The oil reservoirs are the result of a complex succession of geological, physical, chemical, and biological process acting during the geological past. Those processes have created a unique distribution of rocks with petrophysical properties that make up the current petroleum reservoirs. Although these processes may be modeled, we do not understand completely all the details and their interactions. Moreover we do not have the complete data set to provide a unique actual distribution of facies (rock types) and their petrophysical properties within the reservoir. Nevertheless, we can try to create numerical models that represent the result of all these processes and the characteristics of the reservoir rock. Therefore, we attempt to make numerical models consistent with the conceptual idea of the geological setting and also honor all the available information. Thus, reservoir modeling is a multidisciplinary task which involves diverse disciplines such as geology, geophysics, geostatistics and stochastic simulation.