Gas purge microsyringe extraction coupled to comprehensive two-dimensional gas chromatography for the characterization of petroleum migration

Several nitrogen-containing compounds play important roles as molecular indicators in petroleum migration but are usually absent or present at very low concentrations, especially when oils have been subjected to severe biodegradation or water washing. In this study, we use thiophenic compounds (such as dibenzothiophene and its alkylated homologs) as alternative oil migration indicators because they appear to exhibit strong absorption/adsorption interactions with solid organic/mineral phases due to their molecular structure, similar to organic nitrogen compounds. To improve the detection sensitivity of trace analytes, a gas purge microsyringe extraction (GP-MSE) device was developed and coupled to comprehensive two-dimensional gas chromatography/time-of-flight mass spectrometry (GC × GC-TOFMS) for the simultaneous analysis of biomarkers and trace nitrogen- and sulfur-containing compounds in crude oils. Twenty oil samples collected from the Hashan region of the Junggar Basin in China were classified as having different biodegradation levels and confirmed to be primarily sourced from the Fengcheng Formation with similar maturities and depositional environments. In addition to the commonly used oil migration parameters derived from the distribution of organic nitrogen compounds, several geochemical parameters based on the ratios of 4,6-/1,4-, 4,6-/3,6- and 4,6-/2,7-dimethyldibenzothiophenes (DMDBTs) were proposed to indicate the oil migration direction. The conclusions on oil migration obtained from organic nitrogen compounds and DMDBTs were in good agreement and further supported by other geological evidence. An oil migration model for the region was developed by combining the geochemical data with the geological information.