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

摘要

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 N1O1 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; C20-30/C15-45-DBE1-O-2 0.4; and DBE5-6/DBE1(2-22)-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: DBE9-12/DBE15-18-N-1, C20-28/C29-40-DBE12-N-1, C20-30/C31-50-DBE15-N-1, DBE9-12/DBE4-20-O-1 and C-20-(30)/C31-50-DEE8-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 C20-28/C29-40-DBE12-N-1 and C20-30/C31-50-DBE15-N-1 ratios reflected an organofacies control and DBE9-12/DBE4-20-O-1 and C20-30/C31-50-DBE8-O-1 ratios changed with thermochemical sulfate reduction (TSR)- confirmed by delta S-34 variances of OSCs. In light of the multiple controls on the nitrogen and oxygen compounds, DBE9-12/DBE15-18-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.