The Xujiahe Formation is a regional gas reservoir in western Sichuan Depression.Due to the great bu- rial depth

the reservoir generally experienced complicated diagenesis

resulting in tight to supertight reservoirs.Even so

porous reservoirs with relatively good storage capacity were also developed.From observation of more than 30 wells

over 7 000 ordinary thin sections and over 1 000 cast thin sections in the study area as well as the research of related reservoirs

it is demonstrated that volcanic debris dissolved pores are the major storage space of sandstones of of the 2nd and 4th member of Xujiahe Formation

and are the major controlling factor of high quality reservoirs in the area.Dissolution of feldspar deposited through weathering and transportation of parent rock is not the major reason for the forming of sandstone with medium-high porosity.Volcanic debris from syndepositional volcanic eruption contain many "soluble compositions"

which tend to be dissolved and devitrified to form pores during diagenesis

thus controlling the distribution of high-permeability sandstone reservoirs with well-developed secondary dissolved pores.Therefore

the compositions of debries from the syndepositional volcanism and their dissolution intensity are the major factors controlling the development and distribution of high quality reservoirs of the Xujiahe Formation and the formation of sandstone with medium-high porosity.Ganzi-Litang inter-continental rift area where volcanisms were strong during the Late Triassic was the major source for volcanic debris in the study area.