Abstract: The analysis of the petrophysics characterization of clastic reservoir shows that the variation of P-wave velocity was controlled by the sedimentation and diagenesis, and therefor leads to the regular variation of acoustic impedance. The sedimentation was mainly represented by the grain size, sorting and shale content, and the diagenesis mainly includes the compaction,pressolution and cementation. Using the method of classic single impedance threshold only resolves the vertical variation of acoustic impedance. As a result, the paper proposes a more effective method of impedance threshold volume. Through studying example, using the new method can explain the acoustic impedance accurately, which has played good effect in solving the uncertainty of the sand explanation because the areal variation of acoustic impedance caused by the deposition.

Key words: petrophysics, sedimentation, diagenesis, acoustic impedance, threshold volume, lithology interpretation