Origins of Thiadiamondoids and Diamondoidthiols in Petroleum

Sep 15, 2007 - k, dimethyladamantanethiol, 196, C12H20S, 163, DMAT-3. l, dimethyladamantanethiol, 196, C12H20S, 163, DMAT-4. m, dimethyladamantanethio...
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Energy & Fuels 2007, 21, 3431–3436

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Origins of Thiadiamondoids and Diamondoidthiols in Petroleum Zhibin Wei,*,†,⊥ J. Michael Moldowan,† Fred Fago,† Jeremy E. Dahl,† Chunfang Cai,‡ and Kenneth E. Peters§ Department of Geological and EnVironmental Sciences, Stanford UniVersity, Stanford, California 94305-2115; Institute of Geology and Geophysics, CAS, Qijiahuozi, Chaoyang District, P.O. Box 9825, Beijing 100029, P. R. China; and U.S. Geological SurVey, Menlo Park, California 94025 ReceiVed June 14, 2007. ReVised Manuscript ReceiVed July 26, 2007

Thiadiamondoids and diamondoidthiols are orders of magnitude more abundant in oil altered by thermochemical sulfate reduction (TSR) than they are in nonaltered oil. This suggests that thiadiamondoids and diamondoidthiols form during TSR. In order to prove this hypothesis, we perform laboratory TSR experiments on diverse organic compounds using sodium sulfate as an oxidant in the presence of elemental sulfur and deionized water at 200 and 350 °C for 48 and 96 h under acidic conditions (pH ) 4). Our results show that thiadiamondoids and diamondoidthiols can be created from non-sulfur-containing diamondoids by TSR. It seems likely that diamondoid species are organic precursors of thiadiamondoids and diamondoidthiols. In addition, thiocholesterol yields trace quantities of dimethyl-2-thiaadamantanes when heated with montmorillonite at 200 °C, suggesting that these diamondoid derivatives may partly originate by molecular rearrangement of polycyclic sulfides and thiols in the presence of acidic clay minerals since they also exist in crude oil that has not undergone TSR. The present study of these heteroatomic cage compounds improves understanding of TSR and can be used to reduce risk in petroleum exploration.

Introduction Diamondoids are structurally similar to tiny pieces of diamond on the order of a billion billionth of a carat in size.1 The smallest diamondoid is adamantane, consisting of one diamond lattice cage. Diamantane consists of two face-fused cages, triamantane three, and so on. Diamondoids occur in all petroleum but, due to their thermodynamic stability,2 are especially abundant in oil that has undergone thermal cracking. Thiadiamondoids and diamondoidthiols are diamondoid derivatives that contain at least one sulfur atom. While our work shows that these compounds are also present in most petroleum, thiadiamondoid species, particularly alkylated 2-thiaadamantanes, and adamantanethiols occur in very high concentrations in oil that has undergone thermochemical sulfate reduction (TSR).3 TSR is a natural process where petroleum is oxidized by sulfate (generally anhydrite) in deep, hot reservoirs. The products may include H2S, CO2, mercaptans, elemental sulfur, and pyrobitumen.4–6 TSR increases the cost of production while decreasing the value of petroleum. TSR has been documented * Corresponding author: Tel +1 713 431 7382; fax +1 713 431 6310; e-mail [email protected]. † Stanford University. ‡ CAS. § U.S. Geological Survey. ⊥ Current address: ExxonMobil Upstream Research Co., 3120 Buffalo Speedway, Houston, TX 77098. (1) Dahl, J. E.; Liu, S. G.; Carlson, R. M. K. Science 2003, 299, 96–99. (2) Dahl, J. E.; Moldowan, J. M.; Peters, K.; Claypool, G.; Rooney, M.; Michael, G.; Mello, M.; Kohnen, M. Nature (London) 1999, 399, 54– 56. (3) Wei, Z. Ph.D. Dissertation, Stanford University, 2006, 407 pp. (4) Orr, W. L. AAPG Bull. 1974, 58, 2295–2318. (5) Orr, W. L. 95th Annual Geological Society of America Meeting Abstracts, New Orleans,October, Paper 213, 1982, p 580. (6) Worden, R. H.; Smalley, P. C. Chem. Geol. 1996, 133, 157–171.

in the West Alberta Basin of Canada,7 the US Gulf Coast,8 and Saudi Arabia.6 Its occurrence can easily be detected by molecular markers of alkyl 2-thiaadamantanes.9 However, little is known about how these compounds form in subsurface strata or reservoirs or when they begin to occur in sedimentary rocks and/or petroleum. In general, large quantities of mercaptans (thiols) may form by direct reaction between sulfur and petroleum hydrocarbons during TSR. Therefore, the formation of thiadiamondoids might accompany that of diamondoidthiols, if these sulfurization processes only occur during sulfate reduction. However, to date, diamondoidthiols have not been reported in petroleum, and their possible precursors remain unclear. Here we explore the possible reactions of diamondoids during TSR through a series of TSR simulation experiments using various organic compounds in order to ascertain the origins of thiadiamondoids and diamondoidthiols in petroleum. Samples and Methods A series of oil and condensate samples taken from various oil fields in the Gulf of Mexico originated from various depths within the Upper Jurassic Smackover Formation source rock. Bulk data are available elsewhere.3 About 50 mg of each of these samples was spiked with dioctyl sulfide and deuterated diamondoids, including D4-adamantane, D3-1-methyladamantane, D3-1-methyldiamantane, D4-diamantane, D5-ethyldiamantane, and D4-triamantane, as internal standards to quantify thiadiamondoids and diamondoidthiols, and diamondoids, respectively. The samples were then fractionated into saturate, (7) Krouse, H. R.; Viau, C. A.; Eliuk, L. S.; Ueda, A.; Halas, S. Nature (London) 1988, 333, 415–419. (8) Heydari, E.; Moore, C. H. Geology 1989, 17, 1080–1084. (9) Hanin, S.; Adam, P.; Kowalewski, I.; Huc, A. Y.; Carpentier, B.; Albrecht, P. J. Chem. Soc., Chem. Commun. 2002, 1750–1751.

10.1021/ef7003333 CCC: $37.00  2007 American Chemical Society Published on Web 09/15/2007

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Table 1. Tentative Peak Assignments for Alkylated 2-Thiadiamondoids in SIR-GC-MS and MRM-GC-MS Analyses of the OSC Fraction peak no.

peak assignment

MW

molecular formula

base peak

abbreviation

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

5-methyl-2-thiaadamantane 1-methyl-2-thiaadamantane 5,7-dimethyl-2-thiaadamantane 1,5-dimethyl-2-thiaadamantane 1,3-dimethyl-2-thiaadamantane 3,5,7-trimethyl-2-thiaadamantane 1,5,7-trimethyl-2-thiaadamantane 1,3,7-trimethyl-2-thiaadamantane 1,3,5-trimethyl-2-thiaadamantane 1,3,5,7-tetramethyl-2-thiaadamantane tetramethyl-2-thiaadamantane tetramethyl-2-thiaadamantane tetramethyl-2-thiaadamantane tetramethyl-2-thiaadamantane tetramethyl-2-thiadamantane

168 168 182 182 182 196 196 196 196 210 210 210 210 210 210

C10H16S C10H16S C11H18S C11H18S C11H18S C12H20S C12H20S C12H20S C12H20S C13H22S C13H22S C13H22S C13H22S C13H22S C13H22S

93 93 93 93 93 107 107 107 107 121 121 121 121 121 121

5-MTA 1-MTA 5,7-DMTA 1,5-DMTA 1,3-DMTA 3,5,7-TMTA 1,5,7-TMTA 1,3,7-TMTA 1,3,5-TMTA 1,3,5,7-TeTMTA TeMTA-1 TeMTA-2 TeMTA-3 TeMTA-4 TeMTA-5

Table 2. Tentative Peak Assignments for Alkylated Adamantanethiols in MRM-GC-MS Analysis of the OSC Fraction peak no.

peak assignment

MW

molecular formula

base peak

abbreviation

a b c d e f g h i j k l m n o p

1-adamantanethiol 2-adamantanethiol methyladamantanethiol methyladamantanethiol methyladamantanethiol methyladamantanethiol methyladamantanethiol methyladamantanethiol dimethyladamantanethiol dimethyladamantanethiol dimethyladamantanethiol dimethyladamantanethiol dimethyladamantanethiol dimethyladamantanethiol trimethyladamantanethiol trimethyladamantanethiol

168 168 182 182 182 182 182 182 196 196 196 196 196 196 210 210

C10H16S C10H16S C11H18S C11H18S C11H18S C11H18S C11H18S C11H18S C12H20S C12H20S C12H20S C12H20S C12H20S C12H20S C13H22S C13H22S

135 135 149 149 149 149 149 149 163 163 163 163 163 163 177 177

1-AT 2-AT MAT-1 MAT-2 MAT-3 MAT-4 MAT-5 MAT-6 DMAT-1 DMAT-2 DMAT-3 DMAT-4 DMAT-5 DMAT-6 TMAT-1 TMAT-2

aromatic, and organic sulfur compound (OSC) fractions through two-layer liquid chromatography on silver nitrate-impregnated silica gel by sequential elution using hexane, dichloromethane, and acetone, respectively. Gas chromatography–mass spectrometry (GC-MS) was run on the saturate fraction using a Hewlett-Packard 5890 Series II gas chromatograph interfaced to a Micromass Autospec-Q mass spectrometer, operating in a selected ion recording (SIR) mode. The ionization was by electron impact at 70 eV. The GC was equipped with an HP-1 MS fused silica capillary column (60 m × 0.25 mm i.d., 0.25 µm thickness of methyl silicone film). Hydrogen was the carrier gas with a head pressure of 15 psi. Samples were injected at 50 °C while holding constant temperature for 1 min; the oven temperature was subsequently increased to 80 °C at 15 °C/min, then to 290 °C at 2.5 °C/min, and finally to 320 °C at 25 °C/min, where it was held for 25 min. Metastable reaction monitoring (MRM)-GC-MS of thiadiamondoids and diamondoidthiols was performed on the OSC fraction. The temperature program was 50 °C for 1 min, 50–240 °C at 2 °C/min, 240–320 °C at 10 °C/min, and 320 °C for 25 min. The mass spectrometer was run in a parent f daughter mode. The monitored transitions were m/z 154 f 79, 168 f 93, 182 f 93, 196 f 107, 210 f 121 for alkylated 2-thiaadamantanes and m/z 168 f 135, 182 f 149, 196 f 163, 210 f 177 for alkylated adamantanethiols. Alkylated 2-thiaadamantanes were identified by comparison of their GC retention times and characteristic mass spectra reported in the literature.9–11 The tentative peak assignments of alkylated 2-thiaadamantanes are (10) Dessort, D.; Montel, F.; Caillet, G.; Lescanne, M. The 6th Middle East Geosciences Conference and Exhibition, CEO, Bahrain, 2004.

given in Table 1. The identification of a homologous series of adamantanethiols was based on the mass spectral data of 1- and 2-adamantanethiols reported by Dolejšek et al.12 and Greidanus.13 The tentative peak assignments of adamantanethiols are given in Table 2. The quantitation of thiadiamondoids and adamantanethiols was achieved by integration of their peak areas with respect to dioctyl sulfide, assuming the response factor is 1. The aromatic fraction was spiked with d10-phenanthrene and analyzed by SIR-GC-MS to quantify dibenzothiophenes in these samples. Laboratory simulations of TSR were carried out using various organic compounds (ca. 100 mg) and branched/cyclic fractions in T316 stainless steel vessels at temperatures of 200 and 350 °C for 48 and 96 h. These compounds included adamantane, 1,3-dimethyladmantane, diamantane, tridecane, 1-tridecene, pristane, squalene, squalane, decalin, cyclohexene, toluene, phenanthrene, stearic acid, 5R-cholestane, cholesterol, abietic acid, and β-carotene. The initial experimental pH was about 4. Excessive amounts of sodium sulfate (ca. 1 g) were used to represent inorganic sulfate minerals. Elemental sulfur (ca. 4 mg) and deionized water (2 mL) were added, since hydrolysis of elemental sulfur occurs at higher temperatures above 200 °C, producing H2S and sulfate.14 The reaction products were spiked with D3-1-methyladamantane and analyzed by GC-MS in a full(11) Galimberti, R.; Zecchinello, F.; Nali, M.; Gigantiello, N.; Caldiero, L. The 22nd International Meeting of Organic Geochemists (IMOG) SeVille, Spain,abstracts book part 1, 2005, pp 229–230. (12) Dolejšek, Z.; Hála, S.; Hanuš, V.; Landa, S. Collect. Czech. Chem. Commun. 1964, 31, 435–449. (13) Greidanus, J. W. Can. J. Chem. 1971, 49, 3210–3215. (14) Robinson, B. W. Earth Planet. Sci. Lett. 1973, 18, 443–450.

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Figure 1. Partial total ion chromatogram (TIC) and mass chromatograms of products from alteration of (a) adamantane and (b) 1,3-dimethyladamantane by sodium sulfate in the presence of elemental sulfur and deionized water at 350 °C for 4 days. These diagrams show the formation of 1- and 2-adamantanethiols and methyl-2-thiaadamantanes (m/z 168), dimethyl-2-thiaadamantanes and methyladamantanethiols (m/z 182), and trimethyl2-thiaadamantanes and dimethyladamantanethiols (m/z 196) during TSR simulation using adamantane and 1,3-dimethyladamantane as reductants. Unknown peaks are labeled with “•”. I.S. ) D3-1-Methyladamantane. Note: MA ) methyladamantane; AT ) adamantanethiol; TMA ) trimethyladamantane; MAT ) methyladamantanethiol; DMAT ) dimethyladamantanethiol; MTA ) methyl-2-thiaadamantane; DMTA ) dimethyl2-thiaadamantane; TMTA ) trimethyl-2-thiaadamantane.

scan mode to examine the formation of thiadiamondoids and diamondoidthiols. In addition, cyclohexene sulfide, (1R)(–)-thiocamphor, and thiocholesterol were heated with montmorillonite K10 at temperatures of 200 and 350 °C for 48 and 96 h. The reaction products were also analyzed by GC-MS. Montmorillonite K10 was purchased from Aldrich Chemical Co. Its cation exchange capacity (CEC) is about 80–120 mequiv/ 100 g, and it has a surface area of 220–270 m2/g. Results and Discussion Formation of Thiadiamondoids and Diamondoidthiols. Reaction of adamantane and sodium sulfate at 350 °C for 48 h yields small quantities of adamantanethiols, including 1-adamantanethiol and 2-adamantanethiol in the presence of elemental sulfur and deionized water (Figure 1a). However, 2-adaman-

tanethiol is more abundant than 1-adamantanethiol. This is surprising because one might expect the tertiary carbon atom at position 1 in adamantane to be more reactive than the secondary carbon atom at position 2. One possible explanation for the dominance of 2-adamantanethiol is that there are more secondary carbon atoms (6) and associated with hydrogen atoms (12) in adamantane than tertiary carbon atoms (4) and associated with hydrogen atoms (4). Thus, although reaction at the tertiary carbon is energetically favored, there are more secondary carbon atoms and associated hydrogen atoms available for reaction. Under similar conditions, several homologous series of alkylated 2-thiaadamantanes as well as alkylated adamantanethiols are generated from oxidation of 1,3-dimethyladamantane by sodium sulfate (Figure 1b). The generated alkylated 2-thiaadamantanes include 1-methyl-2-thiaadamantane, 5-methyl-2-thi-

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aadamantane, 5,7-dimethyl-2-thiaadamantane, 1,5-dimethyl-2thiaadamantane, and 3,5,7-trimethyl-2-thiaadamantane. Among these 2-thiaadamantane species, 5,7-dimethyl-2-thiaadamantane and 3,5,7-trimethyl-2-thiaadamantane are dominant, and their yields are 0.03% and 0.02%, respectively. This provides evidence for the predominance of trimethyl-2-thiaadamantanes in petroleum.3 Trace amounts of methyladamantanethiol (0.007%), dimethyladamantanethiol (0.003%), and trimethyladamantanethiol (0.002%) also form in the reaction of 1,3-dimethyladamantane and sodium sulfate under TSR conditions (Figure 1b). However, these thia- and thiol-cage compounds are not found in the products from thermal alteration of other compounds (e.g., n-alkanes, n-alkenes, isoprenoids, unsaturated isoprenoids, cyclic hydrocarbons, unsaturated cyclics, aromatics with smaller aromatic rings, polycyclic aromatic hydrocarbons, fatty acids, biomarkers, biomarker precursors, compounds with functionalities) by sulfate during laboratory TSR simulations. Thus, thiadiamondoids and diamondoidthiols must at least partly originate from diamondoid species. Interestingly, alkylated 2-thiaadamantanes are not created in the reaction of adamantane and sulfate. Only adamantanethiols form under the experimental conditions. In contrast, the reaction of 1,3-dimethyladamantane with sulfate gives both alkylated 2-thiaadamantanes and adamantanethiols. This suggests that thiadiamondoids can only form from alkylated diamondoid species rather than from parent diamondoids (e.g., adamantane, diamantane, triamantane). However, our data show that any diamondoid species can yield small amounts of diamondoidthiols under our experimental conditions. Figure 2 shows possible schemes for the formation of thiadiamondoids and diamondoidthiols in petroleum. Sulfate alone cannot oxidize organics, since the high-energy S–O bonds need a catalyst for activation.15,16 As illustrated in Figure 2a, it is possible that sulfate is reduced to elemental sulfur as small amounts of H2S are generated by sulfur hydrolysis at reaction temperatures.14 Elemental sulfur or polysulfides5,15 may then directly attack adamantane in the tertiary position to form 1-adamantanethiol or in the secondary position to form 2-adamantanethiol. However, the yields of these adamantanethiols are extremely low (0.003% for 1-adamantanethiol and 0.21% for 2-adamantanethiol). This might be due to high stability of adamantane that is relatively resistant to thermal alteration by reduced sulfur species under these experimental conditions. It is likely that adamantanethiols form from alkylated adamantanes during sulfur hydrolysis and TSR through a similar type of reaction path (Figure 2b). With respect to the formation of 2-thiadiamondoids from alkylated diamondoids, it is unlikely that sulfur can directly replace the carbon atom in the cage structure of diamondoids. However, cage cleavage might occur in the presence of sulfur species since sulfur radicals derived from sulfate reduction initiate the cleavage of C–C bonds.17,18 Therefore, it is possible that two C–C bonds at secondary carbon atoms in the cage are involved in bond cleavage during the reaction. Controlling Factors for Thiadiamondoid and Diamondoidthiol Formation. For our experiments, oxidation of diamondoids by sulfur species cannot be detected at 200 °C. This can be explained by the high dependence of TSR reaction (15) (16) (17) 54–56. (18)

Toland, W. G. J. Am. Chem. Soc. 1960, 82, 1911–1916. Kiyosu, Y.; Krouse, H. R. Geochem. J. 1990, 24, 21–27. Schmid, J. C.; Connan, J.; Albrecht, P. Nature (London) 1987, 329, Lewan, M. D. Nature (London) 1998, 391, 164–166.

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Figure 2. Possible schemes for the formation of (a) adamantanethiols and (b) thiaadamantanes during thermochemical sulfate reduction (TSR) in petroleum. R ) nCH3 (1, 2, 3, ...); x ) 1, 2, 3, ....

rate on temperature and its high activation energy.19–21 Goldhaber and Orr19 indicate that direct reduction of sulfate by organics is orders of magnitude slower below 300 °C than above 300 °C. In contrast, small quantities of diamondoids reacted to form thianes and thiols at 350 °C. However, only minor variations were observed in the yields of these thia- and thiolcage compounds as the reaction times were increased from 48 to 96 h. Furthermore, pH is also an important controlling factor for sulfate reduction. A substantial increase in sulfate reduction occurs as pH decreases.19,21 Running our TSR experiments at pH ) 4 produced small quantities of thiadiamondoids and diamondoidthiols. Other factors that may influence the reaction rates of TSR, e.g., pressure, initial concentrations of H2S and sulfate, and solubility of hydrocarbons, were not considered in the present study. Other Possible Origins for Thiadiamondoids and Diamondoidthiols. Figures 3 and 4 present the mass chromatograms of thiadiamondoids and diamondoidthiols in MRM-GC-MS analysis. Our results show that trace amounts of thiadiamondoids and diamondoidthiols are detectable in non-TSR-altered oil and condensate samples. Diamondoids form by multistep rearrangements of petroleum precursor molecules in the presence of acidic (19) Goldhaber, M. B.; Orr, W. L. In Variavamurthy, M. A., Schoonen, M. A. A., Eds.; Geochemical Transformations of Sedimentary Sulfur; American Chemical Society Symposium Series 612; American Chemical Society: Washington, DC, 1995; pp 412–425. (20) Machel, H. G. Sediment. Geol. 2001, 140, 143–175. (21) Cross, M. M.; Manning, D. A. C.; Bottrell, S. H.; Worden, R. H. Org. Geochem. 2004, 35, 393–404.

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Figure 5. Plots showing the correlation of thiadiamondoids or diamondoidthiols with diamondoids in Upper Jurassic Smackover Formation oil and condensate samples from Gulf of Mexico.

Figure 3. MRM-GC-MS mass chromatograms of alkylated 2-thiaadamantanes in the OSC fraction separated from the State Line oil sample.

Figure 4. MRM-GC-MS mass chromatograms of alkylated adamantanethiols in the OSC fraction separated from the State Line oil sample.

clay minerals, such as montmorillonite in sedimentary rocks.3 Because TSR did not affect these oil samples, it is possible that the thia- and thiol-cage compounds formed by acid-catalyzed rearrangement of polycylic sulfides and thiols in the sedimentary rocks during late diagenesis or catagenesis.9 Our experimental results also show that trace amounts of dimethyl-2-thiaadamantanes are produced by montmorillonite K10-catalyzed rearrangement of thiocholesterol at 200 °C. During early or late diagenesis, incorporation of sulfur into organic compounds with double bonds and functionalities (e.g., aldehydes, ketones, alcohols) may account for the presence of these polycyclic sulfides.22–24 However, the formation of thiadiamondoids and diamondoidthiols by TSR possibly becomes more significant

as clay minerals lose their catalytic activity through severe transformation during late catagenesis, as supported by the fact that no thiadiamondoids are generated from thiocholesterol in the presence of montmorillonite K10 at 350 °C. Further Evidence for Origins of Thiadiamondoids and Diamondoidthiols in Petroleum. Most of the Smackover Formation oil and condensate samples contain both thiadiamondoids and diamondoidthiols. However, these compounds are most abundant in TSR-altered oils, suggesting that a sulfurization process is involved in their formation. Further evidence to support this notion is provided by the δ34S ratios of thiaadamantanes ranging from +21 to ( 22‰ in Smackover oil samples from the Gulf of Mexico, which are close to those of source sulfate minerals (+18 to +24‰).9 Figure 4a shows a linear correlation between the concentrations of diamondoids and thiadiamondoids as well as those of diamondoids and diamondoidthiols, further corroborating the hypothesis that thiadiamondoids and diamondoidthiols originate from diamondoids via sulfurization during TSR. Alkylated adamantanes may be the precursors for 2-thiaadamantanes, as supported by a good correlation between the concentrations of diamondoids and 2-thiaadamantanes in petroleum.3 Similarly, 2-thiadiamantanes may form by sulfurization of alkylated diamantanes. Because only small amounts of diamondoids can be altered by TSR under high-temperature conditions,3 it is not surprising that in most cases the concentrations of diamondoids are orders of magnitude higher than those of thiadiamondoids and diamondoidthiols (Figure 5). It is well known that sulfur is also a dehydrogenating agent that may abstract hydrogen from saturated hydrocarbons to produce aromatic hydrocarbons and thiophenes.4 However, there is no correlation between the concentrations of thiadiamondoids/ diamondoidthiols and dibenzothiophenes (Figure 6). This suggests that thiadiamondoids and diamondoidthiols are produced by mechanisms that differ from those for alkyldibenzothiophenes,10,11 which are primarily formed by cyclization of longchain unsaturated hydrocarbons upon reaction with inorganic sulfides (H2S or polysulfides).25 (22) Sinninghe Damsté, J. S.; Rijpstra, W. I. C.; Kock van Dalen, A. C.; de Leeuw, J. W.; Schenck, P. A. Geochim. Cosmochim. Acta 1990, 53, 1343–1355. (23) Sinninghe Damsté, J. S.; de Leeuw, J. W.; Kock van Dalen, A. C.; de Zeeuw, M. A.; de Lange, F.; Rijpstra, W. I. C.; Schenck, P. A. Geochim. Cosmochim. Acta 1987, 51, 2369–2391. (24) Amrani, A.; Aizenshtat, Z. Org. Geochem. 2004, 35, 909–921.

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Figure 6. Plots showing the correlation of thiadiamondoids or diamondoidthiols with alkyldibenzothiophenes in Upper Jurassic Smackover Formation oil and condensate samples from Gulf of Mexico.

It is also noteworthy that the abundance of diamondoidthiols has an excellent correlation with that of thiadiamondoids. This suggests that thiadiamondoids and diamondoidthiols have similar origins or their formation mechanisms are similar. Thiadiamondoids are typically about an order of magnitude more abundant than diamondoidthiols in petroleum (Figure 5). Conclusions Our analysis shows that heteroatomic cage compounds, including thiadiamondoids and diamondoidthiols, occur naturally

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in various amounts in petroleum. Higher abundance of these compounds occurs in oil and/or condensate that have experienced TSR alteration. Our laboratory experiments show that diamondoids are potential organic precursors for thiadiamondoids and diamondoidthiols. Most thiadiamondoids in petroleum fluids may originate from alkylated diamondoids during TSR, while diamondoidthiols form from any diamondoid species during TSR. The formation of thiadiamondoids might follow a mechanism similar to that of diamondoidthiols. The formation of these compounds involves sulfurization accompanied by hightemperature abiogenic sulfate reduction in petroleum reservoirs. However, our results also suggest that thiadiamondoids and diamondoidthiols may also be partly formed by rearrangement of polycyclic sulfides and thiols catalyzed by acidic clay minerals in sedimentary rocks during diagenesis and catagenesis. The TSR-associated sulfurization is the predominant process for the formation of thiadiamondoids and diamondoidthiols in petroleum fluids rather than acid catalysis during diagenesis and catagenesis. Acknowledgment. This work was supported by the Stanford Molecular Organic Geochemistry Industrial Affiliates, the Stanford University School of Earth Sciences McGee Fund, AAPG Grantin-Aid, Shell Funds, and National Science Foundation of China (Grant No. 40573034). We thank two anonymous reviewers and Dr. Michael Klein for their insightful comments to improve the manuscript. EF7003333 (25) Adam, P.; Phillippe, E.; Albrecht, P. Geochim. Cosmochim. Acta 1998, 62, 265–271.