Deep architecture of hyperextended marginal basin and implications for hydrocarbon exploration: A case study of Qiongdongnan Basin

The passive continental margin basin in the northern South China Sea is an important oil and gas base in China’s offshore area. This paper reveals depositional filling characteristics and stratigraphic patterns of hyperextended continental margin rift basins based on coupling analysis of the deformation of deep crust and basin filling in the passive continental margin through comprehensive analyses of massive seismic and drilling data from the Qiongdongnan Basin of South China Sea. The results show that the Qiongdongnan Basin, located on the western tip of the northwest sub-basin of the South China Sea, is once a failed continental margin rift basin and presents now as a neck and distal zone along the central depression belt after lithospheric detachment and thinning. The rifting period contains three stretching stages. During the early stage, the formation of isolated rifted basin is characterized by high-angle positive faults. During the detachment active stage, the formation of detachment rifted basin is characterized by low-angle detachment faults with their active time obviously characterized by an east-west migration. And during the late rifting-depression stage, the depocenter is located in the center of the depression. The migration characteristics of differential deformation tectonic movement define the unique deep structure pattern of the Qiongdongnan Basin and lead to obvious differences in depositional fillings and stratigraphic patterns in different tectonic units, especially in depositional systems, where fan delta deposits dominate the main detachment fault side and braided delta deposits occupy the opposite rolling anticline. These factors restricted the distribution of source rocks and deep reservoirs during the rifting period, which in turn constrained geological conditions of hydrocarbon generation and accumulation. Therefore, the deep architecture of hyperextended continental marginal rift basins established based on non-transient breaking-up process of the lithosphere and depositional filling characteristics have important guiding significance for hydrocarbon exploration in the Qiongdongnan Basin, and may serve as reference for the study of deep stratigraphic pattern and depositional filling in passive continental margin basins.