--> The Sand-Rich Lobe-Complexes in the Central Canyon, Deepwater Qiongdongnan Basin, Northwestern South China Sea: Insights From New Wells

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The Sand-Rich Lobe-Complexes in the Central Canyon, Deepwater Qiongdongnan Basin, Northwestern South China Sea: Insights From New Wells

Abstract

The deepwater Qiongdongnan Basin is emerging as the new prolific gas province in the northwestern South China Sea. The basin underwent Paleogene rift episode and Neogene post-rift episode, with the shelf-slope sedimentary system developed since Middle Miocene. The Central Canyon developed by gravity flow during Middle Miocene through Pliocene, extending from west to east along the long axis of the basin with length of 480km and width of 15–30km. Recently, several wells in the canyon with cores and loggings provide new insights into the canyon filling pattern and turbidite lobes distributions. Integrated with high-resolution 3-D seismic attributes imaging, seismic-well calibration and core-logging analysis, the paleomorphology of the canyon, channel architecture in the canyon and sand-rich lobes stacked patterns were delineated in details. Indicated by the new heavy mineral correlations and analysis of trace elements and the age of Zircon in the sediments of the canyon with surrounding provenances, the Central Canyon were primarily filled from the distal sediments of the paleo-Red River, although at the same time the sediments from onshore South Vietnam contributed much to the sandstones. Five wells in the canyon showed the sand-rich lobes distributing along the canyon wall, both on the northern and on the southern side. At the swerving section of the canyon from EW to NE, the sandstone lobes developed more due to the decreasing energy of the turbidite flow. The 100km-long western section of the canyon in Lingshui Subbasin, was generally filled by 2–5 stacked sandstones layers with maximum thickness of 160m, interbedded with thin mudstones. Single layer of sandstone lobes was 10–70m thick, and 5–40 square kilometers. The sandstones are primarily fine grained and fining-upward with massive bedding. The porosity of sandstones was 22–32% with good permeability due to the shallow burials less than 2500m below seabed and less compaction. Laterally, the lobe-complexes were generally incised by the channels or later Mass Transport Deposits(MTDs) and separated from each other forming independent stratigraphic traps, especially at the upper lobes. The basal lobe-complexes were shown to be amalgamating to be the lateral hydrocarbon migration pathways. The architecture of lobe-complexes in the west section of the Central canyon have implication for the turbidite reservoirs delineation and trapping mechanism in the eastward section.