Evolution regularity of the Neogene shallow water delta in the Laibei area, Bohai Bay Basin, northern China

According to the characteristics of sedimentary microfacies, unique vertical depositional sequences and well logging response, the authors propose that a shallow water delta was widely developed in the Neogene of the Laibei area, Bohai Bay Basin of northern China. Based on seismic minimum amplitude slices, well logging data, test analytical data and so forth, detailed research on the evolution of the shallow water delta of the Neogene Lower Member of Minghuazhen Formation was conducted. The results indicate that the third-order sequence base level controls sandbody types. During a period of low base level, a distribu- tary channel sandbody of shallow water deltaic plain was developed. With base level rising, the sandbody type gradually changed into a subaqueous distributary channel sandbody and a sheet sand can be found as well. The fourth-order sequence base level controls mediumshort term evolution of the sandbody. Within a sequence, due to the rising and falling of base level, the sandbody assemblages are identified as an upward-coarsening type, an upwardfining and a compound type respectively. Regionally, from the Laibei Low Uplift to the Huang- hekou Sag, the shallow water delta evoluted from a dendritic shape, to a cuspate shape and finally to a sheet shape.