Abstract: As one of the important controlling factors over the distribution of high-quality reservoirs,volcanic edifices have been the major study objects of explorers for gas reservoirs.The first member of the Yingcheng Formation in Yingshan Sag of northern Songliao Basin,being dominated by acidic volcanic rocks and having a porosity less than 5% with intensive later alteration and less explored with drilling,was studied based on seismic data (the continuity of seismic events,amplitude intensity),layered properties,lithology and logging data,to establish boundary marks of volcanic eruption cycles and stages.Measures such as coherence volume interpretation,trend-surface analyses and manual identification,were used to discern volcanic edifices of different eruption stages and to determine their significances of controlling the quality of reservoirs based on their layout relations with hydrocarbon distribution.The result shows that six stages can be recognized in the volcanic rocks of the member from bottom up.Three massive edifices (the east,central and west edifices) were formed as the volcanic rocks erupted while migrating from east to west along faults in the area.The quality of the reservoirs there was conditioned by the preservation state of the edifices and the internal lithologic distribution.The middle parts of the central and west edifices were found to be well preserved with favorable porosity and permeability (due to the highly developed fractures and dissolved pores) for hydrocarbon to accumulate.But the eastern part of the east edifice was less preserved after a later uplifting and erosion as well as weathering and leaching,and its reservoir properties were poor due to the filling-up of dissolved pores and fractures by the overlaying sedimentary tuff and secondary minerals,thus being considered a less desired exploration target for gas.

Key words: volcanic rock, volcanic edifice, deep gas reservoir, Yingcheng Formation, Yingshan Sag, Songliao Basin