Depositional Characteristics And Hydrocarbon Significance Of Deepwater Fan, In The Northwestern Sub-Basin, South China Sea

The development of hydrocarbon exploration theories and technologies have made the deepwater hydrocarbon to be a hotspot since the late1990s. The breakthrough of deepwater hydrocarbon in the Northern South China Sea(NSCS) proved once again that deepwater fan identified base on seismic reflections can be the favorable exploration target..The Northwestern Sub-Basin(NWSB) located in the deep-sea plain of NSCS became a target area which was on the way of deepwater hydrocarbon explorations transform continental slope to basin plain. The theory of "Source-Conduit-Sink" points out that sediments finally converge in NWSB which have potential impact on deepwater sedimentation patterns and associated fan development.Based on large amounts of seismic reflections and limited drilling data, tectonic-sedimentary framework of NWSB was built and a deepwater fan system in NWSB was identified. We analyzed the depositional characteristics, spatial-temporal evolution of this deepwater fan system, and built a depositional model. We combined deepwater fan in NWSB, Pearl River Deepwater Fan System(PRDFS) with Red River Submarine Fan(RRSF) to be the Deepwater Fan System of NSCS, and summarized its development pattern. Finally, hydrocarbon significance of deepwater fan system in NWSB was forecasted.Channelized deepwater fan in NWSB have confined channel complexes zone with channel avulsions in the upper fan, channel-levee systems with internal channels frequent lateral migration in the middle fan and channel-lobe systems with internal channels died out in the lower fan. Non-channelized deepwater fan shows two-way down-lap character and without internal channels. The deep-water fan systems can be divided into the Late Miocene(Ⅰ), Miocene(Ⅱ and Ⅲ) and Quaternary (Ⅳ and Ⅴ) five period, multi-period deep-water fans vertically superposed and laterally migration. Late Miocene, tectonic subsidence made the Shelf break of NSCS had a large-scale displacement toward the land, to promote lot of sediments from the Pearl River and Red River Provenance System delivered into the NWSB through the canyon-channels (mainly the eastern section of the Qiongdongnan Central Canyon), which conducive to the formation of deep-water fan. Subsequently, with the sea-level fluctuations, the development of multi-phase fan vertically superposed. The Shuangfeng seamount is the spreading ridge of NWSB. Fan body Ⅰ, Ⅱ, Ⅲ stacked on the north of the seamount when the subsidence rate was rather higher on northeast part of the basin in Late Miocene. After the late Pliocene, the subsidence rate was rather higher on southwest part and fan body Ⅲ turn to southward.The subsidence rate of intraslope depressions and basin plain significantly different, as well as fluctuations in sediment flux of the Pearl River, Red River Provenance System on Late Miocene. We supported that there was a sedimentary transformation adjustment process from slope to basin on Late Miocene. Before Late Miocene, deepwater fan system of NSCS located in intraslope depressions, as PRDFS deposited in Baiyun Sag. Late Miocene, sediment bypassing, erosions and slumps were commonly on continental slope. After Late Miocene, deepwater fan system deposited on basin plain(like NWSB deepwater fan system), only a short life cycle RRSF deposited on the northwest slope which effected by dextral strike-slip of Red River fault zone.Compared with neighboring areas with middle-high exploration level, the source, reservoir and cap conditions are very good in NWSB. Miocene and partly Oligocene marine hydrocarbon source rock have good hydrocarbon potential. Deepwater fan sands have good reservoir properties. The regional transgression marine mudstone has strong sealing ability. Furthermore, faults and sand bodies combination to be good conductive conditions and can form lithologic oil/gas pools. The NWSB shows a promising exploration targets.