The Numerical Simulation And Analysis Of Main Controlling Factors On Paleo-Pearl River Delta Sedimentary Processes

A significant sea level falling lead to the elongation of paleo-Pearl River and the development of a widespread shelf-edge delta complexes at 21Ma in the Baiyun Sag within the Pearl River Mouth basin,South China Sea.The hydrocarbon exploration has proved that the delta complexes not only controlled the formation of deep submarine fans,but also formed sandbodies in distributary channels and river mouth bars of delta,which are favorable reservoirs for oil and gas.However,limited by the poor resolution of seismic data and sparse wells,the 3D geometry and connectivity of sandbodies are difficult to depict and describe precisely.Besides,it is still not clear about how sediment grain size and external hydrodynamic regimes(e.g.wave and tide)influence the morphology,distributary networks,stratigraphy and sandbody distribution of the delta complexes,which hasn't been investigated systematically and analyzed quantitatively.Thus,based on sequence stratigraphy,this study divided the whole evolution of delta complex into two stages,including shelf delta in highstand system tract and shelf-edge delta in lowstand system tract.Then,we simulated the sedimentary processes of the deltas in two stages by Delft3D and made a comparative analysis on the sedimentary characteristics between the two different types of deltas with the consideration of the influences from sediment grain size and external hydrodynamic conditions..This study will be helpful for well undertanding the evolution of paleo-delta and providing some practical information for the furture petroleum exploration.The main results of this study are shown as follows:Wave impinges and erodes the delta front,and then distributes the sediments to form sandbars in the front and flanks of delta.The mud is suspended and transported to deep sea by waves,resulting in smooth delta front and inhibiting the formation of river mouth bar and the bifurcation of channels.Tidal currents intrude into delta and form rugose delta front,which is dissected by tidal channels and characterized by straight river channels.Compared with shelf delta,shelf-edge delta bears stronger wave effect and has a confined tide-influenced area due to steeper topography.On the condition that wave and tide coexist,the covering area of delta will be reducd and the delta planform become more elongated with decreasing sediment grain size.Besides,the increasing mud content of the supplied sediment would make the sediment to be more cohesive,which inhibits the bifurcation of channels and the formation of river mouth bar,resulting the channels and levees to be more stable and thus decreasing the number and sinuosity of channels.The sandbodies of the river delta,with poor continuities,mainly concentrate in the channels and river mouth bars.The sand content of the whole delta is about 20%-60%,and the net-channel lithologic traps are much more well developed.Tide-influenced delta has similar sand content with river delta,but tidal currents improve the connectivity and continuity of sandbodies,corresponding to the flanging-channel lithologic trap.Wave-influenced delta has the highest average sand content of 40%-80%,and the sand content in delta front can exceed 90%,corresponding to the sheet-sand lithologic trap.In general,the sandbodies of shelf-edge delta is much more elongated than those in shelf delta.In summary,highstand shelf delta and lowstand shelf-edge delta have different morphologies,stratigraphy structures and sandbodies distribution under different hydrodynamic conditions and sediment grain sizes.Distinct exploration strategies should be made for different types of paleo-Pearl River delta,which were formed at sealevel highstand and lowstand,respectively.