Focused Fluid Flow Structures In The Deepwater Basins Of Northern South China Sea Margin

Basins developed in northern continental margin of South China Sea(SCS)in Cenozoic evaluated from syn-rifting stage before 23.8 Ma to post-rifting stage.The fluid flow in deepwater basins were ongoing along with the seafloor spreading of SCS and followed tectonic activities and changing palaeo-oceanography environment..Based on abundant three dimension seismic data and core data,three typical fluid flow structures were studied,including formations of polygonal faults in deepwater in early Miocene,corrosion and collapse of carbonate in Dongsha Massif in early Miocene,and migrating canyons from middle Miocene to now in the continental margin.Lots of polygonal faults have been found in deepwater basins of the northern South China Sea,such as Baiyun sag of Pear River Mouth basin,Huaguang sag,Changchang sag and Beijiao high of Qiongdongnan basin.Polygonal faults in northern South China Sea are layer-bound,meso-scale normal faults with short length,short interval,high dip angle,and poor directionality.Using tectonic evolution,relative sealevel change and sequence stratigraphy framework in deepwater basins,the study show that polygonal faults developed in tectonically quiet and deepwater sediments.The development of polygonal faults in Baiyun sag of Pear River Mouth basin and Huaguang sag of Qiongdongnan basin is considered to result from dewatering and hydrofracturing in clay minerals.The impact of tectonic stress and gravitation spreading leads to the steady orientation directionality of polygonal faults in Changchang sag and Beijiao high of Qiongdongnan basin.The wide spreading existence of polygonal faults in Neogene sequence has a significant influence in fluid flow,formation pressure forecast and drilling evaluation.Polygonal faults developing in fine grained sediments of deepwater basins showed a rapidly increasing relative sea level.Liuhua carbonate platform was developed in Dongsha Massif in early Miocene with maximum thickness of more than 600 m.A lot of sinkholes were imaged in upper carbonate platform based on coherent slices and three dimension visualization from seismic data.The sinkholes were larger depressions with diameter generally ranging from 200 to 700 m,the maximum more than 900 m,and depth varying from 10 to 90 m.Dongsha event is considered to be the key factor leading to the occurrence of sinkholes.Concretely,basal magmatic activities might provide acidic hydrothermal fluid to dissolve the carbonate rock.NWW oriented faults produced by Dongsha event served as pathways for fluid flow and rooms for fluid exchange.In the end of late-Miocene,exposure caused by Dongsha event accelerated the dissolution rate of carbonate rocks.With continuous corrosion,the dissolved cavern became larger and larger until the carbonate strata over the cavern couldn't hold the upper loading,and then carbonate strata collapsed and the sinkhole formed.The development of thick carbonate platform in Dongsha Massif indicated a steadily ascending relative sea level.It suggests a tectonic subsidence in deepwater regions of Pearl River Mouth Basin in early Miocene.The formations of widespread sinkholes reflected sequences ever exposed extensively caused by Dongsha Movement,which played an important part in late digenesis of carbonate rocks in late Miocene.Massive canyons develop in continental slope of northern South China Sea.Four phases of canyons interlayered by normal sediments were identified,including phase?(13.8-12.5 Ma),phase ?(12.5-10.5 Ma),phase ?(10.5-5.5 Ma),phase ?(5.5 Ma-~).The canyons present the character of northeast oriented migration parallel to the margin and north-south migration perpendicular to the margin in the order of negative sequences.The north-South migration of canyons represented a drop of relative sea level in accordance with the change of shelf breaks.The developing scales also enlarged from phase?to phase?and subsequently shrank to phase ?.Characteristics of canyons discussed above were inferred to relate with the change of relative sea level.The initial of paleo-canyons in 13.8 Ma was inferred to result from a sudden drop of sea level over the world caused by the enlargement of Antarctic Ice Sheets.Canyons happening in 12.5 Ma also were induced by the falling sea level.Canyons occurring between 10.5-5.5Ma depended on the dropping relative sea level,basin subsidence and the basal uplifts caused by tectonic activities.The formation of Arctic Ice Sheet was inferred to lead to the commencement of canyons in 2.7 Ma.The enlarging and shrinking range of canyons were caused by the change of sea level and ocean current circulation in South China Sea.