1. Introduction
The Ordos Basin is one of the main natural gas producing areas in China. Several giant gas fields have been discovered in the basin including the Sulige, Yulin, Wushenqi, Zizhou, Mizhi, Shenmu, Daniudi, Dongsheng and Jingbian (Huang et al. Reference Huang, Fang, Liu, Fang and Huang2015; Dai et al. Reference Dai, Zou, Hu, Li, Li, Tao, Zhu, Ni, Yang, Huang, Shi, Huang, Zhang, Liu, Xie, Li, Qin, Li, Zhu, Luo, Zhao, Yang, Li, Wang, Jiang, Gong, Tao, Liao, Yu, Gong, Fang, Wu, Meng, Wang and Liu2016b; Wang et al. Reference Wang, Shao, Li, Li, Lv and Liu2016; Fan et al. Reference Fan, Yang, Lenhardt, Wang, Han, Li, Li and Zhao2019). It is widely accepted that natural gas in these gas fields, except for the Lower Palaeozoic Jingbian gas field, is generated from the Carboniferous–Permian coal measure strata (Liu et al. Reference Liu, Chen, Liu, Li, Han and Guo2009, Reference Liu, Zhang, Kong, Feng, Fang and Peng2016; Han et al. Reference Han, Tao, Hou and Yao2017a; Lv et al. Reference Lv, Wang, Li, Liu and Li2017; Wu et al. Reference Wu, Liu, Zhu, Li, Liu, Chen and Ni2017). The controversy over the origins of the natural gas in the Lower Palaeozoic Jingbian gas field is due to the abnormal geochemical characteristics of the natural gas (Li et al. Reference Li, Zhang, Luo and Hu2008; Liu et al. Reference Liu, Chen, Liu, Li, Han and Guo2009; Han et al. Reference Han, Tao, Hou and Yao2017a; Wang et al. Reference Wang, Pang, Zhao, Wang, Hu, Zhang and Zheng2017b).
When carbon isotope values increase with an increase in carbon numbers in alkane gases, it is described as a positive series (δ13Cmethane < δ13Cethane < δ13Cpropane < δ13Cbutane). This is usually characteristic of organic gases. When the carbon isotope series is the opposite (δ13Cmethane > δ13Cethane > δ13Cpropane > δ13Cbutane), it is described as a negative or full reversal series and is usually characteristic of inorganic gases (Dai et al. Reference Dai, Xia, Qin and Zhao2004, Reference Dai, Ni, Huang, Gong, Liu, Feng, Peng and Han2016a; Li et al. Reference Li, Li, Li, Zhang, Cui and Zhu2018). However, if carbon isotope values appear in neither of the abovementioned series, but are arranged in a disordered form, it is described as a partial carbon isotopic reversal. The carbon isotope values of the Lower Palaeozoic Jingbian gas field show a partial reversal, and this is why the origins of the natural gas in the Lower Palaeozoic Jingbian gas field are controversial (Li et al. Reference Li, Zhang, Luo and Hu2008; Liu et al. Reference Liu, Chen, Liu, Li, Han and Guo2009; Han et al. Reference Han, Tao, Hou and Yao2017a; Wang et al. Reference Wang, Pang, Zhao, Wang, Hu, Zhang and Zheng2017b).
In summary, a carbon isotopic reversal is mainly caused by the following mechanisms:
(1) Mixing action, which refers to the mixing of organic and inorganic alkanes, coal-derived gas and oil-derived gas, and gases with ‘identical type and different sources’ or ‘identical sources in different periods’ (Dai et al. Reference Dai, Xia, Qin and Zhao2004, Reference Dai, Ni, Huang, Gong, Liu, Feng, Peng and Han2016a; Zou et al. Reference Zou, Tao, Han, Zhao, Ma, Li, Bai and Gao2018);
(2) Fractionation, which includes migration fractionation and water-solution fractionation (Zhang & Krooss, Reference Zhang and Krooss2001; Xiao, Reference Xiao2012);
(3) High thermal maturity (Fuex, Reference Fuex1977; Tilley & Muehlenbachs, Reference Tilley and Muehlenbachs2013; Xia et al. Reference Xia, Chen, Braun and Tang2013);
(4) Bacterial oxidation, oxidation–reduction reactions with transition metals and water or thermochemical sulfate reduction (TSR) (Burruss & Laughrey, Reference Burruss and Laughrey2010; Zumberge et al. Reference Zumberge, Ferworn and Brown2012; Liu et al. Reference Liu, Worden, Jin, Liu, Li, Gao, Zhang, Hu and Yang2013).
As mentioned above, the reason for the carbon isotopic reversal is still unclear, which leads to the dispute over the origins of the natural gas in the Lower Palaeozoic Jingbian gas field (Li et al. Reference Li, Zhang, Luo and Hu2008; Liu et al. Reference Liu, Chen, Liu, Li, Han and Guo2009; Han et al. Reference Han, Tao, Hou and Yao2017a; Wang et al. Reference Wang, Pang, Zhao, Wang, Hu, Zhang and Zheng2017b). Therefore, it is necessary to study the reasons for the carbon isotopic reversal.
The natural gas shows a carbon isotopic reversal in the southern Jingbian gas field, which has a higher thermal maturity. Comparative study of the geochemical characteristics of the natural gas in the southern and northern parts of the Jingbian gas field has rarely been carried out, but it can help to identify the impacts of thermal maturity on natural gas. Moreover, such a comparative study can cover the whole Jingbian gas field, avoiding the drawbacks of previous studies that only focused on either the southern or northern parts of the Jingbian gas field (Li et al. Reference Li, Zhang, Luo and Hu2008; Liu et al. Reference Liu, Chen, Liu, Li, Han and Guo2009, Reference Liu, Zhang, Kong, Feng, Fang and Peng2016; Wang et al. Reference Wang, Pang, Zhao, Wang, Hu, Zhang and Zheng2017b).
2. Geologic setting
The Ordos Basin has an Archaean and Proterozoic metamorphic crystalline basement covered by Mesoproterozoic, Neoproterozoic, Palaeozoic, Mesozoic and Cenozoic sediments with an average thickness of ~6000 m (Ding et al. Reference Ding, Dai, Zhu, Zhang, He, Li and Wang2016; Dai et al. Reference Dai, Zou, Hu, Li, Li, Tao, Zhu, Ni, Yang, Huang, Shi, Huang, Zhang, Liu, Xie, Li, Qin, Li, Zhu, Luo, Zhao, Yang, Li, Wang, Jiang, Gong, Tao, Liao, Yu, Gong, Fang, Wu, Meng, Wang and Liu2016b; Yang et al. Reference Yang, Fan, Han and Loon2017; Fan et al. Reference Fan, Yang, Loon, Yin, Han and Zavala2018; Du et al. Reference Du, Zhao, Wang, Yu, Xiao, Xie, Du and Su2019). The Lower Palaeozoic strata are mainly marine carbonates, and the Upper Palaeozoic–Mesozoic strata are mainly terrestrial clastic rocks (Dai et al. Reference Dai, Zou, Hu, Li, Li, Tao, Zhu, Ni, Yang, Huang, Shi, Huang, Zhang, Liu, Xie, Li, Qin, Li, Zhu, Luo, Zhao, Yang, Li, Wang, Jiang, Gong, Tao, Liao, Yu, Gong, Fang, Wu, Meng, Wang and Liu2016b). The Ordos Basin can be divided into six tectonic units, i.e. the Yimeng Uplift, Weibei Uplift, Jinxi Fold Belt, Xiyuan Thrust Belt, Yi-Shaan Slope and Tianhuan Depression (Liu et al. Reference Liu, Chen, Liu, Li, Han and Guo2009; Yang et al. Reference Yang, He, Qiu, Jin, Sun and Jin2014; Han et al. Reference Han, Tao, Hu, Ma, Liu, Feng and Peng2017b; Li et al. Reference Li, Chang, Yin, Sun and Song2017; Shi et al. Reference Shi, Chang, Yin, Li and Mao2019) (Fig. 1). The Lower Palaeozoic Jingbian gas field, located in the central Yi-Shaan Slope, extends in a N–S direction with a large longitudinal span (Dai et al. Reference Dai, Zou, Hu, Li, Li, Tao, Zhu, Ni, Yang, Huang, Shi, Huang, Zhang, Liu, Xie, Li, Qin, Li, Zhu, Luo, Zhao, Yang, Li, Wang, Jiang, Gong, Tao, Liao, Yu, Gong, Fang, Wu, Meng, Wang and Liu2016b; Xu et al. Reference Xu, Shi, Xie, Manger, Mcguire, Zhang, Wang and Xu2016; Zhou et al. Reference Zhou, Ji, Xu, Che, Niu, Wan, Zhou, Li and You2016).
Fig. 1. Contour map of R o values and geological tectonics in the Ordos Basin, China (modified after Dai et al. Reference Dai, Ni, Huang, Gong, Liu, Feng, Peng and Han2016a).
All the reservoirs of the gas fields in the Ordos Basin are Upper Palaeozoic terrestrial clastic rocks, except for the Jingbian gas field, which is mainly produced from the Lower Palaeozoic marine carbonate (Li et al. Reference Li, Zhang, Luo and Hu2008; Liu et al. Reference Liu, Chen, Liu, Li, Han and Guo2009, Reference Liu, Zhang, Kong, Feng, Fang and Peng2016; Han et al. Reference Han, Tao, Hou and Yao2017a). The source rocks are mainly Carboniferous–Permian coal measure strata (Dai et al. Reference Dai, Zou, Hu, Li, Li, Tao, Zhu, Ni, Yang, Huang, Shi, Huang, Zhang, Liu, Xie, Li, Qin, Li, Zhu, Luo, Zhao, Yang, Li, Wang, Jiang, Gong, Tao, Liao, Yu, Gong, Fang, Wu, Meng, Wang and Liu2016b; Han et al. Reference Han, Tao, Hou and Yao2017a; Lv et al. Reference Lv, Wang, Li, Liu and Li2017; Wu et al. Reference Wu, Liu, Zhu, Li, Liu, Chen and Ni2017). In the Lower Palaeozoic Jingbian gas field, the gas reservoirs correspond to the Carboniferous Benxi Formation aluminous mudstone with continental argillaceous rocks as the cap rocks; laterally, they are sealed by the Ordovician clayey gypsum dolomite (Li et al. Reference Li, Zhang, Luo and Hu2008; Liu et al. Reference Liu, Chen, Liu, Li, Han and Guo2009, Reference Liu, Zhang, Kong, Feng, Fang and Peng2016; Dai et al. Reference Dai, Zou, Hu, Li, Li, Tao, Zhu, Ni, Yang, Huang, Shi, Huang, Zhang, Liu, Xie, Li, Qin, Li, Zhu, Luo, Zhao, Yang, Li, Wang, Jiang, Gong, Tao, Liao, Yu, Gong, Fang, Wu, Meng, Wang and Liu2016b; Han et al. Reference Han, Tao, Hou and Yao2017a).
In this paper, the Jingbian gas field is divided into southern and northern parts based on thermal maturity in order to analyse the effect of thermal maturity on the carbon isotopes. The Jingbian gas field is adjacent to the Sulige, Wushenqi, Daniudi, Yulin, Dongsheng and Shenmu gas fields in the north and the Zizhou and Mizhi gas fields in the northeast (Li et al. Reference Li, Zhang, Luo and Hu2008; Liu et al. Reference Liu, Chen, Liu, Li, Han and Guo2009; Dai et al. Reference Dai, Zou, Hu, Li, Li, Tao, Zhu, Ni, Yang, Huang, Shi, Huang, Zhang, Liu, Xie, Li, Qin, Li, Zhu, Luo, Zhao, Yang, Li, Wang, Jiang, Gong, Tao, Liao, Yu, Gong, Fang, Wu, Meng, Wang and Liu2016b; Han et al. Reference Han, Tao, Hou and Yao2017a). The southern part of the Jingbian gas field has a much higher thermal maturity than the northern part and does not border other gas fields (Fig. 1). Exploration of the Jingbian gas field has increased recently, with many wells being drilled in the southern part.
The geothermal gradient and terrestrial heat flow values for the southern Ordos Basin are obviously larger than those of the northern Ordos Basin (Ren et al. Reference Ren, Yu, Cui, Qi, Chen, Cao and Yang2017). The difference indicates that the southern part has a higher degree of thermal evolution and thermal anomalies. The reason for such a difference is that in early Cretaceous time, thermal activity in the deep lithosphere of the Ordos Basin increased, and the thermal activity in the southern Ordos Basin was much stronger than that in the northern part (Ren et al. Reference Ren, Zhang, Gao, Cui, Xiao and Xiao2007). The abnormal degree of thermal evolution in the southern Ordos Basin has something to do with the intensive tectonic activity of the Qinling orogenic belt during the late Yanshan period. The intense intra-continental orogeny and intra-continental subduction during the Yanshan period may have caused the mantle uplift in the southern Ordos Basin, resulting in an abnormal geothermal field (Ren et al. Reference Ren, Zhang, Gao, Cui and Liu2006; He & Zhang, Reference He and Zhang2018).
During the middle and late Yanshan Movement, accompanied by strong orogenic movement around the Ordos Basin, volcanic activity was frequent in the south. Nepheline syenite was discovered in Well Long 1 with a thickness of 154.6 m, and magmatic rock was found in Well Long 2 with a thickness of 384.5 m in the southern area (Ren et al. Reference Ren, Zhang, Gao, Cui and Liu2006). This indicates that volcanic activity occurred in the southern region, which directly leads to the higher thermal maturity in the southern region than in the northern region. K-bentonite was discovered in the south, which also provides evidence for volcanic-magmatic activity events in the southern region (Wang et al. Reference Wang, Zhou, Wang, Jing and Zhang2015). In the south, a large number of tuffs were also developed, indicating that the crater may be in the Qinling orogenic belt adjacent to the southern part of the basin (Zhang et al. Reference Zhang, Yang, Peng, Yang, Zhang and Shi2009). Seismic sections and gravity and magnetic anomalies through the south also prove that there are magmatic rocks buried in the deep part of the south (Zhang et al. Reference Zhang, Wu, Guo, Chen, Zhao, Ding, Huang, Zhang and Chen1985; Zhang, Reference Zhang1989).
The results of simulation of the geothermal history of the basin suggest that the palaeo-geothermal gradient in late Mesozoic time is higher, and the palaeo-geothermal gradient in the south is higher than that in the north, with abnormally high values appearing in the south (Ren et al. Reference Ren, Zhang, Gao, Cui and Liu2006, Reference Ren, Zhang, Gao, Cui, Xiao and Xiao2007). The thermal structure of the sedimentary layers in the south and north of the basin is quite different, and the temperature at the bottom of each layer in the south is higher than that in the north. The degree of thermal evolution gradually increases from north to south, and the sediments enter the over-mature stage in the south Jingbian gas field (Yang et al. Reference Yang, Zhang, Liu and Meng2012; Feng et al. Reference Feng, Liu, Huang, Gong and Peng2016).
The south Jingbian gas field was a depositional centre with maximum burial in late Palaeozoic time (Feng et al. Reference Feng, Liu, Huang, Gong and Peng2016). The south contains sandstone reservoirs interbedded with shale of higher thermal maturity (Li et al. Reference Li, Ren, Gao, Guo, Lin, Li, Li and Bai2015). This assemblage provided favourable conditions for the cracked gas to migrate to the sandstone reservoir in the south Jingbian gas field (Wu et al. Reference Wu, Dong, Yu and Liu2015).
3. Samples and methods
Twenty-one gas samples were analysed from the northern part and 15 gas samples were collected from the southern part of the Jingbian gas field. The northern gas sample data were compiled from the literature (Dai et al. Reference Dai, Zou, Hu, Li, Li, Tao, Zhu, Ni, Yang, Huang, Shi, Huang, Zhang, Liu, Xie, Li, Qin, Li, Zhu, Luo, Zhao, Yang, Li, Wang, Jiang, Gong, Tao, Liao, Yu, Gong, Fang, Wu, Meng, Wang and Liu2016b), and the southern gas samples were collected and analysed in this study.
These gas samples were collected in high-pressure cylinders (pressure resistant up to 15 MPa) at pressures of ~5 MPa after the vessels had been repeatedly flushed and refilled. Measurements of components and carbon isotope values were conducted at the PetroChina Research Institute of Petroleum Exploration and Development (RIPED) in Beijing, China. To determine the components of the natural gas, an Agilent HP7890A gas chromatograph (GC) was equipped with a flame ionization detector and a thermal conductivity detector. The GC oven temperature started at 30 °C for 10 min, and then rose to the maximum (i.e. 180 °C) at 10 °C/min and was maintained for 20–30 min. A capillary column was used to separate single gas hydrocarbon compositions (PLOT Al2O3, 50 m × 0.53 mm).
A mass spectrometer (Finnigan MAT Delta S) was used to analyse the carbon isotopes. The temperature started at 33 °C, then rose from 33 °C to 80 °C at 8°C/min, then to 250 °C at 5 °C/min, where it was held for 10 minutes. Each gas sample was measured three times and averaged, yielding an analytical precision of ±0.3 ‰.
The carbon isotope values of natural gases have been established by many scholars before (Dumke et al. Reference Dumke, Faber and Poggenburg1989; Coplen et al. Reference Coplen, Brand, Gehre, Gröning, Meijer, Toman and Verkouteren2006), but owing to many years of distribution, the international reference materials are almost exhausted. Therefore, new standard materials for carbon isotope measurement of natural gases should be established (Dai et al. Reference Dai, Xia, Li, Coleman, Dias, Gao, Li, Deev, Li, Dessort, Dulerc, Li, Liu, Schloemer, Zhang, Ni, Hu, Wang and Tang2012; Huang et al. Reference Huang, Fang, Liu, Fang and Huang2015). The international standard materials NG-1 (coal-derived gas), NG-2 (biogas) and NG-3 (oil-derived gas), which were co-developed by China and laboratories from the USA and Germany (Dai et al. Reference Dai, Xia, Li, Coleman, Dias, Gao, Li, Deev, Li, Dessort, Dulerc, Li, Liu, Schloemer, Zhang, Ni, Hu, Wang and Tang2012), were employed for the carbon isotope analysis. The calibrated values of the NG-1, NG-2 and NG-3 standards are as follows:
$$\eqalign{ & {\rm{NG - 1}}: \cr & {\delta ^{{\rm{13}}}}{{\rm{C}}_{{\rm{methane}}}} = - 34.18 \pm 0.10\permil ,\;{\delta {{\rm{13}}}}{{\rm{C}}_{{\rm{ethane}}}}{\rm{ = - 24}}.{\rm{66}} \pm 0.11\permil , \cr & {\delta {13}}{{\rm{C}}_{{\rm{propane}}}}{\rm{ = - 22}}.{\rm{21}} \pm {\rm{0}}.{\rm{11}}\permil ,\;{\delta ^{13}}{{\rm{C}}_{i{\rm{ - butane}}}} = - 21.{\rm{62}} \pm 0.12.\permil , \cr & {\delta ^{13}}{{\rm{C}}_n}_{{\rm{ - butane}}}{\rm{ = - 21}}.{\rm{74}} \pm .13\permil ,\;{\delta ^{13}}{{\rm{C}}_{{\rm{carbondioxide}}}} = - 5.00 \pm 0.12\permil \cr} $$
$$\eqalign{
& {\rm{NG - 2}}: \cr
& {\delta ^{{\rm{13}}}}{{\rm{C}}_{{\rm{methane}}}}{\rm{ = - 68}}.{\rm{89}} \pm {\rm{0}}.{\rm{12}}\permil \cr} $$
$$\eqalign{&{\rm{NG - 3:}} \cr & {{\rm{\delta }}^{{\rm{13}}}}{{\rm{C}}_{{\rm{methane}}}}{\rm{ = - 43}}{\rm{.61 \pm 0}}{\rm{.09\permil, }}\ {{\rm{\delta }}^{{\rm{13}}}}{{\rm{C}}_{{\rm{ethane}}}}{\rm{ = - 40}}{\rm{.24 \pm 0}}{\rm{.10\permil, }}\cr & {{\rm{\delta }}^{{\rm{13}}}}{{\rm{C}}_{{\rm{propane}}}}{\rm{ = - 33}}{\rm{.79 \pm 0}}{\rm{.09\permil}} \cr} $$
The carbon isotope values of the natural gases were measured on-line. Results are reported in the δ notation in per mil (‰) relative to the Vienna Pee Dee Belemnite (VPDB).
4. Results
4.a. Natural gas components
The geochemical parameters of the natural gas are listed in Table 1. Gas components are expressed in vol. % in this paper. Gases in both the southern and northern parts of the Jingbian gas field are dominated by methane, with relatively lower proportions of heavy hydrocarbon gas (heavy hydrocarbon gas means alkane gas with a carbon number greater than 1; in this study it refers to C2H6–C4H10) and non-hydrocarbon gas.
Table 1. Components and carbon isotope values of natural gas from the northern and southern parts of the Jingbian gas field
The content of heavy hydrocarbon gas is 0.20–2.21 % (0.98 % on average) in the northern part of the Jingbian gas field and 0.20–0.73 % (0.38 % on average) in the southern part of the Jingbian gas field. The south is much lower than that of the north. In other words, the distribution range of the south (0.20–0.73 %) is a subset of the north (0.20–2.21 %). It is worth noting that the content of ethane is 0.16–1.76 % (0.83 % on average) in the northern part and 0.19–0.59 % (0.32 % on average) in the southern part, revealing that the north is almost 2.6 times greater than the south in terms of the average value (Fig. 2). The content of C2H6 and C3H8 in the north is obviously higher than that in the south. The content of C4H10 (including i-C4 and n-C4) in the north can be detected and measured; however, most of the content of C4H10 in the south is zero. Owing to the relatively higher thermal maturity in the south, the heavy hydrocarbon was cracked, leading to the lower content of heavy hydrocarbon in the south compared with the north.
Fig. 2. Average gas components of natural gas in the northern and southern Jingbian gas field, Ordos Basin (data from Table 1).
The CO2 content is higher than in some basins. The 3He/4He ratio of noble gas in the Jingbian gas field is 3.49–6.71 × 10−8, indicating a crustal source origin (Dai et al. Reference Dai, Zou, Hu, Li, Li, Tao, Zhu, Ni, Yang, Huang, Shi, Huang, Zhang, Liu, Xie, Li, Qin, Li, Zhu, Luo, Zhao, Yang, Li, Wang, Jiang, Gong, Tao, Liao, Yu, Gong, Fang, Wu, Meng, Wang and Liu2016b). The carbon isotope values of CO2 range from −4.6 ‰ to −0.4 ‰, which is much larger than the critical value (−8 ‰) of inorganic CO2 (Dai et al. Reference Dai, Song, Dai and Wang1996), suggesting an inorganic origin. A relatively thick limestone unit was developed in the Benxi (C2b) and Taiyuan (P1t) formations in the area, and it is likely that CO2 was the product of thermal cracking of these inorganic carbonates (Feng et al. Reference Feng, Liu, Huang, Gong and Peng2016). The CO2 content of the north ranges from 0.1 % to 5.95 %, with an average value of 3.34 %. The CO2 content of the south ranges from 2.43 % to 14.65 %, with an average value of 5.82 %. The CO2 content of the north is a subset of the south. Because the thermal maturity of the south is higher than that of the north, the thermal cracking of inorganic carbonates in the south produced more carbon dioxide than in the north.
4.b. Carbon isotope compositions
The δ13Cmethane values range from –35.9 ‰ to –31.8 ‰ with an average of –33.8 ± 0.10 ‰ in the northern part of Jingbian gas field, and from –35.2 ‰ to –29.1 ‰ with an average of –32.2 ± 0.10 ‰ in the southern part of Jingbian gas field. The δ13Cethane values range from –31.9 ‰ to –24.6 ‰ with an average of –27.8 ± 0.11 ‰ in the northern part of Jingbian gas field, and from –37.8 ‰ to –32.5 ‰ with an average of –35.5 ± 0.11 ‰ in the southern part of Jingbian gas field. For the northern and southern parts, the interval and mode of the δ13Cmethane values are similar, but the δ13Cethane values are different. The δ13Cethane values in the southern part are much lower than those in the northern part. Moreover, a carbon isotopic reversal, namely δ13Cmethane > δ13Cethane < δ13Cpropane, occurs in the southern part of Jingbian gas field, while a positive carbon isotope series, namely δ13Cmethane < δ13Cethane < δ13Cpropane, is dominant in the northern part of Jingbian gas field.
A line chart of carbon isotope values, which can directly show the carbon isotopic reversal, is widely used to identify the origins of natural gas. There is a linear relationship between the carbon isotopes and the reciprocal of the compositions of thermogenic gases generated from the same source rocks (Chung et al. Reference Chung, Gormly and Squires1988; Rooney et al. Reference Rooney, Claypool and Chung1995). According to the line chart of carbon isotope values (Fig. 3), the δ13Cmethane values of the northern and southern parts distribute almost in the same interval and mode with no obvious difference. However, the δ13Cethane values are different, being much lighter in the southern part. It can be seen from the chart that the carbon isotope values in the southern Jingbian gas field have reversed (δ13Cmethane > δ13Cethane < δ13Cpropane) compared with the northern part.
Fig. 3. Relationship between carbon number 1/Cn and δ13Cn values of natural gas in the northern and southern Jingbian gas field: (a) northern Jingbian gas field; (b) southern Jingbian gas field (data from Table 1).
5. Discussion
5.a. High thermal maturity
The relationship between the content of heavy hydrocarbons (C2+) and the δ13Cethane – δ13Cmethane values can be used to identify the correlation between carbon isotopes and thermal maturity (Fig. 4). In the southern Jingbian gas field, owing to higher thermal maturity, heavy hydrocarbon gases are cracked, leading to a lower content of heavy hydrocarbon gases than in the northern part (Xia et al. Reference Xia, Chen, Braun and Tang2013; Dai et al. Reference Dai, Ni, Huang, Gong, Liu, Feng, Peng and Han2016a). The carbon isotopic reversal occurs in the southern Jingbian gas field compared with the northern Jingbian gas field (Feng et al. Reference Feng, Liu, Huang, Gong and Peng2016; Han et al. Reference Han, Ma, Tao, Huang, Hou and Yao2018; Li et al. Reference Li, Li, Li, Zhang, Cui and Zhu2018). The δ13Cethane – δ13Cmethane values in the southern part are below zero along the y-axis, revealing a rare phenomenon of δ13Cmethane > δ13Cethane. In contrast, the δ13Cethane – δ13Cmethane values in the northern part are above zero (δ13Cmethane < δ13Cethane), with no carbon isotopic reversal. The southern and northern parts of Jingbian gas field are similar in geological setting but different in thermal maturity (Li et al. Reference Li, Zhang, Luo and Hu2008; Dai et al. Reference Dai, Ni, Huang, Gong, Liu, Feng, Peng and Han2016a; Feng et al. Reference Feng, Liu, Huang, Gong and Peng2016; Han et al. Reference Han, Tao, Hou and Yao2017a). Therefore, high thermal maturity is hypothesized to be the main reason for the carbon isotopic reversal in southern part of the Jingbian gas field.
Fig. 4. Relationship between heavy hydrocarbon (C2+) and δ13Cethane – δ13Cmethane of natural gas in the northern and southern Jingbian gas field (data from Table 1).
Figure 4 suggests that the carbon isotopic reversal occurs with the increase in thermal maturity. However, the impacts of thermal maturity on δ13Cmethane and δ13Cethane values are still unclear. Solving this issue may provide important clues as to how the thermal maturity induced the carbon isotopic reversal. To clarify the impact of thermal maturity on δ13Cmethane and δ13Cethane values, charts showing the correlation between heavy hydrocarbons (C2+) and δ13Cn are plotted (Fig. 5).
Fig. 5. Relationship between heavy hydrocarbon (C2+) and δ13Cn values of natural gas in the northern and southern Jingbian gas field (data from Table 1). (a) δ13Cmethane versus C2+; (b) δ13Cethane versus C2+.
As we can see from the chart (Fig. 5b), the δ13Cethane values are obviously different in the southern and northern parts, being much lower in the southern part. In other words, natural gases from the northern Jingbian gas field distribute above the southern Jingbian gas field in the y-axis direction. As mentioned above, the southern and northern parts are similar in geological setting but different in thermal maturity (Li et al. Reference Li, Zhang, Luo and Hu2008; Dai et al. Reference Dai, Ni, Huang, Gong, Liu, Feng, Peng and Han2016a; Feng et al. Reference Feng, Liu, Huang, Gong and Peng2016; Han et al. Reference Han, Tao, Hou and Yao2017a). Therefore, higher thermal maturity leads to relatively lighter δ13Cethane values in the southern Jingbian gas field. As we can see from Figure 5a, the δ13Cmethane values increase with thermal maturity. However, the extent of the increase in δ13Cmethane values is obviously less than the extent of the decrease in δ13Cethane values. This means that when thermal maturity reaches a critical value, its impact on δ13Cmethane and δ13Cethane values is different (Zumberge et al. Reference Zumberge, Ferworn and Brown2012; Xia et al. Reference Xia, Chen, Braun and Tang2013; Han et al. Reference Han, Ma, Tao, Huang, Hou and Yao2018). Under the condition of higher thermal maturity (the southern Jingbian gas field), the relationship between thermal maturity and carbon isotope values is no longer consistent with the conventional carbon isotope kinetic fractionation model (Tang et al. Reference Tang, Perry, Jenden and Schoell2000; Ni & Jin, Reference Ni and Jin2011). When thermal maturity reaches a critical value (the southern Jingbian gas field), δ13Cethane values become relatively more depleted with thermal maturity. Although δ13Cmethane values increase with thermal maturity, the extent is relatively smaller (Han et al. Reference Han, Ma, Tao, Huang, Hou and Yao2018). Finally, the rare phenomenon where δ13Cmethane > δ13Cethane occurs.
δ13Cethane – δ13Cmethane is related to thermal maturity in a negative correlation, and the difference value decreases gradually as thermal maturity increases (Liu et al. Reference Liu, Chen, Liu, Li, Han and Guo2009, Reference Liu, Wu, Wang, Jin, Zhu, Meng, Huang, Liu and Fu2019; Han et al. Reference Han, Ma, Tao, Huang, Hou and Yao2018). A negative difference value indicates a carbon isotopic reversal. The relationship between δ13Cn and δ13Cethane – δ13Cmethane is plotted to identify the genetic types, thermal maturity and carbon isotopic reversal of the natural gas (Fig. 6). Almost all the gas samples from the southern part show carbon isotopic reversals compared to those from the northern part. The δ13Cethane values in the northern part are obviously higher than those in the southern part, with no overlaps in the x-axis direction (Fig. 6b). However, in the y-axis direction, from north to south, with the increase in thermal maturity, the δ13Cethane – δ13Cmethane value is gradually less than zero, which suggests carbon isotopic reversal (δ13Cmethane > δ13Cethane). High thermal maturity leads to the carbon isotopic reversal of natural gas in the southern part. As mentioned above, when the thermal maturity reaches a critical value, with the increase in thermal maturity, the δ13Cethane value will become depleted while the δ13Cmethane value will continuously increase (Xia et al. Reference Xia, Chen, Braun and Tang2013; Feng et al. Reference Feng, Liu, Huang, Gong and Peng2016; Han et al. Reference Han, Ma, Tao, Huang, Hou and Yao2018). However, the extent of the increase in δ13Cmethane value is smaller than the extent of the decrease in δ13Cethane value. As shown in Figure 6, the abscissa interval of Figure 6b is obviously wider than that of Figure 6a. The different impacts of high thermal maturity on δ13Cmethane values and δ13Cethane values will finally lead to a carbon isotopic reversal (δ13Cmethane > δ13Cethane).
Fig. 6. Relationship between δ13Cn values and δ13Cethane – δ13Cmethane of natural gas in the northern and southern Jingbian gas field (data from Table 1). (a) δ13Cethane – δ13Cmethane versus δ13Cmethane; (b) δ13Cethane – δ13Cmethane versus δ13Cethane.
When the thermal maturity reaches a critical value, it will lead to the cracking of the liquid hydrocarbon generated early and the residual kerogen. The mixing of the cracked gas leads to the carbon isotopic reversal in the southern Jingbian gas field. Other scholars made similar conclusions in their studies of the Ordos Basin. According to Xia et al. (Reference Xia, Chen, Braun and Tang2013), high thermal maturity leads to cracking, and the cracked gas then mixes with the primary gas, eventually leading to a carbon isotopic reversal (Fig. 7).
Fig. 7. Relationship between δ13Cn values and R o of natural gases in the Ordos Basin (modified after Xia et al. Reference Xia, Chen, Braun and Tang2013).
This was also confirmed by the δ13Cethane – δ13Cpropane versus Ln(C2H6/C3H8) diagram, and the Ln(C2H6/C3H8) versus Ln(CH4/C2H6) diagram. The diagram identifying the cracked and primary gases was proposed by Prinzhofer & Huc (Reference Prinzhofer and Huc1995). It is based on the results of Berhar et al.’s (Reference Behar, Kressmann, Rudkiewicz and Vandenbroucke1992) simulation experiment, combined with the characteristics of the primary gas of Angola and cracked gas of Kansas. All the gas samples from the Jingbian gas field were plotted onto the diagram, the gases in the south and north distributing in different zones (Fig. 8a). Liu et al. (Reference Liu, Wu, Wang, Jin, Zhu, Meng, Huang, Liu and Fu2019) modified and updated the diagram based on data from Chinese basins, including the Turpan-Hami Basin, Sichuan Basin and Liaohe Basin. Gas samples from the Jingbian gas field were also plotted onto this diagram (Fig. 8b). As we can see from the figure, gas samples from the north distribute in the kerogen cracked gas field (refers to primary gases). However, those of the south distribute in the mixed cracked gas and primary gas field. This further confirms that higher thermal maturity causes the cracking of primary gas, and the mixing between primary and cracked gases finally leads to the carbon isotopic reversal in the southern Jingbian gas field of the Ordos Basin.
Fig. 8. Relationship between δ13Cethane – δ13Cpropane versus Ln(C2H6/C3H8) and Ln(C2H6/C3H8) versus Ln(CH4/C2H6) identifying primary gas and mixed cracked and primary gas (modified after Behar et al. Reference Behar, Kressmann, Rudkiewicz and Vandenbroucke1992; Prinzhofer & Huc, Reference Prinzhofer and Huc1995; Liu et al. Reference Liu, Wu, Wang, Jin, Zhu, Meng, Huang, Liu and Fu2019; data from Table 1).
5.b. Mixing action
Supposing natural gas C is mixed with A and B of different origins, according to the material balance principle, the content of C should conform to the following equation (Jenden et al. Reference Jenden, Draza and Kaplan1993; Wu et al. Reference Wu, Luo, Zhang and Liu2016; Han et al. Reference Han, Ma, Tao, Huang, Hou and Yao2018):
$$C_n^C = {f_A} \times C_n^A + (1 - {f_A}) \times C_n^B$$
The carbon isotopes should conform to the following equation:
$${\delta ^{13}}C_n^C = [\,{f_A} \times C_n^A \times {\delta ^{13}}C_n^A + (1 - {f_A}) \times C_n^B \times {\delta ^{13}}C_n^B]/C_n^C$$
When combining the above equations, f A can be removed to get the following equation:
$${\delta ^{13}}C_n^C = (C_n^A{\delta ^{13}}C_n^A - C_n^B{\delta ^{13}}C_n^B)/(C_n^A - C_n^B)\\ - C_n^AC_n^B({\delta ^{13}}C_n^A - {\delta ^{13}}C_n^B)/(C_n^A - C_n^B) \times (1/C_n^C)$$
The above equation can be further simplified as:
$${\delta ^{13}}C_n^C = a/C_n^C + b$$
where, n is the carbon number, 
$C_n^C$
is the content of C, 
$C_n^A$
is the content of A, 
$C_n^B$
is the content of B, f A is the fraction of A, 
${\delta ^{13}}C_n^C$
is the carbon isotope value of C, 
${\delta ^{13}}C_n^A$
is the carbon isotope value of A, and 
${\delta ^{13}}C_n^B$
is the carbon isotope value of B.
It can be seen from the above equation that for a mixture of gases generated from two different source rocks, there is a linear relationship between carbon isotope values and the reciprocal of the composition content (Coplen et al. Reference Coplen, Kendall and Hopple1983; Wu et al. Reference Wu, Luo, Zhang and Liu2016). Therefore, the chart of correlation between δ13Cethane and 1/C2H6 can be used to verify the mixing of natural gases in the Jingbian gas field. Natural gas in the northern part conforms to the theoretical model of mixed natural gases with different origins (Fig. 9), which is consistent with the confirmed research results (Li et al. Reference Li, Zhang, Luo and Hu2008; Liu et al. Reference Liu, Chen, Liu, Li, Han and Guo2009; Huang et al. Reference Huang, Fang, Liu, Fang and Huang2015; Dai et al. Reference Dai, Zou, Hu, Li, Li, Tao, Zhu, Ni, Yang, Huang, Shi, Huang, Zhang, Liu, Xie, Li, Qin, Li, Zhu, Luo, Zhao, Yang, Li, Wang, Jiang, Gong, Tao, Liao, Yu, Gong, Fang, Wu, Meng, Wang and Liu2016b; Wang et al. Reference Wang, Pang, Zhao, Wang, Hu, Zhang and Zheng2017b).
Fig. 9. Relationship between δ13Cethane values and 1/C2H6 of natural gas in the northern and southern Jingbian gas field (data from Table 1).
In contrast, there is no linear relationship between δ13Cethane and 1/C2H6 in the southern Jingbian gas field. As mentioned above, the southern and northern parts have a similar geologic setting but different thermal maturity (Li et al. Reference Li, Zhang, Luo and Hu2008; Dai et al. Reference Dai, Ni, Huang, Gong, Liu, Feng, Peng and Han2016a; Feng et al. Reference Feng, Liu, Huang, Gong and Peng2016; Han et al. Reference Han, Tao, Hou and Yao2017a); high thermal maturity is the reason for the abnormal phenomenon. In the southern part, owing to relatively higher thermal maturity, the δ13Cethane values and 1/C2H6 values deviate from the mixed genetic theory model.
The mixing of organic and inorganic gases could also lead to the carbon isotopic reversal. The Ordos Basin is a stable cratonic basin, and tectonic movements and faults are not developed (Zhao et al. Reference Zhao, Wilde, Guo, Cawood, Sun and Li2010; Qiu et al. Reference Qiu, Liu, Mao, Deng, Wang and Wang2015; Dai et al. Reference Dai, Zou, Hu, Li, Li, Tao, Zhu, Ni, Yang, Huang, Shi, Huang, Zhang, Liu, Xie, Li, Qin, Li, Zhu, Luo, Zhao, Yang, Li, Wang, Jiang, Gong, Tao, Liao, Yu, Gong, Fang, Wu, Meng, Wang and Liu2016b; Wang et al. Reference Wang, Chang, Yin, Li and Song2017a; Guo et al. Reference Guo, Liu, Wang, Deng, Mao and Wang2018). Therefore, the inorganic mantle gases could not migrate to reservoirs and mix with the organic gases. Dai et al. (Reference Dai, Ni, Qin, Huang, Gong, Liu, Feng, Peng, Han and Fang2017) have also confirmed that the Ordos Basin has no inorganic mantle gas injection. The possibility of the mixture with inorganic mantle gases is excluded.
5.c. Rayleigh fractionation
As the thermal maturity continues to increase, gases may undergo Rayleigh fractionation (Tang et al. Reference Tang, Perry, Jenden and Schoell2000; Valentine et al. Reference Valentine, Chidthaisong, Rice, Reeburgh and Tyler2004; Blaser et al. Reference Blaser, Dreisbach and Conrad2015; Laskar et al. Reference Laskar, Yadava and Ramesh2016; Chang et al. Reference Chang, Shi, Han and Li2017). In this process, 12C, with bond strengths lower than those of 13C, is cracked in priority. Consequently, the δ13Cethane values increase gradually in accordance with the following formula (Galimov, Reference Galimov2006):
$${{\rm{\delta }}^{13}}{{\rm{C}}_{{\rm{2,t}}}} = {\rm{ }}{{\rm{\delta }}^{13}}{{\rm{C}}_{2,0}}-1000({\rm{\alpha }}-1){\rm{ln}}{{\rm{C}}_{{\rm{2,0}}}} + {\rm{ }}1000({\rm{\alpha }}-1){\rm{ln}}{{\rm{C}}_{{\rm{2,t}}}}$$
After further processing, the above formula can be simplified as:
$${{\rm{\delta }}^{{\rm{13}}}}{{\rm{C}}_{{\rm{2,t}}}}{\rm{-}}{{\rm{\delta }}^{{\rm{13}}}}{{\rm{C}}_{{\rm{2,0}}}}{\rm{ = 1000(\alpha -1)ln}}{{\rm{C}}_{{\rm{2,t}}}}{\rm{/}}{{\rm{C}}_{{\rm{2,0}}}}$$
where, δ13C2,0 is the carbon isotope of primary ethane, ‰; δ13C2,t is the carbon isotope of ethane after fractionation, ‰; C2,0 is the initial ethane content, %; C2,t is the content of ethane after fractionation, %; and α is the carbon isotope fractionation factor.
According to the above formula, a linear relationship can be observed between δ13Cethane values and contents of ethane during Rayleigh fractionation. In other words, the δ13Cethane values will gradually increase with the decrease in ethane content. A chart showing the correlation between δ13Cethane values and lnC2H6 is plotted for the southern and northern parts (Fig. 10).
Fig. 10. Relationship between δ13Cethane values and lnC2H6 of natural gas (data for Sichuan Basin from Wu et al. Reference Wu, Luo, Zhang and Liu2016; data for Jingbian gas field from Table 1).
As we can see from Figure 10, there is no negative correlation between δ13Cethane values and lnC2H6 in the Jingbian gas field, which is different from the natural gas of the Cambrian and Sinian systems that experienced Rayleigh fractionation verified in the Chuanzhong Uplift of the Sichuan Basin (Wu et al. Reference Wu, Luo, Zhang and Liu2016). Therefore, Rayleigh fractionation does not occur in the natural gas of the Jingbian gas field. The abnormal carbon isotope values of the natural gas of Jingbian gas field are not induced by Rayleigh fractionation.
Owing to higher thermal maturity in the southern Jingbian gas field, heavy hydrocarbon generated early did crack (Xia et al. Reference Xia, Chen, Braun and Tang2013; Dai et al. Reference Dai, Ni, Huang, Gong, Liu, Feng, Peng and Han2016a; Feng et al. Reference Feng, Liu, Huang, Gong and Peng2016; Wang et al. Reference Wang, Pang, Zhao, Wang, Hu, Zhang and Zheng2017b), but the extent of cracking did not reach the stage of Rayleigh fractionation. In other words, if Rayleigh fractionation had occurred, the δ13Cethane values should gradually increase, but the actual situation is that the δ13Cethane values in the southern region gradually decrease. Therefore, the southern Jingbian gas field did not experience Rayleigh fractionation.
This can also be confirmed from Figure 11 (Bernard et al. Reference Bernard, Brooks and Sackett1978; Whiticar, Reference Whiticar1999). Gases from the northern and southern Jingbian gas field show the characteristics of thermogenetic gas with the same kerogen type, which suggests that they are generated from the same source rocks (Fig. 11). However, the R o of the southern part is obviously higher than that of the northern part. It can also be observed that gas samples from both the southern and northern parts do not show the trend of migration fractionation or bacterial oxidation, suggesting that carbon isotopic reversal in the southern Jingbian gas field is not induced by migration fractionation or bacterial oxidation.
Fig. 11. Relationship between δ13Cmethane and C1/C2+3 of natural gas from the northern and southern Jingbian gas field (modified after Bernard et al. Reference Bernard, Brooks and Sackett1978; Whiticar, Reference Whiticar1999; data from Table 1).
6. Conclusions
The Jingbian gas field is divided into southern and northern parts by thermal maturity, with higher maturity in the southern part. For the southern and northern parts, the interval and mode of the δ13Cmethane values are similar, but the δ13Cethane values are much larger in the northern part than in the southern part. Carbon isotopic reversal (δ13Cmethane > δ13Cethane) occurs in the southern part.
Carbon isotopic reversal in the southern part is attributed to higher thermal maturity and mixing between cracked gases and primary gases. When thermal maturity reaches a critical value, δ13Cethane values becomes relatively more depleted with maturity. Although δ13Cmethane values will increase with thermal maturity, the extent will be relatively smaller. Finally, the rare phenomenon where δ13Cmethane > δ13Cethane occurs. High thermal maturity leads to the secondary thermal cracking of the liquid hydrocarbon generated early and the residual kerogen. Mixing of the cracked gases with primary gases also leads to carbon isotopic reversal in the southern part.
Both of the above mechanisms contain a common premise, which is high thermal maturity. High thermal maturity may affect δ13Cn values. On the other hand, high thermal maturity also leads to cracking, and the mixing of the cracked gases with the primary gases eventually leads to carbon isotopic reversal.
Acknowledgements
I appreciate my tutor, Jinxing Dai, an academician at the Chinese Academy of Sciences, for his guidance and support of my work. We wish to thank Professor Chad Deering, Editor, for his invaluable work and kindness. We are also grateful to the constructive comments on this manuscript by Dr. Mi and an anonymous reviewer. This work was financially funded by the National Natural Science Foundation of China (Grant No. 41772120), National Natural Science Foundation of China (Grant No. 41272139) and the Chinese National Science and Technology Major Project ‘Formation conditions, enrichment regularity and exploration potential of tight oil’ (No. 2016ZX05046-001).
