New Geochemical Proxies of Nitrogen- and Oxygen-Containing Compounds Characterized by ESI (-) FT-ICR MS

Nitrogen- and oxygen-containing compounds in oil and source rocks have garnered considerable attention due to their preservation of significant geochemical signals. Three distinct types of crude oils (saline, brackish, and freshwater lacustrine) from the Jizhong Depression (Bohai Bay Basin, China) were characterized utilizing negative-ion electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Saline lacustrine oils (type II kerogen) exhibited higher abundances of N-1 compounds. Conversely, freshwater lacustrine oil (type III kerogen) demonstrated higher abundances of the O-1 class species, accompanied by enriched N-1 and the O-1 class species with a high double-bond equivalent (DBE), which can be attributed to the abundance of oxygen functional groups and polycyclic aromatic structures present in type III kerogen. The high abundance of C-27-DBE4-O-1 in low-maturity saline lacustrine oils may have an interconnected relationship with methyltrimethyltridecylchromans (MTTCs) and/or carotenoid pigments, which is strongly correlated with water salinity. A higher ratio of C27-30-DBE5-O-2/C29-35-DBE6-O-2 (likely steranoic acid/hopanoic acid) for the low-maturity saline lacustrine oils indicates the predominant derivation of organic matter from planktonic and benthic algae. The new parameters (DBE18-20/DBE6-8-N-1, DBE12-15/DBE1-O-2, C-27-DBE4-O-1/DBE4-O-1, and C27-30-DBE5-O-2/C29-35-DBE6-O-2) exhibit strong correlations with conventional biomarker parameters with C-19/C-23 tricyclic terpane, sterane/hopane, Pr/Ph, and gammacerane/C-30 hopane, enabling the assessment of organic matter sources and sedimentary environments. The values of DBE15-20-N-1/N-1, C30-45-DBE1/DBE1-O-2, and EOPFA (EOPFA = (C-22 + 6*C-24 + C-26)/(4*C-23 + 4*C-25)-DBE1-O-2) exhibit heightened sensitivity to biomarker maturity proxies and serve as reliable indicators for assessing crude oil maturity, with a specific applicability to low-maturity oils. The presence of numerous fatty acids and steranoic acids with low thermal stability, as important components of lipids in algae and featuring unique biological framework structures in oils, makes important contributions to the formation of low-maturity oil. This study showed a great potential of ESI FT-ICR MS in petroleum geochemistry.