Abstract: To determine the distribution characteristics and internal structure of a distributary-mouth bar type shallow-water delta reservoir, this study integrated current sedimentation, rock core, well logging, and dynamic data and performed hierarchy analysis on the NmⅡ-1 layer of oilfield A in Bohai Sea. The study established a grading system for the hierarchy of the distributary-mouth bar type shallow-water delta and describes the spatial configuration among its internal single sandbodies. Distributary-mouth bars were identified as the main structural unit of this class of shallow-water deltas, of which the level 5 structural units are complexes formed from distributary-mouth bars of the same period, level 4 units are single distributary-mouth bars, and level 3 units are internal accretion bodies of the distributary-mouth bars. Three main types of single-phase sandbody structural interfaces were identified:mudstone, argillaceous siltstone, and silty mudstone. The length, width, and thickness of distributary-mouth bars of single origin are about 600~1 300 m, 400~800 m, and 2.5~7.0 m respectively, arranged vertically in partially overlapped, main-body overlapped and main-body superimposed configurations, and laterally in isolated, edge sideway joined and main-body sideway joined patterns. The overall evolution is "early foreset deposit with lateral migration-late gradual overlapping aggradations." Three mud interlayers of varied genesis developed in the distributary-mouth bars:foreset inclined layer along the source direction, sideway overlapped layer, and vertically accreted horizontal layer perpendicular to the source. The sideway overlapped layer developed in the proximity of the source, whereas the vertically accreted horizontal layer developed at the lake basin region to the far side. The interlayer thickness of the study area is generally 0.4~1.2 m, with extension less than 300 m.

Key words: Shallow-water delta, distributary-mouth bar, contact pattern, deposition process, reservoir hierarchy