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Comparison of Carbonate Reservoirs Within the United States for CO2 Sequestration


The U.S. Geological Survey (USGS) has been conducting a nationwide assessment of technically accessible storage resources for carbon dioxide (CO2) geosequestration in the onshore and State water areas of the U.S. as mandated by the 2007 Energy Independence and Security Act (Public Law 110–140). This assessment identifies 202 individual storage assessment units (SAUs) found within 36 sedimentary basins, based upon geologic and hydrologic characteristics. Carbonate reservoirs make up many of these SAUs, and are located throughout the U.S. in varying sizes and lithologic composition. In many cases, these carbonate reservoirs are considered strong candidates for CO2 storage by the scientific and municipal communities due to their geographic location, depth from surface, unique porosity and permeability, salinity, and seal. But differences in carbonate reservoirs can be significant, and the properties of these rocks can vary greatly, even within a single formation, due to differences in depositional environment, sedimentation, and diagenetic processes and subsequent rock alterations. Four carbonate reservoirs evaluated in the assessment are compared here, and include the: Salina Group & Middle Silurian Composite SAU of the Michigan Basin; Lower Paleozoic Composite SAU of the Permian Basin; Lower Ellesmerian SAU of the Alaskan North Slope; and Sunniland Formation SAU of the South Florida Basin. This comparison looks at the depositional environment, lithology, type and numerical value of porosity and permeability, reservoir size, and seal for each SAU, in order to compare, contrast, and evaluate the suitability of carbonate reservoirs for CO2 sequestration. Results show that significant differences can exist between carbonate reservoirs, and the size of the reservoir is not the driving component to having a large CO2 storage resource. Rather, the type, value, and distribution of the porosity and permeability within the reservoir, as well as the associated net-porous interval thickness, has a major impact on the overall storage capacity and suitability of the reservoir for CO2 storage. In addition, the presence of interbedded low-permeability layers, such as evaporites, can also affect the suitability and size of the storage resource. Such factors are governed by depositional environment, resulting lithology, and subsequent diagenetic alterations resulting from changes in climate, sea level, and/or basin structure.