The effect of thermochemical sulfate reduction on formation and isomerization of thiadiamondoids and diamondoids in the Lower Paleozoic petroleum pools of the Tarim Basin, NW China

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 H2S) to a greater degree than by TCA. TSR may have oxidized hydrocarbons to oxygenated species and finally to CO2 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. (C) 2016 Elsevier Ltd. All rights reserved.