Bohai Oilfield Research Institute, Tianjin Branch, CNOOC Ltd., Tianjin 300459, China 

 To elucidate the sedimentary characteristics and reservoir development patterns of the near source sandy braided river delta in the Laizhou Bay Sag of the Bohai Bay Basin, an analysis was conducted using seismic, drilling, logging data, core observations, and grain size analysis. This study summarized the sedimentary character? istics of the upper part of the 3rd member of the Shahejie Formation (E2 S U 3 ) in the K Oilfield. The results indicate that in the E2 S U 3 of the K Oilfield, three subfacies can be identified: the braided river delta plain, braided river del? ta front, and the braided river front delta. Six sedimentary microfacies are further distinguished: braided river channels, alluvial plains, underwater distributary channels, mouth bars, sheet sands, and underwater interdistribu? tary channels. These microfacies exhibit distinct particle size distribution characteristics. A complete set of trans? gressive and regressive cycles is evident in E2 S U 3 . Initially, sedimentation was dominated by underwater distributary channels with sand reservoir thicknesses ranging from 4 to 9 m. Mouth bars developed at the channel ends with res?ervoir thicknesses of 3 to 8 m. The distributary channels, having the widest planar distribution, constitute the key reservoirs in the area. Subsequently, as the lake level gradually rose, the extent of mouth bar development peaked. After reaching the maximum flooding surface, the lake level slowly declined, leading to the development of the braided river delta plain subfacies at the top of E2 S U 3 , where the sandstone thickness reduced to 2 to 5 m.