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Isotopic Composition of Abiogenic Gas Produced in Closed-System Fischer-Tropsch Synthesis: Implications for the Origins of the Deep Songliao Basin Gases in China
In this study, closed-system Fischer-Tropsch synthesis was conducted at 380°C and 30 MPa for 72 h with magnetite as a catalyst. The isotopic composition of the closed-system Fischer-Tropsch synthesis gas and the composition of known abiogenic gas were systematically studied, and the deep Songliao Basin gas was also investigated. The results show that closed-system Fischer-Tropsch synthesis of gaseous hydrocarbon isotopes exhibits a partial reverse order, which includes the reverse order of methane and ethane such as δ13C-C1>δ13C-C2<δ13C-C3 and δ2H-C1>δ2H-C2<δ2H-C3. Furthermore, experimental data on the control of NaBH4 content indicates that the carbon isotopes demonstrate a reverse order on condition that the H2/CO2 (mole ratio) is equal to or greater than 4.0; meanwhile, the hydrogen isotopes show a normal order. The deep Songliao Basin hydrocarbon gas component is similar to thermogenic gas and has a trend of a transition to oceanic hydrothermal system abiogenic gas. In addition, the deep Songliao Basin gas isotopic pattern is different from both Lost City and Kidd Creek where the deep Basin gas carbon isotopic pattern has a reverse order, and the hydrogen isotopic pattern has a normal order. Therefore, the deep Basin gas might be a mixture of the oil-type gas and the coal-formed gas, which could be the cause of the isotopic reverse.
Generally, hydrocarbons are mainly derived from microbial decomposition of organic matter [1–3] and organic matter thermal degradation [2, 4]. However, abiogenic hydrocarbons are produced by chemical reactions that do not directly involve organic matter and are present in trace amounts in high-temperature volcanic/geothermal fluids and magma systems [5–7]. The researchers found that a large proportion of abiotic hydrocarbons (up to 90 vol.%) associated with low-temperature gas-water rock interactions were present in Precambrian crystalline shield, submarine peridotite hydrothermal system, continental ophiolite, and serpentinized ultramafic rocks in peridotite blocks [8–12]. Abiogenic gas associated with continental serpentinized ultramafic rock systems have been found in many countries from North America, Europe, and Asia to Oceania [8, 13–18]. Typical characteristics of the gas include a high concentration of methane (CH4, usually greater than 80 vol. %), variable amounts of hydrogen (H2) and C2+ alkanes (ethane, propane, and butane), and typical combinations of stable C and H isotopes of CH4, which overlap only partially with biological (thermal) gases [9, 19]. In fact, abiogenic gas is related to a process known as Fischer-Tropsch synthesis (FTS) [20–22].
Fischer-Tropsch synthesis (FTS) was first developed by German chemists Franz Fischer and Hans Tropsch in 1926, and it is a chemical process that converts carbon monoxide from coal into liquid hydrocarbon-based fuels and lubricants . Usually, it can be defined as a heterogeneous catalytic reduction of carbon dioxide using molecular hydrogen, which is widely considered to be a process that may lead to the presence of organic compounds in meteorites, submarine hydrothermal systems, and igneous rocks [24–29]. Organic compounds produced by Fischer-Tropsch synthesis (FTS) are used to explain the existence of hydrocarbons in igneous rocks and hydrothermal fluids, and they are involved in a variety of geological processes, including the production of methane and hydrocarbons deep in the crust, which provides nutrients for microorganisms in underground and hydrothermal environments (; Szamtmari, 1989; [22, 27, 31–33]).
It has previously been observed that some abiogenic CO2 and biogenic hydrocarbon gas reservoirs have been found successively in the Xujiaweizi fault depression of the Songliao Basin [34, 35]. Most of these gases are produced by source rocks and have carbon isotopic reversals (; ) . Their genetic origin has been debated for a long time (e.g., Guo et al. 1997; [37–41]), mainly because of their common carbon isotopic reversals. Previous studies have found that the deep natural gas in the Songliao Basin is dominated by hydrocarbon gas. The carbon isotope composition of methane is relatively heavy (>-30.0‰), and the carbon isotopic reversal trend is general, so it is believed that there are abiogenic alkane gases in the Xujiaweizi fault depression, which is considered to be related to mantle degassing (; Guo et al. 1997; [43, 44]). Others have argued that the mixing of different types of natural gas formed by organic matter in the same formation is the main reason for those results . In this study, the experimental data and the isotopic composition of abiogenic gas were systematically studied with closed-system Fischer-Tropsch synthesis and pyrolysis at 380°C and 30 MPa, and the deep Songliao Basin gas was also investigated to provide more information for exploring the sources of the deep Songliao Basin gas.
2. Geological Setting
Songliao Basin is a complex faulted basin in which the Lower Cretaceous is dominated by faulted sedimentation and contains a natural gas reservoir, and the Upper Cretaceous is dominated by depression sedimentation and contains a petroleum reservoir . The Xujiaweizi fault depression is located in the north part of the Songliao Basin (Figure 1(a)). From younger to older, the deep strata of the Xujiaweizi fault depression comprise the Denglouku Formation, Yingcheng Formation, Shahezi Formation, and Huoshiling Formation (Figure 1(b)) . Among these, the deposition periods of the Huoshiling and Yingcheng formations were the main developing times for volcanic rocks in fault depression, the deposition period of Shahezi Formation was the main developing period for hydrocarbon source rocks in the fault depressions, and the Denglouku Formation was the regional cap rock of the natural gas in the deep fault depression .
The Xujiaweizi fault depression mainly develops two sets of source rocks of the Lower Cretaceous Shahezi Formation (K1sh) and Yingcheng Formation (K1yc). The source rocks of Shahezi Formation are widely distributed, mainly in the central, western, and northern parts of the fault depression, with a thickness of more than 200 m, whereas source rocks in the Yingcheng Formation are mainly distributed in the Xuzhong area and the southern part of the fault depression with a maximum thickness of 160 m. The TOC average values of Shahezi Formation and Yingcheng Formation are 2.43% and 1.41%, respectively . The organic matter types are mainly type III, and the vitrinite reflectance Ro average values are 2.36% and 2.24%, respectively . Both sets of source rocks are in stages of high to overmature evolution. The main deep gas reservoir types include the Denglouku structural gas reservoir, Yingcheng Formation volcanic lithologic gas reservoir and basement lithologic gas reservoir. The Xujiaweizi fault depression is the most abundant hydrocarbon gas reservoir in volcanic rocks around the world  with proven gas reserves exceeding 250 billion cubic meters that have been discovered since the milestone well XS1 was drilled in 2002. Gas fields, such as Wangjiatun, Songfangtun, Changde, Nongan, and Qingshen, have been discovered in the Xujiaweizi fault depression and its surroundings.
3. Fischer-Tropsch Synthesis
Abiogenic synthesis of hydrocarbons has been discussed since 1940 . Many methods have been described, and although the researchers have not yet reached a conclusion, it is generally believed that hydrocarbons could be produced by reduction of CO2 via an aqueous Fischer-Tropsch synthesis (FTS) reaction. The Fischer-Tropsch reaction is a common industrial process invented by German scientists Franz Fischer and Hans Tropsch in the 1920s. The mass balance equation is as follows:
Although the first study of FTS began in the early 20th century, the mechanism of hydrocarbon formation from CO and H2 is still controversial because the whole process is a very complex binding chain that is simultaneously and continuously reacting on the surface of changing metal-oxide-carbide [50–52]. The composition of FTS products differs essentially from the equilibrium composition [53–55]. FTS is a process controlled by kinetics, and the distribution of products depends on the properties of catalysts and synthesis conditions. The main initial step is to adsorb H2 and CO on the metal surface. The activity and selectivity of catalysts depend mainly on the properties of CO adsorption-desorption-dissociation. Compared to H2, CO can be adsorbed on metal and oxide surfaces . The simple rules of FTS indicate that more olefins and carbon dioxide are formed on iron catalysts, whereas more alkanes and water are produced on nickel and cobalt catalysts . In hydrothermal systems, carbon dioxide is the most likely reactant, and many laboratory experiments [57–63] have shown that saturated hydrocarbons and other organic compounds can be generated by FTS reactions with carbon dioxide as an indirect carbon source. In addition, it has been suggested that in this case, the first stage of the FTS reaction is to form CO by reducing CO2 by H2:
4. Experiments and Methods
4.1. Experimental Conditions and Materials
The Xujiaweizi fault depression is a gas-bearing fault depression with the highest exploration degree in the Songliao Basin. The buried depth of the gas reservoir is generally 3000-4000 m with an average of approximately 3500 m . The paleogeothermal gradient in the Xujiaweizi fault depression is higher than the present average value of 4°C/100 m with a maximum of 5°C/100 m at the end of the Cretaceous (~65 Ma) [65, 66]. The maximum temperatures experienced by reservoirs and source rocks thus occurred before the end of the Cretaceous, except in some areas where they may have been influenced by volcanic activity [66, 67]. In this study, the chosen laboratory conditions (380°C and 30 MPa) of the Fischer-Tropsch synthesis experiments might be representative of the thermodynamic conditions of the Songliao reservoir. In addition, two types of material were selected to carry on the Fischer-Tropsch reaction, including the sodium materials, which are NaHCO3 and NaBH4. NaHCO3 is prepared by CO2 that is taken from the Fangshen-9 Well in the Songliao Basin, China. It has been established that the CO2 in the Fangshen-9 Well has an abiogenic origin. Therefore, the purpose of choosing CO2 in the Fangshen-9 Well to simulate the formation process of deep gas in the Songliao Basin was to make the simulation as realistic as possible, allowing for the exploration of the origins of the gas. The CO2 and NaOH solution is fully reacted and crystallized to obtain NaHCO3. After these two kinds of materials are mixed, the NaHCO3 heating decomposition produces water that reacts with NaBH4 to produce the hydrogen. At a certain temperature and pressure, the hydrogen and carbon dioxide undergo Fischer-Tropsch synthesis. Magnetite is a ubiquitous component of ultramafic-hosted hydrothermal systems (Alt & Shanks, 2003), which is why it was selected for this series of experiments. Magnetite was used as the catalyst in the experiment, and the chemical reaction equation was as follows:
4.2. Experimental Procedure
The pyrolysis experiment was conducted in a closed system following the procedures described in detail by Tao . All pyrolysis experiments were performed in gold cell reactors (). Approximately 5 mg of NaBH4, 5 mg of magnetite, and 40 mg of NaHCO3 prepared by CO2 were taken from the Fangshen-9 Well in the Songliao Basin, China, and were loaded into one gold tube, which was then welded on one end. The gold tube was flushed with argon for approximately 15 minutes to ensure complete removal of air and then sealed in an argon atmosphere using arc welding. After that, the sealed gold was placed in a stainless-steel pressure cooker, and then approximately 10 ml of water was placed in a container connected to a pressurized water line. The pressure device consisted of an air compressor and a booster pump that drives high pressure water into an autoclave. The sample in the autoclave was heated to a target temperature in a single oven. During pyrolysis, the pressure in the autoclave was adjusted by adding water from the pump or removing water from the autoclave through a leak valve. The experiment was carried out at a temperature of 380°C and a pressure of 30 MPa for up to 72 h.
4.3. Product Analysis
The pyrolysis products of Fischer-Tropsch synthesis include hydrocarbon gases and nonhydrocarbon gases. In the analysis of hydrocarbon composition, the gold cell was placed in a vacuum system and pierced with a needle, and the gas products were released and collected by a Toppler pump for quantitative analysis. Then, the composition of the gas products was analyzed using Agilent 6890N-Wasson gas chromatography with a PoraPLOT Q capillary column ( id). The oven temperature was maintained at 70°C for 6 min, then increased from 70°C to 130°C at 15°C/min, from 130°C to 180°C at 25°C/min, and then maintained at 180°C for 4 min. Nitrogen was used as a carrier gas, and experiments were carried out at 180°C using FID and TCD detectors. C1-C5 hydrocarbons, H2S, H2, and CO2 (detection limits of CO2 and H2: 40 ppmv; H2S: 150 ppmv; HC: 10 ppmv; precision 2%; 10% at the detection limit) were quantitatively analyzed by using the external standard method.
Gas carbon isotope analysis was performed by gas chromatography-isotope ratio mass spectrometry (GV IsoPrime™ GC-IRMS) with a capillary column (). The temperature program was as follows: using helium as a carrier gas, the oven temperature was kept at 60°C for 3 min, rose to 180°C with a 25°C/min heating program, then held at 180°C for 10 min. Gas samples were analyzed in duplicates, and the stable carbon isotopic values are reported in the δ-notation in per mil (‰) relative to VPDB. Precision for individual components in the molecular δ13C analysis is ±0.3‰.
GC-IRMS (Finnigan Delta Plus XL) with a capillary column (HP-PLOT, ) was used to analyze the hydrogen isotope of hydrocarbon gases (C1-C3). The temperature program is as follows: the temperature was held at 50°C for 7 min, and then the temperature rose to 180°C at a rate of 30°C/min. The stable hydrogen isotopic ratio (δ2H) values are reported in the δ-notation in per mil (‰) relative to VSMOW and the reproducibility and precision of isotope values are expected to be ±3‰.
5. Results and Discussion
5.1. Gas Isotope Composition
The Fischer-Tropsch synthesis experiments were carried out under closed-system gold tube-high pressure vessels at 380°C and 30 MPa. The gas yields and gas isotopic compositions are shown in Table 1. The results show that the carbon isotopes (PDB, ‰) and hydrogen isotopes (SMOW, ‰) of gaseous hydrocarbons exhibited normal order or partial reverse order, which is the reverse order of methane and ethane, such as and (Figure 2). Hu et al.  also observed that the carbon isotopes have a reverse order or partial reverse order, whereas the Fischer-Tropsch synthesis experiments were carried out under a closed system with an iron-based catalyst for CO and H2 at 270-300°C, 0.7-2.0 MPa. Other scholars have reported that hydrogen isotopes show in a hydrothermal system; however, the carbon isotopes of gaseous hydrocarbons are heavier with an increase in carbon number, and carbon isotopes have a normal order [21, 70]. Fu et al.  indicated that carbon isotopes do not have a reverse order in a closed system with an iron-based catalyst for CO and H2 at 400°C, 50 MPa. In addition, Fischer-Tropsch synthesis experiments were carried out in an open system with an iron-based catalyst for CO and H2 at 260-300°C, 3 MPa, by Taran et al. ; the results showed that the carbon isotopes of gaseous hydrocarbons were reversed only for a low conversion rate of CO, and they considered that other processes (such as a simple mixing of two or more end members) or other P-T conditions of the carbon reduction could be responsible for the “inverse” isotopic trend found in meteorites and some natural gases. To this end, FTS experiments were carried out while controlling the NaBH4 content (Table 1). According to Zhang & Duan , the ethane was probably oxidized when the mantle-derived gas was migrated to the crust. Mantle fluids rose to the boundary of lithosphere-asthenosphere, and the composition of the fluids changed from the mixture of H2O-CH4-H2-C2H6 to the mixture of H2O-CO2-CO. The mantle fluids H2/CO2 (mole ratio) are generally less than 1.2 under the conditions of high temperature and high pressure. The FTS experimental gaseous hydrocarbons with the control of NaBH4 carbon content and hydrogen isotope patterns are shown in Figure 3. When the H2/CO2 (mole ratio) is less than 1.2, the gas carbon isotopes are a normal order and show a linear increase in carbon numbers; when the H2/CO2 (mole ratio) is more than 1.2, the carbon isotopes of methane are heavier than ethane, and the carbon isotopes of C2+ are heavier and show a linear increase in carbon numbers. When the H2/CO2 (mole ratio) is equal to or greater than 4.0, the carbon isotopes have a reverse order. Compared to carbon isotopes, the hydrogen isotopes show when H2/CO2 (mole ratio) is less than 4.0, and the hydrogen isotopes of propane are close to ethane with an increase in H2/CO2 (mole ratio). When H2/CO2 (mole ratio) is equal to or greater than 4.0, the hydrogen isotopes of propane are heavier than ethane, and the hydrogen isotopes show a normal order.
5.2. The Characteristics of Abiogenic Gas
Sherwood Lollar et al.  used stable isotope signatures to suggest that CH4 and higher hydrocarbon gases (ethane, propane, and butane) at Kidd Creek mine on the Canadian Shield are produced abiogenically by water-rock interactions, such as surface-catalyzed polymerization [24, 53], metamorphism of graphite-carbonate bearing rocks [75–77], and other gas-water-rock alteration reactions, such as serpentinization [20, 78–83].
According to Sherwood-Lollar et al. , the isotopic pattern of abiogenic gas of Precambrian Shield sites in Canada is shown in Figure 4(a). It was indicated that δ13C of methane is the heaviest, which is distributed in -32‰~-36‰ (PDB); however, δ13C of ethane is in -36‰~-39‰ which is the lightest in hydrocarbon gas, and C1-C2 shows a depletion trend while C2-C5 shows a consistent trend of isotopic enrichment in 13C with increasing molecular weight. The pattern of hydrogen isotopic variation had consistent 2H enrichment with increasing molecular weight, which is a positive sequence (Figure 4(b)). Proskurowski et al. (2008) pointed out that the carbon isotope compositions of C1 to C4 hydrocarbons from Lost City fluids are increasingly negative (δ13C ranges from -9‰ to -16‰, PDB) with increasing chain length (Figure 4(a)). The hydrogen isotopic composition of Lost City C1 to C3 hydrocarbons shows a similar, although less defined, trend in which molecules with longer chain lengths have similar or slightly lower δ2H values (-120‰ to -170‰, SMOW) relative to shorter-chain alkanes (Figure 4(b)). As shown in Figure 4, the carbon and hydrogen of the closed-system FTS gaseous hydrocarbon are distributed between the Kidd Creek and Lost City.
5.3. The Deep Gas in the Songliao Basin and Abiogenic Gas
5.3.1. Gas Component
The composition of the deep Songliao Basin hydrocarbon gas [41, 84], Fischer-Tropsch experimental gas (Table 1), and the abiogenic gas of Kidd Creek  is shown in Figure 5. As shown in Figure 5, the deep Songliao Basin hydrocarbon gas is similar to thermogenic gas but has a different variation with abiogenic gas and has a trend of a transition to oceanic hydrothermal system abiogenic gas (Proskurowski et al. 2008). Therefore, the deep Basin gas might be a mixture of the oil-type gas and the coal-formed gas, which could be the cause of the isotopic reverse; the closed-system FTS gas is similar to the hydrothermal system and has a trend of transition to the Kidd Creek (Figure 5).
5.3.2. Isotopic Characteristics
Previous studies have found that natural gases in the deep strata of the Songliao Basin are dominated by alkane gases, the carbon isotope composition of methane is heavier (>-30.0‰) and the carbon isotopic reversal trend is general . The carbon isotopes of the deep Songliao Basin hydrocarbon gas become more depleted in 13C with increasing molecular mass (), which is a reverse order . Wang et al.  suggested that natural gases from the Songliao Basin show two different distribution patterns of δ13C values due to differences in precursors and mechanisms of hydrocarbon formation as well as the kinetic isotope fractionation of alkane carbon isotopes. Alkanes formed by the degradation of sedimentary organic matter show lighter δ13C-C1 values (-30.2‰~-58.3‰) with a normal distribution of δ13C values for methane homologues. Abiogenic natural gases show heavier δ13C-C1 values (-30.5‰~-16.7‰) with a reverse distribution of δ13C values similar to alkanes from the Murchison meteorite and polymerization, whereas the δ2H values are featured by a normal distribution. Figure 6 shows the carbon-hydrogen isotope variation with carbon numbers (data from Proskurowski et al. 2008; [11, 84]). As shown in Figure 6, the deep Songliao Basin gas isotopic pattern is different for both Lost City and Kidd Creek in which the deep Basin gas carbon isotopic pattern has a reverse order, and the pattern of hydrogen has a normal order. The abiogenic gas of Kidd Creek only shows , yet the carbon isotopes of C2+ have a normal order, and the carbon and hydrogen isotopes of the Lost City are the reverse order.
5.3.3. Isotope Fractionation
According to previously reported data ([21, 69, 71, 72]; Proskurowski et al. 2008; [84, 85]), the CO/CO2 and methane carbon isotope fractionation diagram was added up for different natural environments and experimental conditions (Figure 7). Overall, the CO2 and methane carbon fractionation is between 15‰ and 25‰ in the natural environments, and under experimental conditions, the CO/CO2 and methane carbon isotope fractionation changed greatly between 8‰ and 40‰, which is related to the conversion rate [69, 72]. The conversion of CO/CO2 and carbon isotope fractionation has a negative correlation. It should be noted that the methane carbon isotope variation is limited to the deep Songliao Basin gas which seems to have nothing to do with the CO2 carbon isotope (Figure 7). The results suggest that the conversion of mantle CO2 to synthesize methane is low, and there is a supplement of organic CO2, which implied that there is a complicated relationship between the two.
During the formation of abiogenic gas, hydrogen isotope fractionation also occurred (Figure 8). As shown in Figure 8, the carbon isotope and hydrogen isotope fractionation all occurred in abiogenic synthesis to methane, and the carbon isotope fractionation increased with the conversion as the hydrogen isotope fractionation decreased with the conversion. It is estimated that the methane carbon isotope fractionation was in the 20‰ and 25‰, and the early hydrogen isotope fractionation was 170‰ while the late fractionation was 80‰. The carbon isotope and hydrogen isotope fractionation of the condensate oil was 20‰ and 180‰. Closed-system Fischer-Tropsch synthesis experimental data (Table 1) shows that a carbon isotope of methane has increased with a decrease in C1/C2, which means that the carbon isotope fractionation is reduced and is probably related to high conversion and polymerization [11, 84].
Closed-system Fischer-Tropsch synthesis and pyrolysis were carried out at 380°C and 30 MPa; the experimental data and the isotopic composition of abiogenic gas were systematically studied, and the deep Songliao Basin gas was also investigated in this study, producing following preliminary conclusions:
(1) The results show that carbon isotopes and hydrogen isotopes of Fischer-Tropsch synthesis gaseous hydrocarbons exhibit normal order or partial reverse order, which is the reverse order of methane and ethane such that and .
(2) It is suggested that carbon isotopes of gaseous hydrocarbons showed a reversal or a reverse order only at low conversion rates of CO2; when the H2/CO2 (mole ratio) is equal to or greater than 4.0, the carbon isotopes show a reverse order, while the hydrogen isotopes show a normal order.
(3) The gas component and isotopic pattern suggests that the deep Songliao Basin gas might be a mixture of oil-type gas and coal-formed gas, which could be the cause of the isotopic reverse.
The experimental data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
This work was financially supported by the Chinese Academy of Sciences Key Project (Grant Nos. XDB10010202 and XDB10030404), the National Natural Science Foundation of China (Grant Nos. 41572350 and 41503049), the National Key R&D Program of China (Grant No. 2017YFA0604803), the Western Light Project and the Key Laboratory Project of Gansu (Grant No. 1309RTSA041).
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