Deepwater Depositional Features of The Miocene Zhujiang Formation in Baiyun Sag, Pearl River Mouth Basin,The North China Sea

Sequence stratigraphy theories played an important role in the discovery of the submarine fan in LW3-1 gas field, Baiyun Sag, Pearl River Mouth Basin. However, with further exploration and rational developments, large-scale analyses of seismic stratigraphy and sequence stratigraphy haven’t met refined descriptions. By using seismic data, logging, petrology, thin sections and SEM, we made a detailed analysis of depositonal features of Miocene Zhujiang Formation centering on depositional processes. Eleven lithofacies were indentified on the basis of detailed core analysis, including mega-thick and thick massive sandstones, thick normal-graded sandstones, thick inverse-graded and inverse-normal graded sandstones, parallel-bedding sandstones, cross-bedding sandstones, slumped and deformed sandstones, thin sandstone interbeds, thin siltstone-mudstone interbeds, thick siltstones, thick mudstones, wavy and flaser-bedding globigerina limestone facies. They could be integrated into five lithofacies associations, i.e., sandstone associations, siltstone-mudstone interbed associations, mudstone associations, siltstone associations and Deepwater limestone associations. Three depositional types were identified, namely submarine fan, basin facies and Deepwater traction current. Six microfacies were further recognized within the fan, including the main channels in the inner fan, distributary channels in the middle fan, inter-channels, levee and the outer fan. The lower Zhujiang Formation, mainly sandstone associations, was inner fan and inner-middle fan deposits of the basin fan and slope fan. The middle part, mainly mudstone associations, was outer fan deposits. With the transgression, the submarine fan was finally replaced by basinal pelagic deposits dominated by mudstone association, siltstone association, and Deepwater limestone associations. During the weak gravity flow activities, the lower channels, the middle-upper outer fan and basin deposits were strongly modified by the Deepwater traction current. The discovery of the Deepwater traction deposition in Miocene Zhujiang Formation would be of great importance. It could be inferred that the Deepwater traction current had been existing after the shelf-break formation since Late Oligocene (23.8Ma) in Baiyun sag, influencing and controlling sediment compositions, distribution, and depositional processes. And it would have a great enlightenment to the palaeo-oceanic current circulation in northern South China Sea.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California