Dolomite Reservoir Characteristics And Controlling Factors Of Longwangmiao Formation,Northwestern Sichuan Basin

Based on the previous studies of Longwangmiao Formation,this paper,used the theory of carbonate reservoir sedimentology and geochemistry as theoretical guidance,and through the field geological section and drilling section macroscopic observation,in addition,combine microscopic observation of rock thin slices under microscope and analysis of porosity and permeability of dolomite.We identify different types of dolomite,and make rock and mineral analysis and geochemical analysis of typical dolomite.Such as,cathode luminescence,scanning electron microscope,electron probe,major elements,trace elements,rare earth elements,carbon and oxygen isotopes etc..Studies of strata and sedimentary facies,basic reservoir characteristics,and diagenesis were carried out in the Leshan,Nanjiang regions and the Central Sichuan basin.Emphasis was placed on the dolomiteization and dissolution of the formation of reservoirs,and the model of dolomitization and karst of the Longwangmiao Formation in the study area was established,and the major controlling factors for reservoir formation were analyzed.Get the following results to know:(1)There is a great contrast in the strata of the Longwangmiao Formation between Northwest Sichuan basin and Central Sichuan basin.Vertically,the lower part of Longwangmiao Formation major develops interplatform tidal flat,and the upper part develops thick layer interplatform beach and thin layer interplatform tidal flat.Horizontally,the lower part of Longwangmiao Formation,from west to east,mixed tidal flat gradually change into interplatform tidal fla,and the upper part major develops interplatform beach,and develop lagoon in low-lying area.(2)Dolomite porosity of the Longwangmiao Formation in Northwest Sichuan is relatively good and has certain reservoir properties.However,reservoir performance of Northwest Sichuan basin compared with the Middle Sichuan area.The main diagenetic sequence is:granular limestone ? seabed fibrous cementation ?dolomitization in the penecontemporaneous? dissolution in the penecontemporaneous ? dentate cementation ? mechanical compaction ? pressure dissolution ? tectonic disruption ?epigenetic dissolution ? burial dolomitization?burial dissolution ? tectonic disruption.(3)The mainly dolomitization of Longwangmiao Formation consists of evaporative dolomitization and refluxing dolomitization.Buried dolomitization are relatively weak,and less influence on the reservoir.(4)Longwangmiao Formation has undergone three different stages of dissolution:karstification during syngenetic-quasi-contemporaneous stage,karstification during epigenetic karstification,karstification during burial.The karstification in the contemporaneous-quasi-contemporaneous stage is widely developed in Longwangmiao Formation.The epigenetic karstification is mainly concentrated in the Central Sichuan region,and the Leshan and Nanjiang regions have weak effects.The burial period is different in the Longwangmiao Formation.(5)The reservoir of the Longwangmiao formation is controlled by sedimentary facies,dolomitization and dissolution.Sedimentary facies controlled the material base of the reservoir development.Early dolomitization,on the hand,formed a certain reservoir space,on the other hand,protected the early reservoir space to be destroyed,but excessive dolomitization reduced the porosity of the reservoir.The different stages of karstification are important factors for the formation of reservoirs.