POTENTIAL OF SUBSEAFLOOR ATLANTIC UNITS FOR GEOLOGIC SEQUESTRATION OF CARBON DIOXIDE

Deep saline aquifers are one type of geologic CO2 “sink.” These require depths sufficient to maintain CO2 at or near supercritical phase, integrity of an overlying seal, and storativity sufficient to prevent displacement of saline water into freshwater zones. Assessment completed for DOE-sponsored Southeast Carbon Sequestration Partnership shows that the Carolinas are underlain by fractured crystalline rocks or thin sequences of sediments, leaving few options for onshore geologic sequestration of CO2. Cretaceous-age strata, 25-175 km offshore in the western Atlantic, show promise for subseafloor (>1km depth) CO2 sequestration. One potential sink is a Lower Cretaceous unit that lies 1-3 km below the seafloor over a 90 km2 area. An overlying, 200-400 m thick shale at depths between 0.8-2.5 km provides the seal for this ~200-1500 m thick unit. Stratigraphic data are primarily from USGS-compiled seismic data collected by many groups between ~1960 -1980 (Hutchinson et al., 1996). Lack of extensive drilling in the western Atlantic is good for seal integrity, but results in few available hydraulic property data. Core data from COST well GE-1 indicate porosity of the Lower Cretaceous unit of ~20%. Modeling studies conducted at UT show increased dissolution of CO2 in saline connate brine with increasing formation pressure. Water column height (50-1000 m) overlying the seafloor enhances suitability of the potential subseafloor sinks because of the added pressure. Future studies will be directed towards more specific source-sink matching and capacity estimations to assess the economic viability of subseafloor sequestration. Sequestration of CO2 in offshore geologic sinks is a global issue. Norway hosts the only offshore geologic sequestration site and it has sparked international debate. Their experience may influence subseafloor geologic sequestration options for the U.S. Legal, regulatory, and policy implications of subseafloor geologic sequestration of CO2 are unresolved at this time.