Abstract
Subsurface CO2 storage is a key strategy to reduce greenhouse gas emission, but leakage of CO2 along natural fluid pathways may affect storage formation integrity. However, the internal structure and the physical properties of these focused fluid conduits are poorly understood. Here, we present a three-dimensional seismic velocity model of an active fluid conduit beneath the Scanner Pockmark in the Central North Sea, derived from ocean-bottom seismometer data. We show that the conduit, which manifests as a pipe structure in seismic data, is separated into two parts. The upper part, extending to 260 m depth, i.e. 110 m below the seafloor, is characterised by seismic velocities up to 100 m/s slower than the surrounding strata. The deeper part is characterized by a 50 m/s seismic velocity increase compared to background velocity. We suggest that the upper part of the pipe structure represents a network of open fractures, partly filled with free gas, while the reason for the velocity increase in the lower part remains speculative. These observations suggest that active pipes can be internally heterogeneous with some intervals probably being open fluid pathways and other intervals being closed. This study highlights the complexity in evaluating focused fluid conduits and the necessity of their detailed assessment when selecting CO2 storage sites.
Highlights
The increasing concentration of greenhouse gases in the atmosphere is one of the major challenges of the 21st century
The region beneath the pockmark is characterized by a decrease in seismic P-wave velocity (Vp) inside the pipe structure compared to the area surrounding it (Fig. 7)
From the results of our high-resolution seismic experiment, we conclude that the pipe structure observed in seismic reflection data beneath the Scanner Pockmark is a real geological feature
Summary
The increasing concentration of greenhouse gases in the atmosphere is one of the major challenges of the 21st century. Pipes and chimneys are the seismic expression of vertical, strata-cutting, focused fluid conduits with chaotic seismic facies and reflections with increased or reduced seismic ampli tude (Moss and Cartwright, 2010; Løseth et al, 2011; Andresen, 2012; Cartwright and Santamaria, 2015; Karstens and Berndt, 2015). Both are columnar and can reach from a few tens of meters in diameter to more than 2 km in diameter. Based on the multi-channel seismic (MCS) data presented in this study and the results of Bottner et al (2019), we will label the focused fluid conduit beneath the Scanner Pockmark as a pipe structure in the following
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