CENOZOIC SEQUENCE STRATIGRAPHY AND SEISMIC FACIES ANALYSIS OF XISHA TROUGH BASIN IN NORTHERN SOUTH CHINA SEA

The Xisha Trough Basin is a large Cenozoic deep-water sedimentary basin with lower exploration degree, which is located in the western part of the north slope of the South China Sea. New high-precision multichannel seismic data combined with geological character of the surrounding area were used to analyze the sequence stratigraphy of the basin. 8 seismic reflection interfaces were identified in the study area. Combined with seismic amplitude cyclic changes, 3 seismic super sequences and 8 seismic sequences were divided for the Cenozoic of the study area, and the top-bottom contact relationship, reflection characteristics, formation thickness, velocity and Ps of each seismic sequence were described in detail. 5 types of typical seismic facies were identified within the sequence framework:parallel and sub-parallel facies, wedge-shaped divergent facies, foreset facies, chaotic configuration and channel-filled facies. Based on the analysis of seismic facies and sedimentary facies, the sedimentary characteristics and sedimentary history of each sequence were analyzed, it suggested that since Eocene, the study area showed the characteristics of nonmarine facies lake basin sedimentary system. Oligocene period, the basin suffered transgression and accepted the littoral and shallow marine deposition. Early Miocene, bathyal deposits were developed widely in central of the basin. In the late middle Miocene, the sea level decline and slope-bathyal environment reduced, Xisha Trough turbidite channel brought its prototype. From late Miocene to Quaternary, it came into stable regional subsidence stage in the study area, and bathyal-abyssal mudstone were developed mainly. In addition, as a result of uplifted area in the north and south edges of the study area provided provenance, nearshore submerged aprons and delta and fan delta also developed inside the sequences. Controlled by the terrain elevation difference the slope fans and turbidite bodies formed in steep slope belt or fault lower plate.