Occurrence and Origins of Thiols in Deep Strata Crude Oils, Tarim Basin, China

Abstract

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.

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

“…Thiols in crude oil are thought to form when TSR-generated H 2 S reacts with petroleum hydrocarbons. High concentration of H 2 S in the associated natural gas created the necessary conditions for thiol formation. , More importantly, once again, the local burial and thermal history created appropriate conditions for the preservation of these thermally labile thiols. In other words, TSR alteration gradually ceased because of the uplift and cooling of the formation after the Early Cretaceous.…”

Identification and Geological Implication of Thiols in the Condensate Oils

“…The column was eluted with 6 mL n -hexane/DCM (2/1, v / v ), 12 mL of n -hexane/DCM (1/2, v / v ), and 6 mL of DCM in sequence to yield eluting solvents sequentially to yield the initial fraction, middle fraction, and final fraction. The previous results indicated that all thiol derivatives generated in the reaction were enriched in the middle fraction. , Hereinafter, the thiol derivatization products, by default, refer to the corresponding enriched fractions (middle fraction), which were characterized using FT-ICR MS.…”

Review on Sulfur Compounds in Petroleum and Its Products: State-of-the-Art and Perspectives

“…Similar to thiadiamondoids, diamondoidthiols have also been found to be orders of magnitude more abundant in TSR-altered oils than in unaltered oils. These compounds have a distribution similar to that of their thiadiamondoid analogs. , Zhu et al investigated the molecular composition of thiols in several deep strata condensate oils from the Tarim Basin that suffered severe TSR alteration. It was determined that the carbon number and DBE ranges of the thiols are between 4–20 and 0–10, respectively.…”

“…(a) Proposed formation pathways for low-molecular-weight thiols with carbon numbers ≤ 3; (b) proposed formation mechanism for thiols with carbon numbers > 3 and diamondoidthiols. Reprinted with permission from ref . Copyright 2019 American Chemical Society.…”