Favorable Stratigraphic Conditions for Carbon Sequestration Exist in the Rocky Mountain Basins

Ronald Drake II and Sean Brennan

The United States Geological Survey (USGS) was directed by the 2007 Energy Independence and Security Act (Public Law 110-140) to assess the potential geologic storage resources for carbon dioxide (CO2]) within the United States. The USGS, following its published, wholly probabilistic methodology for this national assessment, began these assessments in the Rocky Mountain States in the Fall of 2010. 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 by geologic criteria, primarily on the basis of depth, rock properties, and regional extents of the storage and seal formations. The methodology requires that the storage formation be between 3,000 and 13,000 ft below ground surface. This minimum required depth ensures that CO2 will be supercritical. Within the Rocky Mountain basins, a significant portion of the Mesozoic section (such as the Dakota Sandstone), fits within this interval. However, when rock properties indicate that CO2 could be stored at depths greater than 13,000 feet, a separate deep SAU is assessed. The areal extent of the storage formation and overlying seal are required to be continuous and regional in extent. Most Rocky Mountain basins contain thick laterally extensive marine shales like the Pierre Shale or Mowry Shale that would inhibit flow to superjacent strata. In some cases, the stratigraphy includes the potential for multiple or stacked seals. During the assessment, the seal is evaluated for leakage potential and a minimum seal thickness is defined. This minimum seal thickness must exist over the extent of the storage formation and no portion of the SAU may exist where the seal is too thin or nonexistent. A final consideration is the salinity of the water within the pore space of the storage formation; based on available salinity data and geologic models, estimates are made as to how much of the SAU contains water that has less than 10,000 parts per million total dissolved solids. Much of the stratigraphic section in the Rocky Mountain area is composed of porous and permeable sedimentary rock, though much of the rock contains low salinity waters. The USGS Carbon Sequestration assessments have shown that the stratigraphy of the Rocky Mountain basins is conducive to potential CO2 sequestration, with some restrictions.

AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012