Abstract: The permeability of mudstne determines its sealing ability.Mud content,mined composition,grain texture and formation fluid properties are the main factors influencing permeability and sealing ability.The distribution and continuity of mudston are controlled by sedimentary facies.The data of displa.cement pressure,thickness and area distribution have Shawn that mudston of permeability less then 0.003 and and thickness large than 3m may provide an effective sealing for mid-law gas reservoir.Mudston of low permeability and high sealing ability characteristic resistivity response on well logging curves.Thus resistivity log is an effective Method to determine cover thickness and correlate horizontal continuity.The mudston in upper Palaeozoic group in Share-Gan-Ning Basin has the characteristics of huge thickness,good continuity and high sealing ability,and is an ideal cover of gas reservoir.