Development and Evolution Pattern of Paleo-Karst in Slope of Northern Tazhong Area

Starting development from Tumuxiuke formation to the end of Lianglitag No.4 member deposition of the Upper Ordovician in slope of northern Tazhong area,the paleo-karst process is so long and its feature is so abundant and obvious that can be comprehensively identified by the characteristics of geology,drilling,conventional logging,imaging logging and seismic data processing.The vertical structure of paleo-karst can be divided into surfacial karst zone,vertical percolation zone,radial flow karst zone and deep tranquil flow zone from the top to the bottom,among which the deep tranquil flow zone can be further divided into upper and lower subzones and one transitional zone.With the lowering of paleo relief,the upper karst zone trends to thinning and even undevelopment.Thus the quality of fractured-vuggy reservoir formed by the paleo-karst is controlled by paleo relief and sea level changes.The study suggests that the karst slope is favorable prospecting area,the radial flow karst zone and surfacial karst zone are of best reservoir quality,vertically,and the greatiest potential for petroleum exploration is the Yingshan formation of Lower-Middle Ordovician in the studied area.