Depositional Architecture And Reservoir Differences In The Ledong-Baodao Segments Of The Central Canyon In The Qiongdongnan Basin

Submarine canyon deposits have drawn attention due to their significance on sourceto-sink analysis and hydrocarbon exploration.The Central Canyon which has the distinct depositional architecture and reservoir distribution compared with other slopeperpendicular canyons,runs parallel to the paleo-shelf break.Based on high-resolution 3D seismic and well data,this study emphasizes the spatial variability in the depositional architecture and reservoir quality of the canyon fillings along the canyon pathway,and the controlling factors are also discussed.The development of the Central Canyon in the study area initiated since 11.6 Ma.The significant incisions terminated between 8.2Ma and 6.98 Ma,and then the canyon evolved into the filling stage.Finally,the canyon was full filled at 4.2 Ma.The canyon developed along the thalweg of a multiple stepped palaeotopography with a slope-parallel descending trend eastwards,which accumulated large-scale and high-energy turbidity flows during the sea-level fall.The formation of the palaeotopography was mainly resulted by regional tectonic subsidence and the formation of the Ledong Submarine Fan.Analysis of the canyon infillings indicates multiple sediment sources including an axial sediment source from the Central Vietnam and the western Hainan Island and a canyon-side source from the southern Hainan Islands.The former source supplied relatively coarse-grained turbidites,but the latter supplied finegrained mass transport deposits(MTD)and its cogenetic turbidites.The canyon evolved through three stages.Stage 1 is characterized by significant incisions that are responsible for the formation of the canyon.Subsequently or contemporaneously,the extending direction changes of the canyon at the junction of the Ledong and Lingshui Depressions,as well as in the eastern Lingshui Depression,probably gave rise to the lateral erosions which triggered canyon margin failures.Stage 2 is characterized by the predominant axial fills.Along the canyon pathway,bypass facies association gradually evolved into the sinuous channel complexes at the first.Then,the canyon floor broadened and the bypass facies association gradually evolved into the channel-overbank deposits.They are believed to be controlled by the topographic changes.When the canyon evolved into the stage 3,large amounts of MTDs that sourced from the northern continental slope flowed into the canyon.The MTDs not only ponded the axial turbidites at its up-dip direction but also generated cogenetic turbidity flows through the interaction with ambient water.Therefore,geomorphology of the thalweg slope adjusted quickly and finally evolved into a multiple stepped slope,i.e.,three steps separated by two ramps.And the multiple stepped slope controlled the formation of the subsequent multiple-stages submarine fan.All in all,multiple sediment supplies and topographic changes play the predominant roles in controlling the spatial variability in the depositional architecture.As the results of the sedimentary processes influenced by topographic changes,unconfined channel sandstones occurred in the Ledong-Lingshui segment and show continuous distribution.Whereas,the overbank sandstones located in the Lingshui segment and the western Songnan-Baodao segment are characterized by relatively isolated distributions.Moreover,heterogeneity of the sandstone increases along the canyon pathway.Although the texture of sandstones has the primary control on the spatial variability in the reservoir quality,effects of compaction and cementation on reducing permeability cannot be ignored.This study provides direction for the hydrocarbon exploration and exploitation in the Central Canyon and other analogous canyons.Also,this study documents adjustment mechanisms of the thalweg slope in detail for the first time,which can further contribute to the the spatial variability in the depositional architecture and reservoir quality of the deep-water canyon.