Research Of Weathered Crust Karst Reservoir Of Leikoupo Formation Of Middle Triassic In Longgang Area,Central Sichuan Basin

Industrial airflow of T₂l₄³has been captured in many wells of Longgang area of central Sichuan basin,which makes the exploration of Leikoupo Formation in this area a hot spot since 2008.In this paper,Leikoupo Formation in Longgang area is taken as the target,with emphasis on T₂l₄³.According to the theories and methods of sedimentary petrology,petroleum geology and reservoir geology,combining with outcrop,core,thin section,drilling,logging and seismic data,and reference to previous research results,the characteristics of sedimentary facies,weathered crust karst and reservoirs of Leikoupo Formation in Longgang area are studied.And controlling factors in the study area are clarified.The distribution of favorable reservoirs is predicted.The study shows that the weathered crust karst reservoirs on the top of Leikoupo Formation in Longgang area are mainly developed in T₂l₄³.The lithology of T₂l₄³is mainly dolomite,limestone,gypsum and their transitional types and the thickness in the study area is mainly between 60m and 70m.Grain-beach,dolomite flat,limestone flat,gypsum dolomite flat,gypsum lagoon and so on are developed in Longgang area.The most favorable sedimentary facies belt is grain-beach,followed by dolomite flat.Weathering crust karst is easy identified on outcrop,core,thin slice,logging and drilling.The thickness of weathering crust karst in the study area is between 30m and70m.The karstification is characterized by weak weathering and dissolution,strong karst filling and strong gypsum dissolution.The main reservoir rock types in the study area are granular dolomite,algal dolomite and muddy to micritic dolomite.The reservoir space types are mainly intergranular solution pore,intragranular solution pore,intercrystalline solution pore,and fracture is developed.Reservoir physical properties are characterized by low porosity and low permeability.Fractures can greatly improve reservoir permeability.Reservoir types in the study area are mainly fractured-porous reservoirs.Shoal granules,dolomite granules and muddy to micritic dolomite provide material basis for reservoir development.The best lithology is granular dolomite and powder crystal dolomite.Reservoirs are mainly in the vertical vadose belts near the top of Leikoupo Formation and are well developed in relatively high palaeogeomorphology?such as residual hills and karst flat?.According to porosity,permeability,pore type,sedimentary facies and other parameters,the reservoirs in the study area are divided into four basic types.Type-?reservoirs?porosity 2%?6%?are dominant in the study area and favorable reservoir is type-??porosity 6%?10%?.The total thickness of reservoirs is between 15m and 30 m.The single layer thickness is thin and the number of layers is large.The favorable reservoirs are mainly in Longgang 36-Longgang 37-Longgang 168well area,Longgang 19-Longgang 21 well area and Longgang 22-Longgang 23 well area of the north,central and south-central of the study area.