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 were then coded to indicate whether the wells included within the cell are predominantly oil-producing, gas-producing, are both oil- and gas-producing , or are dry or the type of production 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 were current as of 2005 when the cell maps were created in 2007.