Multistage progradational clinoform-set characterisation and evolution analysis of the Early Oligocene in the Baiyun Sag, Pearl River Mouth Basin, South China Sea

Abstract

Abstract Conventional seismic-based techniques for analysis of the progradational stages usually proceed qualitatively according to the facies identified by 2D seismic or cross-well sections. In this study, we try to apply a more quantitative and practical approach to depict the distribution and stacking patterns of the progradational clinoform set (PCS) based on the integrated analysis of well-tie seismic stratigraphy, seismic sedimentology and seismic geomorphology. The Baiyun Sag, a subbasin of the Pearl River Mouth Basin in the South China Sea, provides a suitable example for characterising the clinoform sets with typical progradational seismic reflections. The characterisation results indicate that there were three PCSs and at least nine stages of clinoform that developed during the deposition of the upper member of the Enping Formation (Lower Oligocene). The spatial-temporal distribution of the PCSs can be easily detected by the energy half-time seismic attribute based on the manual interpretation of top and bottom interfaces of each clinoform. Integrated with the progradational seismic facies, progradational angles-based rose diagrams, and seismic geomorphology analysis, the characteristics of each PCS are depicted with stable and gradually increasing sediment supply along the NW-SE direction (azimuths of 130°–160°). The three PCSs display obviously continuous prograding towards the basin centre, whereas several clinoforms (PCS-1-a and PCS-1-b; PCS-3-a and PCS-3-b) exhibit a retrogradational characteristic within one PCS. The overall progressive characteristics, which include having some short period regressive stacking patterns, exhibit a combined effect of sea level change of the South China Sea and the increasing sediment supply intensity from the South China Block.