A method can include accessing a model of a subterranean region where the model includes a fault that corresponds to a fault in the subterranean region; via a first side of the fault, outwardly radiating therefrom cells of the model with a first type of radiation and, via a second, opposing side of the fault, outwardly radiating therefrom cells of the model with a second type of radiation; and, based at least in part on the outwardly radiating of the first type of radiation and the outwardly radiating of the second type of radiation, assigning a portion of the cells of the model a sidedness property value with respect to the fault that indicates that each cell in the portion of the cells is assigned to the first side of the fault or assigned to the second side of the fault.