Contrastive Study On Carboniferous Volcanic Lava And Pyroclastic Rock Reservoirs In Chepaizi Area,Junggar Basin

Predecessors have conducted many studies on the Carboniferous volcanic reservoirs in this Chepaizi area.Most of them have studied volcanic lava and volcaniclastic rocks together in fracture identification and reservoir evaluation.Since volcaniclastic rocks are a transitional type between volcanic lava and sedimentary rocks,the rock genesis,composition and rock properties are quite different from volcanic lava,and their storage space types and physical characteristics are quite different from lava.With the development of more volcanic lava in the east wing of Chepaizi area,the widely developed volcaniclastic rock reservoir in the west wing has attracted more and more attention from scholars.However,due to its many differences,it is necessary to find new Research methods to evaluate reservoirs and guide exploration and production.Through the study of cores and thin slices,there are three main types of volcanic lava reservoir lithology,including basalt,andesite and basaltic andesite.Volcanic lava develops in strips near the eastern crater.Volcanic lava reservoirs have various types of storage spaces,which are divided into three types:primary pores,secondary pores and fractures.Unique stomata and almond dissolution pores are effective storage spaces for volcanic lava.According to the analysis of porosity and permeability samples of volcanic lava,it is considered that volcanic lava belongs to ultra-low porosity and ultra-low permeability.The three types of rock have little difference in porosity and permeability,and the porosity and permeability are relatively average.The core and thin slice data show that there are five main types of volcaniclastic reservoir rocks in Chepaizi area,including volcanic breccia,tuff,sedimentary tuff,tuffy fine sandstone,and tuffy mudstone.There is more development in the west wing,which is vertically stacked with volcanic lava and unevenly distributed.Secondary dissolved pores and structural fractures in volcaniclastic rock reservoirs are the main effective storage spaces.Through studying the porosity and permeability of volcaniclastic rocks,it is believed that the whole volcaniclastic reservoirs have ultra-low porosity and low permeability,with strong heterogeneity of porosity and permeability,and large differences in porosity and permeability of various lithologies.Volcanic breccia and tuff have good permeability.By comparing the storage space,porosity and permeability characteristics of volcanic lava and volcaniclastic rock reservoirs,it is found that volcanic lava reservoirs have more space types,better oil-bearing properties,and the porosity and permeability of volcanic lava reservoirs are slightly better than physical properties pyroclastic rocks,but the volcanic lava has little difference in lithology,volcanic clastic reservoirs have strong heterogeneity,and correlation analysis of porosity and permeability also shows that volcanic lava has a stronger correlation.Through the evaluation and comparison of single factor coefficient and comprehensive coefficient,the two types of reservoir classification are compared and found that volcanic lava reservoirs are superior to volcaniclastic rocks in the evaluation of single factor and comprehensive coefficient.According to the predecessors’ classification standards for the Carboniferous reservoirs in this area,combined with the current oil test and production data modification,new reservoir evaluation standards are obtained,and the reservoirs are divided into three categories.The type Ⅰ reservoirs have the best oil content,and the type Ⅱ reservoirs secondly.Class Ⅲ reservoirs are basically oil-free.Each well is evaluated according to the new standard.The development thickness and characteristics of each type of reservoir are analyzed.It is found that the development frequency of volcanic lava in Type Ⅰ reservoirs is significantly higher than that of volcanic fragments In clastic rocks,and only volcanic clastic rocks,only volcanic breccia and tuff develop Class Ⅰ reservoirs,which are mainly controlled by the characteristics of reservoir space development and rock mechanical properties.