Geochemical characteristics and significance of heteroatom compounds in lacustrine oils of the Dongpu Depression (Bohai Bay Basin, China) by negative-ion Fourier transform ion cyclotron resonance mass spectrometry

Abstract Saline and freshwater lacustrine oils from the Dongpu Depression (Bohai Bay Basin) were characterized by negative-ion electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). ESI-FT-ICR MS detected a large range of nitrogen and oxygen compounds in the oils, mostly comprising N 1 , O 1 , O 2 and N 1 O 1 species. The low mature oils are dominated by O 1 species with double bond equivalent of 4 (DBE = 4; mainly alkylphenols), O 2 species with DBE = 5–6 (i.e., hopanoid, secohopanoic and steroid acids) and DBE = 1 (i.e., fatty acids), indicating a genetic relationship between oxygen compounds and their precursors. The nitrogen and oxygen compounds were significantly influenced by maturity, with the lower maturity oils analyzed enriched in O 2 species, particularly fatty acids and 4–5 ring naphthenes with biologically sourced hydrocarbon skeletons: ratios of O 2 /N 1  > 0.7; C 20–30 /C 15–45 -DBE 1 -O 2  > 0.4; and DBE 5–6 /DBE 12–22 -O 2  > 6.0 may be useful indicators of low maturity. With increasing maturity, the condensation degree of the compounds increased and the carbon number decreased. The relative abundances of N 1 and O 1 species with the following specific DBE carbon number ranges were particularly sensitive to maturity: DBE 9–12 /DBE 15–18 -N 1 , C 20–28 /C 29–40 -DBE 12 -N 1 , C 20–30 /C 31–50 -DBE 15 -N 1 , DBE 9–12 /DBE 4–20 -O 1 and C 20–30 /C 31–50 -DBE 8 -O 1 . In some instances the FT-ICR MS proxies provided a more reliable indication of high maturity levels than traditional sterane based maturity parameters. However, several of these species were also responsive to other influences, namely C 20–28 /C 29–40 -DBE 12 -N 1 and C 20–30 /C 31–50 -DBE 15 -N 1 ratios reflected an organofacies control and DBE 9–12 /DBE 4–20 -O 1 and C 20–30 /C 31–50 -DBE 8 -O 1 ratios changed with thermochemical sulfate reduction (TSR)– confirmed by δ 34 S variances of OSCs. In light of the multiple controls on the nitrogen and oxygen compounds, DBE 9–12 /DBE 15–18 -N 1 ratio seems unaffected by organofacies or TSR and was considered the most reliable maturity proxy displaying a similar or better maturity relationship than Ts/(Ts + Tm). Aside from secondary alteration effects, the freshwater samples showed several compositional differences to the saline samples, albeit from a limited data set, including generally higher O 2 and N 1 (particularly low MW) product abundances, lower abundances and quite different distribution of O 1 products. These distinctions suggest ESI FT-ICR MS analysis has potential for distinguishing these sample types.