The geomechanics session at GHGT-13 in Lausanne has covered a variety of topics including the value of studying natural analogues. Useful insights into CO2 migration within formations can be gained especially where fractures provide natural conduits for geochemical alteration. Exposures of the Jurassic Entrada formation in Utah provide an excellent field example where this phenomenon occurs. CO2 charged brines are well known in this area of the western United States. The iron oxide mineral haematite gives this rock formation a distinctive red hue. Pressurised acidic formation waters, caused by the presence of CO2, bleach the host sandstone adjacent to faulted sections forming white bands on either side of fractures that can be up to several cms wide. Highly porous layers within the formation have also been bleached forming distinctive white bands which are not evident in adjacent less permeable layers. These observations can provide evidence of CO2 migration patterns that can be used to calibrate models used to predict fluid migration through fractures and permeable formations.
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