Keywords: 
• Xushen gasfield /
• K1d2 /
• K1yc1 /
• cap rock /
• seal ability /
• seal period

Abstract: Through drilling and seismic data, development and distribution of caprock of Xushen gas field was studied. It was considered that caprock of Xushen gas field is mainly mudstone caprock of K1d2 and volcanic caprock on top of K1yc1, and the former which distributes in the whole of depression is caprock of gas reserved in K1yc4 volcanic breccia in the south of depression and caprock of gas reserved in K1yc3 volcanic in the north of depression, the later which distributes in the south of depression is caprock of gas reserved in volcanic. Through test and logging data, displacement pressure of caprock and reservoir and excessive pressure of gas reservoirs were studied. It was considered that displacement pressure of caprock in 12 gas reservoirs is larger than the sum of displacement pressure of gas reservoirs. Their seal gas ability effectiveness is good, and it is favorable for gas to accumulate and preserve. Through the study of evolution of seal gas ability of caprock and relation between formation period of seal gas ability and massive expulsion gas period of seal source rock, it was considered that formation period of seal gas of K1d2 caprock and K1yc1caprock are sedimentary period of K1q1,2 and K1d4 and they are earlier than massive expulsion gas period of gas source rock(Sedimentary period of K1 sh). Their seal gas time effectiveness is good. It is favorable for largescaled gas accumulation. 