| Dongpu Depression in Bohai Bay Basin is one of the most typical saline lacustrine petroleum depressions,where some hydrocarbons generated in the early stage.NSO compounds are significant mediums to reveal their genetic mechanism,which however is limited by regular techniques.Fourier transform ion cyclotron resonance mass spectrometry?FT-ICR MS?provides a promising and effective approach for elaborating characterization of these compounds because of its super high resolution and mass accuracy.Here in this paper,the FT-ICR MS and compound specific sulfur isotope methods are first employed to reveal the component and distribution,analyze the main controlling factors and geochemical significance of NSO compound,and explore the genetic and evolution mechanism of the hydrocarbons in Dongpu Depression.The results show that the heteroatom class species,N1,N1O1,O1,O2 and O3,are assigned in saline lacustrine oils with predominance of N1,O1 and O2 by negative-ion FT-ICR MS.The N1species are abundant in DBE values of 9,12 and 15 in all studied sample.Alkylphenol?DBE=4?and fatty acids?DBE=1?are the predominant compounds in O1 and O2 species of crude oils respectively.Hopanoic acid and sterol acid with biological skeleton structures are detected in O2 species of immature oils indicating lower temperature genetic mechanism of immature oils.N1,N1O1,N1S1,O1,O1S1,O2,O2S1,S1,S2 are assigned in crude oils by positive ion FT-ICR MS with a predominant component of S1species,followed by O1S1.DBE values of 1,3,6 and 9 are generally the most abundant compounds in the S1 species of studied samples.Relatively higher amount of O2 species and lower content of nitrogen compounds are observed in the saline lacustrine oils.Molecular weight of lower mature oils distributes much wider than that of mature oils.With increasing maturity,the molecular weight goes narrow,the content of single element compounds such as N1 and S1 in crude oils increases while multi-elements compounds decreases such as O1S1 species,and the double-bond equivalence?DBE?of the NSO coumpounds increases while carbon number decreases.Part of the mature and higher mature oils that concentrated with heavier sulfur content?>20‰?demonstrate large amount of labile thermal compounds of S1 species and lower content of O1 species,indicating thermochemical sulfate reduction?TSR?have been occurred widespread.Oils probably influenced by migration effect as the DBE9 of N1 species?mainly carbazole compounds?generally concentrated in higher mature oils.Maturity has the most impact on the NSO compounds.Based on the analysis above,several ratios of NO species,including DBE9–12/DBE15–18-N1,DBE9–12/DBE4–20-O1,C20–30/C31–50-DBE8-O1,C20–28/C29–40-DBE12-N1 and C20–30/C31–50-DBE15-N1 are proposed as thermal maturity indicators for evaluating the immature to mature oils,of which the DBE9–12/DBE15–18-N1 works best.The composition and distribution of O1,S1 and relatively higher abundance of sulfur-isotopic compositions reveal that most of the oils in northern saline lacustrine environment have suffered TSR effect.Amount of labile thermal compounds such as fatty acids,naphthenic acid with special biological structures in oxygen compound and DBE values less than 9 in sulfur compound have close relationship with non-hydrocarbon and asphaltic components,which are contributing to the early generation hydrocarbons as well.There are two genetic mechanisms that responsible for the immature oils in northern saline lacustrine:biolipids and sulfur-rich macromolecules for early hydrocarbon generation.The sulfur compounds in analyzed oils are formed by both inter-and intra-molecular sulfurization.Ratios of O2/N1?>0.7?,C20–30/C15–45–DBE1–O2?>0.4?,DBE5–6/?DBE0–26–O2?>6.0?are proposed to identify the immature oils,which probably provide theoretical support for similar oil exploration. |
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