Fine-grained and coarse-grained Paleogene sublacustrine fan systems in Fushan Depression, Beibuwan Basin, South China Sea: implications for sedimentary characteristics and depositional processes

The Fushan Depression is a hydrocarbon-bearing half-graben rift sub-basin located in the southeast of the Beibuwan Basin, South China Sea. The sublacustrine fan systems in the Paleogene Liushagang Formation have recently become important targets in this depression, while the depositional process and detailed characterization of these systems are little known. Analysis of drilled cores, wire-line logs, and 3-D seismic survey suggest that the Fushan Depression develops two different types of sublacustrine fan systems. The fine-grained sublacustrine fan system located in the western area is mainly composed of fine-to medium-grained light gray sandstones interbedded with layers of silt-rich shales. This kind of sediment slumped from the distal bar of the delta front due to high sediment pore pressures at the slope edge as well as the activity of the Meitai Fault. By contrast, the coarse-grained sublacustrine fan system formed in the eastern area is characterized by medium to coarse gray sandstones and conglomerates with the development of various deformed lenticular beddings, deformed pillow structures, and micro-faults. Further study suggests that its sedimentation process is closely related to the geomorphology of flexure slope-break belt. The slope changed abruptly from 3 degrees similar to 5 degrees to 7 degrees similar to 9 degrees across the flexure slope point, increasing the accommodation space under the flexure belt. The coarse-grained sublacustrine fan formed when coarse-grained sediment spillover the slope-break toward the increased accommodation space. As the reservoir quality of these sublacustrine fan systems is well controlled by the sedimentary characteristics and sedimentation processes, our results should be of great significance for the development of future exploration.