System and methods for defining non-linear petrofacies for a reservoir simulation model are provided. A cross-plot visualization of selected petrophysical properties from a three-dimensional (3D) geocellular grid array of petrophysical properties representing a reservoir rock formation is presented to a user via a display of a computing device. Upon receiving user input for defining a non-linear petrofacies region of the 3D geocellular grid within the presented cross-plot visualization, data points in the cross-plot visualization that are within the boundaries of the petrofacies region are identified. The identified data points are associated with the petrofacies region. Hydraulic rock properties are assigned to one or more cells of the 3D geocellular grid based on the data points associated with the petrofacies region.