It is known that densities in formations are usually assumed to be constant for gravity model calculations. This also implies that formations are homogeneous and isotropic. However, the formations are usually heterogeneous and densities vary depending on heterogeneity. For this reason, densities should be taken into account as variables. Some scientists consider densities as variables in each formation in model calculations. In other words, density is defined as a function of the required parameters. In fact, functional change is regular. However, density is an irregular variable that depends on the change boundaries of seismic velocity. In this study, it is aimed to take density into account as a variable by using detected seismic velocity boundaries at which seismic velocity changes for each formation. In addition to main formations in model geometry in 3D inversion calculations, another formation was defined. This additional formation has been described by using a combination of all of the change boundaries of seismic velocity present in each formation in a specific order. The density calculated for the additional formation estimated the variation of density between the change boundaries of seismic velocity. This variation is added to the mass densities that are calculated for the description number of each zone. So, lower-density layer comprising oil can be determined by this method. The reliability of the results of the method depends on the reliability of seismic velocity boundaries. Moreover, the increasing number of seismic velocity boundaries leads to the increasing resolution of density variations.