The deep water regions off the coast of Nigeria feature a large expanse of seafloor that is speckled with pockmarks. New data suggest that these pockmarks were mainly created by the rapid formation of methane gas hydrates. How these pockmarks formed is important for measuring methane gas released from the seabed and for identifying the potential for sedimentary failure, which is a major concern for deep, offshore oil industries. Until recently, scientists were unsure of the processes behind the pockmark features.
Using new data, Sultan et al. identified the primary mechanisms for the formation of pockmarks in the deep water off Nigeria as rapid formation of gas hydrate in the center and hydrate dissolution at the border of the structure. The researchers’ work is a companion paper to another publication from 2010, which suggested that slow hydrate dissolution alone was responsible for pockmark formation.
During the Guineco-MeBo expedition, the current team collected more than 60 new core samples of gas hydrate-bearing sediments that were extracted from the seafloor with a drill rig and corers. The rig drilled down to a maximum of 57 meters below the seafloor. The team then combined that data with in situ measurements and pore water analyses at various depths in the region.
They found three lines of evidence for rapid gas hydrate formation: positive temperature anomalies in the gas hydrate occurrence zone (GHOZ), free gas trapped in cracks near the seafloor’s surface in the GHOZ, and the coexistence of gas hydrate and free gas at similar depths. Although slow gas dissolution is also prevalent in the studied region, the scientists found that it was responsible for the structure and look of the overall subseabed but not responsible alone for the pockmark features. (Journal of Geophysical Research: Solid Earth, doi:10.1002/2013JB010546, 2014)
—Jessica Orwig, Freelance Writer
Citation: Orwig, J. (2015), Rapid gas hydrate forms pockmarks in Nigeria’s seafloor, Eos, 96, doi:10.1029/2015EO023907. Published on 12 February 2015.