Seafloor expression of fluid migration as a result of end-member processes: case studies from the North Sea and the central Atlantic margin

Abstract:

In offshore settings, the vertical migration of fluids may generate different structures on the seafloor and in the subsurface that can be readily imaged by multibeam and seismic data, respectively. On the floor, circular to elliptical depressions up to kilometers wide and hundreds of meters deep have been described as pockmarks. They are commonly interpreted as the result from the upward migration of water-hydrocarbon mixtures that lead to overpressure increase and consequent seal breaking and fluid expulsion, which will ultimately produce pockmark depressions at the seabed surface. Nonetheless, fluid expulsion may also lead to the formation of positive extrusive structures at the seabed, called mounds, when the water-hydrocarbon mixture is able to fluidize the muddy component of encasing sediments. This process is similar to the one producing mud volcanoes, which may occur both onshore and offshore. Notably, some circular depressions resembling pockmarks, which may occur at the seabed as isolated features or organized in trails, have been instead interpreted to result from the interaction of gravity-driven processes and bottom currents interacting with fluid migration. These “pockmark-like” depressions therefore share with fluid escape pipes a similar 2D (plain view) geometry, but instead conceal a different 3D architecture resulting from the vertical stacking of sediment waves, likely deposited by supercritical flows as suggested by the upslope direction of migration. In this latter case, the presence of coarser sediments vertically staked in the sediment wave troughs favor the migration of fluids (if present) whichever their origin. All those examples represent a morphologic convergence produced by end-member processes. Using two seismic datasets from offshore Scotland (Loyal Field dataset) and Brazil (Ceará Basin dataset), we show some clear examples of similar morphologic convergence with the aim to highlight how different processes have the ability to shape the seafloor producing similar features, thus suggesting that a continuum of processes may interact each other to favor the migration of fluids in marine settings.