The Characteristics of Natural Fractures in Tight Sandstone Reservoirs of Chang-7 Member in Ningxian-Heshui Area,Ordos Basin

Based on the data of outcrops,cores and imaging logs,structural fractures and diagenetic fractures develop in the tight sandstone reservoirs of Chang-7 member of Yanchang formation in Ningxian-Heshui area in Ordos basin,of which the high-angle shear fractures are dominant,with four sets of orientations like EW,SN,NW-SE and NE-SW,dominated by NE-SW and EW fractures.The big variation of fractures is affected by the lithology,bedding thickness,sedimentary micro-facies and rock heterogeneity,among which the rock heterogeneity is the main factor affecting such a reservoir's fracture development degree.Also,the development of natural fractures of Chang-7 member is subjected to the effect of current stress field.The preferential open NE-SW fracture system will be the main seepage channels.The high rock brittleness index is favorable for volumetric fracturing,forming complex fracture network system in it.