Reservoir Characteristics Of The Third Member Of Lei 4th Member Of Leikoupo Formation In Longgang Area, Sichuan Basin

As an important kind of oil and gas reservoir,the near-surface karst carbonate is typically developed in the third sub-member of the fourth member of the Middle Triassic Leikoupo Formation(Lei43 Submember)in the Longgang Area of the Sichuan Basin,The longgang area is generally under hot climate conditions.1t is characterized by the restricted platform,including the shoal and lagoon.Grain dolostones is the best reservoir of Lei43Submember.The space of the reservoir including residual intergranular pore,intergrain pore,intragranular dissolved pore and intercrystalline pore,fractures are more developed in part of the area.Based on existing three dimensional seismic and drilling data,integrating the filling and sedimentary evolution characteristics of Leikoupo and Xujiahe Formations,we analyse the seismic thickness of the interval from the erosional unconformity surface on the top of Leikoupo Formation to the cast at the bottom of the third member of Xujiahe Formation(Xu3 Member),in order to reconstruct the paleogeomorphology of the near-surface karst.There are a total of three secondary geomorphologic units,namely karst highland,karst transitional zone,and karst basin.They have distinct karst water development patterns and hydrological conditions,which determine the karstification intensity and reservoir quality.Among them,the karst highland suffered strong erosion and thus has poor reservoir capacity due to its long-term exposure beyond the water table,which resulted in severe denudation.As for the karst transitional zone,it is featured by superimposition of multiple periods of strong karstification,which resulted in the high-quality reservoir in the karst monadnockbut poor reservoir in the karren.In terms of the karst basin,which is the drainage area experiencing weak karstification,the reservoir capacity is generally poor,with exception to some good reservoir developed in the shoal.