Major reservoirs in the Songliao Basin (SB) are composed of volcanic rocks below 3000 m (9843 ft) of buried depth. Gas accumulations are mostly found in the buried volcanic highs, which in general correspond to paleovolcanic centers. Porosity in the volcanic rocks depends on both primary and secondary processes. The best porosity is preferentially developed in a proximal facies near the central part of each volcanic edifice. Porosity and permeability decrease with depth of burial for both volcanic and nonvolcanic sections, but their porosity-depth trends differ. Lava and welded ignimbrite slowly lose porosity with burial depth because they solidified by cooling, and their groundmasses (75%) are poor in quartz and calcite precipitation, thus preserving porous space. In contrast, the associated sandstone, conglomerate, tuff, and tuffite are more sensitive to overburden pressure, suffering more intense compaction and cementation. As a result, porosity and permeability of lava and ignimbrite exceed that of the other rocks, and thus, they are the best reservoir rocks below burial depths of ca. 3000 m (9843 ft) in the SB. The paleovolcanic domes are rich in both lava rocks and fractures of diverse origin, and the topographic highs provide favorable locations for gas migration and accumulation.

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