Based on drilling and testing data, the distribution and seal ability of gas cap rock in Xujiaweizi Depression are studied. It is considered that there are mainly 4 kinds of gas cap rocks in Xujiaweizi Depression, including mudstones, volcanic rocks with high and low interval transit time and argillaceous conglomerates. Mudstones are regional cap rocks, which mainly distributed in K₁d2 and strata above it. Volcanic rocks with high interval transit time are local cap rocks, which mainly distributed on the top of K₁yc1. Volcanic rocks with lowinterval transit time mainly distributed in K₁yc1 and K1 yc3, and argillaceous conglomerate cap rocks are mainly in K₁ yc4. The seal abilities of mudstone, volcanic rocks with high interval transit time and argillaceous conglomerate cap rocks are strong, while the seal ability of volcanic rocks with low interval transit time is weak. Based on the study of the distribution relation between cap rocks and gas, the controlling effect of cap rocks on gas accumulation and distribution in Xujiaweizi Depression is concluded and mainly displayed in the following 3 aspects: ①Cap rocks in K₁d2 and the top of K₁ yc1 control the vertical distribution of gas;②The difference of distribution horizons of cap rocks leads to different reservoirseal assemblage and different enrichment degree of gas in the south and north of depression; ③The difference of volcanic cap rocks with high and lowinterval transit time causes that gas mainly distributed on the top of volcanic rock
bodies.