PRELIMINARY REGIONAL CHARACTERIZATION UPDATE OF THE TUSCALOOSA MARINE SHALE IN SOUTHERN MISSISSIPPI: IMPLICATIONS FOR CO2 SEQUESTRATION

The U.S. Geological Survey (USGS) Carbon Dioxide (CO₂) Storage Resources Assessment Team recently completed an assessment of the national CO₂ storage potential (https://pubs.usgs.gov/circ/1386/). To better understand the distribution of geologic storage resources for anthropogenic CO₂, the USGS is conducting finer-scale studies on reservoirs and their respective seals. This specific study focuses on regional characterization of the Tuscaloosa marine shale (TMS), in Mississippi (MS), which was identified in the USGS CO₂storage assessment as a seal for potential sandstone storage reservoirs within the underlying lower Tuscaloosa Formation.

Previous work by the authors compared X-ray diffraction (XRD) results of TMS cuttings samples from southern MS to TMS core samples from the Cranfield CO₂sequestration site obtained by the University of Texas at Austin Bureau of Economic Geology (BEG). Samples for this study were collected from wells located ~15-120 km away from the Cranfield site. Comparison of XRD data revealed major mineral components in the TMS are quartz, illite, and kaolinite. However, evaluation of TMS seal integrity requires additional information, such as mercury injection capillary pressure (MICP) analyses for porosity and permeability values.

MICP analyses for four cuttings samples of dark fissile shale from the TMS in southern MS have porosities from ~5-7%; median pore throats (nm) and pore radii (nm) from ~5.0-5.5 and ~24.6-36.4, respectively; Swanson permeabilities (mD) from ~1.0x10^-4-1.7x10^-4; and high capillary entry pressures (MPa) from ~20-30. Previous research by BEG found similar MICP properties for the Cranfield site. BEG also noted that TMS samples containing interbedded sandstone have lower porosities and permeabilities as well as higher capillary entry pressures due to calcite cementation than the mudstone samples.

Similar low porosity and permeability of the TMS and high entry capillary pressures indicated by our study and BEG may suggest potential for high sealing capacity to retain CO₂ in areas other than the Cranfield CO₂ injection site. Additional MICP and XRD analyses of new core samples recently collected by the USGS from the TMS, in addition to capillary pressure calculations, will provide further insight into the characterization of the formation as a regional CO₂ seal.