The Sedimentary Characteristics of the Central Canyon in the Deep-water Area, Qiongdongnan Basin

Zhang Daojun; Wang Yahui; Zuo Qianmei; Wang Zhenfeng; He Weijun

Abstract：The Central Canyon is situated in deep-water(300-3500m) of the Qiongdongnan Basin of the northern South China Sea, shows a S-shaped NE-trending as a whole,which originates from the east margins of the Central sag of the Yinggehai basin and terminates in the Xisha trough ,with a length of more than 570 km and a width of about 6-19 km . The natural gas reservoir of the Central Canyon was discovered in 2010. It contains an OGIP of 750 billion cubic meters and recoverable reserves of 139 billion cubic meters in the Late Miocene-Early Pliocene period. The formation and evolution of the Central Canyon are controlled by tectonics, sea level fluctuation, and special palaeogeomorphology presented a feature of altitude higher in the south and north but lower in the middle place as a whole.

Based on the sequence stratigraphic framework analysis, the Central Canyon system mainly develops from 5.5 Ma (S30) to 3.8 Ma (S28), which comprise several stacked sand_rich and mass transport deposits, composite sequences constructed of numerous higher parasequence sets.

About 290 m-thick channel complex in the Central Canyon shows best reservoir quality in amalgamated meandering channel-fill sand-bodies composed of fine-sandstone with high average porosity (>27%) and excellent permeability (500mD). The Central Canyon reservoir architecture is spectacularly imaged by 3-D seismic techniques, both in time sections and through a variety of amplitude extractions, while an extensive program of core and wire-line log acquisition and analysis has enabled high-resolution definition of the channel-fill sediments. The Canyon was initiated by the tectonic event in the Late Miocene and the introduction of coarse sediments to the interior of the basin possibly at times of relative sea level fall. Initially, there was significant erosion, showing V, U, W and composed shapes in vertical. Subsequent canyon infill commonly commenced with debris flows, slumps, and slides, sometimes overlying basal lags, progressed to turbidity flow deposits and low sinuosity turbidity channel with a large set of sandstone in packages, which is featured moderate - good continuity, strong amplitude reflection characteristics in the seismic profile. Meanwhile a series of small incised record in the phase axis reflection corresponds to a series of small turbidity channel, reflecting current flow migrating in the canyon; Amplitude extractions and horizon slices in the canyon display a low- to moderate -sinuosity map patter. Bright amplitudes on the inside bends are associated with shingled seismic reflections observed on vertical seismic profiles. In general, Several depositional elements have been observed in association with the sinuosity channel within the study area, including channels being infilled with mudstone, point bar, associated levees and mass transport complexes in some segment. This pattern was commonly repeated following reincision, which may have occurred several times. Secondly, with the rapid rise of relative sea level, large sets of deep-sea mudstone and turbidity flow deposits overlaid them. They show a poor- medium continuity and moderate amplitude reflection characteristics. Mass transport deposits often appear in the eastern part of the Canyon, exhibiting messy seismic reflection characteristics; Ultimately, With decreasing of the erosion power, a set of upward-decreasing net-to-gross sand ratios and deep-sea mudstone deposited on the top of the Canyon in the Early Pliocene. The different characteristics of Canyon in different times can be considered in terms of range of relative sea level, amount of Sediment supplying and the intensity of tectonic action. We propose the development of depositional model for the Central Canyon initiation, growth, abandonment, and lithofacies and architectural element distributions in space and time. This model imparts an important predictable control on reservoir and seal geometries.

Key words: Qiongdongnan basin, Deep-water canyon, Miocene-Early Pliocene, Turbidities

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013