Sedimentary characteristics of shallow-water delta and responses features in palaeoenvironment: a case study from the lower part of Neogene Minghuazhen Formation (Bonan area, Bohai Bay Basin, E China)

Shallow-water deltas are important reservoirs for hydrocarbon resources, and there are many oil and gas discoveries found in the sands in China. Many large shallow-water delta sedimentary systems were found in the Bohai Bay Basin in the Late Miocene. Several classic shallow-water deltas are developed in the early-stage sequence (SQmL2) of the lower part of Neogene Minghuazhen Formation in the Bohai Bay Basin which have attracted much attention in recent years. We choose the SQmL2 as an example to reveal the relationships between paleoenvironment and shallow-water delta facies and architecture. Analysis of 3D seismic data and logging data indicated that two types of sedimentary facies, including shallow-water delta and shallow lake facies, are developed during SQmL2. The shallow-water delta facies mainly consist of subaqueous distributary channel with mouth bar rarely developed. The deltas are mainly comprised of relatively thin (1–4 m) fine- to medium-grained sandstone. The combinations of distributary channels on planer view are mostly branch-shaped and lobe-shaped. The shallow-water deltas developed in SQmL2 are controlled by paleogeomorphology, sediment supply, and paleoclimate. The flat topography and sediment supply are favorable for sediments moving basinward for a long distance and forming channelized sand bodies. As the climate gets humid, the lake level rises and shallow lake area expands, resulting the sand bodies move landward and become much thinner. The lake level has changed from shallow to deep and to shallow, and correspondingly, sand body of subaqueous distributary channels has changed from lobe shape to branch shape and to lobe shape.