Volcanic reservoirs are usually characterized by complex rock types and higher heterogeneity.The paper systematically studied the diagenesis and porosity evolution of volcanic reservoirs from the Yingcheng Formation in the Xujiaweizi Fault Depression

Songliao Basin

via core observation and identification of casting thin slice.The result shows that all valid reservoir lithologies

including rhyolite and crystal tuff

underwent six diagenetic stages

while different types of diagenesis

stages

and evolution patterns occurred to different reservoir lithologies.That is why the porosity evolution and physical properties of those rocks were heterogeneous.In the study area

dissolution is the most effective diagenesis process to the formation of valid reservoir

including organic acid dissolution and CO

2

-bearing acidulous water dissolution.And the secondary pores mostly developed in the acid volcanic rocks after organic acid dissolution played a decisive role in forming valid reservoirs.In terms of the controlling factors for dissolution

some of the most important ones may be the extent of original primary porosity system

the intensity of fracture activity and tectonic cracking in later periods

the top/bottom unconformities of volcanic rocks

etc.