Characteristics and sedimentary model of a reticular shallow-water delta with distributary channels: lower member of the Neogene Minghuazhen Formation in the Bozhong area of the Huanghekou Sag, China

Abstract

The Bohai Bay Basin entered a late stage of structural depression during the sedimentary depositional period of the lower member of the Minghuazhen Formation in the Bozhong area of the Huanghekou Sag, China. Numerous analyses of geological and geophysical data have shown that the climate was relatively humid in the Bozhong area. Additionally, the basin basement subsided at a stable rate, creating a broad basin with gentle slopes. The research area in this study was close to the sediment source, which supplied it with a sufficient amount of sediment. The lake shoreline changed relatively frequently, and the lake was deep, which allowed shallow-water deltas to develop widely. The reticular shallow-water delta in the research area has the following sedimentary characteristics: (1) the fine-grained lithology, low component and textural maturities, and the cumulative probability particle size curve suggest tractive flow deposition. (2) Deltas with low sandstone contents and widely distributed red mudstone are widespread. (3) Chaetosphaeridium and biological disturbances are common. (4) The lake level fluctuates frequently. (5) Delta-front subfacies are mainly developed in the Bozhong area; they show a reticular planar distribution, comprise composite sandbodies, and are superposed by multi-stage underwater distributary channels. (6) The river channel is narrow and developed a large number of main river channels in different directions. (7) The underwater distributary channel microfacies are distributed within multibranched and bifurcated channels and generally show a discontinuous positive rhythm; however, the estuary dam sandbody is not well developed. This work suggests that a reticular shallow-water delta sedimentary system formed under humid climate conditions in the Bozhong area and is characterized by its near-source type, fine granularity, wide distribution, and vertically and horizontally overlapping of underwater distributary channel sandbodies. During the exploration process, this sedimentary system can provide a reasonable guide with which to predict the sandbody distribution in a lithologic reservoir.