4.1. Formation and decomposition of diamondoids during hydrothermal pyrolysis of an HD23 oil
Hydrothermal pyrolysis of the HD23 oil shows that both adamantanes and diamantanes were newly generated and decomposed. Still, their yield curves are partially different from the anhydrous13: First, diamondoids were generated in a broader range of EasyRo with higher yields at < 1.7% EasyRo during the hydrothermal experiments than the anhydrous (Fig. 6), indicating that water promoted the yields of diamondoids at low EasyRo ( < ~ 2.0%). With increasing EasyRo, the differences in the yields of diamondoids between the two became smaller. Among diamondoids, adamantanes show an increase in their yields from 0.48–2.1% EasyRo (Fig. 6a), and the range is wider than the range of 1.0–2.1% for the anhydrous experiments. Similarly, diamantanes began to be generated at 0.79% EasyRo from hydrothermal pyrolysis experiments, much lower than 1.7% EasyRo for the anhydrous pyrolysis experiments (Fig. 6b). Second, the decomposition of diamantanes and TeMA from the hydrothermal experiments occurred at EasyRo > 3.0% and EasyRo > 2.5%, obviously lagging behind that from the corresponding anhydrous experiments at EasyRo > 2.5% and 2.1%, respectively (Fig. 6b and Fig. 3). This may indicate that water can delay the decomposition of high molecular weight diamondoids during oil thermal cracking.
Larger yields of diamondoids from hydrothermal pyrolysis than the anhydrous (Fig. 6) can be explained as follow. As the result of ionic reactions, hydrothermal pyrolysis of organic matter generates more considerable amounts of intermediate olefinic and isomeric hydrocarbons than the anhydrous pyrolysis50. In turn, the olefins and isomeric hydrocarbons will be hydrogenated by rapid free radical reactions, raising the yields of diamondoids during hydrothermal pyrolysis. That is, combining ionic and free radical reactions can accelerate isomerization and cyclization of these olefinic hydrocarbons to generate the relatively high yields of diamondoids under hydrothermal pyrolysis.
It is necessary to discuss which one, hydrothermal or anhydrous pyrolysis, has the products representing maturation of natural samples, considering the more significant differences in EasyRo for decomposition of diamantanes and yields of diamondoids between the two. The EasyRo for the generation and decomposition of the 3-+4-MD in this study are close to that of natural samples from both coals and rocks, that is, ca. 1.2% EasyRo vs 1.1% Ro for the generation and 3.0% EasyRo vs ca. 4.0% Ro for the decomposition7. In contrast, EasyRo obtained from anhydrous pyrolysis are deviated more from the natural samples, 1.5% for the generation and 2.5% for the decomposition13,51,52. Ro values are approximately equal to the calculated EasyRo values at EasyRo < 1.5 ~ 2.0%. The differences between Ro and calculated EasyRo are slightly more significant at EasyRo > 1.5 ~ 2.0%, likely due to the change in the chemical composition of solid kerogen with higher maturity levels53. This result suggests that hydrothermal pyrolysis has the products closer to the cracking of natural samples, which is supported by the gas produced from the hydrothermal pyrolysis more similar to the natural gas than anhydrous pyrolysis23. Moreover, water is ubiquitous in petroleum reservoirs and may provide H and O involved in petroleum generation and evolution38,54, suggesting hydrothermal pyrolysis may represent the maturation of natural samples better than the anhydrous.
4.2. Diamondoids as proxies for thermal maturity
It is widely accepted those isomerization ratios such as MAI. MDI, EAI, DMAI-1, TMAI-1, TMAI-2, DMDI-1 and DMDI-2 can be used to determine the thermal maturity of highly mature crude oils(Ro > 1.1%)9,10,30,32,34, and they can be applied for different maturity ranges16. Isomerization-related diamondoid ratios are unaffected by thermal maturity levels with EasyRo < 2.0% in anhydrous pyrolysates and used as proxies of thermal maturity at > 2.0% EasyRo16. In this study, MDI, EAI, DMAI-1 and DMDI-2 can be applied to reflect maturity at much lower EasyRo from hydrothermal pyrolysis: 1.47–3.5% EasyRo for MDI with R2 of 0.8717 (Fig. 7b), 0.86–2.5% EasyRo for EAI with R2 of 0.8412 (Fig. 7c), 1.08–3.5 % EasyRo for DMAI-1 with R2 of 0.8502 (Fig. 7e) and 1.08–3.5 % EasyRo for DMDI-2 with R2 of 0.9304 (Fig. 7d). This supports that MDI is an effective proxy of maturity at > 1.3% Ro for either source rock extracts9 or hydrothermal pyrolysates10. However, consistent with Fang et al. (2012)13, MAI in this study seems not related to EasyRo (Fig. 7a), and thus cannot be used as a proxy to assess the thermal maturity of oils. MDI, EAI, DMAI-1, DMDI-2 can serve as reliable maturity indicators with broad EasyRo ranges mainly > 1.0%. In contrast, at EasyRo < 1.0%, diamondoid-related proxies including MDI, EAI, DMAI-1, DMDI-2 show no correlations with EasyRo, suggesting that they cannot be used to determine the maturity of oils and thus source rocks. The previous observation supports this proposal that diamondoid concentrations and distributions are dependent on the source rocks instead of maturity within the oil window55.
Other isomerization ratios (e.g., DMAI-2, TMAI-2 and TMAI-1) show good correlations with thermal maturity in the higher EasyRo ranges of 2.08–3.5% with R2 of 0.9617, 0.9752 and 0.8581 (Fig. 7g-i). These ratios seem controlled by the parent organic matter during the generation stage of diamondoids (EasyRo < 2.0%), and thus may reflect the source feature rather than maturity16. They can be used to reflect maturity only at higher maturity levels (> 2.0% EasyRo) as found in Fang et al. (2013)16 and this study. However, unlike other studies, DMDI-1 does not correlate well with EasyRo values in this study (Fig. 7f), probably due to the relatively sizeable analytical error associated with low concentrations of dimethyldiamantanes in the pyrolysates.
4.3. Diamondoids as proxies for the extent of oil cracking
Oil cracking involves the thermal breakdown of heavy hydrocarbons to smaller ones, or the process of ultimately converting oil to hydrogen-rich gas and carbon-rich pyrobitumen56. In our hydrothermal pyrolysis, we found that the extent of oil cracking (EOC; i.e., the percentage of liquid hydrocarbon converted to gas and pyrobitumen, or EOC= (1-Mc/M0)×100, Mc and Mo are residual and initial liquid hydrocarbons, respectively) can rapidly increase to 90% with the rise in EasyRo from 0.48–1.81% (Fig. 8a). Oil cracking occurs at slower rates with further increasing maturation as reflected in the increase in EasyRo from 1.81% (480℃) to 3.5% (600℃) and relatively stable EOC around 90–95%. However, at the high maturity (above 500℃) almost all of the liquid hydrocarbons have been consumed, so the error is around ± 5% from 2.19% (504℃) to 3.5% (600℃) in the oil pyrolysis experiments.
Oil cracking involves the thermal breakdown of heavy hydrocarbons to smaller ones or ultimately converting oil to hydrogen-rich gas and carbon-rich pyrobitumen. In our hydrothermal pyrolysis, we found that the extent of oil cracking (EOC; i.e., the percentage of liquid hydrocarbon converted to gas and pyrobitumen, or EOC= (1-Mc/M0)×100, Mc and Mo are residual and initial liquid hydrocarbons, respectively) can rapidly increase to 90% with rising in EasyRo from 0.48–1.81% (Fig. 8a). Oil cracking occurs at slower rates with further increasing maturation, as reflected in an increase in EasyRo from 1.81–3.5% and relatively stable EOC around 90–95%.
EOC can also be calculated as (1- C0/Cc)×1005, in which 3-+4-MD is assumed not to have newly been generated or decomposed during oil cracking (C0 and Cc are concentrations of 3-+4-MD before and after oil cracking). However, an increase in the 3-+4-MD occurs at ca. 1.2% EasyRo. The decrease in the 3-+4-MD yield is observed at 3.0% EasyRo during oil thermal cracking experiments (Fig. 9a), suggesting the assumption does not apply (Fig. 9b). This finding is supported by other pyrolysis experiments8,13,16, lending usage of (1-C0/Cc)×100% is suspect. Based on our results, Dahl’s formula for EOC is only applicable to a very narrow range of maturity (EasyRo < 1.2%), and gives higher values than those obtained from our hydrothermal experiments (Fig. 8a). The differences between the two results become progressively smaller with the increasing extent of oil cracking with the values from 6–21% at EasyRo from 0.48–1.81% and from 2.5–6% at EasyRo from 1.81 to 3.0%. Obviously, the (1-C0/Cc) ×100% should be changed to [1-C0/(Cc-Cnew gener)]×100% at 1.2% < EasyRo < 3.0%, but the Cnew gener is difficult to obtain. Fortunately, we find that the calculative EOC (EOC1 =[1- C0/Cc]×100) shows a good positive linear correlation with the actual EOC (EOC2=[1-Mc/M0]×100) with equation of EOC2 = 1.2402×EOC1–28.952 and R2 value of 0.9593 at EasyRo < 3.0% (Fig. 8b). This reveals that although Dahl’s method may overestimate the extent of oil cracking, especially in highly cracked samples due to the new generation of 3- + 4-MD, the method can be corrected and new calculation formula can be used to reflect actual EOC.
The bridgehead-methylated diamondoids are thermodynamically more stable than other methylated diamondoid species33. On this basis, some diamondoid isomerization ratios (MAI, MDI, DMAI-1, DMAI-2, TMAI-1, TMAI-2, EAI, DMDI-1, DMDI-2) are used as maturity indicators. Figure 10a & b shows that there is a good positive correlation between diamondoid isomerization ratios (EAI and DMDI-2) and EOC2 with regressive equations as follow, where EAI is applicable in the range of EasyRo < 1.81% (Fig. 10a).
EAI = 0.0015 EOC2 + 0.2635 r² = 0.6355 (EasyRo < 1.81%) (2)
DMDI-2 = 0.0024 EOC2 + 0.3177 r² = 0.7271 (3)
This implies that these parameters might help assess the extent of oil cracking (EOC2). Note that the isomerization ratio of DMDI-2 has a good correlation with EOC2 throughout the EasyRo range examined, indicating that it may be a reliable proxy for a wide range of maturity.
On the other hand, the concentration ratios of diamondoid pairs are expected to eliminate the effect of matrix changes during the thermal cracking of oil. Some diamondoid concentration ratios (As/Ds, MAs/MDs, DMAs/DMDs, and DMAs/MDs) appear positively correlated with EOC2 at EasyRo from 0.48–2.1% (Fig. 10c-f) with regressive equations as follow.
As/Ds = 0.0115 EOC2 + 1.7478 r² = 0.6052 (EasyRo < 2.1%) (4)
MAs/MDs = 0.0083 EOC2 + 0.2543 r² = 0.8507 (EasyRo < 2.1%) (5)
DMAs/MDs = 0.0131 EOC2 + 0.5355 r² = 0.8054 (EasyRo < 2.1%) (6)
DMAs/DMDs = 0.0225 EOC2 + 0.8348 r² = 0.6159 (EasyRo < 2.1%) (7)
However, the above diamondoid isomerization ratios negatively correlate with EasyRo values of > 2.1% when admantanesadamantanes enter the decomposition stage. The above equations established from hydrothermal pyrolysis are proposed to be used as proxies of the extent of oil cracking during 0.48–2.1% EasyRo for natural petroleum reservoirs.
4.4. New generation of diamondoids and thiadiamondoids during TSR?
4.4.1. Occurrence of TSR in the experiments
In pyrolysis experiments containing elemental S, MgSO4 or CaSO4·2H2O and ZS1-L oil, H2S was generated and may have been derived from 1) cracking of ZS1-L oil, 2) elemental sulfur hydrolysis, 3) thermochemical reduction of MgSO4 or CaSO4·2H2O. The H2S is not mainly from cracking of ZS1-L oil because thermal decomposition of ZS1-L oil with a sulfur content of 0.18% can only generate 0.056mmol/g H2S. Thus, H2S must have mainly derived from the reduction of elemental S and MgSO4. Elemental S may react with water at temperatures as low as 200℃ in the following disproportionation reaction47,57,58:
4S + 4H2O → SO42− + 3H2S + 2H+ (8)
An alternative production pathway for the exceptionally high yields of H2S was via the classical aqueous reaction of elemental S and hydrocarbon shown in Eq. 9 (Orr, 1974; Schmid et al., 1987; Goldhaber and Orr, 1995; Seewald, 2003):
4S + 1.33(—CH2—) + 2.66H2O → 4H2S + 1.33CO2 (9)
The H2S/S0 molar ratio can be used to determine the amount of H2S from the conversion of elemental S (Table 2). The H2S/S0 molar ratio based on Eq. 8 and Eq. 9 will approach 0.75 and 1, respectively. Group 1 experiment at the lowest temperature of 336 ℃ (0.57% EasyRo) shows that nearly all H2S was derived from elemental S as supported by the H2S/S0 molar ratio around 0.71and by δ34SH2S value of -5‰ (Table 2), which is close to that of elemental S (-6.3‰). Furthermore, the increasing production of CO2 did not start until above 408℃, when the H2S/S0 molar ratio began to be greater than 0.75. This disconnect in thermal production implies H2S was not produced via Eq. 8 from 336–384 ℃ (0.57–0.79% EasyRo) in Group 1. With the temperature increasing, the H2S/S0 molar ratio gradually increases until it reaches a maximum of 1.36 at 528°C (2.62% EasyRo), and then gradually decreases to 1.12 (Table 2). Meanwhile, the δ34S value of H2S show rise from − 5‰ to -2.45‰, getting closer to the δ34S value of MgSO4 (δ34S of + 3.75‰), suggesting that the H2S may have significantly derived from the reduction of MgSO4 in the aqueous experiments with reaction as follows:
SO42− + 2H+ + CH4→CO2 + H2S + 2H2O (10)
It can be expected that with the increase of temperature, more MgSO4 was involved in TSR reaction, or TSR has proceeded to higher degrees. If S can react with hydrocarbons to release H2S without generation of SO42−, the molarity of elemental S is about 12.79 mmol (26.8 mg), which is lower than H2S from 504℃ to 600℃. Hence, the conversion of elemental S is insufficient for the generation of H2S from 504℃ to 600℃. MgSO4 was involved in the reaction. Moreover, calculation of all 103mg 12.86 mmol/g MgSO4 and 26.8mg12.79 mmol/g elemental sulfur in the experiment is 25.8865 mmol/g H2S, which is much higher than the maximum yield of H2S (17.43mmol/g) occurs at 528 ℃. The yield of H2S and the low magnitude of the influence of MgSO4 on the δ34SH2S confirm that only limited amounts of MgSO4 were involved in the TSR reactions. Some early formed H2S would be expected to react with hydrocarbons to form OSCs such as thiols, (poly)sulfides, thiophenes, and benzothiophenes59,60, thus H2S shows a decreasing trend at EasyRo = 2.5–3.87% (Table 2).
However, Group 2 experiments are reactions between elemental S and hydrocarbons with no CaSO4·2H2O involved. Firstly, the maximum value of the H2S/S0 molar ratio in Group 2 is around 0.9 at the first EasyRo = 1.13 and then slightly decreases from 0.93 to 0.65 with EasyRo from 1.13 % to 1.69 %. Secondly, the δ34S of H2S generated in Group 2 ranged from − 5.79‰ to -6.79‰, within ± 1‰ of elemental S (-6.3‰). Finally, Group 2 produced a very high amount of CO2 (1.71mmol/g at 1.13% EasyRo) at the first desired time compared to the meager yields produced by Group 1 above 360℃(Table 2). This indicates H2S was created via the classical aqueous reaction of elemental S and hydrocarbon shown in Eq. 9. Moreover, the presence of elemental S can accelerate the rate of hydrocarbon thermal chemical alteration (TCA) due to organic sulfur compounds (e.g., thiols and sulfides) that form through the reaction of H2S or polysulfides with hydrocarbons and subsequently thermally degrade leading to the formation of sulfur radicals that in turn enhance TCA reactions61.
Therefore, it can be concluded that the positive δ34S value of H2S and the increase of CO2 and H2S/S0 molar ratio for the Group 1 experiments, as shown in Table 2, may be ascribed to the TSR reaction between sulfate and hydrocarbons at elevated temperature rather than the reaction between hydrocarbons and H2S or elemental sulfur, as shown Group 2 experiments. Here, TSR experiments represent Group1 experiments in this study.
4.4.2. Generation of diamondoids during TSR
The presence of TSR reaction significantly increases the yield of diamantanes relative to the blank non-TSR experiments (Fig. 5). Here, at 1.47% EasyRo (456 ℃), a higher yield of diamantanes detected in the TSR system (218.94 µg/g) was higher than that of the thermal chemical alteration (TCA) (only 156.07µg/g), indicating that diamantanes must have newly generated during TSR (Fig. 5a). On the other hand, the TSR reaction accelerates the generation of diamantanes comparing with TCA (Fig. 5a). For example, diamantanes are shown to be predominantly generated in the EasyRo range of 0.57–1.81% with maximum yields of 240 µg/g at 1.81 EasyRo that is before TCA experiments that much lower diamantanes yields of 72.74 µg/g at 0.79% EasyRo to 182.1 µg/g of 3.1% (Fig. 2). Elemental S can substantially lower the onset temperature of hydrocarbon thermal chemical alteration and appears to reduce the activation energy of low-sulfur oil thermal chemical alteration by approximately 92 kJ mol− 1 61. The observed acceleration of diamantanes generation is possibly due to sulfur-derived radical species or H2S formed via TSR or disproportionation reaction that enhances the formation of diamantanes.
Furthermore, diamantanes show a rapid decrease after the maximum yields at EasyRo > 1.81% (Fig. 5a), which is significantly lower than that (EasyRo > 3.0%) of thermal chemical alteration (TCA) (Fig. 2). Similarly, diamantanes remain stable at up to 550 ℃ during TCA while the temperature is 480 ℃ during TSR at the same heating rates of 20 ℃/h (Fig. 5d). This result may be due to the catalysis of S radical (i.e., from H2S), which can accelerate the decomposition of HC or OM.
The mechanism for generating diamondoids during TSR may be through free radical reactions, a mechanism similar to their generation from high temperature cracking of alkanes during the experiment simulation62,63. Consequently, we considered that the sulfur-derived radical species or H2S during TSR have a facilitative effect on the cleavage of high molecular-mass fractions, resulting in the new generation of diamondoids from TSR experiments in the present study. Meanwhile, hydrogen exchange between water and organic matter also proceeds via sulfur-derived radical species (i.e., from H2S)50, leading to demethylation and isomerization of hydrocarbon to form diamondoids. Briefly, TSR can lead to the generation of diamondoids through free radical reactions.
4.4.3. Generation of thiadiamondoids during TSR
Thiadiamondoids are diamond-like compounds with a sulfide bond located within the cage structure (Fig. 1), and were suggested to be formed from reactions of adamantanes with sulfur species64. Thiaadmantane and methyl thiaadmantanes isomers were detected at 1.81% EasyRo when the yields of diamantanes reached a maximum value during the hydrothermal pyrolysis of ZS1-L oil under TSR condition (Fig. 1). To our knowledge, this is the first successful laboratory synthesis of thiaadmantanes from a petroleum sample via TSR. Although previous laboratory experiments have successfully synthesized thiaadmantanes, thiaadmantanes were only detected from reactions of reduced S or CaSO4 with pure diamondoids64,65. Based on these laboratory experiments, Wei et al. (2007b)64 proposed that diamondoids appear to be the only precursors of thiaadmantanes during TSR (Fig. 11a). However, our results indicate that thiaadmantanes and diamondoids may have been generated simultaneously, likely not via reactions with diamondoids based on the following aspects (Fig. 11). Firstly, during TSR experiments at EasyRo of 1.81%, both diamantanes and corresponding thiaamantanes were formed, and thiadamantanes show positive correlations with the corresponding diamantanes (2-TA vs D; M-2-TA vs MD; DM-2-TA vs DMD; TM-2-TA vs TMD) from (Fig. 12a &b) with a higher yield of diamantanes during TSR compared with hydrothermal pyrolysis or anhydrous pyrolysis (Fig. 12a). The experimental results indicate that diamondoids and thiadamantanes may have been formed simultaneously, which is consistent with case studies showing the positive relationships between diamondoids and thiadiamondoids concentrations from oils and condensates from the Tarim Basin and Gulf of Mexico Basin25,49. In contrast, if diamondoids are the only precursor of thiaadamantane64, conversion of significant amounts of diamondoids to thiaadmantanes may lead to a negative correlation between the yields of diamondoids and thiaadmantanes. Secondly, C–C bonds in the cage structure of diamondoids have been proposed to be hard to break up due to their thermal stability30–32, it is more energy-favorable to form thiaadamantanes from other non-cage compounds. Thus, it is reasonable for thiaadamantanes to have been generated during the formation of diamondoids. Considering that diamondoids can be generated from pyrolysis of all four fractions13–16, a non-diamondoid source of thiaadamantanes is proposed here as shown in Fig. 11b.
However, thiaadmantanes only form at 1.81% EasyRo (480 ℃) not at other experiments from 336 ℃ to 600 ℃, under the TSR condition. One possible explanation is that thiaadamantanes are formed in relatively high-temperature conditions and are expected to decompose higher EasyRo. Xiao et al. (2019)66 proposed that thiaadamantanes show slight to moderate cracking at EasyRo of 1.81% and thus have far less thermally stable than diamondoids. Similarly, thiadiamondoids were found to be thermally degraded at temperatures > 180 ℃ in TSR-altered oils from the Smackover and Norphlet formations of the US Gulf of Mexico49,67. The reservoir temperatures of 180 ℃ can correspond to the equivalent vitrinite reflectance values of about 1.9% based on the thermal history of the Norphlet Sandstone in Mobile Bay, northern Gulf of Mexico68. Our TSR experimental results are generally consistent with this field observation. Another possible explanation is the difficulty of detecting thiaadmantanes due to low concentrations and loss during the complex sample preparation.
Notably, TSR resulted in the new generation of diamondoids (Fig. 5), and thus had a significant effect on the distribution and concentration of diamondoids. Thus, in TSR-altered oils, diamondoid-related maturity proxies have been altered significantly (Table 3), and thus cannot be used to indicate EOC.