Recurrent slope failure and submarine channel incision as key factors controlling reservoir potential in the South China Sea (Qiongdongnan Basin, South Hainan Island)

Abstract

High-resolution multi-beam bathymetry, 3D and 2D seismic reflection profiles from the South China Sea are used to investigate the morphology, characteristics, origin and implications to petroleum systems of recurrent slope failure in the Qiongdongnan Basin, northern South China Sea. Seven Late Miocene-Holocene mass-transport deposits (MTDs) and numerous submarine canyons were identified on the continental slope and rise, providing new insights on the evolution of an enigmatic region of the South China Sea. This paper defends that the interpreted MTDs were triggered by a combination of high sedimentation rates and local tectonic uplift. By comparing the stratigraphy of the study area with local sea-level curves, we show that a significant portion of shelf-edge deposits effectively bypassed most of the continental slope during the Miocene-Quaternary to accumulate as large MTDs on its lower part (i.e. on the 'basin-floor'), independently of sea-level changes. Our work has implications to the current sequence stratigraphic models for continental margins, and sheds new light on the reservoir potential of Miocene units in the South China Sea. Hence, regions where base-of-slope fans are expected to occur are, in the study area, occupied by large-scale recurrent MTDs sourced from the shelf edge. Stratigraphically, recurrent slope instability resulted: a) in abrupt episodes of accommodation space creation on the shelf-edge, b) on a moderate reduction in accommodation space at the base of the continental slope, and c) in a complete separation between shelf and distal slope depositional systems, with most stratigraphic markers on 3D seismic data being diachronous across the continental margin. As MTDs also comprise the bulk of channel-fill deposits in large submarine canyons, we propose that the reservoir potential of channel-fill deposits in the South China Sea is closely dependent on the nature of the sediment (i.e. net-to-gross ratio) eroded and transported by these same MTDs.