The storage assessment unit (SAU) is the fundamental unit used in the National Assessment of Geologic Carbon Dioxide Storage Resources project for the assessment of geologic CO2 storage resources. The SAU is shown here as a geographic boundary interpreted, defined, and mapped by the geologist responsible for the assessment interval. Individual SAUs are defined on the basis of common geologic and hydrologic characteristics. The resource that is assessed is the mass of CO2 that can be stored in the technically accessible pore volume of a storage formation. The technically accessible storage resource is one that may be available using present-day geological and engineering knowledge and technology for CO2 injection into geologic formations and therefore is not a total in-place resource estimate. The SAU boundary is defined geologically as the limits of the geologic elements that define the SAU, such as limits of reservoir rock, geologic structures, depth, and seal lithologies. The only exceptions to this are SAUs that border an international, or Federal-State water boundary. In these cases, the international or Federal-State water boundary forms part of the SAU boundary. Drilling-density cell maps show the number of wells that have been drilled into the SAU. Each 1-square-mile cell has a count for the number of unique well boreholes drilled into the SAU. For a given sedimentary basin, the National Assessment of Geologic Carbon Dioxide Storage Resources project identifies SAUs containing the potential for storage and sequestration of carbon dioxide. Proprietary well header data from IHS ENERDEQ through 2010 were queried to determine which wells were drilled into specific SAUs. The coordinates of wells are proprietary and cannot be released; however, counts of the number of wells per square mile are presented in the well drilling density data layer.