Delta Deposition of Chang 4+5 of Danba Area in Ordos Basin

Chang4+5 reservoir of Yanchang Formatiori is the major exploration target zone in Danba Area of Ordos Basin.Based on core and log data and sandstone percentage map in 300 m well space,the depositional microfacies, sandstone distruibution and prolific feature are analyzed comprehendsively for charaterization of Chang 4 + 5 reservoirs of Yanchang Formation in.Chang4 + 5 reservoirs is subdivided into lower Chang4 + 5 and upper Chang4 + 5 according to the depositional cycles and shale marker.Lower Chang4 + 5 is considered to be front delta deposition.It consists of submarine distributary channel,sheet sand and interdistributary bay which alternates each other laterally and orientes in NNE direction.The sandbody is NW wide along lake shoreline as the result of rework of lake waves,reflection it has good continuity and the scale is larger.Upper Chang4 + 5 belongs to delta plain deposition which consists of distributary channel sandstone and overflooded mudstone.The sandbody is NE oriented too but with high planary length-width ratio in comparison to sandbodies of front delta.Indicate that sandbody distribution is controlled significantly by channel,it has good continuity along ancient flow direction of NE.Lower Chang4 + 5 Reservoir properties and oil bearing is better than Upper Chang4+5