Unconfined Flow Deposits in Front Sandbodies of Shallow Water Deltaic Distributary Systems: Examples from the Yellow River Mouth Sag, Offshore Bohai Sea, China

Zhang, Xintao; Tian, Lixin; Zhou, Xinhuai; Niu, Chengmin

Abstract: Bohai Basin is characterized by stable tectonics with steady and slow subsidence,which formed in low-relief,low gradient, and large-scale shallow water depositional setting during the Neogene.in this study we document the architecture,contour, sandbody overlapping and sedimentary environment of 260 individual sandbodies deposited in shllow water delta, and two types of shallow water delta were recognized whose sandbody is dominated by distributary channel in the first one and distributary bar in the other one.

Distributary channel of shallow water delta is characterized by amalgamated coarse, sandy channel deposits with poor preservation of overbank facies. Channel ranges are generally smaller in the middlel areas, but their sizes are variable. The deposits include distributary, meandering and multiple channels which show different degrees of lateral migration. Framework sandbodies are distributary channels and levees which have a width 3 to 4 times of the channels. Distributary channels are of the most distal channelized features and can be both subaerial and subaqueous.Such a process was considered as the dominant depositional mechanism during lowstands lake. Distributary bar of shallow water delta is characterized by poor interconnection of sandbodies in the distal areas because the channel-fill bodies are sparse, small and not deeply incised. Along with the lake level rising, the channel sandbodies were reworked into lobate and pool-shaped sandbodies with a vertically coarsening upward trend. Framework sandbodies are sand bars which may have evolved to sand bank or swamp later. The depositional process is similar to modern "floodouts"of the Dongting Lake and Poyang Lake in central China. Deltaic processes are thought to have the primary depositional mechanism in the distal sectors of the shallow water delta during highstands period of lake level.

Recently many oilfields of Neogene with nearly 4.0 hundred million cubic meter oil have been found in shallow water delta reservoir. Particularly, distributary bar sandbodies should be the focus targets in exploration for adding new reserves and increasing productions in this region.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013