Keywords: 
• reservoir /
• petrophysical parameters /
• lower limit /
• distribution function curve method /
• Zhongbu Gas Field

Abstract: It's important to exactly determine the lower limit of petrophysical parameters for reservoir in calculation of reserves.Laboratory test is commonly used to get the lower limit values.However,the test is complicated and takes a long time.As for intensively anistropic reservoir,a large amount of data should be obtained which means much more time and costs.Therefore an easy,fast and economic method is necessary to be provided instead of traditional test.Distribution function is one of the most important mathematic characteristics of geological body.Distribution curve is often used to analyse the distribution regularity of the studied object.The distribution function curve method presented in this paper is a new statistic method used to determine the lower limit of petrophysical parameters for reservoir.Based on making full use of data from well logging interpretation available in oil-gas field,the disribution curves for effective and non-effective gas-bearing reservoirs are drawn in the same coordinate system respectively.Theoretically,if there exists a point of intersection for the two curves,the corresponding frequency must be zero.Actually,there is a superimposed region because of the mixed effects for the effective and non-effective gas-bearing reservoir.Under the condition of no seperation of mixed effects,the value corresponding to the intersection point for the two curves is taken as the lower limit.This method is applied to the determination of lower limit for the gas-bearing resevoir of Zhongbu Gas Field in Ordos Basin.The results coincides well with the values obtained by methods of accumulative production capacity,experience statistics and laboratory test,which demonstrates that this method is successful and valuable to be extended.