Study on Tight Oil in Chang7 of Yanchang Formation in the Maling Province of Ordos Basin

The tight oil refers to the oil that accumulates in oil shale or interbedded tight sandstone reservoirs adjacent to source rocks and with surface-air permeability less than 0.3mD.Generally,this accumulated oil has not yet experienced large-scale and long-distance migration,and tuus its accumulation mainly develops in a semi-deep to deep lake zone,typically in the oil shale and tight sandstone of Chang-7internal.Tight oil in Ordos Basin is characterized by a wide distribution,superior conditions of source rocks,tight sandstone reservoirs,complex pore-throat structures,poor physical properties,high oil saturation,better oil properties and low pressure coefficient.The tight oil shows a density of 0.84~0.86g/cm3 and a main peak at nC15in the whole hydrocarbon chromatogram.The tight oil consists mainly of light hydrocarbons,low content of aromatic hydrocarbons,and has an equilibrial ratio of Pr/Ph,varing from 0.85to 1.10,and-31.7‰ ofδ13CPDB for the whole crude.Its terpane distribution is characterized by relatively abundant rearranged hopanes,low content of gammacerane,and high ratios of Ts/Tm,The steranes in the crude mainly consist of rearranged steranes,whileααα-20Rsterane isomers display an asymmetricVtype distribution pattern,representing a typical lacustrine-derived oil.According to the characteristics ofhigh C30-rearranged Hopane and extremely low C30 Hopane of the Chang 71tight oil,and the characteristics of low C30-rearranged Hopane and extremely high C30 Hopane of the Chang 72tight oil in combination with other geochemical parameters in oil-source correlation,Chang 71、Chang 72reservoir tight oil has a close relationship with Chang 7black mud shale and oil shale source rock in the Maling region.