1. A method for building a predictive or forward model adapted for predicting a future evolution of a reservoir, comprising: receiving seismic measurements, electromagnetic (EM) measurements, gravity measurements, and reservoir pressure measurements; generating a first result by inverting the seismic measurements, wherein the first result comprises an artifact; generating a second result by inverting the EM measurements constrained by the first result; generating a refined result by constraining the first result by the second result to reduce the artifact; generating, by a fluid flow simulator, a fluid flow model based on the reservoir pressure measurements; generating a pressure, water saturation, and conductivity spatial maps by constraining an inversion of the EM measurements using the fluid flow model from the fluid flow simulator coupled to an EM simulator by Archie's saturation equation; obtaining a first density of the reservoir from the gravity measurements; obtaining a second density of the reservoir from the seismic measurements; estimating average rock compressibility in the reservoir by combining the first density and the second density; generating a map of fluid contacts in the reservoir by integrating the EM measurements and the gravity measurements, wherein the EM measurements are sensitive to water/oil contacts, and wherein the gravity measurements are sensitive to gas/oil contacts; generating, by a processor, a reservoir model and associated parameters based upon the refined result, the pressure, water saturation, conductivity special maps, the average rock compressibility, and the map of fluid contacts; and receiving, by a reservoir simulator, the reservoir model and, responsive thereto, generating the predictive or forward model.