Paleokarstification and reservoir distribution in the Middle Triassic carbonates of the 4~(th) member of the Leikoupo Formation,northeastern Sichuan basin

The hydrocarbon breakthrough in the 4th member of the Leikoupo Formation shows important geological significance for petroleum exploration.Through comprehensive analysis of drilling cores,thin section,well logging data,fluid inclusion,carbon,oxygen and strontium isotope,three types of paleokarstification,including syndepositional,epigenic,and buried ones,are identified in the residual deposits under the unconformity surface at the top of Leikoupo Formation.They are considered as the key factors that control the reservoir formation of the 4th member of the Liekoupo Formation.The distribution of the reservoirs formed in the early stage is controlled by syndepositional paleokarstification.These reservoirs are limited in lateral by distribution of high-energy beach facies.Epigenesis paleokarstification is a critical geologic event that leads to the formation of the reservoir.The related reservoirs are vertically limited within 90m below the unconformity surface.Based on vertical and lateral distribution patterns of weathering karstification,it is suggested that karst-related reservoirs are commonly occurred vertically in debris-support breccia limestones and dolostones that distributed within surface karst area,or along half-filling high-angle dissolved seams and in small to medium-sized solutional pores in the vertical direction,and half-filling small to medium-sized solutional pores,dissolved seam,broken crack in the cave top and river breccia.Laterally,these reservoirs generally developed at karst slope,and karst hammock are found as the most advantageous geomorphic units for the formation of reservoirs.Burial karstification is related with dissolution of barbonte minerals resulted from hydrothermal fluids and organic acid,and it is considered as the key factor for reservoir optimization.