莺歌海盆地乐东区典型滨浅海滩坝砂体构型及成因模式OA

The shallow marine beach-bar deposits are crucial reservoir types for natural gas development in the Ledong 15D Gas Field,Ledong Area,Yinggehai Basin,South China Sea.To reveal the complex internal architecture of the reservoir sandbodies,the gas group Ⅲ of the first member of the Yinggehai Formation(Ying 1 Member)was selected as the study target.Methods including petrographic analysis,laser particle size analysis,well log response analysis,and seismic attribute analysis were employed to investigate the sedimentary characteristics of the gas group Ⅲ in the Ying 1 Member,clarify criteria for identifying single bar boundaries,conduct architectural analysis,study the genesis of the architecture,and establish architectural models.The results indicate that the sediments of the gas group Ⅲ in the Ying 1 Member exhibit high compositional maturity and are dominated by fine-grained sands,developing contiguous beach-bar sandbodies distributed parallel to the shoreline.The beach-bar sandbodies are clas-sified into three hierarchical levels:composite beach-bar,single bar,and accretionary unit.The criteria for identifying single bar boundaries can be summarized into four types:inter-bar beach sand,inter-bar shallow marine mud,differences in well log curve morphology and thickness,and elevation differences between adjacent bars.The sub-gas group Ⅲ-2 of the Ying 1 Member developed three stages of large-scale bars laterally stacked seaward,while the sub-gas group Ⅲ-1 is dominated by isolated small-scale bars superimposed on earlier bar deposits.The single bars exhibit three stacking patterns:isolated,ver-tically superimposed,and laterally migrating.The architectural characteristics are controlled by hydro-dynamic zonation of wave action:isolated bars develop in the swell zone,vertically superimposed bars form in the breaker and surf zones,and laterally migrating bars dominate in the swash zone.Influenced by high-frequency sea-level fluctuations and hydrodynamic differentiation,the internal accretionary units and interbeds within a single bar are distributed nearly horizontally nearshore and progressively stack seaward at low angles offshore.These results provide a basis for reservoir prediction and well pattern optimization during gas field development.