Abstract:

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 CO₂-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.

Key words: diagenesis, dissolution, valid reservoir, volcanic rock, Yingcheng Formation, Songliao Basin