Multichannel reflection seismic data from the Lower Congo Basin, profile GeoB08-288, GeoB08-298, and GeoB08302

Active high intensity gas seepage is documented for the first time at the seaward edge of the salt occurrence in the southern Lower Congo Basin. Microbial methane release from the seafloor occurs on the crests of two 800 m high ridges formed by fault-propagation folding. Intense uplift is documented since the end of the Miocene by distinct onlapping reflections on the landward flank of these ridges. A paleo-pockmark structure suggests an onset of seepage coincident with this deformation period. High-resolution seismic imaging reveals methane migration along strata from Oligocene/Miocene fan deposits towards the ridge crests where large gas accumulations form beneath a discontinuous Bottom Simulating Reflection (BSR). Detailed mapping revealed that free gas and gas hydrate occurrences below and above the base of the gas hydrate stability zone are closely linked to sedimentary strata in the flanks of topographic ridges. Gas transport through the gas hydrate stability zone originates from the shallowest area of the BSR directly beneath the seafloor seep sites, suggesting pressure controlled venting. These sites represent the most seaward salt-related gas seepage features documented in the area and illustrate the initiation of long-lasting seepage at the front of an area of compressional tectonics at a passive continental margin.