Characteristics Study Of Aromatic Compounds In Tight Oil In Jimsar Sag

The Jimsar Sag is an important target of onshore tight oil exploration in China in recent years.This paper focuses on the study of the geochemical characteristics of source rocks,reservoir diagenesis and reservoir space,the relationship between reservoir configuration,source and,accumulation mechanisms of the organic matter.This paper combines the theory and the organic geochemical analytical methods of the 17 tight oil samples using fractionation,chromatography-mass spectrometry(GC-MS),to describe the geochemical characteristics of the tight oils from the upper and lower sweet spot of the Lucaogou Formation in the Jimsar sag.The aromatic compounds were analysed by the qualitative and quantitative methods to illustrate the distribution characteristics and geological significances of these compounds in the upper and lower spot of the Lucaogou Formation.In comparison with the physical properties of the tight oil samples from upper and lower sweet spot,the oils in lower sweet are characterized by deeper burial depth and higher density,higher viscosity and lower wax content than the oils in the upper sweet spot.For the bulk geochemical characteristics,the oils in the upper and lower sweet spot are characterized by high aliphatic hydrocarbon content,less aromatic hydrocarbon and asphaltene content,in which the asphaltene content is slightly higher than the aromatic content in the organic matter.The oils in the upper sweet spot has more aliphatic hydrocarbons and less asphaltene than in the lower sweet spot of the Lucaogou Formation.The maturity parameters in the aromatic hydrocarbons showed that the oils in the upper and lower sweet spot are both in mature stage,in which the lower sweet spot tight oil was slightly higher maturity than that of the oils in the upper sweet spot.However,no significant maturity difference was observed between the oils in the upper and lower sweet spot.The triaromatic steranes,aromatic isoprenoids and dehydroxylation vitamin E show the organic matter in the tight oil of the Lucaogou Formation was deposited in the brackish-saline water under the reducing conditions,The sedimentary environments of the oils in the lower sweet spot are more saline and reducing than in the upper sweet spot.Pr/Ph and aromatic isoprenoids ratio(AIR)show that the tight oils in the upper sweet spot of the Lucaogou Formation in the Jimsar Sag has transient and occasional photic zone euxinia,while the tight oils in the lower sweet spot has more persistent photic zone euxinia.The characteristics of naphthalene and phenanthrene compounds indicate that the biological precursors of theh tight oils in the upper and lower sweet spot were mainly dominated by bacteria,algae and other lower aquatic organisms with minor higher plants.The triarylsteranes indicate that organic matter in the tight oils in the upper and lower sweet spots may be derived from some algalsources,such as green algae and blue-green algae.This may be associated with the airborne nutrients containing Fe and Mg,which were brought by volcanic ash and are beneficial to the surge of cyanobacteria and algae,The biomarker characteristics of aromatic hydrocarbons indicate that sedimentary environments and sources of the organic matter are the reasons for the different bulk geochemical characteristics between the oils in the upper and lower sweet spots,while influence of the thermal maturity is minor.