Characteristics of lithology and lithofacies of volcanic reservoirs in Xushen gas field

Both acidic and intermediate-basic volcanic rocks are highly developed in the deep blocks Wangshen-1 and Dashen-3 of the Xushen gas field,Songliao Basin,making it difficult to recognize lithologies and lithofacies.Lithologies of the volcanic rocks were identified through integrated core observation,thin section analysis and bulk rock analysis.On the basis of this,conventional log data,imaging log data,borehole log data and seismic data were integrated to recognize lithologies in the whole well interval.Four facies and 11 subfacies have been identified in the volcanic rocks.The 4 facies are eruptive facies,effusive facies,volcanic conduit facies,and volcanic sedimentation facies.And the 11 subfacies include pyroclastic fall deposits,pyroclastic-bearing lava flow and pyroclastic surge deposits of the erruptive facies;lower,middle and upper effusive subfacies of the effusive facies;volcanic neck,subvolcano and subtle eruptive breccias of the volcanic conduit facies;re-transportation and extraclast-bearing subfacies of the volcanic sedimentation facies.