Tight Sandstone Microfacies and Reservoir Heterogeneity of Chang 3 of Yanchang Formation in Weibei Area,Ordos Basin

Based on the comprehensive core analysis,its experiment and logging data,combined with regional geological background and sand-body prediction by seismic data,the microfacies characteristics and its effects on reservoir heterogeneity of Chang 3in Weibei area are discussed.The results show that the delta front underwater distributaries channel of shallow braided delta,characterized by multiphase superposition on vertical section and rapid changes horizontally,is the main sedimentary microfacies.The thickness of this channel is between 8-23.5m,and the channel thickness of each single phase has a big difference.The scale of these channels differs significantly in eastern and western part of research area,and varies quickly along the river.The superposition sand channel has strong heterogeneity on physical property and oil saturation,mainly influenced by the scale of single phase channel.In thicker single phase channel the physical property and oil saturation gets fairly good,while as thickness of channel decreasing,it is much worse due to calcite cementation and boulder-clay.The distribution of physical property and oil saturation is controlled by underwater distributary channel.With wider channel and larger scale sand body,the porosity of single phase channel will be better and the horizontal heterogeneity will be weaker.