THE SEDIMENTARY CHARACTERISTICS AND FORMATION MECHANISM OF DEEP-WATER CHANNELS IN YINGGEHAI AND QIONGDONGNAN BASINS, SOUTH CHINA SEA

Multiple types of deep-water channels are developed in the Miocene-Pliocene strata in Yinggehai and Qiongdongnan basins, the South China Sea. These deep-water channels are highly heterogeneous, varying in run-out length, incision depth, sinuosity, and internal lithologies and grain sizes. These heterogeneities challenge the prediction of sandstone and mudstone distribution in the channel, and consequently, bring the risks of failure on the assessment of oil and gas exploration. Integrated studies of field outcrops, cores, image well-logging, and seismic data are carried to reveal the geometric, structural, flow dynamics, source-to-sink features of the deep-water channel deposits. This work has four significant outcomes. (1) Two primary formation mechanism of deep-water channels are identified, including gravity-flow downcutting incisions and negative topography caused by the extensional process when the sedimentary body slides downslope (here named extensional slide). (2) Deep-water channels formed by along slope gravity-flow downcutting incision are characterized by short extension length and deep incision depth and internally are filled by slumps in early stage and mudstone in late stage. The incisions along the strike direction of basin center form long extending and deeply incised channels, which are internally infilled by thick sandstone at the base and mudstone at the top. (3) The extensional force induced by the slides moving downslope can form linear and kidney-shaped grooves and gullies. Repeat and long-lasting activities of large-scale of extensional slides can connect and stack the isolated grooves and gullies into large-scale slope canyon-channels along slope. The lithologies of channels are more heterogeneous with varying sand mud ratio and highly dependent on the sediment sources. (4) The deep-water channels in Yinggehai and Qiongdongnan basins usually are formed by the interplay of downcutting and extensional slide. The extensional slides can transform the gravity-flow transport and incision from along slope dip direction to strike direction, whereas the downcutting process can smooth the surface of extensional slides and rework the slump deposits within it.