Abstract: Under certain circumstances that poresity in reservoir is constant. Rt-φ crossplot is difficult to divide effcciive thickness in oil reservoir, but Rt-SP crossplot is better. This paper infers the piinciple of Rt—SP crossplot from Aremes forinula. When SP log is effected by circumstances.as quality is reduced. By means of correcting mud filirate resistivity under unified temperature. the correction formula is derived . and the method to correct SP log is proposed. As a example. Rt—SP crossplot is applied to the south of Hong Shan Zui oil field to divide the effective thicknesse. The result is good.