通过对徐家围子断陷天然气藏盖层分布及封闭能力的研究，得到徐家围子断陷天然气藏主要有泥岩、高声波时差火山岩、低声波时差火山岩和泥质砾岩4 种盖层。泥岩主要分布在登二段以上地层中，为区域性盖层。高声波时差火山岩盖层主要分布在营一段顶部，为局部盖层。低声波时差火山岩盖层分布在营一段和营三段内部，泥质砾岩盖层分布在营四段，二者均为夹层。泥岩、高声波时差火山岩和泥质砾岩盖层封闭能力强，低声波时差火山岩盖层封闭能力弱。通过盖层与天然气分布之间的关系研究得到，徐家围子断陷盖层对天然气成藏与分布的控制作用主要表现在以下3 个方面：①登二段和营一段顶部盖层控制着天然气的纵向分布；②盖层分布层位差异造成断陷南北具有不同的天然气储盖组合，天然气富集程度不同；③高声波时差与低声波时差火山岩盖层之间的差异造成天然气主要分布在火山岩体顶部。

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.