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.