Development characteristics and controlling factor analysis of the Neogene Minghuazhen Formation shallow water delta in Huanghekou area, Bohai offshore basin

Based on a combination of high resolution 3-D seismic, drilling and well logging and core data, this study focuses on describing the depositional features of the Neogene Minghuazhen Formation shallow water delta in Huanghekou area(HHKA), Bohai offshore basin and discussing the evolution and controlling factors of shallow water delta sandbody. An obvious meandering fluvial delta system developed in sequence 1(SQ1) of the Neogene in HHKA with thinner sandbody of distributary channels and poor development of mouth bar. The sequence texture obviously influences the vertical development and stacking pattern of sandbodies and controls the distribution of sandbodies in plain view as well. In shallow water lacustrine basins, relative topographic height difference leads to change of distribution of accommodation space, and sandbodies of distributary channels usually develop well in local low-lying areas where accommodation space increases. The delta is dominated by distributary channel sandbodies during the early period of base level rising. Sandbodies contact with each other in a lateral stacking pattern and are characterized by a fan shape in plain view. Distributary channels gradually narrow and tend to shift during the mid-late-period of base level rising, while the sandbodies are characterized by a net shape in plain view. During the period of base level slow falling, the multistory/multilateral channel sandbodies dominated the inner front of shallow-water delta and the delta sand dispersal distributes as a lobe shape.

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.