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Carbon and Hydrogen Isotopic Reversals in Highly Mature Coal-Derived Gases: A Case Study of Paleozoic Gases in the Southern Ordos Basin, China
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.
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  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 .
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 , (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 ; 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 . This basin is a petroliferous cratonic basin with stable subsidence, depression displacement, visible torsion, and multiple cycles of deposition . 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  and providing an unprecedented opportunity to improve our understanding of the geochemistry of overmature gas systems.
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].
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.
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 . The regional seal of the upper Paleozoic reservoirs consists of stable, laterally distributed, Shangshihezi and Shiqianfeng formation lacustrine mudstones .
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 .
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.  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.
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. .
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.
4.1. 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  makes the generation of CO2 possible at any temperature.
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.  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).
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.  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  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.  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 , 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)).
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).
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% . 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 . The δ13C-C2H6 content of Ordos gases increases with decreasing wetness (Figure 6(b)).
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  modified the diagram after Bernard et al.  and Faber and Stahl  to identify primary gases and gases altered by secondary effects. Based on previous work, Dai et al.  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 . 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.
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 . Such gases are typically characterized by δ13C-CH4>δ13C-C2H6>δ13C-C3H8>δ13C-C4H10, with δ13C-CH4> − 25‰ . 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 . Mantle-sourced helium in all cases has ratios of >0.1 . 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 , (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  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; ). 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 . 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.
(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 . 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.  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.
Other models have been proposed to explain carbon isotopic reversal in overmature shale gases, including mass transport , 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.  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  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.  and Erdmann and Horsfield , 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  showed that the results of Dieckmann et al.  and Erdmann and Horsfield  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.
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 , 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 .
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).
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  and redox-active transition metals . 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 .
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 .
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 . 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. .
(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
The data used to support the findings of this study are included within the article.
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.
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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