THE GEOLOGIC SEQUESTRATION POTENTIAL FOR CARBON DIOXIDE WITHIN THE DENVER BASIN OF COLORADO, WYOMING, AND NEBRASKA

The United States Geological Survey (USGS) was directed by the 2007 Energy Independence and Security Act to assess the potential geologic storage resources for carbon dioxide (CO₂) within the United States. Utilizing a probabilistic methodology for a national CO₂ sequestration assessment, the USGS has determined that several Cretaceous formations within the Denver Basin of Colorado, Wyoming, and Nebraska contain geologic storage potential.

The basic unit of assessment for the USGS methodology is the storage assessment unit (SAU), which consists of a storage formation and an overlying regional seal formation. The SAUs are defined using geologic criteria that include rock properties, formation depth, and regional extents of the storage and seal formations. The methodology requires a storage formation depth range of 3,000 to 13,000 ft below ground surface. This minimum required depth ensures that CO₂ will be in a supercritical phase. Within the Denver Basin, a significant portion of the Cretaceous section fits within this depth interval.

The storage formation and overlying seal are required to be continuous and regional in areal extent. The Denver Basin contains thick, laterally extensive, marine shales (Pierre, Carlile, Graneros, Mowry, and Skull Creek Shales) which could inhibit CO₂ flow to overlying strata. During this assessment, the seals were evaluated for leakage potential and a minimum seal thickness of about 50 to 75 ft was assigned. This minimum seal thickness must exist over the extent of the storage formation in the SAU. A final consideration is the salinity of the water within the pore space of the storage formation. Based on available salinity data, estimates were made as to how much of the SAU contains water that has less than 10,000 parts per million total dissolved solids.

Five SAUs were identified within the Denver Basin that have porous and permeable sedimentary units, the required competent overlying regional seals, and meet the other requirements of the assessment methodology. Most of these SAU reservoirs are or have been targets of hydrocarbon exploration and production. The USGS carbon sequestration assessment of the Denver Basin has shown that there is potential to store in the subsurface, approximately 3.7 metric gigatons (Gt) of CO₂ in technically accessible Cretaceous formations.