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.