Formation evaluation is one of the essential elements in hydrocarbon exploration. Rock physics is one of the key components in exploration, development, and hydrocarbon production in which it provides correlation between geologic reservoir parameters and seismic properties. The aim of this study is to analyze how clay mineral content can affect the Amplitude Versus Offset (AVO) seismic response on sandstone reservoir based on the Gassmann equation in rock physics modeling. In this study, rock physics modeling was conducted from two wells data (X-1 zone C and X-2 zone Q) in Balikpapan Formation, Kutai Basin, with a mixture of clay and quartz using Gassmann equation. The rock physics modeling process consisted of modeling the minerals, the fluids, as well as the rock frame. Those three were assembled with Gassmann equation to calculate Velocity-P (Vp). Velocity-S (Vs), and Density Bulk (ρb). Acoustic impedance (AI) and shear impedance (SI) were calculated to get R (θ) values from Zoeppritz approximation. With the seismic forward modeling, the synthetic seismogram was produced by using R (θ) values from modeling to know the AVO seismic response. In conclusion, our experiment showed that zone C in well X-1, with Illite as clay content, has higher modeling values compared to real data. While zone Q in well X-2, with Smectite as clay content, has lower modeling values compared to real data. Thus, the R (θ) values from rock physics modeling in zone Q gave more negative impedance contrast which affect in clearer bright spot on AVO seismic response, compared to real data. While in zone C, the R (θ) values from rock physics modeling did not gave insignificant change in impedance contrast so that the AVO seismic response were not affected.