Distributions and Applications of Nitrogen- and Oxygen-Containing Compounds in the Saline Lacustrine Source Rocks of the Baoding–Raoyang Sag, Bohai Bay Basin, Eastern China

This study is the first to use high-resolution mass spectrometry technology to conduct a detailed analysis of nitrogen- and oxygen-containing compounds in immature–low-mature saline lacustrine source rocks in the Baoding–Raoyang Sag of the Bohai Bay Basin, revealing their distribution patterns and developing new methods for maturity assessment and oil-source correlation based on these compounds. There has been a lack of research on the distributions and geochemical applications of nitrogen- and oxygen-containing compounds in immature–low-mature saline lacustrine source rocks. Nitrogen- and oxygen-containing compounds in the tertiary saline, brackish-freshwater lacustrine source rocks with differing maturities and related crude oils in the Baoding–Raoyang Sag (Bohai Bay Basin, China) were analyzed using negative-ion electrospray ionization (ESI), Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), and gas chromatography–mass spectrometry (GC–MS). The results suggest that when Ro ≤ 0.47%, O2 species prevail in saline lacustrine source rocks with no or little N1, N1O1, and O1 species. When Ro > 0.47%, the relative abundance of nitrogen-containing compounds (e.g., N1 and N1O1 species) increases with increasing maturity, but the abundance of oxygen-containing compounds (e.g., O2 species) significantly decreases. The freshwater lacustrine source rocks are richer in oxygen-containing compounds than the saline lacustrine source rocks. There are good correlations between DBE17/DBE12-N1, DBE10/DBE5-O1 and vitrinite reflectance, maturation indices C29 sterane ααα20S/(20S + 20R), and C23 βα/αα-tricyclic terpane, which can be used as auxiliary indicators for maturity evaluation and have particular advantages in saline lacustrine source rocks when there is little vitrinite and/or the vitrinite reflectance fails. Oil-source rock correlation of saturated hydrocarbons and nitrogen- and oxygen-containing compounds indicates that the mature first member of the Shahejie Formation at burial depths greater than 3000 m is the major contributor of the low-mature oil in the study area. The hydrocarbon generation model of the saline lacustrine source rocks in the Baoding–Raoyang Sag is thereby re-established, offering important guiding significance for the exploration of low-mature oil in this and analog areas.