Sedimentary characteristics of the delta sandstones from the Chang-9 oil reservoirs in the Yanchang Formation,Ordos Basin

The integration of well logs and core examination show that the braided delta and meandering delta are the major delta facies types in the Chang-9 oil reservoirs of the Upper Triassic Yanchang Formation,Ordos Basin. The braided delta sandstones in the western part of the Basin are built up of braided distributary channel and braided subaqueous distributary channel sandstones,and characterized by developed scour surface,trough cross-bedding,tabular cross-bedding and parallel bedding,fining-upward sequences,and two-step and/or three-step grain-size probability curves. The meandering delta sandstones in the eastern part of the Basin assembled by subaerial distributary channel,subaqueous distributary channel and channel-mouth bar sandstones display the sedimentary structures such as scour surface,trough cross-bedding,tabular cross-bedding,parallel bedding and ripple lamination,fining-upward (occassionally coarsening-upward)sequences,and two-step and/or three-step (occasionally four-step) garin-size probability curves. The braided distributary channel,braided subaqueous distributary channel and subaerial distributary channel sandstones are best developed,followed by subaqueous distributary channel and channel-mouth bar sandstones in the Chang-9 oil reservoirs of the Yanchang Formation in the Ordos Basin.