One approach to mitigating climate change associated with burning fossil fuels is to store the emitted carbon dioxide (CO2) beneath the seabed, for example in former natural gas reservoirs beneath the North Sea. It is critical that such storage sites do not leak, so it is important to build an understanding of how such leakage could occur.
The location and potential intensity of CO2 leakage at the seafloor are critically dependent on the distribution of fluid pathways in the sub-seafloor geology, and on the permeability of these pathways.
A feature that may represent a type of fluid pathway are so-called “chimney” structures, which are common in the North Sea. These features are visible on seismic sections, and cross-cut the sediment layering in the sub-seafloor. Many of these are linked to depressions on the seabed known as pockmarks, which can reach hundreds of metres in diameter. Natural gas migrating upwards from deeper strata is likely to have transited through these chimney and pockmark structures into the overlying water column at some point in geological time.
During RRS James Cook cruise 152, we will conduct a geophysical field programme to investigate in detail the pathways feeding one such pockmark that is actively venting gas, apparently from an accumulation several tens of metres below the seafloor. We will use sound waves to make images of these pathways, to determine their three-dimensional physical properties, and to estimate how readily they transmit gas to the seabed.
One of the techniques we will use to acquire these images involves the use of ocean bottom seismometers (pictured left) – these are placed on the seafloor to detect and record artificially-generated seismic signals that pass through the subsurface rock layers and reveal information about the subsurface geological structures – including (we hope!) detailed data on these seafloor chimneys.