Controlling mechanisms of giant deep water pockmarks in the Lower Congo Basin

• Published in: Marine and petroleum geology Vol. 83; pp. 140 - 157
• Main Authors: Wenau, S., Spieß, V., Pape, T., Fekete, N.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01-05-2017
• Subjects: Gas seepage; Lower Congo Basin; Multichannel seismics; Pliocene Congo Fan; Pockmark formation; Salt tectonics; Multichannel seismics; Pliocene Congo Fan; Salt tectonics; Gas seepage; Pockmark formation; Lower Congo Basin
• ISSN: 0264-8172; 1873-4073
• DOI: 10.1016/j.marpetgeo.2017.02.030

Effective seal breaching is a major contributor to methane seepage from deep sea sediments as it ensures the migration of gas and liquid hydrocarbons from buried reservoirs to the seafloor. This study shows two giant pockmarks on the lower slope of the Lower Congo Basin associated with salt-tectonic faulting and the buried Pliocene Congo deep sea fan. The progressive burial of Pliocene fan deposits results in mobilization of methane from gas hydrates at the Base of the Gas Hydrate Stability Zone which migrates through the hemipelagic seal towards the seafloor along salt-induced faults. Seal-breaching in this part of the Lower Congo Basin relies solely on salt-tectonic faulting contrasting with upslope seafloor seepage settings where polygonal faulting within the hemipelagic seal occurs. Dedicated 2D and 3D seismic and acoustic surveying allows the detailed reconstruction of the evolution of pockmarks which appear to have been active for the last 640 kyr. We also show indications that the modern seafloor depression formed due to reduced sedimentation in the vicinity of active seepage. The presented seafloor seepage features illustrate the mode of gas release from the Pliocene fan in the Lower Congo Basin, which contrasts with previously investigated seepage environments further upslope.
•Polygonal faulting in hemipelagic sediments is absent in the deep-water Lower Congo Basin.•Seafloor seepage relies solely on salt-tectonic faulting for seal breaching.•Shallow gas reservoirs form as the BGHSZ moves upwards during sedimentation.•Giant pockmark have been active for about 640 kyrs.•Pockmark formation relies on non-deposition induced by active seepage over time.