Abstract: Based on the experimental analysis of caprocks and reservoir rocks of natural gas, the authors of this paper study the influential factors over the quality of natural-gas caprocks and the time of their formation in the Jiyang Depression. The analysis data of the mineral compositions of argillites reveal that the quality of natural-gas caprocks is dominated by sedimentary environment, compaction and diagenesis levels of argillites.Furthermore, the experimental analysis on the expulsion pressure and the breaking pressure of the reservoir rocks suggest that the caprocks could be taken as capping beds as the suffient sludges deposit and become stratified sequences.