Preliminary evaluation of Cenozoic basins in Arizona for CO2 sequestration potential
Jon E. Spencer, Senior Geologist, Arizona Geological Survey
The U.S. Department of Energy (DOE) established a national program to evaluate the feasibility of separating carbon dioxide (CO2) from industrial sources and pumping it underground for long-term storage or disposal. This was done in response to concerns that CO2 emissions from fossil-fuel combustion, and from other industrial processes such as cement production from limestone, are increasing solar energy absorption and thus increasing atmospheric temperatures. Underground disposal of CO2 is known as “sequestration.” It is uncertain sequestered CO2 will react with geologic materials and so be effectively disposed, or will remain in a gaseous or liquid state and so may be recovered after a period of storage. A major aspect of the DOE program is to evaluate subsurface geology to determine the potential of underground rock formations for long-term CO2 sequestration.
The West Coast Regional Carbon Sequestration Partnership (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. This report represents an overview of the AZGS initial assessment of CO2 storage potential in Arizona’s Cenozoic basins.
Initial WESTCARB studies at the Arizona Geological Survey are directed at determining Cenozoic basin volume and volume below 800m depth, with the purpose of reducing the number of basins subjected to further evaluation for carbon-sequestration potential. Basin volume below 800m depth is important because CO2 will remain in a liquid state at pressures corresponding to depths greater than 800m. Successful sequestration requires both adequate permeability and porosity for large-volume CO2 injection, and an impermeable cap rock that will prevent movement of CO2 to shallower depth. Basin stratigraphy and sediment characteristics will be evaluated by further study of the largest and deepest basins.
Basin and Range Physiographic and Tectonic Province
Southern and western Arizona is part of the Basin and Range tectonic and physiographic province, which is characterized by numerous small mountain ranges and intervening basins. This region extends southward to central Mexico and northward through southeastern California, Nevada, western Utah, and southern Idaho (Dickinson, 2002). The numerous basins and ranges are commonly separated by normal faults, although in almost all of Arizona, the faults are inactive and largely or entirely buried (Spencer and Reynolds, 1989). These late Cenozoic basins are filled with generally poorly lithified, porous and permeable sand and gravel, but also include silt and clay, limestone, marl, basalt, and the evaporites halite, gypsum, anhydrite. While the permeable sediments are potential targets for CO2 sequestration, the evaporites and lacustrine limestone and marl are potential cap rocks.
Depth to bedrock
Figure 1. Cenozoic sedimentary basins in Arizona (dark gray) and depth-to-bedrock contours with 800m contour interval. Very shallow basins and sediment veneers were excluded from basins.
Gravity surveys are effective in determining approximate depth to bedrock in the Basin and Range province because the upper Cenozoic basin-filling sediments are less dense than the bedrock that makes up underlying basin floors and adjacent ranges. Gravity surveys done largely by University of Arizona faculty and students more than 30 years ago, with a total of ~22,000 measurement stations, were utilized to make a depth-to-bedrock map (Oppenheimer and Sumner, 1980, 1981). Depth-to-bedrock contours were modified by incorporating information derived from 321 deep drill holes, few of which penetrated to depths greater than 500 m. The greatest uncertainties in basin contours are for deep basins because small variations in density estimates for basin-fill or underlying bedrock can have an especially large influence on calculated depth of deep basins. As a consequence, depths greater than approximately 1000m are especially uncertain.
The basin-depth contours on the depth-to-bedrock map of Oppenheimer and Sumner (1980, 1981) were digitized by the U.S. Geological Survey and incorporated into a geodatabase for a new depth-to-bedrock map by the Arizona Geological Survey (Richard et al., 2007). These contours were modified to incorporate additional drill-hole data and gravity modeling studies (see references in Richard et al., 2007) as well as improved representation of basin margins on the updated geologic map of Arizona (Richard et al., 2000). Numerous sources of uncertainty include the unmodeled gravitational signature of volcanic rocks interbedded or overlying basin sediments, complex surface topography in incised basin fill not incorporated into gravity models or bedrock depth estimates, and variable bedrock density that produces overestimates of basin depth where bedrock density is low and underestimates basin depth where bedrock density is high (see discussion in Richard et al., 2007).
To a first-order approximation, basin-fill sediments in Arizona are porous and permeable while bedrock is not. Hence, voluminous groundwater is derived from basin-fill sediments but little is derived from bedrock. Groundwater quality generally declines with depth in Arizona’s Cenozoic basins, with higher total dissolved solids at greater depth. Thus, the deeper parts of Cenozoic basins are potential targets for CO2 sequestration, both because they are porous and permeable and because contained groundwater is typically too saline for human consumption or agriculture. As noted above, effective CO2 sequestration must be done at depths greater than 800m because associated pressures at such depths will keep CO2 in a high density, essentially liquid state, which results in a much more efficient use of pore space.
Figure 2. Calculated volume of Cenozoic sedimentary basins in Arizona.
Figure 3. Calculated volume below 800m depth for Cenozoic sedimentary basins in Arizona that extend below 800m depth.
Figure 4. This map shows the 88 Cenozoic basins for which volume calculations were done. Contiguous basins were divided at areas of shallow bedrock. The ten basins with the greatest basin volume below 800m depth are shown in red. These basins are the subject of ongoing investigations.
ESRI® ArcMap™ (v. 10) software was used to create a digital representation of basin depths for all of Arizona, with the digital representation derived from the contour map of Richard et al. (2007; Figure 1). This representation was then used to calculate basin-sediment volumes in 88 Cenozoic basins in the Basin and Range province and along the southwest margin of the Colorado Plateau. Total basin-sediment volume was calculated at 42,247 km3, with 49% of the sediment volume in the largest ten basins (Figure 2). Fifty-seven basins were determined to extend below 800m depth, with a total basin volume of 12,655 km3 below 800m depth (Figure 3). Ten of Arizona’s deep sedimentary basins contain 71% of the total deep-basin volume (Figure 4). These basins are now the focus of continuing study to determine if they contain shallow, impermeable cap rocks that would prevent upward CO2 diffusion and escape to the atmosphere, and if deeper sedimentary units are sufficiently porous and permeable to be candidates for industrial-scale CO2 sequestration.
Dickinson, W.R., 2002, The Basin and Range province as a composite extensional domain: International Geology Review, v. 44, p. 1-28.
Oppenheimer, J.M., and Sumner, J.S., 1980, Depth-to-bedrock map, Basin and Range province, Arizona: Tucson, University of Arizona, Department of Geosciences, Laboratory of Geophysics, 1 sheet, scale 1:1,000,000 (available as Arizona Geological Survey publication NP-14).
Oppenheimer, J.M., and Sumner, J.S., 1981, Gravity modeling of the basins in the basin and Range Province, Arizona, in Stone, Claudia, and Jenny, F.P., eds.: Tucson, Arizona Geological Society Digest 13, p. 111-116.
Richard, S.M., Reynolds, S.J., Spencer, J.E., and Pearthree, P.A., 2000, Geologic map of Arizona: Arizona Geological Survey Map 35, scale 1:1,000,000.
Richard, S.M., Shipman, T.C., Greene, L.C., and Harris, R.C., 2007, Estimated depth to bedrock in Arizona: Arizona Geological Survey, Digital Geologic Map DGM-52, layout scale 1:100,000, with 9 p. text.
Spencer, J.E., and Reynolds, S.J., 1989, Middle Tertiary tectonics of Arizona and the Southwest, in Jenney, J.P., and Reynolds, S.J., eds., Geologic evolution of Arizona: Arizona Geological Society Digest, v. 17, p. 539-574.