Abstract: Similar to the Sichuan Basin,the northwest region of Guizhou Province is of stiff and stable basement.Only middle and lower petroleum systems have been preserved in the northwest of Guizhou.In the middle of Guizhou,only lower petroleum system is accessible.As to cap formation of lower system,gypsiferous rock of Loushanguan Formation as well as mudstone of Jindingshan and Niutitang Formations play important roles.They are the keys for exploration potential preservation and serve as overall seal.Though multi-stages of tectonic movements take place during the late stage,gypsiferous rock of Loushanguan Formation is still preserved and seals the lower system.Mudstone cap formation also has strong seal ability with breakthrough pressure over 15.97 MPa and belongs to type I cap formation.Pressure seal may exist in the northwest of Guizhou because that,though the Middle Guizhou Uplift up rises greatly during the late stage,Cambrian seal system is not yet destroyed and can work as effective seal.The most favorable place for accumulation is the Dafang-Jinsha tectonic belt and its northern and southern slopes.