Scholarly article on topic 'Secondary origin of negative carbon isotopic series in natural gas'

Secondary origin of negative carbon isotopic series in natural gas Academic research paper on "Earth and related environmental sciences"

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{"Carbon isotopic series" / "Negative carbon isotopic series of secondary origin" / "Shale gas" / "Coal-derived gas"}

Abstract of research paper on Earth and related environmental sciences, author of scientific article — Jinxing Dai, Yunyan Ni, Shipeng Huang, Deyu Gong, Dan Liu, et al.

Abstract The carbon isotopic series of alkane gases were divided into three types: (1) positive carbon isotopic series: δ13C values increased with increasing carbon numbers among the C1–C4 alkanes, which is a typical characteristic for primary alkane gases; (2) negative carbon isotopic series: δ13C values decreased with increasing carbon numbers among the C1–C4 alkanes; and (3) partial carbon isotopic reversal, which had no increasing or decreasing relationship between the δ13C values and carbon numbers. Negative carbon isotopic series were further divided into primary and secondary origins. The primary is a typical characteristic of abiogenic gases, while the secondary is a result of the secondary alteration imposed on biogenic gases usually observed in over-mature shale gas and coal-derived gas. Previous research has proposed several possible explanations for negative carbon isotopic series of secondary origin, such as secondary cracking, diffusion, and the Rayleigh fractionation of ethane and propane through redox reaction with the participation of transition metal and water at 250–300 °C. After a comparative study, the authors found that the negative carbon isotopic series of secondary origin for both shale gas and coal-derived gas appeared in areas where source rocks (shales) were at an over-mature stage, but not in areas where source rocks (shales) were only at a high-maturity stage. As a result, high maturity (>200 °C) was the main controlling factor for the occurrence of negative carbon isotopic series of secondary origin in thermogenic gases. Within this maturity interval, secondary cracking, diffusion, and Rayleigh fractionation of ethane and propane could happen separately or together.

Academic research paper on topic "Secondary origin of negative carbon isotopic series in natural gas"

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Journal of Natural Gas Geoscience xx (2016) 1—7

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Original research paper

Secondary origin of negative carbon isotopic series in natural gas*

Jinxing Dai*, Yunyan Ni, Shipeng Huang, Deyu Gong, Dan Liu, Ziqi Feng, Weilong Peng,

Wenxue Han

PetroChina Research Institute of Petroleum Exploration and Development, Beijing 100083, China

Received 15 December 2015; revised 4 February 2016 Available online ■ ■ ■

Abstract

The carbon isotopic series of alkane gases were divided into three types: (1) positive carbon isotopic series: S13C values increased with increasing carbon numbers among the Cj—C4 alkanes, which is a typical characteristic for primary alkane gases; (2) negative carbon isotopic series: S13C values decreased with increasing carbon numbers among the Cj—C4 alkanes; and (3) partial carbon isotopic reversal, which had no increasing or decreasing relationship between the S13C values and carbon numbers. Negative carbon isotopic series were further divided into primary and secondary origins. The primary is a typical characteristic of abiogenic gases, while the secondary is a result of the secondary alteration imposed on biogenic gases usually observed in over-mature shale gas and coal-derived gas. Previous research has proposed several possible explanations for negative carbon isotopic series of secondary origin, such as secondary cracking, diffusion, and the Rayleigh fractionation of ethane and propane through redox reaction with the participation of transition metal and water at 250—300 °C. After a comparative study, the authors found that the negative carbon isotopic series of secondary origin for both shale gas and coal-derived gas appeared in areas where source rocks (shales) were at an over-mature stage, but not in areas where source rocks (shales) were only at a high-maturity stage. As a result, high maturity (>200 °C) was the main controlling factor for the occurrence of negative carbon isotopic series of secondary origin in thermogenic gases. Within this maturity interval, secondary cracking, diffusion, and Rayleigh fractionation of ethane and propane could happen separately or together.

Copyright © 2016, Lanzhou Literature and Information Center, Chinese Academy of Sciences AND Langfang Branch of Research Institute of Petroleum Exploration and Development, PetroChina. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: Carbon isotopic series; Negative carbon isotopic series of secondary origin; Shale gas; Coal-derived gas

1. Introduction

Some distribution regularities are observed in carbon isotopic series of alkane gases: (1) positive carbon isotopic series: S13C values increased with increasing carbon numbers among the C1—C4 alkanes (S13C1 < S13C2 < S13C3 < S13C4), which is atypical character for biogenic alkane gases; (2) negative carbon isotopic

* This is English translational work of an article originally published in Natural Gas Geoscience (in Chinese).The original article can be found at:10. 11764/j.issn.1672-1926.2016.01.0001.

* Corresponding author.

E-mail address: djx@petrochina.com.cn (J. Dai). Peer review under responsibility of Editorial Office of Journal of Natural Gas Geoscience.

series: S13C values decreased with increasing carbon numbers among the C1—C4 alkanes (S13C1 > S13C2 > S13C3 > S13C4); and (3) when the d13C values did not increase/decrease with increasing carbon number, this is called a carbon isotopic reversal [1,2].

2. Negative carbon isotopic series

2.1. Negative carbon isotopic series of primary origin

Negative carbon isotopic series of primary origin were observed in abiogenic gases from inclusions in magnetite, found in the volcanically active area of Yellowstone Park in the U.S., the Mid-Ocean Ridge of the North Atlantic, and meteorolite of Australia (Table 1) [3—7].

http://dx.doi.org/10.1016/j .jnggs.2016.02.002

2468-256X/Copyright © 2016, Lanzhou Literature and Information Center, Chinese Academy of Sciences AND Langfang Branch of Research Institute of Petroleum Exploration and Development, PetroChina. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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2 J. Dai et al. / Journal of Natural Gas Geoscience xx (2016) 1—7

2.2. Negative carbon isotopic series of secondary origin

In recent years, large-scale distributed alkane gases (especially shale gas) with negative carbon isotopic series were observed in some over-mature sedimentary basins; i.e., shale gas from the Wufeng—Longmaxi Formation in the southern Sichuan Basin, China (Table 2) [8,9], Fayetteville shale gas from the Arkoma gas field in the U.S. [10], and shale gas from the Horn River gas field in the Western Canadian Sedimentary Basin (Table 3) [11]. These shale gases have high wetness values and are derived from source rocks with high TOC values and maturities (over-mature). As shown in Table 2, the Wufeng—Longmaxi shale gas had wetness and Ro values of 0.34%—0.77% and >2.2% [12] (or of 2.2%— 3.13% [13]), respectively. As shown in Table 3, the Fayetteville shale gas had wetness and Ro values of 0.86%—1.60% and 2%— 3%, respectively. The Horn River shale gas had a wetness value of 0.2% (Table 3). The helium associated with the

Wufeng—Longmaxi shale gas had a R/Ra ratio of 0.01—0.04, indicating a crustal origin (Table 2). Thus, the negative carbon isotopic series in these alkane gases were the result of secondary alteration that is different from that of the negative carbon iso-topic series of primary origin in abiogenic gases. In this study, it is defined as "negative carbon isotopic series of secondary origin".

Large-scale distributed alkane gases with negative carbon isotopic series were not only observed in over-mature shale gas, but also in over-mature coal-derived gas from the southern Ordos Basin in China (Table 4, Fig. 1). Gas source-rocks in this area are coals and dark mudstones in the Carboniferous Benxi (C2b), the Permian Taiyuan (Pit), and the Shanxi (Pjs) formations. Coal seams mainly occur in the Taiyuan and Shanxi formations with the thickness of 2—20 m. The coal measures belong to humic coals with average TOC values and chloroform bitumen "A" of 70.8%—74.7% and 0.61%— 0.80%, respectively. Dark mudstones have average TOC

Table 1

Geochemical parameters of abiogenic natural gas with negative carbon isotopic series of primary origin.

Sample location

5 C/%, VPDB

References

Magma rock in Khibiny massif Russia Mud volcano, Yellowstone Park USA Chimera, Turkey

Lost City in the North Atlantic Mid-Ocean Ridge Australian Murchison meteorite

-3.2 -21.5 -11.9 -9.9 9.2

-9.1 -26.5 -22.9 -13.3 3.7

-23.7 -14.2 1.2

[6] [7]

Table 2

Geochemical parameters of Wufeng—Longmaxi shale gas from the Jiaoshiba and Changning-Weiyuan gas fields in the Sichuan Basin.

Well Formation Molecular composition/% Wetness/% 513C%, VPDB 3He/*He (10-8) R/Ra References

CH4 C2H6 C3H8 CO2 N2 CH4 C2H6 C3H8

JY1 O3w,S1l 98.52 0.67 0.05 0.32 0.43 0.72 -30.1 -35.5 4.851 ± 0.944 0.03

JY1-2 O3w,S1l 98.80 0.70 0.02 0.13 0.34 0.73 -29.9 -35.9 6.012 ± 0.992 0.04

JY1-3 O3w,S1l 98.67 0.72 0.03 0.17 0.41 0.75 -31.8 -35.3

JY4-1 O3w,S1l 97.89 0.62 0.02 1.07 0.65 -31.6 -36.2

JY4-2 O3w,S1l 98.06 0.57 0.01 1.36 0.59 -32.2 -36.3

JY-2 O3w,S1l 98.95 0.63 0.02 0.02 0.39 0.65 -31.1 -35.8 2.870 ± 1.109 0.02

JY7-2 JY12-3 O3w,S1l O3w,S1l 98.84 98.87 0.67 0.67 0.03 0.02 0.14 0.00 0.32 0.44 0.70 0.69 -30.3 -30.5 -35.6 -35.1 -38.4 5.544 ± 1.035 0.04 This study

JY12-4 O3w,S1l 98.76 0.66 0.02 0.00 0.57 0.68 -30.7 -35.1 -38.7

JY13-1 O3w,S1l 98.35 0.60 0.02 0.39 0.64 0.62 -30.2 -35.9 -39.3

JY13-3 O3w,S1l 98.57 0.66 0.02 0.25 0.51 0.68 -29.5 -34.7 -37.9

JY20-2 O3w,S1l 98.38 0.71 0.02 0.00 0.89 0.74 -29.7 -35.9 -39.1

JY42-1 O3w,S1l 98.54 0.68 0.02 0.38 0.38 0.71 -31.0 -36.1

JY42-2 O3w,S1l 98.89 0.69 0.02 0.00 0.39 0.71 -31.4 -35.8 -39.1

S1l 97.22 0.55 0.01 2.19 0.56 -30.3 -34.3 -36.4 [8]

S1l 98.34 0.68 0.02 0.10 0.84 0.70 -29.6 -34.6 -36.1

JY1HF S1l 98.34 0.66 0.02 0.12 0.81 0.69 -29.4 -34.4 -36.1

S1l 98.41 0.68 0.02 0.05 0.80 0.71 -30.1 -35.5

S1l 98.34 0.68 0.02 0.10 0.84 0.70 -30.6 -34.1 -36.3

JY1-3HF S1l S1l 98.26 98.23 0.73 0.71 0.02 0.03 0.13 0.12 0.81 0.86 0.77 0.74 -29.4 -29.6 -34.5 -34.7 -36.3 -35.0

Wei201 S1l 98.32 0.46 0.01 0.36 0.81 0.48 -36.9 -37.9 3.594 ± 0.653 0.03 [8]

Wei201-H1 S1l 95.52 0.32 0.01 1.07 2.95 0.34 -35.1 -38.7 3.684 ± 0.697 0.03

Wei202 S1l 99.27 0.68 0.02 0.02 0.01 0.70 -36.9 -42.8 -43.5 2.726 ± 0.564 0.02

Ning201-HI S1l 99.12 0.5 0.01 0.04 0.30 0.51 -27.0 -34.3 2.307 ± 0.402 0.02 [9]

Ning211 S1l 98.53 0.32 0.03 0.91 0.17 0.35 -28.4 -33.8 -36.2 1.867 ± 0.453 0.03

Zhao104 S1l 99.25 0.52 0.01 0.07 0.15 0.53 -26.7 -31.7 -33.1 1.958 ± 0.445 0.01

YSL1-H1 S1l 99.45 0.47 0.01 0.01 0.03 0.48 -27.4 -31.6 -33.2 1.556 ± 0.427 0.01

Note: Wetness = £(C2-C5)^(C1-C5), %.

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Table 3

Geochemical parameters of shale gas with negative carbon isotopic series of secondary origin in North America.

Basin Formation Molecular composition/% Wetness/% 513C%o, VPDB References

CH4 C2H6 C3H8 CO2 N2 CH4 C2H6 C3H8 CO2

Eastern Arkoma Basin Fayetteville 98.22 1.14 0.02 0.61 1.17 -38.0 -43.5 —43.5 -17.2 [10]

98.06 1.34 0.02 0.58 1.37 -41.3 -42.2 -43.6 -19.5

95.3 1.14 0.02 3.53 1.20 -36.8 -42.0 -42.6 -9.9

95.84 0.82 0.01 3.33 0.86 -36.2 -40.5 -40.6 -8.8

98.01 1.28 0.02 0.69 1.31 -41.3 -42.9 -43.5 -19.9

97.95 1.1 0.02 0.93 1.13 -38.4 -42.8 -43.2 -11.7

93.1 1.25 0.02 5.63 1.35 -35.7 -40.4 -40.4 -10.2

93.72 1.16 0.02 5.1 1.24 -37.7 -41.9 -42.3 -12.5

98.31 1.19 0.02 0.48 1.22 -40.8 -43.6 -43.6 -17.6

97.98 0.96 0.02 1.04 0.99 -41.4 -44.1 - 44.3 -15.7

97.82 1.23 0.03 0.92 1.27 -41.9 -43.2 -45.2 -17.6

96.8 1.51 0.03 1.67 1.57 -39.9 -44.4 -44.6 -11.7

92.38 1.11 0.02 6.49 1.21 -36.4 -41.4 -41.5 -8.9

95.57 1.11 0.02 3.29 1.17 -36.5 -37.9 -39.7

96.28 1.55 0.03 2.14 1.61 -35.9 -39.9 -41.1 -6.2

96.51 1.53 0.03 1.94 1.59 -36.2 -40.2 -40.2 -5.7

96.47 1.31 0.03 2.2 1.37 -37.9 -41.7 -42.0 -4.7

97.08 1.26 0.02 1.64 1.30 -37.3 -41.8 -41.9 -8.9

97.01 1.36 0.02 1.61 1.40 -38.1 -40.4 -41.8 -6.9

Western Canada Sedimentary Basin Horn River 0.20 -27.6 -33.8 [11]

0.20 -32.1 -34.9 -38.8

0.20 -31.3 -34.1 -37.3

0.20 -31.2 -32.0 -35.5

0.20 -30.7 -34.4 -36.9

Table 4

Geochemical parameters of shale gas with negative carbon isotopic series in the southern Ordos Basin.

Well Formation Molecular composition/% Wetness/% 513C/%o, VPDB 3He/4He (10~8) R/Ra

CH4 C2H6 C3H8 C4H10 C5H12 CO2 N2 CH4 C2H6 C3H8

Shi2 P1sh8 96.68 0.73 0.09 0.08 1.41 1.07 0.92 -29.20 -30.70 —31.90 6.64 ± 0.7 0.06

Shi225 P11s2 93.87 0.42 0.03 5.01 0.67 0.48 -28.80 -34.10

Shi48 C2b2 94.89 0.52 0.04 4.29 0.25 0.59 -29.90 -36.50 7.66 ± 1.04 0.07

Shi37 C2b1-2 96.60 0.42 0.03 2.74 0.22 0.46 -30.80 -37.10 -37.30 7.49 ± 1.41 0.07

Shan380 P1sh8 90.58 0.94 0.13 0.02 0.01 1.13 7.18 1.20 -24.50 -28.30 -29.30

Shan428 P1S1 90.20 0.67 0.11 0.02 3.21 5.79 0.88 -28.10 -29.20 -29.30

Su353 P1s1-P1sh8 93.12 1.11 0.17 0.04 0.01 1.86 3.69 1.41 -24.10 -25.60 -28.70

Su243 P1sh8 92.81 0.80 0.14 0.02 0.56 5.51 1.02 -26.20 -28.90 -30.60

values and chloroform bitumen "A" of 2.0%—3.0% and 0.04%—0.12%, respectively [14]. As shown in Fig. 1, these coal-derived gases with negative carbon isotopic series, along with some with carbon isotopic reversals, are distributed in the southern Ordos Basin where the gas source-rocks have a maturity higher than 2.4% Ro. Coal-derived gases with negative carbon isotopic series have a wetness value of 0.46%—1.41% with an average of 0.87% (Table 4) which is lower than that in North America (Table 3) and higher than that in the southern Sichuan Basin (Table 2).

3. Genesis of negative carbon isotopic series of secondary origin

Although the negative carbon isotopic series in marine shale gas have been widely studied, the large-scale occurrence among coal-derived gas has seldom been reported. In the

following text, we summarize the main explanations for this phenomenon and try to determine the controlling factors.

3.1. Shale gas with negative carbon isotopic series of secondary origin are only observed in over-mature shales, and not in low/high mature shales

As shown in Table 2, the Wufeng—Longmaxi shale gases from the southern Sichuan Basin in China were derived from over-mature source-rocks, and all have negative carbon isotopic series of secondary origin. In contrast, shale gases from the U.S. had a much wider maturity interval, especially those from the Barnett shales which are mainly at mature and high-maturity stages (RO = 0.7%—2.0%). Most Barnett shale gases had positive carbon isotopic series, and some had a carbon isotopic reversal [10,15]. Only a handful of Barnett shale gas had negative carbon isotopic series. Positive carbon isotopic

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Fig. 1. Relationship between carbon isotopic series and Ro%.

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series were found in Marcellus shale gas when its wetness was at a high level (14.7%—20.8%), while negative carbon iso-topic series were observed when its wetness was at a low level (1.49%—1.57%) [16]. With decreasing wetness, the gas maturity increased gradually. The Montney shale gases from the West Canadian Sedimentary Basin exhibiting high wetness values usually had positive carbon isotopic series, while those with low wetness values usually had a carbon isotopic reversal. Since the Horn River shale gas had a wetness value lower than 0.2, it had negative carbon isotopic series [11] (Table 3). Fig. 2 is a correlation map between wetness and carbon isotopic distribution in which data from Tables 2 and 3 are plotted. Most negative carbon isotopic series are observed in over-mature shale gases (gases with low wetness), while most positive carbon isotopic series are observed in low-mature to mature shale gases (gases with relatively high wetness). The distribution pattern of carbon isotopes viewed in relation to shale gas maturity indicates that temperature is a controlling factor in the distribution pattern of carbon isotopes.

3.2. Coal-derived gases with negative carbon isotopic series of secondary origin are only generated from overmature coal measures

Four hundred and thirty-three coal-derived gas samples from the Ordos Basin were plotted in the correlation map between carbon isotopic distribution and Ro values (Fig. 1). Negative carbon isotopic series were observed only in gases from the southern Jingbian and Yan'an gas fields in the

southern Ordos Basin, which on a large scale had a wetness value of 0.46%—1.41% and a maturity of 2.2%—2.7% Ro, respectively (Fig. 1, Table 4). The maturity of coal-derived gas from the Ordos Basin decreased northward and reached its lowest level of 0.75% Ro in the Shenglijing gas field. Coal-derived gases from the Shenmu gas field were at the mature stage with a lowest maturity of 1.1% Ro. Among the 55 gas samples from this gas field, 47 samples had positive carbon isotopic series accounting for 85.5% of the total samples, and only 8 samples had weak carbon isotopic reversals. Similar distribution patterns were observed in the Daniudi, Yulin, Zizhou, Jingbian, Wushenqi, Sulige, Dongsheng, and Shen-glijing gas fields. As a result, in an area where mature to high-mature source-rocks are distributed, most gases had positive carbon isotopic series and some had carbon isotopic reversals. In contrast, gases with negative isotopic series were only observed in an area where over-mature source-rocks were distributed. This indicates that high temperature is a controlling factor for the formation of negative isotopic series.

3.3. Negative carbon isotopic series resulting from secondary cracking

Secondary cracking usually happens at the over-mature stage during which the residual oils and wet gases in the shale gas system are cracked simultaneously. The oils are cracked into natural gas whose ethane contains relatively more negative carbon isotopes. Meanwhile, the cracking process also results in a rapid decrease of ethane in primary natural gas. Thus, the mixing of these two types of ethane has finally caused the reversals of carbon isotopes [17].

Fig. 2. The relationship between wetness and carbon isotopic distributions of shale gases from China, U.S., and Canada.

3.4. Rayleigh fractionation

Burruss and Laughrey [18] proposed that the negative carbon isotopic series in some deep-basin gases were the result of the Rayleigh fractionation of ethane and propane through a redox reaction with the participation of transition metal and water at 250—300 °C.

3.5. Diffusion

Diffusion in molecules is affected by their size and weight. The larger the molecules become, the more slowly they diffuse. Since the diameters of molecules increase with the increasing carbon number, gas molecules have a diffusion rate of CH4 > C2H6 > C3H8 > C4H10. CH4, C2H6, C3H8 and C4H10 are composed of 12C and 13C and can be divided into four molecular configurations as follows:

/12CH4;

Crj _v 12/-<12^u 13/-d3/-Trj

2H6—* C CH6, C CH6, C CH6

(1) (2)

c3h8—12c12c12ch8, 12^12^13

c12c13ch8, 12c13c13ch 13c13c13ch8

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Crj _v 12^ 12 ^12^12/-^TT

4H10—* C C C CH10,

12C12C12C13CHi,

12C12C13C13CHi

12C13C13C13CH10; 13C13C13C13CH10

(1) Because 12CH4 is lighter than 13CH4, it diffuses faster than 13CH4. Thus, through diffusion process, carbon isotopes of methane turn to be more positive. (2) Three molecular configurations were observed in ethane with their molecular weight and diffusion rates being 12C12CH6 < 12C13CH6 < 13C13CH6 and 12C12CH6 > 12C13CH6 > 13C13CH, respectively. Thus, diffusion also made the carbon isotopes of ethane turn to be more positive. (3) C3H8 and C4H10 could be divided into four and five molecular configurations, respectively. In a similar way as mentioned in (1) and (2), diffusion made the carbon isotopes of propane and butane turn to be more positive.

Because (1), (2), (3), and (4) have different configurations of 12C

and 13C, gases in the diffusion system (source rocks) had a frac-

tionation degree of (1) > (2) > (3) > (4) and a diffusion rate of

CH > C2H6 > C3H8 > C4H10. Under these dual actions, the primary

1 ^ 1 ^ 1 ^ 1 ^ positive series in carbon isotopes (S C1 < S C2 < S C3 < S C4)

in natural gas will finally transform into the secondary negative

series (S13C1 > S13C2 > S13C3 > S13C4) after a long period.

Hydrocarbons generated from sapropelic source-rocks of different maturities have different migration phases: during the immature to low-maturity stage, they migrate in a water-dissolving phase; during the mature stage, they migrate in an oil-dissolving phase; during the high-maturity stage, they migrate in a gaseous phase; and during the over-mature stage, they migrate in a diffusive phase [19]. Hydrocarbons generated from humic source-rocks migrate in a water-dissolving phase during immature to low-maturity stage, in a gaseous phase during a mature to high-maturity stage, and in a diffusive phase during an over-maturity stage [19]. Because gases from both humic and sapropelic source-rocks at an over-maturity stage migrate in a diffusive phase, it is most effective for diffusion to cause negative carbon isotopic series at such a maturity stage, either in shale gas or in coal-derived gas.

cracking also occur at the over-mature stage. The key factor for the Rayleigh fractionation of ethane and propane through redox reaction is the participation of transition metal and water at 250—300 °C. The most favorable period for diffusion-caused negative carbon isotopes during primary migration was at the over-mature stage. Experimental data from Vinogradov and Galimov [20] indicate that high temperature (>200 °C) will result in a negative carbon isotope series.

The six hypotheses are all related to high temperature. Only at a high temperature will the negative carbon isotopic series of secondary origin occur, as a result of one or several processes mentioned above. In turn, the large-scale occurrence of such isotopic series indicates that hydrocarbons have evolved into the over-mature stage.

4. Conclusion

Negative carbon isotopic series can be divided into primary and secondary origins. The primary is a typical character of abiogenic gases, while the secondary resulted from the secondary alteration imposed on biogenic gases under high temperature. Negative carbon isotopic series of secondary origin are commonly observed in over-mature shale gas or coal-derived gas. The large-scale occurrence of such isotopic series indicates that hydrocarbons have evolved into the over-maturity stage.

Foundation item

Supported by PetroChina Major Projects for Oil and Gas Exploration (2014B-0608).

Conflict of interest

There is no conflict of interest. Acknowledgments

3.6. High temperature (>200 °C)

Vinogradov and Galimov [20] proposed that the equilibrium effects of isotope exchange differs at different temperatures: the carbon isotopic reversal between methane and ethane (S13C1 > S13C2) occurs when the geothermal temperature is higher than 150 °C, and negative carbon isotopic series (S13C1 > S13C2 > S13C3) occur when the geothermal temperature is higher than 200 °C.

Altogether, six possible explanations for negative carbon isotopic series of second origin were discussed in this study. Based on a large database of shale gases from the Wufeng—Longmaxi shales in China, the Barnett, Marcellus, Montney, and Fayetteville shales in the U.S., and the Horn River shales in Canada as well as coal-derived gases from the Ordos Basin, we concluded that negative carbon isotopic series of second origin only occur in over-mature gases. Negative carbon isotopic series caused by secondary

We thank Professor Wenzheng Zhang and senior researcher Qinfen Kong for the generative help of Ro data collection.

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