The model layer facies grids describe geographic variations in lithology mixtures (facies) used in the 3D petroleum systems model of the Williston Basin. The grids provide constraints on the physical properties used in the model simulation, including mechanical compaction, thermal conductivity, heat capacity, radiogenic heat, porosity and permeability trends, diagenesis, and capillary entry pressures. The grids were derived from borehole geophysical log signatures from two key model layers: the middle member of the Devonian-Mississippian Bakken Formation and the Upper Ordovician Icebox Formation. All the other model layers used a single lithology mixture and were not represented by gridded facies maps. This is a child item of a larger data release titled "Data release for the 3D petroleum systems model of the Williston Basin, USA".