Structural style and Basin Formation in Deep-water Area of Northern South China Sea

In the deep-water area of northern South China Sea (SCS) developed a series of sedimentary basins. Active exploration for deep-water hydrocarbon has begun in these areas since this century. The well LW3-1-1 at water depth of 1480m in the BaiYun Sag (BYS) of the Pearl River Mouth Basin in 2006 discovered 56m layer of pure gas, demonstrated the good hydrocarbon potential of the area. Wide-angle seismic profiling has verified the transitional type of crust in the slope areas. The Moho surface shoals step-by-step from 30-29km under the shelf, ~15 km under the slope, and ~12km under the abyssal plain. Moho also rises beneath depocenters, mirroring the shape of sedimentary basement. The crustal thickness at the center of the BYS is <7km. Lower crustal high velocity layer is found in the eastern and central portions of the northern SCS. The pre-Cenozoic basement in northern SCS is the extension of the inland basement and consists of mainly metamorphosed Paleozoic and Mesozoic marine and continental strata, complicated by Yanshanian (J-K) intrusive and extrusive rocks. From geophysical data we inferred that a SW-NE Mesozoic trench-arc system exists beneath the Cenozoic sediments in the northeastern SCS, related to the subduction of the Paleo-Pacific Ocean towards the East Eurasian margin. The stress field in the East Eurasian margin changed abruptly in Late Cretaceous. Rifting started in the entire margin and eventually led to the opening of the SCS in late Early Oligocene. Large sedimentary basins developed in the margins of the SCS. Paleogene lacustrine sediments contain hydrocarbon sources, while traps are mostly found in Neogene marine strata. The structure of the northern SCS shows clear W-E variation, divided into NE-, NEE-, and NE-trending segments by two major NW-SE transfer faults. The Southern Depression of the Qiongdongnan Basin to the west is characterized by NE-trending half grabens. The BYS at the central segment is characterized by NEE-trending composite grabens and down warps with relatively small offset of boundary faults. To the east the Chaoshan Depression is composed of Mesozoic strata under very thin (<1km) Cenozoic cover. The origin of such a W-E variation might be related to the existence of Mesozoic subduction system in the east, which influenced not only the formation of Cenozoic sedimentary basins and the variation of sedimentary facies, but also the thermo- rheological structure of the underlying lithosphere. The BYS from the bottom upwards includes 3 layers, a layer of rifts, a layer of faulted down-warps, and a layer of down-warps. Compared with the 2-layer bull-head structure of the Zhu 1 depression in the shelf, the BYS has one more layer of faulted down-warps. This might indicate that after a short period of brittle rifting the relatively hot lithosphere in the slope has undergone a period of ductile extension. The post-extension sequence in BYS is much thicker than that predicted by thermal subsidence theory. We suspect that in a passive margin the formation mechanism of deep-water basins is different from that of shallow-water basins. A study is ongoing to explore the basin formation mechanism, taking into account of the factors of abnormal lithosphere rheology, active mantle underplating and magmatic heating, lower crust flow, as well as the superposition of later extensional events. The study is supported by NSFC grants 40576027 and 40238060.