3D Congo Pockmarks - Hydrate & Black Hole

Since the first MTU survey to the Lower Congo Basin in 2000 on R/V Meteor Cruise M47-3, subsequently gas hydrate research was proposed as part of the BMBF-funded Geotechnologie program. In the Project CONGO the R/V Meteor Cruise M56 was carried out to survey and sample potential gas hydrate occurrences at the seafloor and to gain a better understanding of the controls of seepage.

As part of MARUM Research Area E (subproject NN), the 3D seismic data set was processed and interpreted.

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. 

At two giant pockmarks on the lower slope of the Lower Congo Basin, associated with salt-tectonic faulting and the burial by the Pliocene Congo deep sea fan was studied. The progressive burial of Pliocene fan deposits results in mobilization of methane from gas hydrates at the Base of the Gas Hydrate Stability Zone (BGHSZ), 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.

Publications

Wenau, Stefan, Spiess V., Pape T., Fekete N. (2017) Controlling mechanisms of giant deep water pockmarks in the Lower Congo Basin. Marine and Petroleum Geology 83, 140-157, https://doi.org/10.1016/j.marpetgeo.2017.02.030.

Zühlsdorff, Lars, Spiess V. (2005) Three-dimensional seismic and acoustic imaging of gas migration and gas hydrate accumulation beneath pockmarks in hemipelagic sediments off Congo, South-West Africa. In: Proceedings of the fifth international conference on gas hydrates (ICGH 5), Norway, 10 pages. (PDF)