Abstract:

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.

Key words: beach bar system, sedimentary characteristics, Yan 7+8 oil reservoir, Ordos Basin