Yields and relative contents of TRS reaction products
Yield of CH4, CO2, and H2S from direct thermal cracking of crude oil are significantly lower than those resulting from cracking of a crude oil and MgSO4 solution under similar experimental conditions (Table 1). Increased yields of gaseous products might be attributed to the involvement of sulfur in the reaction, which may also trigger TSR37,38, converting heavier hydrocarbons into CH4, CO2, and H2S12. Since the activity energy for alteration of hydrocarbons TSR is lower than that for thermal cracking38, gas yields from the crude oil and MgSO4 solution is higher than those for thermal cracking of crude oil alone under the same pyrolysis conditions (temperature and time). Compared to the increase of CH4 yield during thermal cracking of crude oil, the CH4 yield during thermal cracking of crude oil and MgSO4 rapidly increased in the first 72 h at 360 °C, and then slightly decreased (Fig. 1a), which may be related to the oxidation of methane to H2S and CO2 during TSR12,27. The H2 yield from thermal cracking of crude oil gradually increased with time up to 219 h, but the H2 yield from crude oil and MgSO4 decreased from 0.23 ml/g to 0.15 ml/g until 72 h, and remained almost constant at less than 0.15 ml/g as the reaction continued (Fig. 1b). This observation suggests that TSR may have a very limited effect on H2 formation during pyrolysis. During the reaction of crude oil and MgSO4, the yield of H2S and CO2 increased rapidly before until 72 h (Fig. 1c, d), after which the CO2 yield remained almost constant as the H2S yield decreased slightly. Because the presence of a MgSO4 solution introduces sulfur and oxygen into the pyrolysis system, the yield of H2S and CO2 increased12,37. After 72 h, the yield of CH4 and H2S slightly decreased (Fig. 1a, d), indicating that the TSR process consumed these components to some extent and led to the production of CO2 and sulfur27,40,43. The dryness coefficient (C1/C1–5) of the gas formed by thermal cracking of crude oil is low, while the dryness coefficient of gas formed during pyrolysis with TSR is higher, accompanied by an increased CH4 content (Table 1), which further indicates that oxidation of heavy hydrocarbons by TSR will generate CH412,40,43.
Because the source gas of CO2 and H2S is produced during the TSR process, the ratios of CH4/CO2 and (CO2 + H2S)/(CO2 + H2S + ∑C1−5) were used to investigate variations of the chemical composition of gas altered by TSR. As shown in Fig. 2, ratios of (CO2 + H2S)/(CO2 + H2S + ∑C1−5) sharply decrease with increasing CH4/CO2 ratios. The ratio of CH4/CO2 for crude oil, C9, and MN with MgSO4 is less than 1.0, while the ratio of CH4/CO2 for thermal cracking of crude oil is above 1.0 (Fig. 2a). In contrast to the ratios of CH4/CO2 and (CO2 + H2S)/(CO2 + H2S + ∑C1−5) for crude oil, the same ratios for C9 and MN with MgSO4 are less than 1.0, and the ratio of (CO2 + H2S)/(CO2 + H2S + ∑C1−5) decreases as the CH4/CO2 ratio increases. Compared to the wide range of ratios for pyrolysis of crude oil and MgSO4, the ratio of CH4/CO2 and (CO2 + H2S)/(CO2 + H2S + ∑C1−5) for C9 or MN with MgSO4 shows a smaller range (Fig. 2b). These variations of CH4/CO2 and (CO2 + H2S)/(CO2 + H2S + ∑C1−5) caused by thermal cracking of crude oil and different degrees of TSR alteration during pyrolysis are similar to those caused by thermal cracking and TSR alteration of natural gas in gas reservoirs1, 20,21. Thermal cracking of crude oil produced more CH4, while TSR increased the yield of CO2 and H2S. The content of CH4 in alkane gas further increased with increased thermal cracking. In contrast, the CO2 and H2S contents varied at different stages of TSR. Based on the properties of H2S-bearing natural gas and previous simulation results1,6,12,23,40, variations in gas produced by TSR pyrolysis are similar to those of H2S-bearing natural gas altered by TSR.
Fractionation characteristics of carbon and hydrogen isotopes during TSR
The carbon isotopic composition of produced CO2 from crude oil, C9, and MN with MgSO4 at different temperature is negatively correlated with heating time. The δ13CCO2 value gradually decreased from − 2.1‰ (C9, 360 °C, 4 h) to − 28.5‰ (crude oil, 360 °C, 219 h) (Table 2, Fig. 3). The carbon isotopic composition of CO2 from crude oil, C9, and MN with MgSO4 became significantly lighter over time until about 20 h heating time. After that, the δ13CCO2 value for crude oil and C9 with MgSO4 remained mostly constant, which is similar to what happens during thermal cracking of crude oil. The decrease of δ13CCO2 values during the first 20 h can be attributed to a greater fractionation of carbon isotopes at the onset of TSR12. After 20 h, the carbon isotope fractionation gradually reached a balance between CO2 and CH4. Although the δ13CCO2 value for CO2 produced from MN with MgSO4 decreased with heating time, CO2 was relatively more enriched in 13C when compared to crude oil, C9, and MgSO4. In addition to having a heavy carbon isotopic composition, MN was also the most easily oxidised component during TSR. Due to the addition of MgSO4 to MN, MN was quickly oxidised by TSR and converted to gas with CO2 as the main component. Concentrations of heavy hydrocarbon gases were below the detection limit of the instruments. In summary, the transformation of crude oil, C9, and MN by TSR converts 12C-rich hydrocarbons to 12C-enriched CO2, which might be converted to 12C-rich calcite and precipitated in gas reservoirs9,21.
The δ13C value of gas produced from crude oil, C9, and MN with MgSO4 at different temperature and heating times is shown in Fig. 4. With increasing carbon numbers, the carbon isotopes of alkane gas from thermal cracking of crude oil at a pyrolysis temperature of 360 °C became heavier in the order of δ13C1 < δ13C2 < δ13C3.
The δ13C1 values gradually increased over a relatively large range, while both δ13C2 and δ13C3 increased in a more narrow range as heating time increased (Fig. 4a). The carbon isotopic composition of alkane gas produced from crude oil and MN with MgSO4 also became heavier with increasing carbon number in the order of δ13C1 < δ13C2 < δ13C3. However, at the same temperature, with the addition of MgSO4 solution, δ13C1 gradually increased in a relatively narrow range, while both δ13C2 and δ13C3 showed larger increase as the reaction proceeded (Fig. 4b–d). It is obvious that the δ13C1 of alkane gas produced in the presence of a MgSO4 solution is higher than that produced during comparative experiments without MgSO4. In a single thermal system, the δ13C of alkane gas from thermal cracking of crude oil gradually becomes higher with increasing carbon number, and is linearly correlated with the reciprocal of the carbon number (1/n)44. The reduction in the variation of δ13C1 values produced by TSR alteration indicates that these hydrocarbons are rapidly oxidised to CH4 with a δ13C1 value similar to the source material. The TSR process leads to a 13C increase in CH4, producing a carbon isotopic composition more similar to that of crude oil (− 32.8‰). In contrast, CO2 becomes gradually enriched in 12C due to equilibrium fractionation of carbon isotopes between CO2 and CH4. Overall, the variation of δ13C1 is significantly smaller in the presence of MgSO4, during TSR of crude oil, similar to that of H2S-bearing alkane gas in the Sichuan Basin, where natural gas shows a heavy carbon isotopic composition of CH420. The carbon isotope composition of CO2formed in the presence of TSR is lighter than that formed in the absence of TSR, because TSR will oxidise a large portion of the hydrocarbons, leading to more intense δ13CCO2 fractionation. Therefore, δ13CCO2 values related to crude oil cracking are relatively heavier, while δ13CCO2 will be relatively enriched in 12C during pyrolysis (with TSR) in the presence of a MgSO4 solution (Table 2).
Our experiments show for the first time that the carbon isotopic composition of gas produced from C9 with MgSO4 became partially reversed to δ13C1 > δ13C2 < δ13C3 after a heating time of 24 h. The CH4 produced from C9 with MgSO4 shows an extremely small variation in δ13C1 values. The variation of δ13C2 became larger than that of δ13C3 with longer reaction time (Fig. 4e). This partial reversal of the carbon isotope series of alkane gas is similar to that of H2S-bearing alkane gas in the Sichuan Basin, Ordos Basin, and other locations1,16,23. Therefore, it is likely that the partial reversal of the carbon isotope series of alkane gas in H2S-bearing natural gas reservoirs happens when light hydrocarbons are altered by TSR. The isotopic composition of reaction products of pyrolysis with MN could not be detected due to the low content of alkane gases.
Figure 5 shows the variation of δ2H–Cn in alkane gases produced from crude oil at 360 °C, from a mix of crude oil and MgSO4 at 350, 360, and 370 °C, as well as from a mix of C9 and MgSO4 at 360 °C. Similar to the δ13C values, the hydrogen isotopic composition of alkane products from crude oil and a mixture of crude oil and MgSO4 became heavier with increasing carbon number under all conditions, i.e. δ2H–C1 < δ2H–C2 < δ2H–C3. The δ2H–C1 values gradually increased in a relatively large range, and both δ2H–C2 and δ2H–C3 increased in a narrower range as heating time increased (Fig. 5a). The δ2H–C1 value for a mix of crude oil and MgSO4 gradually increased over time, but in a relatively small range for each temperature. The δ2H–C2 and δ2H–C3 values also increased with heating time, but in a much larger range compared with that of δ2H–C1 (Fig. 5b–d). The variation of δ2H–C2 is slightly larger than that of δ2H–C3. The δ2H–C1 values remained almost constant during the TSR alteration of C9, while the δ2H–C2 values show much larger variations. The δ2H–C3 show the largest variations, but a reversed trend of the hydrogen isotopic composition of alkane gas was not observed during pyrolysis (Fig. 5e). These results indicate that the TSR can reduce the variation of δ2H–C1, possibly due to the similar hydrogen isotopic composition of the reaction product (C1 gas) and that of the precursor and/or the involvement of hydrogen derived from water21. The δ2H–Cn fractionation could not be accurately calculated because of a lack of the hydrogen isotopic composition of H2 and H2S. However, TSR of crude oil in the presence of MgSO4 greatly reduced the variation of δ2H–C1, which is consistent with the characteristics of H2S-bearing alkane gas in the reservoirs of the Sichuan Basin, China45. In general, hydrogen isotope fractionation during pyrolysis system may be more complicated than fractionation of carbon isotopes, because hydrogen contributes both to formation of alkane gas and H2, while also providing hydrogen for the formation of H2S. More importantly, the presence of water may provide an important hydrogen during pyrolysis21.
These results suggest that TSR can alter both the carbon and hydrogen isotopic composition of gaseous alkane products, and even lead to a reversed trend of the carbon isotope series of CH4 and C2H6 for a mix of C9 and MgSO4 as the source material. This can result in similar isotopic compositions to those of H2S-bearing natural gas45. Liu et al.20 investigated carbonate gas systems in the Sichuan Basin of China and found different amounts of acid gases (CO2 and H2S) in all marine strata. Both positive carbon isotope series and partially reversed sequence of alkanes were found. The positive carbon isotope series formed during production of sour gases was caused by TSR alteration that preferentially reacted with 12C-bearing heavy hydrocarbons. Therefore, the carbon isotope composition of residual heavy gases became heavier, and the partially-reversed carbon isotope sequence was again converted to be positive series.
In this study, we found that the TSR process played a significant role in the pyrolysis of C9 and MN with a MgSO4 solution, which lead to production of mainly H2S and CO2. TSR is more sensitive to MN, which makes it possible that MN can be completely oxidised into CO2 and H2S (Table 1). The smallest variation of both carbon and hydrogen isotopic compositions was observed for CH4, and was closest to that of the precursor. The heavier carbon isotopic composition of the precursor and C2H6 produced from thermal cracking initially led to a reversed carbon isotope series of the alkane gas (Table 2, M-48). However, the temperature of our experiments (360 °C) was much higher than that in natural geological environments. Therefore, heavy hydrocarbons enriched in 12C tended to become unstable, and were subject to thermal cracking, which led to isotope fractionation. 13C was enriched in the residual heavy hydrocarbons, and the effect of fractionation was larger than that for CH4 during TSR, changing the carbon isotope sequence back to positive.
Gas souring index (GSI) and carbon and hydrogen isotope fractionation of alkane gas
Because H2S can be produced during TSR, the gas souring index (GSI), i.e., H2S/(H2S + ∑C1−5), has been used as an indicator for the occurrence and degree of TSR46. The variation of the GSI at different stages of TSR can be established by statistical analysis of H2S-bearing natural gas samples45. To reproduce the variation of molecular and isotopic compositions of H2S-bearing natural gas during TSR, the GSI and carbon and hydrogen isotope fractionation mechanism of alkane gas were studied during pyrolysis at different temperatures with various heating times.
During thermal cracking of crude oil, a very small amount of H2S with minor CO2 was produced, indicating that no TSR occurred during direct thermal cracking of crude oil. In contrast, the CO2 yield gradually increased with a larger GSI (> 0.6) for TSR involving a mixture of crude oil and MgSO4 solution, and increased rapidly with further increasing GSI (Fig. 6), which is similar to the relationship between GSI and CO2 content of H2S-bearing natural gas20. The δ13CCO2 values remained nearly constant but gradually decreased with increasing GSI during thermal cracking due to the presence of TSR (Fig. 7). This phenomenon can be attributed to TSR, which preferentially incorporates 12C from hydrocarbons into CO2 leading to 12C-enrichment with increasing TSR intensity44.
Figure 8 shows the relationship between the gas souring index and δ13C1, δ13C2, and δ13C3. The direct thermal cracking of crude oil produced only a small amount of H2S due to low sulfur content of the crude oil, resulting in a GSI of less than 0.1. The GSI significantly increased with the addition of MgSO4 solution to the crude oil to above 0.6 under different conditions. Meanwhile, the carbon isotopic composition of alkane gas became larger with increasing TSR intensity, i.e., longer reaction time (Fig. 8). The carbon isotopic composition of the alkane gas revealed that the δ13C1 variation of CH4 produced in the presence of MgSO4 was much lower than that of C2H6. The carbon isotope composition changed more significantly with the increasing GSI and longer heating time. Therefore, during TSR, the variation of δ13C1 became smaller, compared with that of δ13C2 and δ13C3. The δ13C1–δ13C2 difference became higher with increasing GSI, which is completely different from that of natural gas formed by direct thermal cracking of crude oil. During thermal cracking of crude oil, the carbon isotope composition of gaseous products gradually becomes more similar with increasing carbon number45.
The δ2H–Cn values of alkane gas show a variation similar to the δ13Cn values, with increasing GSI during the TSR process. The range of δ2H–C1 values is fairly low, while that of δ2H–C2 is higher, especially for the TSR of C9 whose δ2H–C1 remained almost constant (Fig. 9). Similar to the carbon isotopic composition, the δ2H–C1–δ2H–C2 difference increased as the gas souring index became larger, indicating that TSR can lower δ2H–C1 values and increase those of δ2H–C2, producing an abnormal hydrogen isotope composition of alkane gas, which is only characteristic of thermal cracking of crude oil. Both δ13C1–δ13C2 and δ2H–C1–δ2H–C2 became lower with increasing TSR intensity. However, in the case of oil cracking, δ13C1–δ13C2 and δ2H–C1–δ2H–C2 show the opposite trend with increasing TSR (Fig. 10). This observation may be caused by the small variation of the δ13C1 and δ2H–C1 produced by TSR, which is similar to the isotopic composition of the precursor. The carbon and hydrogen isotopic composition of C2H6 was more heavily affected by the thermal cracking than oxidisation by TSR. Therefore, the variation of its carbon and hydrogen isotope compositions is higher. In addition, the difference in the carbon and hydrogen isotopic composition between CH4 and C2H6 gradually increased with a larger GSI.
Although TSR alteration decreased the variation of both carbon and hydrogen isotope compositions of CH4, as compared with products from direct thermal cracking of crude oil, δ13C2 became larger with increasing δ13C1. The δ13C2 versus δ13C1 plot (Fig. 11a), and δ13C2 versus δ13C3 plot (Fig. 11b), showed positive correlations, with the latter having a higher correlation. The δ13C1 and δ13CCO2 values are negatively correlated (Fig. 11c), possibly because the carbon isotopic composition of CO2 produced by TSR is enriched in 12C34. For CH4, C2H6, and C3H8, δ2H–C1 and δ2H–C2 are positively correlated with temperature and heating time. δ2H–C2 values increase with increasing δ2H–C1 values. The positive correlation between δ2H–C2 and δ2H–C3 is even more significant (Fig. 12). Based on these results, it can be concluded that the oxidation of hydrocarbons by TSR is accompanied by thermal cracking of crude oil, resulting in distinct patterns among the δ2H values of alkane gases. Although TSR effect can lead to smaller variations of carbon and hydrogen isotope compositions of CH4 compared with C2H6 and C3H8, δ2H–C1 values increased with increasing δ13C1 values (Fig. 13), suggesting that direct thermal cracking of crude oil also produced CH4. However, the CH4 produced during pyrolysis of C9 should be primarily produced by TSR.
All TSR simulation experiments of crude oil, C9 and MN under different conditions suggest that the TSR alteration produces CH4 with similar carbon and hydrogen isotopic compositions to those of its precursor and reduced variations of isotopic compositions. The difference between CH4 and C2H6 in carbon isotope and hydrogen isotopic composition increases with increasing TSR intensity, while the carbon and hydrogen isotopic composition of C2H6 and C3H8 became heavier and shows smaller differences with increasing temperature, similar to the results from thermal cracking of crude oil. The production of alkane gas with similar chemical and isotope compositions to H2S-bearing natural gas during these experiments, suggest that TSR can alter the carbon and hydrogen isotopic composition of alkane gas20,21. In addition, a partially reversed carbon isotope series of alkane gas (δ13C1 > δ13C2 < δ13C3) was observed, which further confirms the above conclusion. 12C-enriched CO2 was mainly produced from the oxidation of hydrocarbons by TSR. However, dissolution of CO2 and precipitation of carbonate minerals in aqueous fluids can complicate the carbon isotopic composition in the marine carbonate reservoir45.