Fractionation of hydrocarbons and NSO-compounds during primary oil migration revealed by high resolution mass spectrometry: Insights from oil trapped in fluid inclusions

Abstract

The composition of oil trapped in fluid inclusions (FI), occurring in mineral cements, can provide valuable insights into oil migration. Here, FI oils in a calcite vein (representing expelled fluids) and source rock (SR) extracts (representing retained bitumen) from the Hosszúhetény Calcareous Marl Formation (HCMF) in the Mecsek Mountains area of Hungary were investigated to assess how organic compounds are fractionated during primary migration. Biomarkers analyzed by gas chromatography–mass spectrometry and gas chromatography-stable carbon isotope ratio mass spectrometry were used to demonstrate that the FI oils had been expelled from the HCMF marl (calculated vitrinite reflectance of ~0.74% Rc). Fourier transform-ion cyclotron resonance-mass spectrometry then provided insights into polar compound geochemistry, showing that O1, N1, N1O1 and S1O1 compound classes are preferentially retained in the source rock bitumen, while less polar compounds like aromatic hydrocarbons (HCs) and S1 compounds are assumed to be preferentially expelled. Independent of the compound class, compounds with higher double bond equivalents (DBEs) are enriched in the retained source rock bitumen. Thus, besides the molecular polarity determined by the functional groups, the molecular size and the degree of aromaticity appear to be the most important factors affecting the migration and retention behavior of the petroleum HCs and NSO-compounds in the HCMF. Moreover, the enrichment of high DBE compounds with shorter alkyl chains in the SR extracts infers that shielding effects could have played a major role for compound retention and expulsion during primary oil migration.