Geochemical Distortion on Shale Oil Maturity Caused by Oil Migration: Insights from the Non-Hydrocarbons Revealed by FT-ICR MS

Oil migration may influence the reliability of molecular maturity parameters. Sample extracts from a shaly sequence subjected to intrasource migration, were characterized using FT-ICR MS and GC–MS techniques. The hydrocarbon maturity parameters including sterane isomer ratios and methylphenanthrene indices (MPI) are relatively higher in siltstones than in shales. In contrast, the non-hydrocarbons indicate a lower maturity in siltstones as revealed by the smaller size of molecular cores, the larger length of alkyl side chains, and the more enrichment of Ox species. It is concluded that this discrepancy is attributable to petroleum migration. The thermal-stable isomers of hydrocarbons, used as numerators in the ratios, have stronger mobility consequently leading to a faster migration into siltstones. In contrast, the molecules of non-hydrocarbons with low aromatization and molecular size, and higher alkylation degrees, easily migrate into the siltstones due to relatively lower polarity, precipitability, and absorptivity. Additionally, the hydrophilic matrix surface and high porosity of the strata also facilitate the accumulation of non-hydrocarbons in the siltstones during the early diagenesis. Such findings contribute to a better understanding of oil molecular migration behavior, and suggest a caution for using geochemical indices as maturity parameters when involving petroleum migration.