Abstract:

Study on lithology and petrography of volcanics and mechanism of volcanic activities in the Lower Cre-taceous Yingcheng Formation of the Shengping gas field in the Songliao Basin,indicates that rock types are mainly pyroclastics and lava,with minor pyroclastic lavas,sedimentary pyroclastic rocks,pyroclastic sedimentary rocks and sub-volcanic rocks.Lavas are dominated by rhyolite with minor dacite.Four volcanic eruption-effusion cycles can be recognized in the third member,the major gas-bearing interval,of Yingcheng Formation.The vol-canic activities are characterized by alternation of relatively quiet fissure effusion with relatively strong central eruption that features in high frequency,rapid eruptions,short intermittence,and quick condensation and consoli-dation.Five lithofacies (explosive,effusive,extrusive,volcanic conduit and volcanic-sedimentary facies)and six sub-facies(splash fall lava,pyroclastic,lava flow,volcanic gravity flow,volcanic neck and sub-volcanic) can be recognized.Wells with high gas flow rate are mainly located in areas with the volcanic explosive facies or interbe-ded explosive facies and effusive facies.The volcanic reservoir quality is mainly controlled by lithology,lithofa-cies,thickness and spatial distribution of volcanic rocks,followed by the characteristics of volcanic activity,pa-leogeomorphology,diagenesis and tectonic action.

Key words: mechanism of volcanic activity, lithofacies and reservoir quality, petrography, volcanic rock of the Yingcheng Formation, Shengping gas field, Songliao Basin