Petroleum Sulfur Biomarkers Analyzed by Comprehensive Two-Dimensional Gas Chromatography Sulfur-Specific Detection and Mass Spectrometry

Abstract

An atypical distribution of steroids occurred in oilsand and oil samples in the Jinxian Sag, Bohai Bay Basin, northern China. It is featured by usually abundant C 21 −C 26 short-chain steranes that are derived from the cleavage of the weak C−S bonds at the C-17 and C-20 positions. Sulfur-containing petroleum biomarkers were further analyzed in these sulfur-rich heavy oils by comprehensive two-dimensional gas chromatography (GC × GC) using sulfur-specific detection to locate and confirm sulfur species and GC × GC time-of-flight mass spectrometry (TOF MS) to identify previously unknown sulfur-containing biomarkers. Preliminary GC−MS analysis revealed a series of compounds yielding characteristic fragment ions consistent with thiophenic substructures. For confirmation, GC × GC−sulfur chemiluminescence detection (SCD) was used to confirm the sulfur moiety. The molecular structures of sulfur biomarkers were proposed as unsaturated isoprenoid thiophenes based on fragmentation patterns observed in their 70 eV electron-impact ionization (EI) mass spectra. There were constant neutral losses of 71 and 111 Da from the molecular ions in the 70 eV EI mass spectra throughout a whole series. The endeavor of sulfur-containing biomarker characterization may provide some clues of paleotransformation of biological precursors and geological products in oil/kerogen under thermal stress during burial and resolve some missing links between the biosphere and the geosphere. ■ INTRODUCTIONWhen exploratory wells of Zhao-7 and Zhao-9 were drilled in Jinxian Sag of Bohai Bay Basin, northern China (see the map in Figure 1), extremely high concentrations (as high as 92%) of hydrogen sulfide in the reservoir gas and sulfur contents as high as 14.7% in the associated crude oils were discovered. 1,2 Limited amounts of the oil samples were collected before the wells were sealed for safety concerns.In a previous characterization of the Zhao-7 and Zhao-9 oils by routine gas chromatography−mass spectrometry (GC−MS) analysis, a series of short-chain steroids and their regular carbon numbered analogues, including lanostanes (C 24−25 and C 30 ), regular steranes (C 27−30 ), 4-methyl steranes (C 22−23 and C 27−30 ), 4,4-dimethyl steranes (C 22−24 and C 28−30 ), short-chain steranes (C 23−26 ), abundant pregnanes (C 21−22 ), and androstanes (C 19−20 ), together with S-containing steroids (20-thienylpregnanes and thienylandrostanes) were detected and identified. 1 In addition, C 22−27 monounsaturated sterenes were also detected in relatively high concentrations in an immature oilsand sample in the vicinity of this region. 2 The prevalence of these short-chain steroid analogues, i.e., C 21 −C 26 > C 27 −C 30 , in oil-related samples of this region was believed to relate to the natural sulfurization process, which played a significant role in the formation of these short-chain steroids. High concentrations of benzothiophenes, dibenzothiophenes, and benzonaphthothiophenes are dominant in the aromatic fractions. 1−5 Sulfur-containing steroids were a...

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