Discovery and Molecular Characterization of Organic Caged Compounds and Polysulfanes in Zhongba81 Crude Oil, Sichuan Basin, China

Deep carbonate strata in the Sichuan Basin in China mainly produce natural gas of high maturity. The discovery of uncommon condensate oil in the Zhongba gas field is of particular research interest. Condensate oil from the Zhongba81 well of the Lei3 Member of the Triassic Leikoupo Formation was comprehensively characterized using two-dimensional gas chromatography time-of-flight mass spectrometry. Abundant caged compounds (diamondoids, ethanodiamondoids, and thiadiamondoids), thiols, cyclic, and acyclic sulfides as well as polysulfanes were identified within it. High levels of diamondoids and ethanodiamondoids suggest that the condensate experienced high degrees of thermal cracking. Moderate levels of thiadiamondoids in the condensate and sulfur isotopes in abundant H2S show that the reservoir has undergone moderate thermochemical sulfate reduction alteration. A large number of low-molecular-weight sulfur compounds observed including thiols and polysulfanes have been preserved in the current low-temperature reservoir. More detailed information on the molecular composition of crude oil can facilitate stronger understanding of hydrocarbon accumulation and improve field exploration potential.