Abstract:

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.

Key words: volcanic rocks, volcaniclastic rocks, volcanic lava, cap rock, interval transit time, sealing gas ability, Songliao basin