Research On Reservoir Characteristics Of The Ordovician Lianglitage Formation In Zhonggu 51 Well Block,Tazhong Area

The Ordovician carbonate rocks are widely distributed in Tazhong area.The reservoirs have great oil and gas potential,however,the reservoirs are deep buried,complex,and heterogeneous.Starting with cores and thin sections,this paper studies the sedimentary characteristics,diagenesis,reservoir space and physical properties of the reservoir by synthesizing logging data,logging data and analysis of laboratory data,and then determines the controlling factors of the reservoir,and predicts the distribution of favorable reservoirs.Firstly,three types of lithology are recognized,which are grainstone,micrite,and biogenic limestone.The content of the grainstone in the first member of Lianglitage Formation（Ol₁）and the second member of Lianglitage Formation（Ol₂）is relatively high,whereas the content of micrite in the third member of Lianglitage Formation（Ol₃）-fifth member of Lianglitage Formation（Ol₅）is relatively high.The biogenic limestone is dominated in the Ol₁-Ol₃,and only few biogenic limestone are developed in the fourth member of Lianglitage Formation（Ol₄）and Ol₅.The content of micrite tends to decrease from the Ol₅ to the Ol₁.Secondly,two sedimentary facies are interpreted in the study area,as platform margin and open platform facies.The platform margin facies can be divided into three sub-facies,which are marginal platform shoal,reef mound,and micrite mound.The open platform facies can be divided into three sub-facies,which are inner platform shoal,platform micrite mound,and inter-platform sea.The marginal platform shoal-reef mound complex is mainly distributed along Tazhong I fault zone,while the inner platform shoal is mainly distributed along Tazhong 10 fault zone.The variability of sedimentary facies in the five members indicate oscillation of water depth.Five types of diagenetic processes are identified within the Lianglitage Formation,including cementation,dissolution,replacement,compaction,and pressure dissolution.On the basis of the characteristics of the five diagenetic processes,the corresponding diagenetic environment of each member is analyzed.Ol₅to Ol₃ is mainly dominated by burial diagenetic environment.Ol₂ is formed under atmospheric freshwater diagenetic environment and burial diagenetic environment.Ol₁ is mainly formed under both seawater diagenetic environment and atmospheric freshwater diagenetic environment.Three types of reservoir space are recognized,as of cave,pore,and fracture.Based on the space type,the reservoirs are classified as cave-type,pore-type,fracture-type,and pore-fracture type.By petrophysical analyses of the core,the petrophysical properties of individual members are characterized and compared.The results show that petrophysical properties of Ol₁ and Ol₂ are significantly better than those of Ol₃to Ol₅.The reservoir quality of Lianglitage Formation in the study area is controlled by multiple factors,including sedimentary facies,diagenesis,and tectonic movement.Marginal platform shoal-reef mound complex and inner-platform facies are favorable reservoir.Dissolution is the main constructive diagenetic process.Atmospheric freshwater diagenetic environment is the favorable diagenetic environment.Fractures associated with tectonic movement effectively communicate the pores.Evaluation standard of the reservoir is established based on the integrated characteristics of sedimentary facies,faults,porosity,and permeability.Finally,distribution of potential reservoirs is predicted.