Abstract: SK1 well was a scientific exploration well on central mole track of Jurong fault depression. The 118m shows including oil flecked, oil trace and fluorescence in 24 layers in this well. Abundance shows with low show level and no hydrocarbon reservoirs indicated that hydrocarbon migration in this well was active, but preserve condition was insufficiency. Four cap formations of mudstone and mudstone with gypsum were drilled in this well as K₂p, T₁q, P₂l, S₁g. This paper systematically analyzes the macroscopical, microcosmic and electrical characters of cap formations. Sealing capability of each cap formation was comprehensively evaluated and classified. Through analysis of geological factors which influenced validity of cap formations, it was considered that multi-period tectonic movements and thin continental facies cover formation were main factors that caused relatively poor cap formation sealing conditions.