Cap rocks are the necessary conditions for the accumulation of natural gas and formation 0fgas pools．By means of analyzing the microscopic characters of cap rocks，the authors think thatcap rocks of the shallow gas pools in the Jiyang depression are of low abrupt pressure type．On thebasis of the correlation analysis of parameters of the pore structure，the authors put forward clas-sification standards of mudstone cap beds of shallow gas pools，and in combination with themacroscopic geologic characters and influence factors of the cap rocks，the authors put forwardthe conditions for areas favorable for cap rocks from the view—point of dynamic balance，that is，good micro—confining capacity，great mudstone thinkness，persistence of the mudstone，less devel—oped structural fractures and SO on．And on that basis a synthetical assessment of preservationcondition on part of Neogene gas fields in the Jiyang depression has been made．