- The characteristics of a rock unit that reflect its origin and permit its differentiation from other rock units around it. Facies usually are characterized using all the geological characteristics known for that rock unit. In reservoir characterization and reservoir simulation, the facies properties that are most important are the petrophysical characteristics that control the fluid behavior in the facies. Electrofacies and other multivariate techniques are often used to determine these characteristics. Rock types rather than facies are more likely to be used in reservoir simulation.