Discovery of High-Abundance Diamondoids and Thiadiamondoids and Severe TSR Alteration of Well ZS1C Condensate, Tarim Basin, China

Diamondoid and thiadiamondoid compounds in crude oils are generally related to thermal cracking and TSR (thermochemical sulfate reduction). Diamondoids occur in almost all petroleum, but usually with low concentration (2 mu g/g or less). The abundance of thiadiamondoid reflects the extent of TSR, which is rarely detected >20 mu g/g. Recently, a commercial oil-gas production has been discovered in the deep Cambrian strata of the Tarim Basin, China. The condensate sample conducted with GCXGC-TOFMS (2D gas chromatography/time-of-flight mass spectrometry) analysis revealed that diamondoids series contains 281 components with a total concentration of 187 mg/g, and the thiadiamondoids series contains 267 components with a total concentration of 28 mg/g, respectively. We reported the identification of 14 compounds, including 11 dithiatriamantanes and 3 tetrathiadiamantanes for the first time in natural oils. According to the comprehensive analysis of sulfur and carbon isotopes and hydrocarbon composition of the condensate, these compounds are considered to be residual products of thermal cracking at high temperature and severed TSR alteration. The extremely heavy sulfur isotope of the thiadiamondoid compounds from TSR alteration illustrates that hydrogen-sulfide-enriched gas reservoirs exist in the deep Cambrian strata of the Tarim Basin.