Origin of Adamantanes and Diamantanes in Marine Source Rock

Nov 24, 2015 - source rock were investigated by thermal simulation experiments based on a marine ... Published data show that at a given maturity leve...
0 downloads 0 Views 1MB Size
Download PDF
Article pubs.acs.org/EF

Origin of Adamantanes and Diamantanes in Marine Source Rock Yun Li,† Yuan Chen,† Yongqiang Xiong,*,† Xiaotao Wang,† Chenchen Fang,‡ Li Zhang,† and Jinhua Li† †

State Key Laboratory of Organic Geochemistry (SKLOG), Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, P. R. China ‡ PetroChina Research Institute of Petroleum Exploration and Development, Beijing 100083, P. R. China S Supporting Information *

ABSTRACT: Thermal maturation-related variations in the yields of lower diamondoids (adamantanes and diamantanes) in source rock were investigated by thermal simulation experiments based on a marine shale and kerogens obtained from the shale via isolation and artificial maturation, representing different maturity stages of the oil generation window. The simulations show that lower diamondoids are formed and destroyed during thermal maturation of the shale. For example, adamantanes are generated mainly in the maturity range of 0.8%−1.8% EasyRo, then they begin to degrade at 1.8% EasyRo. Diamantanes are produced mainly during the maturity range of 1.0%−2.2% EasyRo and begin to degrade at 2.2% EasyRo. The mineral matrix of shale may have a strong effect on the destruction of diamondoids, leading to a reduction in the peak yield and a reduction in the maturity level corresponding to the peak yield of diamondoids. A comparison of the diamondoid yields from four kerogens at different maturity levels indicates that the lower diamondoids are derived mainly from secondary cracking of extractable organic matter (bitumens) occurring in the source rock. For instance, at the peak stage of adamantane formation (2.1% EasyRo), 75.6% of the total adamantanes is generated from the cracking of bitumens and the remaining 24.4% is from the primary cracking of kerogens. Similarly, the yield of diamantanes generated from the secondary cracking of bitumens accounts for 87.8% of the total diamantanes at the peak stage of diamantane formation (2.5% EasyRo). Almost no diamondoids are detected in the pyrolysates of more mature kerogen (1.3%EasyRo), suggesting that 1.3% EasyRo is the upper limit of maturity for the generation of diamondoids from kerogen. Diamondoid isomerization ratios are maintained at relatively constant levels during the formation stage of diamondoids, whereas a linear correlation with maturity occurs during the destruction stage, suggesting that isomerization ratios of diamondoids are controlled by their thermal stability just in the destruction stage and are unaffected by hydrocarbon generation and expulsion of source rock at early thermal stages. This finding indicates that these diamondoid indices are a potential tool for evaluating the thermal maturity of source rocks at highly mature stages.

1. INTRODUCTION Diamondoids are rigid three-dimensional diamondlike cage hydrocarbons that occur in many petroleum deposits1−3 and in extracts of coals and sedimentary rocks.4−6 Due to their high resistance to thermal and biological degradation, diamondoidrelated indices have been established to determine the thermal maturity of highly mature source rocks and crude oils,2,7,8 to distinguish source rock facies,5 and to evaluate the extent of oil cracking3,9 and the biodegradation of crude oils.10,11 Although diamondoids are generally considered to be of great significance in petroleum geochemistry and have attracted a great deal of attention, their successful application has not been as extensive as anticipated, possibly on account of a lack of understanding of the origin and formation mechanisms of these compounds in sedimentary rocks and petroleum. Unlike biomarker compounds, which are biosynthesized, lower diamondoids (adamantanes and diamantanes) in nature are currently considered to be geo-synthetic compounds generated mainly by Lewis acid catalyzed rearrangements of polycyclic hydrocarbons12,13 and high-temperature thermal cracking of various organic compounds.14−18 It has been reported that a number of organic compounds (e.g., n-alkanes, n-alkenes, cyclohexane, fatty acids, phytol, stigmastanol, cholesterol, and cedrene) present in sediments and crude oils can be precursors of diamondoids, which are generated as a result of thermal stresses and interactions with suitable catalysts.1,14−16,19−21 © 2015 American Chemical Society

Published data show that at a given maturity level, the concentrations of diamondoids in source rocks with type I and II kerogens are generally 1 order of magnitude higher than that in source rocks with type III kerogens,6 suggesting that the diamondoid-generating ability of different types of organic matter is variable and that, for example, type I and type II organic matter in the presence of clay minerals can produce more diamondoids than type III organic matter in coals.15 In addition, diamondoids generated from different types of source rocks show distinct compositions.5 The organic matter in source rocks includes soluble (bitumen) and insoluble (kerogen) components.22 Wei et al.15,19 suggested that diamondoid precursors might be incorporated into kerogen skeletons via polysulfide, ether, and ester groups, as in biomarkers, and that they might be released from the kerogens when weaker carbon−heteroatom bonds cleave during thermal maturation. This hypothesis presumed that diamondoid moieties were formed during the diagenetic stage. The enrichment of diamondoids at relatively high-maturity stages has been attributed to the decomposition of other components in oils or extracts.3 However, this mechanism can explain diamondoid formation only in terms Received: September 2, 2015 Revised: November 24, 2015 Published: November 24, 2015 8188

DOI: 10.1021/acs.energyfuels.5b01993 Energy Fuels 2015, 29, 8188−8194

Article

Energy & Fuels of the primary cracking of kerogen in source rocks within the thermal oil generation window, but it cannot explain diamondoids generated from secondary cracking of bitumen at higher maturation stages. Our previous studies have indicated that diamondoids can be generated and destroyed during oil cracking, and that all four subgroups (saturated, aromatic, resin, and asphaltene fractions) in oil can produce diamondoids at high-maturity stages.18,23 Therefore, multiple sources and multiple pathways of origin can result in complex variations in the concentrations and distributions of diamondoids in source rocks. Quantitatively evaluating the diamondoid-generating ability of bitumen and kerogen is helpful for understanding which diamondoid precursors are mainly preserved in or formed from, and when they are generated. Fang et al.24 answered those questions based on two simulation experiments of a source rock extract and the corresponding kerogen, indicating that source rock extract is the main contributor of diamondoids during the maturation process. However, as is well-known, the extractable bitumen can be generated from the kerogen during different maturity stages and can be preserved depending on the expulsion efficiency. Consequently, the contributions of the kerogen and bitumen to the generation of the diamondoids may be a dynamic change process. The purpose of this study was to investigate the formation and evolution of lower diamondoids in source rock using a series of laboratory simulation experiments on a marine shale and kerogens from the shale, to assess the contributions of bitumen and kerogen in the source rock to diamondoid yields, and to further discuss the characteristics of diamondoids generated at different maturity levels.

Table 1. Conditions for the Preparation of the Four Kerogen Samples Used in the Simulation Experiments of Diamondoid Generation sample

maturity

temperature (°C)

time (hour)

TOC (%wt)

K1 K2 K3 K4

0.57 0.80 1.00 1.30

− 320 345 365

− 57 65 95

68.12 73.77 69.92 72.56

levels (0.57% Ro, and 0.8%, 1.0%, and 1.3% EasyRo) were performed in sealed gold tubes following Fang et al.18 Briefly, aliquots of 10−50 mg of kerogen or 50−100 mg of shale sample were loaded into a series of gold tubes (length, 40 mm; inside diameter, 4.2 mm; wall thickness, 0.25 mm) before purging with argon for 5 min and sealing under an argon atmosphere. The sealed gold tubes were placed in a series of stainless steel autoclaves, which were heated in an oven at two constant heating rates of 20 °C/h and 2 °C/h. The pressure was maintained at 50 MPa during the heating procedure. Sampling was conducted at ∼24 °C intervals between 336 and 600 °C (total of 12 samples for each heating rate). The autoclaves were removed from the oven after heating and were cooled to room temperature in air. 2.3. Determination of Pyrolytic Products. The chemical composition of gaseous hydrocarbons in the pyrolysates was analyzed using an Agilent 6890N gas chromatography (GC) instrument modified by Wasson ECE Instrument, which was described by Pan et al.27 Briefly, the cleaned gold tube for each temperature point was placed in a vacuum glass system connected to a GC inlet. After the gold tube was pierced with a steel needle, gaseous products were released and introduced into the GC system, through which the analyses of both the organic and inorganic gas were performed in an automatically controlled procedure. Helium with a minimum purity of 99.99% was used as a carrier gas, with a constant flow rate of 1.0 mL/ min. The GC oven temperature was initially held at 40 °C for 6 min, ramped to 180 °C at 25 °C/min, then held at 180 °C for 4 min. Quantification was performed using an external standard method. The gold tube for the analysis of C6−C12 hydrocarbons and diamondoids was cooled for ∼30 min using liquid nitrogen and then rapidly cut in half, and the halves were placed in a 4 mL sample vial filled with methanol (for the analysis of C6−C12 hydrocarbons) or isooctane (for the analysis of diamondoids). The composition of C6− C12 hydrocarbons in the pyrolysates was then analyzed using headspace single-drop microextraction coupled with GC-flame ionization detection.18,28 GC analyses were performed on an Agilent 7890 GC instrument fitted with an HP-PONA fused silica capillary column (50 m × 0.20 mm inner diameter × 0.5 μm). Nitrogen was used as the carrier gas at a flow rate of 1.0 mL/min. The GC oven temperature was held at 35 °C for 5 min, then ramped to 50 °C at a rate of 1.5 °C/min, finally programmed at 8 °C/min to 300 °C, and held at 300 °C for 5 min. Quantification of the C6−C12 hydrocarbons was performed by the integration of the peak areas, and the response factors for individual hydrocarbons relative to the internal standard (nC8D18) were calculated based on the peak area ratios of each hydrocarbon to the internal standard. The determination of diamondoids in the pyrolysates was performed using a gas chromatography−mass spectrometry−mass spectrometry (GC−MS−MS) technique. Quantification of diamondoids by GC−MS−MS has been described in detail by Liang et al.29 Briefly, GC−MS−MS analysis was performed using a Thermo Fisher TSQ Quantum XLS instrument. The GC instrument used was equipped with a programmed-temperature vaporizer (PTV) injector and a DB-1 fused silica capillary column with a 50 m × 0.32 mm inner diameter × 0.52 μm thickness film. These analyses used a PTV splitless mode with an inlet temperature of 300 °C and a split flow at 15 mL/min following 1 min of splitless flow. Helium (99.999% purity) carrier gas was used during analysis in constant flow mode at a rate of 1.5 mL/min. The GC oven temperature was initially set at 50 °C for 2 min, before increasing at 15 °C/min to 80 °C, 2.5 °C/min to 250 °C, and a final increase of 15 °C/min to 300 °C before being held for 10

2. EXPERIMENTAL METHODS 2.1. Samples. The marine shale sample used in this study was collected from the Xiamaling Formation of the upper Proterozoic Qingbaikou Series, occurring in Xiajiagou of Xiahuayuan town of Zhangjiakou city, Hebei Province, North China. The geochemical characterization of the Xiamaling Formation in this area had been described by Zhang et al.25 The shale contains type II kerogen and exhibits a low Tmax value (434 °C), a low vitrinite-like maceral reflectance (Ro) (0.57%), and a relatively high total organic carbon (TOC) content (6.78 wt %). Prior to analyses, surfaces of the shale were cleaned with dichloromethane and then the shale was ground to pass a 100-mesh sieve. In addition to whole-rock samples, four kerogen samples were prepared from the organic-rich marine shale via isolation and artificial maturation; these kerogens were used to represent the kerogens associated with different maturity levels from the same source. The source rock sample was demineralized in a water bath (80 °C) with hydrochloric acid and hydrofluoric acid to isolate kerogen and was then ground to pass a 100-mesh sieve before extraction. The kerogen concentrate was Soxhlet-extracted for 72 h with dichloromethane:methanol (93:7 v/v) to remove free hydrocarbons bound to the kerogen matrix and was then dried at 50 °C for 12 h. The concentrate represents original kerogen prior to oil generation (K1). The other three kerogen samples (K2, K3, and K4) were obtained by artificial maturation of the immature kerogen (K1). The maturation levels of the kerogens represented the main range of the oil-generation window. The artificial maturation process was performed in a vacuum glass tube. Heating temperature and time were set according to the EasyRo method suggested by Sweeney and Burnham26 (Table 1). After Soxhlet-extraction with dichloromethane for 72 h to remove newly produced soluble organic matter, the kerogen samples were used to simulate diamondoid formation from the kerogens at different maturity levels. 2.2. Pyrolysis Experiments. Pyrolysis experiments of whole-rock sample and the four extracted kerogens representing different maturity 8189

DOI: 10.1021/acs.energyfuels.5b01993 Energy Fuels 2015, 29, 8188−8194

Article

Energy & Fuels

Figure 1. Yield curves of adamantanes (a) and diamantanes (b) generated from the artificial thermal simulation of whole-rock and original kerogen (K1). min. Quantification of diamondoid compounds (Table S1 in the Supporting Information) was undertaken using a SRM mode comparison between peak areas for unknowns and two internal standards, namely, n-dodecane-d26 for adamantanes and n-hexadecaned34 for diamantanes.

destruction. Therefore, when diamondoid cracking happens, the concentration of diamondoids in the source rock extract may be unable to make a rapid response because the extract is more susceptive to thermal degradation than diamondoids. Only when the destruction rate of diamondoids is larger than that of the extract will the concentration decrease. Consequently, the beginning of diamondoid cracking obtained based on its concentration in extract will be later than the real one. In this work, we use the “yield”, which represents the absolute quantities of diamondoids generated from each unit of the shale and kerogens. As the weight of the shale or kerogen is a fixed value during the maturation process, the decrease of “yield” indicates that the diamondoid cracking has already happened. As a result, the beginning of diamondoid cracking determined by our methods will be earlier than Wei’s. Figure 1 shows that during the artificial maturation of original kerogen (K1), the formation of adamantanes and diamantanes begins at ∼0.8% and 1.0% EasyRo, respectively. However, with increasing maturity, kerogen displays a larger range and a higher maximum yield of diamondoids than does whole-rock, such that in the case of kerogen, the generation stage of adamantanes and diamantanes is extended to 2.1% and 2.5% EasyRo, respectively, and maximum yields reach 138.5 and 27.6 μg/g TOC, respectively. The yield curves of diamondoids from kerogen are similar to those obtained from oil cracking,18 i.e., the main formation stage is within the range of 1.0%−2.1% (adamantanes) and 1.5%−2.5% (diamantanes), followed by destruction. A comparison between the results of kerogen and whole-rock indicates that in the early formation of adamantanes and diamantanes ( 1.5%), the yields of adamantanes and diamantanes of the whole-rock become lower than K1. This may also be caused by the effect of mineral matrix of shale, which may make the beginning of diamondoid cracking in the source rock earlier than that in K1, leading to a reduction in the peak yield and a reduction in the maturity level corresponding to the peak yield of diamondoids. Figure 2 shows yield curves of methane, C2−C5 gaseous hydrocarbons, and C6−C12 light hydrocarbons from the simulation of the thermal maturation of shale. A comparison of the results with Figure 1 suggests that the formation of

3. RESULTS AND DISCUSSION 3.1. Formation and Destruction of Lower Diamondoids during Shale and Kerogen Pyrolysis. In this study, the evolution of diamondoids was assessed in terms of yields normalized to TOC (in micrograms per gram TOC); i.e., we determined the mass of diamondoids generated at a given maturity level relative to the mass of TOC in the shale or kerogen samples (Table S2 in the Supporting Information). Figure 1 shows yield curves of adamantanes and diamantanes generated from thermal pyrolysis of whole-rock sample and original kerogen (K1). The formation and destruction of the lower diamondoids (adamantanes and diamantanes) in rock can be easily identified in the yield curves. For example, when the maturity is less than 0.8% EasyRo, the yields of adamantanes from the shale and kerogen are very low and diamantanes cannot be detected, indicating that diamondoid concentrations in the initial shale and kerogen samples are very low. With increasing thermal maturity, the yields of adamantanes in the shale increase to 18.4 μg/g TOC at 0.8% EasyRo, which is clearly higher than the initial concentrations of adamantanes in the shale (Figure 1a), showing that the formation of adamantanes in the shale starts at a relatively early stage of thermal maturity (1.3% EasyRo are likely to be derived from the late secondary cracking of bitumen generated by kerogen at different maturation stages. Figure 4 shows the contributions of bitumens to diamonodiod yields, as estimated yields of diamondoids from the four kerogens during maturation. For instance, at the peak

stage of adamantane formation (2.1% EasyRo), 75.6% of the total adamantanes are generated from the secondary cracking of bitumens and 24.4% from the primary cracking of kerogens. Within the fraction generated by the secondary cracking of bitumens, 42.2% of adamantanes are from the bitumen 1 fraction (formed in the early oil generation stage, at 0.57%− 0.80% EasyRo), 34.5% are from the bitumen 2 fraction (formed in the peak oil generation stage, at 0.80%−1.0% EasyRo), and 23.3% are from the bitumen 3 fraction (formed in the late oil generation stage, at 1.0%−1.3% EasyRo). Similarly, at the peak stage of diamantane formation (2.5% EasyRo), the yields of diamantanes generated from the secondary cracking of bitumens accounts for 87.8% of the total diamantanes, while the bitumen 1, bitumen 2, and bitumen 3 fractions contribute 60.5%, 34.4%, and 5.1%, respectively, of the diamantanes generated from the bitumen fraction. Therefore, the expulsion of hydrocarbons from source rocks during the oil generation window influences the concentration of diamondoids in rocks; i.e., high hydrocarbon expulsion efficiency within the oil window leads to a decrease in diamondoid concentrations in source rocks. 8192

DOI: 10.1021/acs.energyfuels.5b01993 Energy Fuels 2015, 29, 8188−8194

Article

Energy & Fuels 3.3. Variations in Diamondoid Indices. Previous studies have indicated that the substitution of more bridgehead carbons into a diamondoid compound is associated with the increased thermodynamic stability of the compound, as compared with the stabilities of homologues.1,9,30 Consequently, isomerization indices of diamondoids have been used to evaluate the thermal maturity of crude oils and source rocks; examples of such indices include the methyladamantane index (MAI) [1-MA/(1MA + 2-MA)], the ethyladamantane index (EAI) [1-EA/(1-EA + 2-EA)], the dimethyladamantane indices (DMAI-1 [1,3DMA/(1,3-DMA + 1,2-DMA)] and DMAI-2 [1,3-DMA/(1,3DMA + 1,4-DMA)]), the trimethyladamantane indices (TMAI1 [1,3,5-TMA/(1,3,5-TMA + 1,3,4-TMA)] and TMAI-2 [1,3,5TMAI/(1,3,5-TMA + 1,3,6-TMA)]), and the dimethyldiamantane indices (DMDI-1 [4,9-DMD/(4,9-DMD + 3,4DMD)] and DMDI-2 [4,9-DMD/(4,9-DMD + 4,8-DMD)]). Figure 5 shows the variations in the diamondoid ratios observed during thermal maturation of the three kerogens representing different degrees of maturity (K1, K2, and K3). Overall, the isomerization ratios display no obvious variations during the main formation stage of diamondoids, indicating that the source or formation mechanism of diamondoids is a control factor that influences diamondoid isomerization ratios in this stage. This result can explain the conclusion by Schulz et al.5 that diamondoid indices (e.g., EAI, DMDI-1, and DMDI-2) are good indicators of source, rather than the maturity stage in the oil window. However, a rapid increase with maturity occurs in the destruction stage of diamondoids. For example, at maturity stages up to 1.5% EasyRo for MAI and EAI, up to 2.0% EasyRo for DMAI-1 and DMAI-2, and up to 2.5% EasyRo for TMAI-1, TMAI-2, DMDI-1, and DMDI-2, these indices show linear correlations with maturity. Thus, the thermal stability of diamondoid compounds becomes a critical factor influencing the diamondoid isomerization ratios when the thermal maturity stage enters the destruction stage of diamondoids. Therefore, these isomerization indices can be used to determine the maturity of only high-maturity source rocks. The values of MAI, EAI, TMAI-1, and TMAI-2 show similar evolutionary trends during the pyrolysis of the three kerogens (K1, K2, and K3), indicating that they are unaffected by hydrocarbon generation and expulsion of source rock at early thermal stages. However, the values of DMAI-1 and DMAI-2 among the pyrolysates of the three kerogens show obvious deviations. The effect of analytical error can be excluded as a possible cause of the deviations, as the C2 -alkylated adamantanes are one of the most abundant components in adamantanes. We therefore infer that difference in source are the main cause of the deviations. The contribution of bitumen 3 to diamantanes is very low (only 4.5%; Figure 4), leading to an absence of information on kerogen K3 in the plots of DMDI-1 versus EasyRo and DMDI-2 versus EasyRo (Figure 5).

3.5% EasyRo for diamantanes). (2) Comparisons of diamondoid yields from kerogens of different maturities show that diamondoids in source rocks are generated mainly from the secondary cracking of bitumens within the source rock at late stages of thermal maturation (>1.3% EasyRo), accounting in our studies for 75.6% of the maximum yield of adamantanes and 87.8% of the maximum yield of diamantanes. (3) The upper limit of maturity for the generation of diamondoids from the primary cracking of kerogens is 1.3% EasyRo. (4) Some diamondoid isomerization indices show relatively constant levels during the formation stage of diamondoids, whereas a linear correlation with maturity occurs in their destruction stage, indicating that these diamondoid indices are a potential tool for the evaluation of the thermal maturity of source rocks at high- and overmature stages.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.5b01993. Peak identification, formulas, and abbreviations of diamondoid compounds in the study (Table S1); yields of individual diamondoid hydrocarbons during the thermal maturation of the shale and K1−K4 kerogens (Table S2); and yields of gaseous and light hydrocarbons during the maturation of the shale (Table S3) (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-20-85290744. Fax: +8620-85290706. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grants 41172115, 41372138, and 41303032), and National Science & Technology Major Project of the Ministry of Science and Technology of China (Grant 2011ZX05008-002-32). This is contribution No. IS2165 from GIGCAS. We are grateful to Dr. Zhang W. B. for his help with GC−MS−MS analysis and Mr. A. Xu, Mr. Y. Li, and Mrs. X. Yang for their assistance with pyrolysis experiments.

■

REFERENCES

(1) Wingert, W. S. Fuel 1992, 71, 37−43. (2) Chen, J. H.; Fu, J. M.; Sheng, G. Y.; Liu, D. H.; Zhang, J. J. Org. Geochem. 1996, 25, 179−190. (3) Dahl, J. E. P.; Moldowan, J. M.; Peters, K. E.; Claypool, G. E.; Rooney, M. A.; Michael, G. E.; Mello, M. R.; Kohnen, M. L. Nature 1999, 399, 54−57. (4) Imuta, K.; Ouchi, K. Fuel 1973, 52, 301−302. (5) Schulz, L. K.; Wilhelms, A.; Rein, E.; Steen, A. S. Org. Geochem. 2001, 32, 365−375. (6) Wei, Z. B.; Moldowan, J. M.; Jarvie, D. M.; Hill, R. Geology 2006, 34, 1013−1016. (7) Li, J. G.; Philp, P.; Cui, M. Z. Org. Geochem. 2000, 31, 267−272. (8) Zhang, S. C.; Huang, H. P.; Xiao, Z. Y.; Liang, D. G. Org. Geochem. 2005, 36, 1215−1225. (9) Wei, Z. B.; Moldowan, J. M.; Zhang, S. C.; Hill, R.; Jarvie, D. M.; Wang, H. T.; Song, F. Q.; Fago, F. Org. Geochem. 2007, 38, 227−249.

4. CONCLUSIONS A marine shale and samples of kerogen obtained from the shale via isolation and artificial maturation were used to investigate the formation and evolution of diamondoids in rock. The main conclusions of the study are as follows. (1) Diamondoids generated from kerogen within source rocks experience an evolutionary process that is similar to that of oil during thermal maturation, including a generation stage (0.8−2.1% EasyRo for adamantanes and 1.0−2.5% EasyRo for diamantanes) and a destruction stage (2.1−3.0% EasyRo for adamantanes and 2.5− 8193

DOI: 10.1021/acs.energyfuels.5b01993 Energy Fuels 2015, 29, 8188−8194

Article

Energy & Fuels (10) Grice, K.; Alexander, R.; Kagi, R. I. Org. Geochem. 2000, 31, 67− 73. (11) Wei, Z. B.; Moldowan, J. M.; Peters, K. E.; Wang, Y.; Xiang, W. Org. Geochem. 2007, 38, 1910−1926. (12) Fort, R. C.; Schleyer, P. V. Chem. Rev. 1964, 64, 277−300. (13) Schneider, A.; Warren, R. W.; Janoski, E. J. J. J. Org. Chem. 1966, 31, 1617−1625. (14) Giruts, M. V.; Rusinova, G. V.; Gordadze, G. N. Pet. Chem. 2006, 46, 225−236. (15) Wei, Z. B.; Moldowan, J. M.; Dahl, J.; Goldstein, T. P.; Jarvie, D. M. Org. Geochem. 2006, 37, 1421−1436. (16) Giruts, M. V.; Gordadze, G. N. Pet. Chem. 2007, 47, 12−22. (17) Dahl, J. E. P.; Moldowan, J. M.; Wei, Z. B.; Lipton, P. A.; Denisevich, P.; Gat, R.; Liu, S. G.; Schreiner, P. R.; Carlson, R. M. Angew. Chem. 2010, 122, 10077−10081. (18) Fang, C. C.; Xiong, Y. Q.; Liang, Q. Y.; Li, Y. Org. Geochem. 2012, 47, 1−8. (19) Wei, Z. B.; Moldowan, J. M.; Paytan, A. Org. Geochem. 2006, 37, 891−911. (20) Gordadze, G. N.; Giruts, M. V. Pet. Chem. 2008, 48, 414−419. (21) Berwick, L.; Alexander, R.; Pierce, K. Org. Geochem. 2011, 42, 752−761. (22) Burnham, A. K.; Braun, R. L. Org. Geochem. 1990, 16, 27−39. (23) Fang, C. C.; Xiong, Y. Q.; Li, Y.; Chen, Y.; Liu, J.; Zhang, H.; Adedosu, T. A.; Peng, P. A. Geochim. Cosmochim. Acta 2013, 120, 109−120. (24) Fang, C. C.; Xiong, Y. Q.; Li, Y.; Chen, Y.; Tang, Y. J. Mar. Pet. Geol. 2015, 67, 197−203. (25) Zhang, S. C.; Zhang, B. M.; Bian, L. Z.; Jin, Z. J.; Wang, D. R.; Chen, J. F. Sci. China, Ser. D: Earth Sci. 2007, 50, 527−535. (26) Sweeney, J. J.; Burnham, A. K. Am. Assoc. Pet. Geol. Bull. 1990, 74, 1559−1570. (27) Pan, C. C.; Yu, L. P.; Liu, J. Z.; Fu, J. M. Earth Planet. Sci. Lett. 2006, 246, 70−89. (28) Fang, C. C.; Xiong, Y.; Liang, Q. Y.; Li, Y.; Peng, P. A. Org. Geochem. 2011, 42, 316−322. (29) Liang, Q. Y.; Xiong, Y. Q.; Fang, C. C.; Li, Y. Org. Geochem. 2012, 43, 83−91. (30) Clark, T.; Knox, T. M. O.; Mckervey, M. A.; Mackle, H.; Rooney, J. J. J. Am. Chem. Soc. 1979, 101, 2404−2410.

8194

DOI: 10.1021/acs.energyfuels.5b01993 Energy Fuels 2015, 29, 8188−8194