Mississippian-age submarine landslides are a risk for seal-failure in Devonian Chattanooga Shale, a potential CO2 storage unit, Tennessee, U.S.A.

Seal failure above shale-gas reservoirs is a major technological risk associated with underground storage of injected CO2 and can escape detection unless thorough sedimentological and stratigraphic investigations are carried out. We address potential seal failure at the top of Devonian Chattanooga Shale due to previously unrecognized, syndepositional (Mississippian) listric faults and submarine landslides in the overlying Fort Payne Formation.

The Fort Payne Formation (Viséan) is a mixed carbonate (crinoidal–bryozoan) and calcareous-dolomitic shale and cherty limestone succession of low-angle clinothem, which prograded ~70 km basinward from south-central Kentucky into Tennessee. Clinothem foreset to proximal bottomset strata include sediment gravity-flow deposits representing slope gullies/channels, sediment waves, and mass-transport deposits (MTDs). Thick bottomsets, which cover more than 17,000 km2 across Tennessee, are dominated by sediment waves and sheeted drifts deposited by bottom-currents. They comprise most of the subsurface Fort Payne strata in the Cumberland Plateau, where the Fort Payne has produced oil and gas for 80 years. Although seismic reflection data are unavailable, well-based isopach maps reveal numerous arcuate-shaped troughs of extremely thin to absent Fort Payne strata interpreted as submarine landslides. They preserve remnants of headwall scarps, evacuation zones, and downflow accumulation zones of MTDs. The underlying Chattanooga Shale was the weak layer along which horizontal shearing took place. Post-failure sedimentary fill includes siltstones and silty sandstones, which compromise the seal.

In early 2010s, DOE funded a field project to assess injection and storage potential of CO2 in the Chattanooga Shale, a shale-gas reservoir and seal in the study area discussed above. Stratigraphic assessment of the shale was extensive but minimal for the Fort Payne Formation. Thus, the extremely thin zones, interpreted herein as slide complexes, were unnoticed. The CO2 injection test-site was placed outside the area affected by submarine landslides. Future projects must consider seal-risk by submarine landslides.