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Thermal maturation-related variations in the abundance and distribution of diamondoids in source rocks were investigated by thermal simulation experiments on extracts and kerogen obtained from a same marine shale, and by quantitative analysis of diamondoids within pyrolysates using highly sensitive and selective gas chromatography–triple quadrupole mass spectrometry (GC–MS–MS). The results indicate that diamondoids within the marine shale are generated mainly during 0.8%–1.7% EasyRo, generally deriving from extractable organic matter within source rocks, and are destroyed after 1.7% EasyRo to be entirely removed by ca. 3.0% EasyRo. Ternary plots of the relative distribution of diamondoids and bivariate diagrams for diamondoid parameters (e.g., DMA/MD vs. DMAI-1, and DMA/MD vs. TMAI-1) show systematic changes in diamondoids with increasing thermal maturity (expressed as EasyRo). [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]

[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]