Outer-shelf to shelf-margin delta and deep-water slope fan systems

the most important petroleum reservoirs in the Pearl River Mouth Basin(PRMB)

were widely developed in the Upper Oligocene of the northern South China Sea.Sequence architecture

sedimentary genetic types

as well as sedimentary evolution and its controlling factors of the shelf-margin deltas and associated slope fans in the Upper Oligocene(Zhuhai Formation) in the southern PRMB were studied based on the integrated analysis of seismic

core and well logging data.The Zhuhai Formation is a composite sequence which can be further divided into four sequences.The shoreline trajectories were mapped by trac-king the changes of shoreline breaks

and the depositional genetic units(transgression

slightly-ascending normal regression

apparently-ascending normal regression and locally-descending forced regression

etc.) were identified.The outer-shelf deltas are characterized by tangential reflections on seismic profiles with the thickness of foresets ranging from100 m to 150 m.In contrast

the shelf-margin deltaic clinoforms

with a thickness of 200-1 000 m

display as S-shaped or sigmoid-tangential seismic reflections

and generally consist of thick beds of delta front bar deposits and large-scale slump and deformed sediments

which are associated with prodelta-slope fan systems.In addition

six deltaic clinoforms were identified in the study area

and their areas tend to extend along their strikes.The composite sequence(CS3) is controlled by both relative sea level changes and tectonic subsidence.The massive sediment supply and regional sea level fall are dominant factors controlling the wide-spread distribution of the shelf-margin delta and prodelta slope fan systems.