North Sea fracture discovered
2012-10-02 06:00 by Anja Reitz
A large fracture, 3km in length, has been discovered during a scientific research cruise in the central North Sea. The discovery was made with the aid of an autonomous underwater vehicle (AUV) in 2011, deployed by University of Bergen from the research vessel G.O. Sars during an ECO2 Project survey to study the potential short-term and long-term impacts of CO2 storage on marine ecosystems.
The AUV was equipped with a synthetic aperture sonar (SAS) measuring the acoustic back-scatter intensity of the seafloor. With this new approach, the large branched seabed fracture was revealed. The intriguing structure was revisited during two ECO2 cruises later in summer 2012, led by the University of Bergen and GEOMAR. Seismic and sonar data obtained during these cruises revealed that the 1 – 10 m wide fracture penetrates 150 - 200 m deep into the sub-surface.
The fracture serves as a conduit for the ascent of methane gas from the deep subsurface which then dissolves in near-surface pore fluids. Where the fracture meets the seabed it is covered with soft sediments and up to 3 m wide patches of bacterial mats. Surface sediment samples were taken at these bacterial mats by a remotely operated vehicle (ROV). Sediment analyses combined with measurements on the sea floor revealed that the microorganisms completely convert the dissolved methane into CO2. When the entire fracture area is considered, about 1 ton of methane-derived CO2 is released into the overlying seawater per year. Similar natural seeps, where methane ascends from sub-seabed geological formations to fuel rich and diverse microbial ecosystems at the seabed, have previously been documented in the North Sea. Together with other available evidence, this indicates that the fracture is a natural structure that formed in the geological past.
The newly-discovered fracture is 25 km north of the Sleipner CO2 storage site. Computer models and observations from monitoring surveys imply that the CO2 stored in the Utsira Sand at Sleipner will never reach the fracture area. Furthermore, the available seismic data show that the fracture is vertically separated from the Utsira Sand by several thick, low permeability sedimentary seals. However, the chemical composition of gases ascending through the fracture indicates that a significant portion originates from deeper geological reservoirs. The ECO2 project will continue to investigate and monitor the fracture in order to evaluate its permeability for methane gas and CO2. According to the ECO2 project coordinator Klaus Wallmann “This discovery shows that there are still surprises awaiting us as we further investigate the seabed, even in waters we think we know well. It demonstrates the importance, both for ongoing and planned storage projects, to map and monitor the seabed using available cutting-edge technologies.”
Klaus Wallmann: firstname.lastname@example.org
Carbon Dioxide Capture and Storage (CCS) is regarded as a key technology for the reduction of carbon dioxide (CO2) emissions from power plants and other industrial sources at the European and international level. The EU hence supports a selected portfolio of demonstration projects to promote, at industrial scale, the implementation of CCS in Europe. Several of these projects aim to store CO2 below the seabed. Even though CO2 separated from natural gas has been safely stored sub-seabed in the North Sea at Sleipner for over 15 years, as implementation develops towards large scale, scientific knowledge about how CO2 affects marine ecosystems needs to be further improved and detailed. Against this background, the EU funded ECO2 project is assessing the likelihood of leakage and impact of leakage on marine ecosystems (http://www.eco2-project.eu/).