Characteristics of Carbonate Interlayer Karst Reservoir of Yingshan Formation in Tazhong Northern Slope,Tarim Basin

Large-scale interlayer karst reservoirs are well developed in the Yingshan formation in Tazhong northern slope of Tarim basin,where the depth of karstification reaches 180 m below the unconformity.But the reservoir characteristics are varied in different areas and the main-control factors are unclear,which become the bottleneck of restricting the petroleum exploration of Yingshan formation.This paper redefines the concept of interlayer karst and analyzes the characteristics of the karst reservoir in Tazhong area.In combination with the structural evolution and the sea level changes,it is suggested that the system of karst is unperfect and the large-scale caves are undeveloped,because the interlayer karst of Tazhong Yingshan formation was formed in the Caledonian II episode,with short-term outcrops,small and isolated caves,and dominant vuggy and fracture-vuggy types of reservoir.While the low dip angle of the formation when outcropping,the bedding plane and water table were almost parallel,so there existed local bed-parallel karstification,the reservoirs occurred in quasi laminar distribution and regional diachroneity.The three stages of the sea level changes and relative stagnant phase of water table resulted in 5 sets of reservoirs developed from No.1 to No.2 members of Yingshan formation.By analyzing the key controlling factors of reservoir and developing the reservoir distribution model,the favorable exploration blocks are comprehensively predicted in this paper.