Identification and Geological Implication of Thiols in the Condensate Oils

Thiols (or mercaptans) are widely present in petroleum but generally in trace amounts. Their formation and transformation are important parts of the geochemical sulfur cycle, but the characterization of these compounds is not comprehensive due to their thermal instability and trace content. In this work, the methylation/demethylation method was performed to separate sulfur compounds in two condensate oils, while thiols therein were derived into alkyl methyl sulfides and also isolated during the process. There were negligible sulfides and thiophenes separated, while the separated thiols accounted for 29.0% and 52.4% of the two condensate oils in total sulfur content, respectively. This indicates that the content of these thiols is abnormally high, thus allowing for a detailed analysis of such species. The separated methylated thiols were analyzed by gas chromatography with a sulfur selective detector and mass spectrometry, and the results were in good agreement with the composition obtained by Fourier transform ion cyclotron resonance mass spectrometry. The specific structures and abundances of individual n-alkyl, iso-alkyl, and monocycloalkyl thiols were identified, indicating carbonium ions may be intermediates of sulfurization during the diagenesis process. This work complements the characterization of thiols in petroleum and provides important evidence for the complex thermochemical sulfate reduction reaction network