Structural controls on the location and distribution of CO 2 emission at a natural CO 2 spring in Daylesford, Australia

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Abstract

Secure storage of CO 2 is imperative for carbon capture and storage technology, and relies on a thorough understanding of the mechanisms of CO 2 retention and leakage. Observations at CO 2 seeps around the world find that geological structures at a local and regional scale control the location, distribution and style of CO 2 emission. Bedrock-hosted natural CO 2 seepage is found in the Daylesford region in Victoria, Australia, where many natural springs contain high concentrations of dissolved CO 2 . Within a few meters of the natural Tipperary Mineral Spring, small CO 2 bubble streams are emitted from bedrock into an ephemeral creek. We examine the relationship between structures in the exposed adjacent outcropping rocks and characteristics of CO 2 gas leakage in the stream, including CO 2 flux and the distribution of gas emissions. We find that degassing is clustered within ˜1 m of a shale-sandstone geological contact. CO 2 emission points are localised along bedding and fracture planes, and concentrated where these features intersect. The bubble streams were intermittent, which posed difficulties in quantifying total emitted CO 2 . Counterintuitively, the number of bubble streams and CO 2 flux was greatest from shale dominated rather than the sandstone dominated features, which forms the regional aquifer. Shallow processes must be increasing the shale permeability, thus influencing the CO 2 flow pathway and emission locations. CO 2 seepage is not limited to the pool; leakage was detected in subaerial rock exposures, at the intersection of bedding and orthogonal fractures. These insights show the range of spatial scales of the geological features that control CO 2 flow. Microscale features and near surface processes can have significant effect on the style and location and rates of CO 2 leakage. The intermittency of the bubble streams highlights challenges around characterising and monitoring CO 2 stores where seepage is spatially and temporally variable. CCS monitoring programmes must therefore be informed by understanding of shallow crustal processes and not simply the processes and pathways governing CO 2 fluid flow at depth. Understanding how the CO 2 fluids leaked by deep pathways might be affected by shallow processes will inform the design of appropriate monitoring tools and monitoring locations.

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APA

Roberts, J. J., Leplastrier, A., Feitz, A. J., Shipton, Z. K., Bell, A. F., & Karolytė, R. (2019). Structural controls on the location and distribution of CO 2 emission at a natural CO 2 spring in Daylesford, Australia. International Journal of Greenhouse Gas Control, 36–46. https://doi.org/10.1016/j.ijggc.2019.03.003

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