Molecular Characterization of NSO Compounds and Paleoenvironment Implication for Saline Lacustrine Oil Sands by Positive-Ion Mass Spectrometry Coupled with Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry

NSO compounds mainly exist in geological bodies in the form of nonhydrocarbons and asphaltenes with abundant geological and geochemical information. Combined with the gas chromatography mass spectrometry (GC-MS) technology, positive-ion electrospray ionization Fourier-transform ion cyclotron resonance MS (FT-ICR MS) was used to understand the composition and distribution characteristics of NSO compounds in the oil sands of the Dongpu Depression and to explore their paleoenvironmental significance. The results show that n-alkanes are characterized by an even carbon number and phytane dominance, suggesting a saline lacustrine environment. Certain abundance of nC(37) and beta-carotane, high gammacerane content, and low diasterane content are detected in the analyzed samples, reflecting the saline-reducing paleoenvironment for the organic matter. Nine types of heteroatom compounds are detected: N-1, N1O1, N1S1, O-1, O1S1, O-2, O2S1, S-1, and S-2. The main compounds are S-1 and N-1 compounds, followed by O1S1 compounds. The double-bond equivalent (DBE) value of S-1 compounds is mainly distributed between 3 and 12, and the carbon number is mainly distributed between 18 and 35. The DBE value of N-1 compounds is mainly distributed between 4 and 14, and the carbon number is mainly distributed in the range 15-35. Among the S-1 compounds, DBE3 compounds (thiophenes) have relatively more sulfur-containing carotenoids (C-40). The abundance of C-40 S-1 and the ratio of pyridine and its homologue DBE4-8/DBE9-12 N-1 compounds show a good contrast with the paleoenvironment indicators of gammacerane/C-30 hopane and diasterane/regular sterane. They can be used as a reference for the paleoenvironment index. Maturity is another factor affecting the distribution of NSO heteroatoms in the oil sands. NSO compounds are enriched in the DBE area with higher condensation, and the main peak carbon shifts forward. As the maturity increases, the relative abundance of N-1 compounds increases, the aromatization intensifies, and carbon is broken into short chains. The research results shed light on the potential application of NSO compounds in petroleum exploration based on FT-ICR MS.