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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 μg/g or less). The abundance of thiadiamondoid reflects the extent of TSR, which is rarely detected >20 μg/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 GC×GC-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.

Thiols (or mercaptans) in crude oils have been considered to be associated with thermochemical sulfate reduction (TSR). However, the molecular composition and the formation mechanism of thiols in crude oils are not clear. This study provides the molecular composition of thiols in several deep strata condensate oils (condensates) from the Tarim Basin. Selective Michael addition products of thiols were characterized using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and comprehensive two-dimensional gas chromatography–MS (GC × GC MS). The carbon numbers and double bond equivalents of the thiols ranged between 4–20 and 0–10, respectively. Abundant diamondoidthiols found in two condensates were from high-temperature reservoirs. Various thiol compounds with multiple heteroatoms (S2, S3, S1O1, S1O2, and S2O1 class species) were detected in two other condensates, and low reservoir temperature made it possible to prevent the depletion of the multiple-heteroatom thiols. The pronounced 34S enrichment in H2S and high abundance of thiadiamondoids indicate that four condensates were originally sourced from Cambrian strata and suffered severe TSR alteration. The pathway of thiol formation was speculated as follows: TSR-generated H2S reacts with petroleum hydrocarbons. Deep Cambrian strata in the Tarim Basin has the potential of being a huge hydrocarbon resource, although the high maturity and TSR alteration may negatively impact the quantity and quality of reserves and lead to a high H2S content. The results presented here provide new insights into the TSR reaction, which may assist deep oil exploration and facilitate evaluation of the extent of TSR alteration.

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