Volcanic rocks in Xujiaweizi fault depression in Songliao basin can be classified into lava and vocaniclastic rocks.The effective reservoir spaces of volcanic reservoirs are pores and fissures. Fissures can not only act as reservoir spaces

but can also interconnect pores of different types and promote dissolution at later stages. A computation model of fissure porosity is developed based on recognition of fissure types. According to logging interpretation of 12wells

the volcanic reservoirs are divided into 3 grades based on their fracture porosity

i.e.grade Ⅰ with porosity above 0.5%

grade Ⅱwith porosity ranging from 0.2% to 0.5%

and grade Ⅲ with porosity below 0.2%. Volcanic rock facies can be subdivided into marginal

transitional and central subfacies. The central subfacies

located near volcanic conduit

has well developed structural and contraction fissures and relatively good poroperm characteristics

so it would be the most favorable reservoir facies belt; the transitional subfacies

having relatively well developed fissures and moderate poroperm characteristics

would be a favorable reservoir facies belt; while the marginal subfacies has poor poroperm characteristics

it would be an unfavorable reservoir facies belt.