Characteristic and Evaluation of Chang 3 Member Reservoir of the Yanchang Formation in the Fuxian Area, North Shaanxi

The oil-bearing Chang-3 member of the Yanchang Formation is one of the major oil producing formations in the Fuxian area,north Shaanxi. Chang-3 member belongs to an environment of delta frontal subfacies. The primary types of the reservoirs include distributary channel sandbodies,river mouth bar sandbodies. The secondary types are front-mat sand and distal bars. Statistics and analysis for 80 simples indicate that mineral maturity of Chang 3 reservoir is low,and textural maturity is medium-low. Reservoir diagensis types are relatively comlex and mainly consist of compaction, cementation and dissolution in the A period of the late stage. Diagenesis evolution can be divided into three stages:compaction and cementation to pore reduction,cementation,infilling and pore reduction and corrosion to pore enlargement. Petrophysical property analyses of 1747 simples from 18 wells show that the physical property of the Chang-3 member reservoir is of low porosity and very low permeable reservoir. Pore types are characterized by residual protozoal intergranular porosity and dissolution porosity. The reservoir is closely controlled by sedimentation and diagenesis. It is suggested that the Chang-3 reservoir can be divided into the high quality excellent,the intermediate quality and the low quality reservoirs. The physical property of the Chang 3² reservoir is bettern that Chang 3¹ and Chang 3³.