Retrograding sedimentary sequence and shore-shallow lacustrine beach bar system are developed in the Mesozoic Jurassic Yan'an Formation in southern Ordos Basin

and the latter has close paragenetic relationship with the meandering river delta around the lake basin of the Yan'an stage.Retrogradation and the destruction of the meandering delta provide clastic input for the development of the low-shallow lake beach bar system.During the depostion of Yan 7+8

An'sai and Zhidan areas in the north of Shaaxi province were in shore-shallow lacustrine environment.Beach bar deposits were widely developed as the major part of sedimentation

and can be divided into four microfacies types including beach sand

sand bar

sand sheet and mudstone.Generally

the beach bar sand bodies occur at the side of the river mouth and the flat open shore-shallow lake environment

and are parallel with the lake shoreline.They are dominated by medium and fine sandstone

and have massive bedding

wavy bedding

inclined wavy bedding and lenticular bedding

etc.Beach sand is usually in sheet-like shape and is relatively thin.Bar sand

which is the most representative deposit of the beach bar system

usually has banded shoreline-parallel distribution

large thickness

and lenticular shape on section

reverse grading at the bottom and transgressive sequences in the middle and upper parts.A series of beach bar sandbodies superimposed with each other in different phases

forming large-scale shoreline-parallel clustered reservoirs with high net-to-gross ratio.The mudstones of shore-shallow lake facies between the beach bars act as lateral barriers or overlying seals of the beach bar sandbodies.The sedimentary characteristics of the shore-shallow lacustrine beach bar system have significant influences on reservoir development and oil/gas accumulation of the Yan'an Formation.