Gas migration phenomena and pockmark evolution in a Gulf of Mexico minibasin

Abstract

Seafloor pockmarks vent potent greenhouse gasses into the ocean, support lush chemosynthetic communities and sequester carbon on the seafloor. However, the processes controlling gas migration in the subsurface, gas escape at the seafloor and pockmark evolution are not well constrained. Here, we investigate the evolution of three large, elongated pockmarks and the subsurface gas migration system driving their evolution, using 3D seismic reflection data from a northern Gulf of Mexico salt-withdrawal minibasin. These pockmarks are unique in that they elongate in opposite directions, deepen away from mounds within them and are located in the center of a salt-withdrawal minibasin, a location where vents are rarely documented. The mounds are characterized by high root mean square amplitude anomalies, which are typical of methane derived authigenic carbonate mounds precipitated via anaerobic oxidation of seeping methane over thousands of years in such setting. The pockmarks are underlain by vertical zones of masked reflections (chimneys or gas migration conduits) occurring across a ∼638 m thick sub-seafloor mass transport complex. Below the chimneys, NW-SE oriented normal faults extend upward, terminating below the mass transport complex. These features are geo-spatially related to deeper multi-layer potential gas reservoir sands observed at 3–5 s TWT of the dataset. We argue that the pockmarks elongated under the combined influence of long-term seepage of methane sourced from deep reservoirs and driven vertically upward along faults and chimneys, and authigenic carbonate precipitation. The carbonates sealed part of the pockmarks, causing a deflection in the foci of methane venting and promoting elongation and deepening away from the intra-pockmark carbonates. These seafloor-subsurface linkages may occur in other marine sedimentary basins around the globe.