carbon isotope reversal (δ¹³C-CH₄>δ¹³C-C₂H₆>δ¹³C-C₃H₈), whereas others have partial reversal with δ¹³C-CH₄>δ¹³C-C₂H₆<δ¹³C-C₃H₈, or there are no δ¹³C-C₃H₈ data (Table 1). The δ¹³C-CH₄ value is less than −25‰ for all except three samples. As the ³He/⁴He value of mantle helium is higher than that of crust-sourced helium, the ratio , a comparison of the ³He/⁴He ratio of sample () against the atmospheric ratio (), can be used to distinguish whether mantle-derived inorganic gas was injected into the natural gas-producing system [40]. Mantle-sourced helium in all cases has ratios of >0.1 [42]. values for gases in the southern Ordos are all <0.1. It is therefore concluded that the helium in the Paleozoic gases of the southern Ordos Basin is not mantle-derived, suggesting an “organic origin” of the associated hydrocarbon gases.

(2) Mixing. Some studies have suggested that the isotopic reversal observed in the southern Ordos Basin was caused by the mixing of coal-derived and oil-type gases from source rock containing type I-II kerogen [3], (Yang et al., 2012), [4, 7]. This cause can easily be excluded. To generate gas mixtures with the light carbon isotopic composition of ethane, the oil-type gas end-member should have δ¹³C-C₂H₆ lighter than the mixed gases. δ¹³C-C₂H₆ values of most samples are less than −32‰, with the lightest being −39.42‰ (Table 1); however, the lightest δ¹³C-C₂H₆ values of oil-type gases found in Paleozoic reservoirs range from −30.0‰ to −32.0‰ [10, 43]. Even with all of the ethane in the mixture being oil-type gas, it would not account for the carbon isotopic characteristics observed here.

(3) Wet-Gas Cracking. This model involves the mixing of primary gas generated directly from kerogen cracking and secondary gas generated from intermediate kerogen products. The wet-gas cracking model produces a positive relationship between ln(C₂/C₃) and δ¹³C-C₂H₆–δ¹³C-C₃H₈ with increasing thermal stress [44] during the secondary cracking process and an initial increase followed by a rapid decrease in iso-butane/n-butane (iC₄/nC₄) ratios with respect to the decrease in wetness (due to the lower stability of iC₄ than nC₄ at high maturity; [15]). The relationship between ln(C₂/C₃) and δ¹³C-C₂H₆–δ¹³C-C₃H₈ in the Ordos data (Figure 8(a)) indicates that secondary cracking of wet gas does not occur in gases of the southern Ordos Basin. If cracking was the main cause of isotopic reversal, the trend of ln(C₂/C₃) would be expected to first decrease and then increase with increasing δ¹³C-C₂H₆–δ¹³C-C₃H₈, as shown for Barnett and Fayetteville shale gases [44]. This trend is not evident here (Figure 8(a)), indicating that wet-gas cracking cannot be the main cause of carbon isotopic reversal in the southern Ordos Basin. There is no obvious rollover pattern of iC₄/nC₄ versus wetness in Ordos Basin data (Figure 8(b)), suggesting that cracking of wet gas may have occurred.

(a)

(b)

Figure 8: (a) Relationship between δ¹³C-C₂H₆–δ¹³C-C₃H₈ and gas wetness (the trending line for primary cracking and secondary cracking was modified from [44]); (b) relationship between iC₄/nC₄ and gas wetness (modified from [44]).

(4) Abiotic Polymerization. Under conditions of ultrahigh temperatures and pressures, similar to those at which inorganic gases are generated, polymerization processes could alter carbon isotope distributions to be similar to those of inorganic gases [45]. If abiotic polymerization occurred, carbon isotopic separation of 1000ln(σC2–C1) would be expected to decrease as thermal stress increased. As more wet gases were polymerized with increasing thermal stress, wetness would be expected to increase. Taking 1000ln(σC2–C1) as an indicator of thermal stress, the dryness indicator C₁/(C₂+C₃) should decrease with decreasing 1000ln(σC2–C1) if the gases were generated by polymerization. Zeng et al. [45] suggested that gases from the Songliao field with isotopic reversals were generated by abiotic polymerization, and the trend for their data is plotted in Figure 9 for comparison. As shown in Figure 9, there is an initial increase and then decrease of C₁/(C₂+C₃) ratios with increasing 1000ln(σC2–C1) values for the Songliao gases. However, for data from the present study, C₁/(C₂+C₃) ratios of Ordos Basin gases continue to increase with increasing 1000ln(σC2–C1) (Figure 9). If the gas had been generated by polymerization, C₁/(C₂+C₃) ratios should decrease in the high-maturity stage. Abiotic polymerization thus cannot be the main cause of carbon isotopic reversal in gases of the southern Ordos Basin.

Figure 9: Plot of isotopic separation 1000ln(σC2–C1) versus C₁/(C₂+C₃), where (modified after Zeng et al., 2008, the dashed line indicated the expected trend for gas generated by abiotic polymerization).

Other models have been proposed to explain carbon isotopic reversal in overmature shale gases, including mass transport [46], water-kerogen redox reactions [47, 48], and other reactions involving transition metals [13, 49]. However, as southern Ordos Basin gases are stored in organic-poor sandstone and carbonates, fractionation through gas movement is unlikely to be the main cause of isotopic reversal. Nor could the other reactions be the cause because the conditions required for these reactions do not occur in the Ordos Basin.

5.2.2. Closed-System Aromatization-Polycondensation Model

It is clear that geological conditions should be taken into account in explaining carbon isotopic reversal of southern Ordos Basin gases. Dieckmann et al. [50] conducted closed-system pyrolysis experiments with a type III kerogen Taglu Sequence sample, demonstrating that large amounts of gas are generated at extreme levels of thermal stress, with δ¹³C values of methane falling to values 10‰ lighter than those of open systems. They suggested that neoformed materials produced during low-maturity stages become active at high maturities () and recombine with residual kerogen or coal to form gases through aromatization-polycondensation reactions. Furthermore, Erdmann and Horsfield [51] have shown that closed-system pyrolysis of type II/III kerogen of the heater formation results in recombination reactions, which act as sources of gas beyond the normal thermal conditions of oil-to-gas cracking, with aromatic compounds, phenols, and short n-alkyl chains being the main products formed from type III kerogen. Aromatization-polycondensation reactions are well known as major processes taking place during coal maturation [52–54]. In the mechanism proposed by Dieckmann et al. [50] and Erdmann and Horsfield [51], some neoformed macromolecules with relatively light carbon isotopic compositions were generated in low-maturity stages and preserved in the closed system. In overmature stages, these neoformed macromolecules react with residual kerogen or coal to form short-chain alkanes depleted in ¹³C. Such reactions could generate alkanes with light carbon isotopic compositions in the overmature stage.

Mahlstedt and Horsfield [36] showed that the results of Dieckmann et al. [50] and Erdmann and Horsfield [51] were applicable to natural maturation and also discovered that the ability of source rocks to generate late gas (the gas generated through aromatization-polycondensation at high maturity) is related to the initial organic-matter structure, depositional environment, and precursor biota. The heterogeneous aromatic and/or phenolic type II/III and type III coals from fluvial-deltaic-terrestrial environments are most favourable for late gas generation (Yang et al., 2015). The C-P source rock of the Ordos Basin contains Shanxi (P₁s), Taiyuan (P₁t), and Benxi (C₂b) coals and mudstones, with these coaly source rocks having been precipitated in such an environment, with the shore and shallow water delta environment gradually transitioning to a tidal flat or terrestrial swamp. Over long periods, large areas of coal seam sediment were formed. The maceral composition of the source rocks includes vitrinite (Table 3), indicating a terrestrial origin. The sedimentary environment of the upper Paleozoic source rocks indicates that they were favourable precursors for late gas generation.

Table 3: Geochemical parameters of upper Paleozoic source rocks of Ordos Basin [55, 56].

Item
Organic carbon (%)
Chloroform bitumen A (%)
Total hydrocarbon (ppm)
Maceral composition (%)

Vitrinite
Fusinite
Inertinite

Max/min
Average

Shanxi formation
Coal
89.17/49.28
73.6
2.45/0.1
0.8
6699.93/519.9
2539.8
90.2/43.8
73.6
54/6.3
24
12.3/0
4.6

Mudstone
19.29/0.07
2.25
0.5/0.0024
0.04
524.96/519.85
163.8
47/8
20.5
87/51.8
72
20.3/0
7.4

Taiyuan formation
Coal
83.2/3.83
74.7
1.96/0.03
0.61
4463/222
1757.1
98.8/21.2
64.2
63.7/1.3
32.1
15.1/0
3.7

Mudstone
23.38/0.1
3.33
2.95/0.003
0.12
1904.64/15
361.6
82/8.3
38
89.3/15.3
53.3
34.5/0.3
8.4

Limestone
6.29/0.11
1.41
0.43/0.0026
0.08
2194.53/88.92
493.2

Benxi formation
Coal
80.26/55.38
70.8
0.97/0.41
0.77

93.3/72
87.2
25.2/6.7
16
2.8/0
1.4

Mudstone
11.71/0.05
2.54
0.44/0.0024
0.065
1466.34/12.51
322.73
47.8/12.3
24.5
59.8/12.3
44
39.5/0.3
18.2

Note: this table has also been used in Liu et al., [57].

The geological setting of the southern Ordos Basin provides conditions similar to those essential for such reactions, including type III coaly source rock, high thermal maturity, and a closed system. Most southern Ordos Basin gases were formed from thick coaly source rocks with high maturities and high quality. Large amounts of gas were generated in short time periods and stored in nearby tight sandstone reservoirs with thick caprocks and with gas migration being hindered by the stable basement. Paleozoic gas thus accumulated near its source. Therefore, southern Ordos Basin gas fields have conditions approximating closed systems in which aromatization-polycondensation reactions may occur.

A special example for comparison is the gas of the Kela2 gas field in the Tarim Basin, which was also generated from overmature () coaly source rocks, but do not display carbon isotopic reversal [16], because the Kela2 gas field is an open system, with neoformed materials having expired before the overmature stage.

If aromatization-polycondensation reactions took place in the southern Ordos Basin gas fields under study here, the gases generated would contain isotopically light methane, ethane, and propane. However, as the amounts of methane > ethane > propane in the residual alkanes generated from kerogen cracking, a small addition of newly formed gas would result in rollover of δ¹³C-C₃H₈, whereas rollover of δ¹³C-CH₄ would require a larger amount. This would result in two trends: (1) with increasing maturity, rollover of δ¹³C-C₃H₈ would occur first, followed by δ¹³C-C₂H₆, and although isotopically light C₁ was added, residual heavy C₁ would dominate the gas mixture with insignificant changes in δ¹³C-CH₄, and rollover of δ¹³C-CH₄ would occur last; (2) as ¹³C₃ became depleted more rapidly than ¹³C₁ and ¹³C₂, reversal of δ¹³C-C₂H₆ and δ¹³C-C₃H₈ would occur first, with reversal of δ¹³C-CH₄ and δ¹³C-C₂H₆ occurring later. The evolution of carbon isotopic compositions in Ordos Basin gas fields appears to follow these two processes (Figures 6(a), 6(b) and 10(a)). The rollover points of δ¹³C-CH₄, δ¹³C-C₂H₆, and δ¹³C-C₃H₈ occur at different wetness and thermal maturities, with wetness of δ¹³C-CH₄, δ¹³C-C₂H₆, and δ¹³C-C₃H₈ being 1.2%, 2.0%, and 5.0%, respectively (Figures 6(a), 10(a) and 10(b)). With respect to increasing maturity in the whole basin, the reversal of δ¹³C-C₃H₈<δ¹³C-C₂H₆ occurs first in the south Sulige field (Ro 2.0%–2.6%), then reversal of δ¹³C-C₂H₆<δ¹³C-CH₄ occurs in the South Jingbian field (), whereas complete reversal occurs only where .

(a)

(b)

Figure 10: (a) δ¹³C-C₃H₈ versus gas wetness (modified from [15]); (b) detailed plot of δ¹³C-CH₄ versus gas wetness with a wetness range of 0%–4%.

5.3. Hydrogen Isotopic Reversal

A unique characteristic of the southern Ordos Basin gas fields is the hydrogen isotopic reversal (δD-CH₄>δD-C₂H₆). Rollover of δD-CH₄ has been observed in overmature shale gases worldwide [13–15]. Although southern Ordos Basin gases show a rollover of δD-C₂H₆ (Figure 11(a)), Figure 11(b) indicates that δD-CH₄ values continue to increase as wetness decreases, which differs from the trend observed for overmature shale gases, although both gas types display hydrogen isotopic reversal (δD-CH₄>δD-C₂H₆).

(a)

(b)

Figure 11: (a) δD-C₂H₆ versus wetness of the gases from different gas fields in the Ordos Basin; (b) δD-CH₄ versus wetness of the gases from different gas fields in the Ordos Basin.

The hydrogen isotopic reversal of methane has been reported in overmature unconventional and conventional gases [13–15]. The temperature of the reservoirs can reach 200–350°C, and at such temperatures, hydrocarbon reactions can be impacted by the presence of water [47] and redox-active transition metals [58]. Most previous studies attributed the phenomenon to isotopic exchange between CH₄ and H₂O at high temperatures, leading to reversal of δD-CH₄ and δD-C₂H₆ and rollover of δD-CH₄ with increasing maturity. Gases that have undergone isotopic exchange between CH₄ and H₂O can be recognized by increasing CO₂ content with decreasing wetness and decreasing δ¹³C-CO₂ with decreasing wetness [15].

Evolutionary trends of CO₂ content and δ¹³C-CO₂ with decreasing wetness are illustrated in Figure 12. CO₂ contents of southern Ordos Basin gases are higher than those of other gases in the Ordos Basin (Figure 12(a)), but there is no apparent relationship between δ¹³C-CO₂ and wetness (Figure 12(b)). Figure 3(b) indicates that CO₂ is of inorganic origin, and its δ¹³C-CO₂ values are much heavier than those of overmature shale gases. These factors indicate that the high CO₂ contents were not generated from reactions of CH₄ and H₂O, and isotopic exchange between CH₄ and H₂O at high temperatures cannot be the cause of hydrogen isotopic reversal in the southern Ordos Basin. Moreover, recent work on the clumped isotopic composition of methane indicates that all hydrogen atoms in methane (and ethane and propane) are from organic matter and that a contribution of hydrogen from water is unlikely [59].

(a)

(b)

Figure 12: (a) CO₂ content versus wetness (modified from [15]); (b) δ¹³C-CO₂ versus wetness (modified from [15]).

The main difference between the hydrogen isotopic compositions of southern Ordos Basin gases and other overmature gases is that rollover of hydrogen isotopes in the former occurs with δD-C₂H₆ whereas the latter occurs with δD-CH₄. This is attributed to simultaneous rollover of carbon and hydrogen isotopes (i.e., both carbon and hydrogen isotopes of methane continue increasing with maturity, whereas those of ethane increase at first and then decrease, indicating that the rollover of carbon and hydrogen isotopes occur through the same process, possibly at the same time).

In the late gas-generation stage, when the first formed moieties recombined with residual kerogen to form methane and ethane, the process should have followed a similar isotopic fractionation rule to that of primary gas generation. That is, the ¹²C–¹²C bonds in all C-¹H molecules have lower activation energy (are easier to break) than those in molecules containing at least one ²H, and the remaining reactants (the residual kerogen) become ²H-enriched, whereas the gas products become ²H-depleted [60]. In the late gas-generation stage, the cracking of earlier-generated C₆₊ moieties would have released alkanes with lighter hydrogen isotopic compositions than those of alkanes originating from late kerogen. These gases with light hydrogen isotopes mixed with the existing gases with much heavier hydrogen isotopes, causing the isotopic rollover of ethane. As the amount of ethane was much smaller than that of methane, a small amount of mixing would have easily caused the hydrogen isotopic composition of the mixed ethane to be lighter, resulting in the rollover. This is consistent with the recent work of Ni et al. [61].

6. Conclusions

(1)Paleozoic natural gases in the southern Ordos Basin display carbon isotopic reversals, and the gases are not of inorganic origin. Modelled carbon isotope trends of mixed gases containing varying proportions of oil-type and coal-derived gases show that carbon isotopic reversals are not caused by the mixing of different types of gases. Moreover, because of the different geological setting between conventional coal-derived gas and unconventional shale gas, the mechanisms for carbon isotopic reversal in shale gas could not be applied to explain the reversal in southern Ordos gases. The wetness of southern Ordos gases decreases rather than increases with incremental increases in thermal stress, indicating that abiotic polymerization was not the cause of the carbon isotopic reversal(2)Given the evolutionary trend of δ¹³C-CH₄, δ¹³C-C₂H₆, and δ¹³C-C₃H₈ and the geological background of the southern Ordos Basin, carbon isotopic reversal in southern Ordos gases is best explained by a closed-system aromatization-polycondensation model, during which some neoformed macromolecules with relatively light carbon isotopic compositions were generated at a low-maturity stage and preserved in the closed system. During the overmature stage, these neoformed macromolecules reacted with the residual kerogen/coal structure to form short-chain alkanes depleted in ¹³C through aromatization-polycondensation reactions. The evolutionary trend of δD-CH₄ and δD-C₂H₆ in the Ordos Basin also suggests that the hydrogen isotopic reversal was caused by aromatization-polycondensation reactions

Data Availability

The data used to support the findings of this study are included within the article.

Disclosure

The manuscript has been presented at the 2019 Goldschmidt as a short abstract: https://goldschmidt.info/2019/abstracts/abstractView?id=2019005642.

Conflicts of Interest

The author declares no conflicts of interest.

Acknowledgments

The author thanks Professor W. Z. Zhang and Q. F. Kong of the Petroleum Exploration and Development Research Institute, Changqing Oilfield Company, PetroChina, for contributing data and supporting sample collection. The author also appreciates the careful reviews and constructive suggestions of the anonymous reviewers. This study was supported by the National Natural Science Foundation for Young Scientists of China (Grant No. 41702161).

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