Compositional Characterization of Expelled and Residual Oils in the Source Rocks from Oil Generation–Expulsion Thermal Simulation Experiments

Petroleum generation–expulsion thermal simulation experiments on a low-maturity type I source rock were carried out. The composition of hydrocarbons and heteroatom-containing compounds in both expelled and residual oils was characterized by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry and gas chromatography-mass spectrometry. A thorough analysis of fractionation effects was presented between expelled and residual oils. The oil composition strongly depended on the pyrolysis temperature. A significant difference in the molecular composition was found between expelled and residual oils at various pyrolysis temperatures. The difference in maturity for expelled and residual oils could be revealed clearly by the difference in the molecular composition of heteroatom-containing compounds. Carboxylic acids in the residual oil cracked quickly with the pyrolysis temperature above 350 °C (% Ro = 1.0) but part of them would survive if they were expelled out of the source rock timely. The variations between the relative abundance of oxygen compounds and nitrogen compounds indicated that the thermostability of oxygen-containing nitrogen compounds was lower than that of neutral nitrogen compounds. The variation trends in double bond equivalents and carbon number distributions of O1, O2, and N1 class species were similar. The aromaticity of heteroatom-containing polar species increased with maturity.