Cell maps for each oil and gas assessment unit were created by the USGS to illustrate the degree of exploration, type of production, and distribution of production in an assessment unit or province. Each cell represents a quarter-mile square of the land surface, and the cells are coded to represent whether the wells included within the cell are predominantly oil-producing, gas-producing, both oil and gas-producing, dry, or the type of production of the wells located within the cell is unknown. The well information was initially retrieved from the IHS Energy Group, PI/Dwights PLUS Well Data on CD-ROM, which is a proprietary, commercial database containing information for most oil and gas wells in the U.S. Cells were developed as a graphic solution to overcome the problem of displaying proprietary PI/Dwights PLUS Well Data. No proprietary data are displayed or included in the cell maps. The data from PI/Dwights PLUS Well Data are current as of 2003.

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".

Formation tops were picked in 16,200 wells from the Williston Basin of North Dakota, South Dakota, and Montana. Each formation top was picked using borehole geophysical log data. Many wells in the Williston Basin do not penetrate the entire stratigraphic section and in many other cases the shallow intervals were not logged. In total, there are 141,096 individual picks composing 29 sets of formation tops. Each top set represents a recognizable stratigraphic marker that can be correlated from well to well. The formation tops serve as foundational observations for building the stratigraphic and structural framework of the 3D petroleum systems model. How each formation top assigns to age, lithostratigraphic unit, and modeled horizon is outlined in the "Williston_Basin_Data_Release_Overview.csv" data table in the parent directory of this data release. This is a child item of a larger data release titled "Data release for the 3D petroleum systems model of the Williston Basin, USA".

The Total Petroleum System is used in the National Assessment Project and incorporates the Assessment Unit, which is the fundamental geologic unit used for the assessment of undiscovered oil and gas resources. The Total Petroleum System is shown here as a geographic boundary defined and mapped by the geologist responsible for the province and incorporates not only the set of known or postulated oil and (or) gas accumulations, but also the geologic interpretation of the essential elements and processes within the petroleum system that relate to source, generation, migration, accumulation, and trapping of the discovered and undiscovered petroleum resource(s).

The Total Petroleum System is used in the National Assessment Project and incorporates the Assessment Unit, which is the fundamental geologic unit used for the assessment of undiscovered oil and gas resources. The Total Petroleum System is shown here as a geographic boundary defined and mapped by the geologist responsible for the province and incorporates not only the set of known or postulated oil and (or) gas accumulations, but also the geologic interpretation of the essential elements and processes within the petroleum system that relate to source, generation, migration, accumulation, and trapping of the discovered and undiscovered petroleum resource(s).