Abstract: The present paper is designed to improve our understanding of the heterogeneity of oil reservoirs and provide the basis for improving the precision of the reservoir description,with emphasis on the division and controls on the flow units in the Chang-3₃ oil reservoirs in the Huachi Oil Field,Shaanxi.Three types of flow units may be recognized for the studied oil reservoirs, which are macroscopically controlled by sedimentary microfacies,and microscopically by diagenesis and reservoir spaces.