Characteristics And Evolution Of Cenozoic Periplatform Deep-water Sedimentary System In The Xisha Area, Northern Continental Margin Of The South China Sea

Geoscientists have focued on the deep-water sedimets recently as deep-water sediments can record some geological information such as the sedimentary dynamics, ancient geological oceanography and so on. Meanwhile, it also received intense attention by the oil industry because it is important reservoir for the oil and gas as well as hydrate resources. However, studies about periplatform deep-water sedimentary systems of the Xisha area in the South China Sea is rare. In that thesis, we used high resolution of 2D and part of 3D seismic data, well data and combined with geophysical interpretation technologies to analyze the characteristics of different deep-water sedimentary system around carbonate platforms in the Xisha area, South China Sea.1. Mound-shaped reflections were identified both in NE-SW and NW-SE oriented seismic profiles, and the mounds are most prominent in the central part of NW–SE oriented seismic profiles with undulating tops that flatten out towards the NW and SE. The mound-shaped reflections are mostly asymmetric with steeper eastern flanks in the NE–SW oriented seismic profiles. They are approximately round in planform. Paleogeographic analysis shows that the Xisha uplift was dominated by tropical shallow carbonate platforms, while the Beijiao depression became a bathyal environment in the middle Miocene. The absolute value of the wave impedance of the sediments is much smaller than that of the LH11-1 Reef Oilfield, indicating that the sediments were not derived from reefs. Integrated analysis suggests that the periplatform sedimentary mounds were generated by the combined work of gravity flow and bottom currents. The different subsidence rates between the QDNB and the Xisha uplift oversteepened the paleo-slope gradient and sediments transported downslope generated mounds on the slope that were deformed consistently to the north. The velocity of bottom current in the middle Miocene may have been high enough to flow into the SCS anderode, transport and redeposit deep-sea sediments.Sediments transported from the Xisha carbonate platforms crept downslope and were laterally reworked by bottom currents.2. Zhongjian deep-water channel developed in the graben lying between Xisha uplift and Guangle uplift, and the channel was separated apart into two branches because of the existence of a paleo-highland. The north branch can be divided into five stages with significant migration, including Sanya Formation(I), Meisha Formation(II), Huangliu Formation(III), Yinggehai Formation(IV) and Ledong Formation(V).The south branch can be divided into four stages(Meisha Formation(I), Huangliu Formation(II), Yinggehai Formation(III) and Ledong Formation(IV)) and with inner high amplitude, continuous reflections. Each stage of it show an erosion-infill-abandon sedimentary cycle. Paleogeography analysis shows that the channel-filled deposits compose of carbonate and reefs debris and volcaniclastic rocks.3. High resolution seismic data shows that there widely exist periplatform channel system in the Xisha area, northern south China sea. Channels around reefs display strong amplitude reflections, and were filled with weak-strong and continuous reflections, chaotic seismic reflection can be found at the bottom of the channels. Small-scale channels on the slope show a serious of V-shaped reflections with incised shallow on the underlying strata. A channel initiated from early Miocene located in the southern slope of the Xisha carbonate platforms. On the east of the Xisha carbonate platform, an unfilled canyon exist.4. Palaeo-geology analysis suggest that the sediment supply mainly come from the Xish and Guangle uplift. The Xisha uplift originated from the south china continental in respond to the expending the South China Sea and the exist of the Xisha Trough, Central Depression and the southern uplift belt of the Qiongdongnan Basin stop the sediment from the south china continental from Miocene. The Guangle uplift is separated with Vietnam uplift by a narrow depression developed along the trace of the East Vietnam Boundary Fault Zone. The inexistence of shelf- slope system of the Vietnam uplift made it impossible for the sediments from Vietnam uplift to be transported into the Guangle uplift. Sedimentary facie show that early-middle Miocene, carbonate rocks, reef clastic and igneous clastic were the main sediment supply for the channels. Late Miocene to present the carbonate platforms decrease and not so much tropical sediments supplied, so the channels mainly composed of deep-sea sediments.5. Palaeogeomorphology of the Xisha area greatly controlled the development the deep-water channel. The low-lying belt between the Xisha and Guangle uplift restrict the flow direction of the channel. The ancient rise separated the Zhongjian deep-water channel into two branches: the southern one to the Zhongjian depression and the northern one to the Huaguang depression. The confined space limited the migration of the channel, however, the northern branch channel migrated unidirectionally because of the open space in the north.6. The third order sea-level changes controlled the erosion-backfilled-abandoned cycle evolution of the Zhongjian deep-water channel, while the second order sea-level changes controlled the component and the scale of it. Bottom current play an important role in the burying of the channel and reworked the sediments of the channel.The third order sea-level changes the same with global eustasis periodically, and the Zhongjian channel developed in regression period when the sea-level fell. The falling and rising of the third order sea-level play an import role in the cycle development of the channel. The second order sea-level changes were against with the global eustasis, and as with the consistent rising of the second order sea-level, carbonate platform became drowned eventually from its flourish stage. As a consequence, the sediment filled in the channel changed from carbonate rocks to deep-sea mudstones. The erosion of gravity flow decrease because of the rising of the second order sea-level, which lead to decreases of the cut depth in the southern branch of Zhongjian deep-water channel.