Seismic responses and forming conditions of the Oligocene shelf-edge delta and deepwater fan system in Changchang-Heshan sag,northern South China Sea

Changchang-Heshan sag is a deepwater sedimentary basin and located on the continental slope in the northern South China Sea.It is an important strategic exploration block after oil and gas explo-ration breakthrough in Baiyun and Liwan sags in adjacent deep-water regions.Guided by the theory of classic sequence stratigraphy and based on the characteristics of the seismic reflection and stratigraphic cycles,the Oligocene Zhuhai Formation could be divided into six third-order sequences(namely ZHSQ1,ZHSQ2,ZHSQ3,ZHSQ4,ZHSQ5 and ZHSQ6 from bottom to top).Six types of seismic facies,including high continuity,weak-medium amplitude sheet seismic facies of shelf,S-shaped and oblique progradational seismic facies of shelf-edge,variable amplitude,medium continuity,mound-shaped bidirectional overlap seismic facies of continental slope and so on,are identified within the Zhuhai Formation.Sshaped and oblique progradational seismic facies represents shelf-edge delta,while mound-shaped bidirectional overlap seismic facies rep