Abstract:

Based on the study of cores, well cuttings, and outcrop sections, the facies of volcanic rocks can be divided into 4 types and 12 sub-facies. Pores and fissures are the two major types of reservoir spaces in volcanic rocks in Xujiaweizi fault-depression. The types of reservoir space in volcanic reservoirs have been found to be controlled by the facies. The volcanic reservoirs found in various facies belts have quite different pores and fissures and their assemblages. Gas cavities, fissures in rhyolitic lamination, and joint fissures have been identified mainly in eruption-overflow facies. The main pore-type identified in explosive facies is intergranular pore; and inter-breccia pore and primary fracture are dominant in extrusive facies.Joint fissures and various primary fractures are mainly developed in volcanic conduit facies.It is found that “unconsolidated formation” often distributes at the base of each cooling unit in explosive explosive facies,which might be resulted from fast covering and conservation of the weathered crusts of the original basement rocks or unclinkered drifting clasts. A large “unconsolidated bodies in rock-dome” can often be found in the inner belt sub-facies of extrusive facies, which are actually of large accumulation bodies of perlite balls. Special attention should be paid to looking for these two-type reservoirs in the intermediate-acidic volcanic rock area. The major tectoclase in this area is a set of conjugated, high-angle (fault plane with dip angle of 50°～60°), NNE and NNW strike, and shear or compressive-shear fractures, which constitute the main migration pathway of oil & gas.

Key words: volcanic rock facies, volcanic reservoir, classification, unconsolidated formation, Songliao basin