Based on the research of the application of four-order units(i.e.oil/gas field,block,development unit,and calculation unit) in reserve estimate,basic principles are proposed for establishing the corresponding relationship of the four-order unit.Upon this relationship,the paper analyses in detail the classification of calculation units,and suggests that in order to maintain the accuracy of reserve calculation,the oil-gas-water systems shall be incorporated or subdivided vertically and horizontally.Vertically,the main concerns shall be the lithologic association,fluid property,oil/gas reservoir type,reservoir distribution,etc.;while horizontally,factors such as oil/gas reservoir scale,complication,and configuration of oil,gas,and water layers,shall be taken into consideration.