The Department of Energy (DOE), including its National Energy Technology Laboratory and West Coast Regional Carbon Sequestration Partnership (WESTCARB), have established national programs to evaluate the technical feasibility of long-term subsurface geologic storage of carbon dioxide (CO2) produced by industrial activity. WESTCARB is a consortium of seven western U.S. States and one Canadian Province that is one of seven regional North American partnerships established to evaluate technical aspects of high-volume CO2 capture and sequestration. Collaborative WESTCARB research programs have included more than 90 public agencies, private companies, and non-profit organizations. The Arizona Geological Survey (AZGS) began work in 2010 on WESTCARB Phase III – Arizona Geological Characterization. As part of WESTCARB Phase III, the AZGS is evaluating the potential for CO2 sequestration in geologic formations that are below a level of 800 meters (m) depth below land surface (bls). Arizona geological characterization is directed at porous and permeable geologic formations with overlying impermeable sealing strata in Cenozoic sedimentary basins in the Basin and Range physiographic and tectonic province, and Paleozoic sedimentary formations in the Colorado Plateau. An initial screening of Cenozoic sedimentary basins with significant depth and volume below the 800 m bls level resulted in ten candidate basins from a total of 88 basins (Spencer, 2011). This report represents a WESTCARB assessment of CO2 storage potential in the Willcox basin, which is one of ten Cenozoic basins in Arizona identified for future study during the preliminary evaluation (Spencer, 2011), and is part of Tasks 2 and 3 of Arizona WESTCARB Phase III (California Energy Commission Agreement Number 500-10-024). Task 2 consists primarily of characterizing basin structure, stratigraphy, lithology, and the nature of seals or a cap rock. This task also includes determining the storage capacity of permeable sediments below 800 m depth. Task 3 is to determine if, and at what depth, saline groundwater exceeds 10,000 milligrams per liter (mg/L) of total dissolved solids (TDS), criteria necessary for injection and storage of CO2. Carbon dioxide storage capacity can be estimated based on the extent of permeable strata underlying impermeable strata that is saturated with groundwater containing a TDS concentration above 10,000 mg/L.