Constraining leakage pathways through the overburden above sub-seafloor CO2 storage reservoirs

The location and potential intensity of any possible CO₂ leakage at the seafloor are critically dependent on the distribution and permeability of fluid pathways in the sediment overburden overlying any putative storage reservoir. Evaluation of seismic reflection data, as part of a European Union funded project (STEMM-CCS), has revealed structures cross-cutting the overburden within the North Sea and Norwegian Sea. These seismically-imaged pipes and chimneys are considered to be possible pathways for fluid flow. Natural fluids from deeper strata have migrated through these structures at some point in geological time. If CO₂ leaking from sub-seafloor storage reservoirs reaches the base of these structures, and if their permeability is sufficiently high, they could act as CO₂ leakage pathways towards the seafloor and overlying water column. To provide a reliable prediction of potential seafloor seep sites, the hydraulic continuity, and especially the permeability of these pathways, needs to be better constrained and quantified.

This paper will describe results from two geophysical cruises to the Scanner and Challenger pockmark complexes in the North Sea. These pockmarks were known to be the locations of vigorous and persistent methane venting, were associated with bright spots at shallow depth, and had chimney structures imaged on seismic reflection data to depths of several hundred metres. The Scanner pockmark is a composite feature involving two overlapping seabed pockmarks, each a few hundred metres in diameter, lying in c. 155 m water depth. These cruises aimed to collect data that could be used to constrain the geometry and internal structures of fluid flow "chimney" structures with the eventual aim of determining the current permeability of the sub-surface. Understanding the flow of methane from different depths in the sub-surface will facilitate an understanding of possible pathways of CO₂.