Early Permian Volcanic Lithology,Lithofacies and Their Relations to Reservoir in Northwestern Margin of the Junggar Basin

Early Permian volcanic rocks in northwestern margin of the Junggar Basin have different rock types and lithofacies.In this paper,data from core,thin sections and element chemistry were used to study volcanic lithology and their relations to reservoir quality.The Jiamuhe Formation is characterized by lava and volcanoclastic rocks,which are mainly basaltic andesites,andesites and quartz andesites.The lithology of volcanic rocks in the Fengcheng Formation are different in different areas.Lava including basaltic andesitoid and tephrite was the main type in Kebai area,while welded pyroclastic rock and volcaniclastic lava of rhyolite,pollenite and andesitoid are the main types in Wuxia area.The high-quality reservoir developed in eruptive breccias and lava such as andesite in the Jiamuhe Formation and in the Fengcheng Formation of the Kebai area,and in welded pyroclastic rock and breccias lava with stone bubbles in the Fengcheng Formation of the Wuxia area.Besides,same volcanic rock would have different reservoir quality because of different volcanic environments.In the studied area,although diagenesis was diverse in different parts,high-quality volcanic rock reservoir developed mainly in the upper effusive sub-facies,air fall,and pyroclastic subfacies in explosive facies.Effusive and explosive facies were common in the Jiamuhe Formation,while effusive facies and pyroclastic flow subfacies in explosive facies were the main facies in Fengcheng Formation of Kebai area and of Wuxia area,respectively.Therefore,distinct explorative targets should be chosen in different layers in different areas.Lithofacies was another important controlling factor for volcanic reservoir besides fracturing in fault zones and leaching under the regional unconformable surface.