Abstract: The degree of fault tightness is one of the important factors for fault sealing control. According to analyses of faulting stress and shale smearing, the fault tightness index (I_FT) is defined as the ratio of faulting stress σ_F to the compressive strength σ_C of fault zone. Where the value of I_FT is bigger than 1, the fault tightens and seals well;otherwise,it seals worse. The study shows that I_FT values distributed along northern faulting in Beisantai field are of non-homogeneity. As long as the I_FT is bigger than 1 in oil reservoir or its overlying stratum, the oil-gas in it could be vertically sealed in incompletely sealed conditions above the reservoir. In addition,the I_FT values are not related to the buried depth during the same geological time,for the deeply buried fault couldn’t mean the bigger I_FT value and good sealing property, but the oil-gas in it still may migrate upward throughthe fault.

Key words: fault, stress, shale, Junggar basin