Comparison of Alkane Distributions in Crude Oils Detected by Gas Chromatography of Whole Oils and Saturated Fractions and Gas Chromatography–Mass Spectrometry of Saturated Fractions

Abstract Alkane-related parameters of oils obtained by both gas chromatography (GC) and gas chromatography–mass spectrometry (GC-MS) were widely used in oil-source correlation. However, it is still unclear whether different detection methods will lead to significant differences in these parameters. The compositions of alkanes in Jurassic oils from the Sichuan Basin were investigated using GC analysis of whole-oil samples and saturated hydrocarbons and GC-MS analysis of saturated hydrocarbons. Comparative analyses of alkane-related parameters obtained by these detection methods were also performed in this study. n-Alkanes in the range nC3–nC35 were detected in oils using whole-oil GC analysis. By contrast, short-chain n-alkanes (nC3–nC13) were not detected by GC and GC-MS analysis of saturated hydrocarbons. The abundances of n-alkanes in the range nC14–nC16 were lower than those detected by whole-oil GC analysis. nC21–/nC22+ ratios and terrigenous/aquatic ratios (TAR = (nC27 + nC29 + nC31)/(nC15 + nC17 + nC19)) obtained by GC analysis of saturated hydrocarbons are similar to those obtained by m/z 85 mass chromatograms of GC-MS and show huge differences from those obtained by whole-oil GC analysis and total ion current of GC-MS. However, the differences between these ratios obtained by GC analysis of saturated hydrocarbons, m/z 85 mass chromatograms, and total ion current of GC-MS are systematic. Pristane/phytane (Pr/Ph) ratios obtained by GC analysis of saturated hydrocarbons exhibit little difference from those obtained by total ion current of GC-MS but show large differences from those obtained by whole-oil GC and m/z 85 mass chromatograms of GC-MS. While, the differences between Pr/Ph obtained by all these methods mentioned above are systematic and can be basically eliminated. Additionally, the different detection methods have no influence on carbon preference index, odd-to-even preference, Pr/nC17 and Ph/nC18.