Molecular Analyses of Petroleum Hydrocarbon Change and Transformation during Petroleum Weathering by Multiple Techniques.

Various analytical techniques are used to study the weathering process of four crude oils, i.e., Iranian light crude oil, Daqing crude oil, Shengli crude oil, and Tahe crude oil. The molecular composition and structural information of -alkanes, polycyclic aromatic hydrocarbons (PAHs), and heteroatom compounds were characterized by gas chromatography-flame ionization detector (GC-FID), gas chromatography-mass spectrometry (GC-MS), and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), respectively. The results showed that the weathering loss of -alkanes was related to the molecular weight, and the low-molecular-weight (LMW) -alkanes were more volatile. The loss degree of LMW naphthalene and alkylation homologues in PAHs was also higher. With the increase in the alkylation degree, the weathering resistance ability of PAHs was enhanced. In the negative-ion ESI FT-ICR MS mode, a total of 16 classes of compounds were detected for neutral nitrogen compounds and acidic compounds in the four crude oils. With the increase in weathering time, the relative abundances of NO, NO2, and O3S compounds gradually increased. In particular, the NO and NO2 compounds with different condensation degrees increased significantly. These results indicated that in addition to the volatilization of hydrocarbon compounds, nitrogen compounds were also oxidized to a certain extent during the weathering process. The provided information would enrich the understanding of the short-term weathering process of petroleum hydrocarbons.