The separation process of lithology and fluid reservoirs is an important part of characterizing the hydrocarbon reservoir; in this case the reservoir also acts as the source rock. Reservoir characterization involved identification of both lithology and fluid content of reservoir. As field "X" reservoir has no clean formation, using acoustic impedance parameters it is hard to identify clearly. It still has a high ambiguity. In impure reservoir formation, crossplots of Acoustic Impedance (AI) and Shear Impedance (SI) could not separate the lithology as well as fluid content clearly. Poisson Impedance (PI) has been implemented to solve the problem. The AI - SI crossplot is rotated to get the lithology-fluid trend which is associated to an equation of PI (c) = AI - c SI. The accuracy of the PI value depends on the value of c (optimization factor of rotation). Therefore, the c value was calculated through the TCCA (Target Correlation Coefficient Analysis) method. Analysis of sensitivity c parameter was performed in the well of field "X." The parameters of Zp, Zs and density are obtained from simultaneous inversion. PI models able to show clearly the separation of rock lithology as well as gas content of reservoir. Lithology impedance (LI) as a result of the PI-GR correlation is able to separate sand and shale very well. Also, from the PI-SW correlation the hydrocarbon content is very well separately identified. The hydrocarbon zone was shown to be 3150-3260 m depth.