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Abstract: This study investigates the origin and evolution of adamantanes and diamantanes in petroleum, based on the results of pyrolysis experiments on different group components of crude oil and quantitative analyses of diamondoids in the pyrolysates. Results show that the formation and evolution of lower diamondoids in petroleum occurs in three main stages: (1) early generation during the formation of oil (< ca. 0.8–1.0% EasyRo), (2) generation during the cracking of oil (>1.0% EasyRo), and (3) destruction during the late period of oil cracking. The lower diamondoids that formed during the early generation stage include free diamondoids present in the original oil and diamondoids released or transformed from group fractions of oil at relatively low levels of maturity (< ca. 0.8–1.0% EasyRo). During the process of oil cracking, all four group fractions (saturated, aromatic, resin, and asphaltene fractions) can produce adamantanes and diamantanes. Overall, adamantanes are generated primarily within the maturity range 1.0–2.3% EasyRo, and diamantanes within the maturity range 1.6–2.7% EasyRo. Group composition of the oil is probably a major factor controlling the yield and distribution of diamondoids in the main generation stage. The yield ratios of diamondoids can be used as maturity indices to assess the thermal maturity of oil, while the isomerization ratios of diamondoids are potential source facies indices for the generation stage of diamondoids. The destruction of diamondoids is observed at thermal maturity ranges >2.3% EasyRo in adamantanes and >2.7% EasyRo in diamantanes. The thermal stability of diamondoid compounds becomes a critical factor influencing the isomerization index of diamondoids during the destruction stage. The isomerization ratios of some diamondoids (e.g., MAI, DMAI-1, DMAI-2, TMAI-1, TMAI-2, and EAI) are well correlated with maturity within certain maturity ranges, indicating that isomerization indices provide good estimates of the thermal maturity of highly cracked oils. Bivariate diagrams of concentration versus isomerization indices of diamondoids are introduced to assess the maturity of oils; these diagrams provide more refined maturity assessments over a wider range of maturities than are available from single diamondoid parameters only. [Copyright &y& Elsevier]

Abstract: In this study, headspace single-drop microextraction (HS-SDME) coupled with gas chromatography-flame ionization detection (GC-FID) was tested to determine C6–C12 light hydrocarbons (LHs) in petroleum and aqueous samples. Several significant experimental parameters, such as drop solvent type, drop volume, sample solution ionic strength, agitation speed and extraction time were optimized. Under optimum extraction conditions, specifically, a 1.5μl microdrop of n-hexadecane, 30min extraction of a 5ml aqueous sample placed in a 10ml vial, and stirring at 1000rpm at room temperature, the reproducibility and accuracy of this method were found to be satisfactory. Two examples using this method indicated that HS-SDME is a simple, efficient and promising technique for the determination of volatile C6–C12 LHs in complex matrices. [Copyright &y& Elsevier]

Diamondoid compounds are widely used to reflect thermal maturation of high mature source rocks or oils and oil cracking extents. However, diamondoids and thiadiamondoids were demonstrated to have newly been generated and decomposed in our hydrothermal pyrolysis of crude oil and TSR experiments. Our results show that adamantanes and diamantanes are generated primarily within the maturity range 0.48–2.1% and 1.2–3.0% EasyRo, respectively. Their formation is enhanced and the decomposition of diamantanes obviously lags at elevated temperatures compared with anhydrous experiments. MDI, EAI, DMAI-1, DMDI-2 may serve as reliable maturity proxies at > ca.1.0% EasyRo, and other isomerization indices (TMAI-1, TMAI-2 and DMAI-2) are effective for the highly mature organic matter at EasyRo > 2.0%. The extent of oil cracking (EOC) calculated from the broadly used (3- + 4-) MD method (Dahl et al. in Nature 399:54–56, 1999) is proven to overestimate, especially for highly cracked samples due to the new generation of (3- + 4-) MD. Still, it can be corrected using a new formula at < 3.0% EasyRo. Other diamondoid-related indices (e.g., EAI, DMDI-2, As/Ds, MAs/MDs, DMAs/DMDs, and DMAs/MDs) can also be used to estimate EOC. However, these indices cannot be applied to TSR-altered petroleum. TSR is experimentally confirmed to generate diamantanes and thiaadmantanes at 1.81% EasyRo likely via direct reactions of reduced S species with hydrocarbons and accelerate the decomposition of diamantanes at > 2.62% EasyRo compared with thermal chemical alteration (TCA). More studies are needed to assess specific mechanisms for the formation of thiadiamondoids under natural conditions. [ABSTRACT FROM AUTHOR]

Diamondoid compounds are widely used to reflect thermal maturation of high mature source rocks or oils and oil cracking extents. However, diamondoids and thiadiamondoids were demonstrated to have newly been generated and decomposed in our hydrothermal pyrolysis of crude oil and TSR experiments. Our results show that adamantanes and diamantanes are generated primarily within the maturity range 0.48–2.1% and 1.2–3.0% EasyRo, respectively. Their formation is enhanced and the decomposition of diamantanes obviously lags at elevated temperatures compared with anhydrous experiments. MDI, EAI, DMAI-1, DMDI-2 may serve as reliable maturity proxies at > ca.1.0% EasyRo, and other isomerization indices (TMAI-1, TMAI-2 and DMAI-2) are effective for the highly mature organic matter at EasyRo > 2.0%. The extent of oil cracking (EOC) calculated from the broadly used (3- + 4-) MD method (Dahl et al. in Nature 399:54–56, 1999) is proven to overestimate, especially for highly cracked samples due to the new generation of (3- + 4-) MD. Still, it can be corrected using a new formula at < 3.0% EasyRo. Other diamondoid-related indices (e.g., EAI, DMDI-2, As/Ds, MAs/MDs, DMAs/DMDs, and DMAs/MDs) can also be used to estimate EOC. However, these indices cannot be applied to TSR-altered petroleum. TSR is experimentally confirmed to generate diamantanes and thiaadmantanes at 1.81% EasyRo likely via direct reactions of reduced S species with hydrocarbons and accelerate the decomposition of diamantanes at > 2.62% EasyRo compared with thermal chemical alteration (TCA). More studies are needed to assess specific mechanisms for the formation of thiadiamondoids under natural conditions. [ABSTRACT FROM AUTHOR]

We attempted to differentiate the effects of thermal chemical alteration (TCA) from thermochemical sulfate reduction (TSR) on formation and isomeric distribution of thiadiamondoids and diamondoids in oils and condensate from deeply buried Cambrian and Ordovician carbonate reservoirs in the Tarim Basin, China. Diamondoids and thiadiamondoids with 1–3 cages were detected in all the oils analyzed. Total thiadiamondoid concentrations reflect extent of TSR. Diamondoid concentrations and most of the diamondoid-based maturity proxies and the source rock lithology proxy increase with increasing extent of TSR, indicating that they are controlled by the extent of the TSR process (or reaction with H 2 S) to a greater degree than by TCA. TSR may have oxidized hydrocarbons to oxygenated species and finally to CO 2 and newly generated and concentrated diamondoids and organic sulfur compounds. The most extensive TSR-altered ZS1C condensate contains 3.2 wt% total dibenzothiophenes, 0.4 wt% thiadiamondoids and 5.9 wt% diamondoids together with higher polymantanes including tetramantanes and pentamantanes in association with polythiadiamondoids, including dithiaadamantanes, trithiaadamantanes and tetrathiaadamantanes. These polythiadiamondoids have been detected only from heavily TSR-altered petroleum in this study and North American samples and may be derived from the sulfurization of hydrocarbons, including diamondoids, under high temperature and highly reducing conditions during advanced stages of TSR. The polymantanes may have been generated from alkylation of lower-cage diamondoids and subsequent homologation, or from free radical reactions. This study indicates that TSR may have led to the generation and isomerization of thiadiamondoids and diamondoids, and thus diamondoid-based maturity and lithology proxies cannot be used to reflect maturity and lithology in the TSR active areas, and that the occurrence of polythiadiamondoids can be used to indicate heavy TSR alteration. [ABSTRACT FROM AUTHOR]

Abstract: In this study, a simple solvent dilution followed by highly selective and sensitive gas chromatography–triple quadrupole mass spectrometry (GC–MS–MS) detection was employed to quantify diamondoids in crude oils. Runtime parameters, i.e., parent and daughter ions, collision energy (CE) and scan time, were optimized to obtain maximum selectivity and sensitivity for target analytes. Under optimum conditions, the reproducibility and accuracy of the method were tested and found to be satisfactory. Comparison of GC–MS–MS and GC–MS methods for the determination of diamondoids indicates that GC–MS–MS yields higher sensitivity (method quantitation limits of 0.08–0.37μg/goil) and better selectivity than GC–MS (method quantitation limits of 0.78–8.44μg/goil) due to the elimination of matrix ion interferences using the selected reaction monitoring (SRM) mode. In addition, quantitative data confirm that group separation has a considerable effect on the quantification of diamondoids and the effect appears to depend on multiple factors. Two crude oils (TZ261 and TD2) from the Tarim Basin, China were used to evaluate the GC–MS–MS method. The results prove that the GC–MS–MS method is a promising tool for quantitative analysis of diamondoids in crude oils, especially for oil samples with low diamondoid concentrations. [Copyright &y& Elsevier]

Abstract: In this study, headspace single-drop microextraction (HS-SDME) coupled with gas chromatography–isotope ratio mass spectrometry (GC–IRMS), was employed to determine compound specific carbon isotopic values (δ13C) of gasoline range hydrocarbons. The reproducibility of the method was found to be satisfactory. By comparison with the δ13C values of the twelve target compounds determined using direct injection of their n-C16 solution, no obvious isotopic fractionation was observed during the HS-SDME procedures. Some parameters that could affect the carbon isotopic fractionation, such as ionic strength of working solutions and inlet split ratio, were examined. The results also suggest that these factors had no significant effect on the carbon isotopic determination of gasoline range hydrocarbons. The application of HS-SDME to a crude oil sample proved that this method could be a promising tool for the determination of carbon isotopic values of gasoline range hydrocarbons in oils or aqueous samples. [Copyright &y& Elsevier]

[Display omitted] • Pyrolysis experiments on four organic matter types simulate diamondoid evolution. • Principal component analysis explains the main factors affecting diamondoid indices. • PCA-Fisher discriminant analysis model objectively identifies oil source types. • PCA-regression analysis model quantitatively predicts oil thermal maturity. Assessing the origins and thermal maturity of oils is an important issue in applied petroleum geochemistry. In this study, we extracted information regarding the sources and thermal maturity of oils from diamondoid data using multivariate statistical analysis based on four series of thermal simulation experiments conducted on Type I, II, and III kerogens and a crude oil. Using principal component analysis (PCA) and discriminant analysis (DA), we established the relationship between diamondoid indices and kerogen type, which was then used to identify the source of oils in some oil fields in China. PCA and regression analysis (RA) were used to determine the quantitative relationship between diamondoid indices and thermal maturity, which was then used to estimate the thermal maturity of the studied oils. Our results indicate that this approach is effective for determining the source and maturity of oils, particularly for condensates. [ABSTRACT FROM AUTHOR]