Carbonate reservoir has been characterized in the field "X" using Simultaneous Inversion to determine lithology and fluid content. The reservoir located in northern East Java Basin in the form of big reef carbonate on Tuban Formation. Carbonate Reservoir has unique characteristics than the others because its diagenesis process which may affect highly heterogeneities. Simultaneous Inversion is implemented in this study as it can generate acoustic impedance, shear impedance and rock density simultaneously. It give the availability of the fluid parameters for the reservoirs, thus, simultaneous inversion gives benefittoward theacoustic inversion which only generates acoustic impedance. We then extract Lame parameters from the 3 outputs that are Lambda Rho which is sensitive to fluid content and Mu Rho which is sensitive to lithology. Vp/Vs ratio can also be generated to sharply identify fluid content. Simultaneous inversion in field "X" is controlled by 2 wells, M01 and M02. In this study we use angle gather as seismic input. Based on analysis of Lambda Mu Rho and Vp/Vs ratio, the reservoir has potential hydrocarbon. The existence hydrocarbon is indicated from cross plot Vp/Vs vs Lambda Rho, where the value of Vp/Vs is about 1.7-1.8 and Lambda Rho is about 40-60 Gpa∗g/cc as well as the Mu rho value is about 40-80 Gpa∗ g/cc from the Mu Rho vs Density's cross plot and the depth is about 8400-8750 ft based on depth-plot from measured depth (calculated from KB). The slicing map shows that the lithology and hydrocarbon fluid tend to spread to North West of field "X".