Characteristics Of Carboniferous Volcanic Weathering Crust Reservoir In Shixi Oilfield, Junggar Hinterland

In recent years,volcanic rocks have gradually become the focus of reservoir research,but more focused on the primary type of it.The study of volcanic weathering crust reservoirs needs to be further studied urgently,Because volcanic weathering crust is the remains of volcanic rocks after long-term weathering,which makes the dense volcanic rocks have the basic conditions for forming reservoirs.Therefore,the identification and structural division of weathering crust of Carbonifero us volcanic rocks,lithology and lithofacies,reservoir space and physical properties,influencing factors and reservoir distribution characteristics of Carboniferous volcanic rocks in Shixi oilfield in the hinterland of Junggar have been deeply discussed in this paper.The formation environment of volcanic rocks in the study area is discriminated by using logging data and chemical analysis,by silica-alkali maps and trace element cobweb maps.Volcanic rocks in the study area were formed in the island arc environment.And then the weathering crust of volcanic rocks is identified by using stratigraphic markers,clay mineral markers,element change markers and well logging markers,and finally,which are combined with the relationship between porosity and depth,the change of logging parameters,the change of element content weathering index and conventional logging curve weathering index to divide the structure of volcanic weathering crust.It is divided into weathered clay layer,strong weathered layer,weak weathered layer and unweathered bedrock from top to bottom.Based on the classification of volcanic weathering crust structure,the dominant lithology and lithofacies of volcanic weathering crust reservoir in the study area is studied by logging identification,thin slice analysis and production data.The dominant lithology is agglomerate,volcanic breccia and breccia tuff,and the dominant lithofacies is overflow facies.It was summarized that the main controlling factors of the weathering crust reservoirs of volcanic rocks by analyzing Porosity and permeability of different lithology and lithofacies,comparing the porosity changes of the structural layers of weathered crust,combining with the results of comparison between the cumulative production of single well and the area of fracture development obtained by the weighted superposition method and the finite element prediction method,considering the production status of each paleogeomorphologic unit and discussing the change of reservoir caused by volcanic rocks in each stage of fracture.According to the influence of main controlling factors on volcanic weathering crust reservoirs,the plane distribution characteristics of weathering crust reservoirs in the study area were studied by combining the dominant lithology(lithofacies),paleogeomorphologic characteristics and fracture distribution characteristics.And in this paper,the vertical distribution of volcanic weathering crust reservoirs in the study area is studied by discussing the regularity of oil layer development and void longitudinal variation in combination with the structure division of volcanic weathering crust.The reservoirs are mainly distributed in the strong weathering layer 20?80m below the carboniferous top surface,and some of the weak weathering layer 80?150m below the carboniferous top surface.