火山岩盖层类型及封气能力——以松辽盆地徐家围子断陷为例

利用钻井、测井和分析测试资料，对徐家围子断陷火山岩盖层类型、识别标志、分布、封闭能力和对天然气成藏与分布的控制作用进行了研究，认为徐家围子断陷火山岩盖层分为火山碎屑岩和火山熔岩盖层2种。火山碎屑岩盖层主要是凝灰岩和火山角砾岩，具有井径扩容、电阻率小和高声波时差特征；火山熔岩盖层主要为流纹岩、凝灰岩和安山岩，具有井径不扩容、电阻率中等和低声波时差特征。利用高声波时差和声波时差值小于56 μs/ft可以分别识别火山碎屑岩盖层和火山熔岩盖层。火山碎屑岩盖层区域分布在断陷南部下白垩统营城组一段火山岩体的顶部，火山熔岩盖层局部分布在营一段和营三段火山岩体内部。火山碎屑岩盖层较火山熔岩盖层具有更强的封气能力，火山碎屑岩盖层控制着徐家围子断陷火山岩中天然气的区域聚集与分布，火山熔岩盖层仅控制着天然气在火山岩体中的局部聚集与分布。

Volcanic Cap Rock Type and Evaluation of Sealing Gas Ability: An Example of Xujiaweizi Depression

By use of drilling, logging and analysis and testing data, the type, feature, identification mark, distribution and sealing ability of volcanic cap rock in Xujiaweizi fault depression and the controlling effect for the accumulation and distribution of gas were studied. It was considered that there were 2 types of volcanic cap rocks in Xujiaweizi fault depression, volcaniclastic rock cap rocks and volcanic lava cap rocks. Volcaniclastic rock cap rocks are mainly tuff and volcanic breccia which are characterized by small resistivity，enlargement of borehole and high interval transit time. Volcanic lava cap rocks are mainly rhyolite, tuff and andesite which are characterized by middle resistivity，normal borehole and low interval transit time. Volcaniclastic rock cap rock and volcanic lava cap rock can be identified respectively by high interval transit time and interval transit time which are lower than 56 μs/ft. Volcaniclastic rock cape rocks are distributed regionally on the top of K₁yc¹ volcanic rock body, while volcanic lava cap rocks are distributed locally in the volcanic rock body. Sealing gas ability of volcaniclastic rock cap rock is stronger than that of volcanic lava cap rock. Volcaniclastic rock cap rocks controlled the regional accumulation and distribution of gas in volcanic rock in Xujiaweizi fault depression, while volcanic lava cap rocks only controlled the local accumulation and distribution of gas in volcanic rock body.