Sedimentary characteristics of the braided deltas in the eighth member of the Lower Shihezi Formation in the northeastern part of the Ordos Basin

The eighth member of the Middle Permian Lower Shihezi Formation is considered as the main gas-producing horizons in the northeastern part of the Ordos Basin,where the reservoir sandstones are mainly controlled by sedimentary facies.The examination of field outcrops and well logs on the basis of sedimentary characteristics,depositional mechanisms,sedimentary facies and sandstone distribution has disclosed that there widely occur the braided deltas,especially the braided-shallow water lacustrine deltas in the study area.The reservoir sandstones in the delta plain facies are interpreted as distributary channel sandstones,while those in the delta front facies are considered as the subaqueous distributary channel sandstones.The channel-mouth bar,sheet sandstone and distal bar sedimentary microfacies are less developed.