WELL LOG ANALYSIS OF THE LOWER CRETACEOUS BIG CYPRESS GROUP FOR THE USGS NATIONAL ASSESSMENT OF GEOLOGIC CARBON DIOXIDE STORAGE RESOURCES IN THE SOUTH FLORIDA BASIN

Thirty-five side-wall electric logs from the Florida Geological Survey were analyzed for the U.S. Geological Survey national carbon dioxide (CO₂) storage resource assessment. Specifically, the investigation focused on the storage potential of the Big Cypress Group in the South Florida Basin within the Dollar Bay, Gordon Pass, and Marco Junction formations. These Lower Cretaceous Albian age units consist of shallow marine limestone, dolostone and anhydrite lithologies. A paucity of regional structure and thickness data for these formations required that the authors conduct well log analyses to produce the necessary georeferenced numerical inputs required by the assessment methodology.

In the USGS assessment of the South Florida Basin, these three formations were separated into two distinct storage assessment units (SAUs) based on the availability of thick, regionally-extensive, impermeable evaporite sealing strata. Following an initial formation identification phase, formation thickness and porosity were determined from gamma-ray, resistivity and neutron formation logs for the units under investigation. Interpolation of well log analyses results yielded depth-to-top of formation and isopach maps for the seal and reservoir formations in each SAU. Depth-to-top of formation is integral to the assessment methodology because the storage formation must be at least 3,000 feet below the surface to maintain CO₂ in the supercritical phase. Isopach maps of prominent evaporite beds facilitated the delineation of areas where the sealing intervals were sufficient for retention of buoyant CO₂. The combination of reservoir depth and seal thickness determined the boundary of the SAUs and therefore the area input of the calculation. Porous thickness isopachs, also an input to the assessment calculation, were generated from the log analyses of the SAUs’ reservoir units. While the logistical and time requirements of the national assessment favor the use of existing datasets and published isopach and structure map data, well log analyses and subsequent interpolations produced quantitative results with an acceptable level of certainty appropriate for a national scale assessment.