Chapter 4: Characteristics of Marine Methane Macroseeps
The ebullition of methane through the seafloor (macroseepage) is a relatively rare occurrence. Such seepage has been proven to affect the seabed’s sediments, its topography, its life, and the seawater in various ways and at various scales. From the results of detailed surveys conducted on three distinct and continuous macroseeps in the North Sea (Tommeliten, Scanner, and Gullfaks) over several decades, using a range of tools and scientific disciplines and comparing the various results from these macroseeps with those of shallow methane macroseeps elsewhere, it is concluded that macroseepage of methane at less than 160-m water depth most probably affects not only the local topography, geochemistry, biogeology, and water column but also the region, including the downstream water column, the seafloor surface, and the seawater surface. In addition, some macroseeps provide methane to the lower atmosphere. It is also suspected that some macroseeps give birth to adjacent microseepage and therefore represent important geobiological systems that can only be understood properly by long-term studies performed at many scales and by cross-disciplinary scientific methods. Marine methane macroseeps are characterized by (1) visual ebullition through seafloor holes; (2) hydroacoustic flares (columnar midwater reflections); (3) ethane concentration anomalies in the water column and adjacent sediment porewater; (4) development of visual, biological, and chemical aureoles surrounding the seep location; (5) anomalies (strong gradients) in chemical, temperature, and biological composition of the water column, especially downcurrent (e.g., pH, eH, carbon dioxide [CO2], oxygen [O2], methane [CH4], sulfate, sulfide); (6) topographical effects (mounds, depressions, pockmarks); (7) carbonate cementation of subsurface sediments surrounding the conduit and adjacent sediments; (8) bacterial mats on the sediment surface adjacent to a seep; (9) upwelling of seawater; (10) downwelling (circulation) of seawater into conduit throats; (11) sea-surface effects, e.g., nutrients coming to the surface because of upwelling; (12) sea-surface slicks and seabirds feeding, downcurrent of the seep; (13) attraction of fish and other macrofauna to the seep; and (14) anomalies in methane concentration in the lower atmosphere above the seep. These effects are listed in order of occurrence, with the most common first (1 and 2) and the less common at the bottom (13 and 14).
Figures & Tables
With the increased resolution power of many geophysical methods, we are seeing direct evidence of seeps on a wide variety of data, including conventional seismic. New methods and technology have also evolved to better measure and detect seeps and their artifacts and reservoir charge and to map migration and remigration routes. In addition, detection of seepage is important for minimizing the risks associated with shallow gas drilling hazards, ensuring platform stability, and preventing well blow-outs.