Abstract:

Correlation among 35 in-basin boreholes and 2 continuous field coring sections resulted that porosity and permeability of pyroclastic rocks (mean 18.7%, 0.32×10^-3μm²) are higher than that of volcanic lava rocks (mean 14.0%, 0.18×10^-3μm²) in the shallow part of basin with burial depth less than 500 meters, while contrarily in the deep part of basin with burial depth more than 2 800 meters, porosity and permeability of pyroclastic rocks (mean 2.6%, 0.05×10^-3μm²) are rather poor than volcanic lava rocks (mean7.3%, 0.07×10^-3μm²).Generally, porosity and permeability of both two types of rocks  decrease with increasing burial depth, however, the changing rate of pyroclastic rocks is remarkably larger than that of volcanic lava rocks. Therefore, volcanic lava rocks become primary reservoir due to higher porosity and permeability than pyroclastic rocks when burial depth range is below 2 500 to 3 000 meters. This is mainly contributed by their differences in diagenesis as volcanic lavas are concreted by cooling so that their framework volume is hardly affected by compaction, while pyroclastic rocks are consolidated by compaction, similarly to sedimentary rocks. Therefore, petroleum exploration in the middle and shallow part of basin (burial depth less than 2 500 meters) could attentively aim at pyroclastic rocks.

Key words: Songliao basin, Cretaceous Yingcheng Formation, pyroclastic rocks, porosity, permeability