Abstract: Althrough the well known formula advocated by Park Jones in 1956 for investigating the oil in place confined within the drainage area around a producing well has long been widely used in both domestic and foreign oil fields,its derivation and validity are often subject to speculation.In this paper a general form of Jones' formula has been derived by introducing in a shape factor C_A.As a result,it shows that the original formula presented by Jones is but a special case in which the drainage area is assumed to be a circle with the well at the centre.If the well studied is of irregular shape and/or located arbitrarily,an appropriate shape factor selected from the works of Dietz must be taken into account.Otherwise an appreciable error may result.It also emphysizes the necessity of producing the well at a constant rate long enough before shutting in for testing in order to reach a pseudo-steady state.The minimum time required may be determined by using the dimensionless time factor which is also related to the shape of the drainage area as well as its relative position in the area.