PRELIMINARY REGIONAL CHARACTERIZATION OF THE TUSCALOOSA MARINE SHALE: 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. The USGS is conducting finer-scale studies on reservoirs with high potential for CO₂ sequestration and their associated seals to better understand the distribution of geologic storage resources for anthropogenic CO₂. This presentation focuses on regional characterization of the Tuscaloosa marine shale (TMS) in Mississippi (MS), which was identified in the USGS assessment as a seal for potential sandstone storage reservoirs within the lower Tuscaloosa Formation.

Previous research by the University of Texas at Austin Bureau of Economic Geology (BEG) analyzed the seal performance of a core retrieved from the TMS 70 m above the Cranfield CO₂ sequestration site injection zone. Analyses using x-ray diffraction (XRD), mercury intrusion capillary pressure, and scanning electron microscopy indicated the TMS would be a good seal for CO₂ sequestration. To augment the Cranfield project-scale work, this study endeavors to more extensively characterize the TMS through mineralogical analyses.

TMS cuttings samples (27) from southwest MS were collected from the MS Office of Geology and submitted for semi-quantitative XRD analyses using the Rietveld method. The analyses revealed mean values (weight %) of 26% quartz, 17% potassium (K) feldspar, 13% plagioclase, 18% illite and illite/smectite, 20% kaolinite and chlorite, 2% pyrite, 5% carbonates, and 1% other minerals. The quartz, kaolinite, illite, and pyrite results were similar to BEG’s study. BEG had higher carbonate (mean 15%), likely because their sampling included calcareous fossil-bearing intervals whereas this study selected shale cuttings, and lower feldspar (mean 10% K-feldspar, 4% albite) values. Dissimilar results may also be due to the wider geographic distribution of this study’s sample set.

Both studies’ XRD results indicate major minerals in the TMS are quartz, illite, and kaolinite, with varying degrees of the remaining minerals, which may suggest similar permeability and porosity across the TMS as evaluated at Cranfield and, subsequently, desirable regional seal performance. However, petrographic and further mineralogical analyses are required to better regionally characterize the TMS.