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Ongoing studies
Methane Seepage in the Lower Congo Basin
Active seepage of gas from the seafloor has been documented in the Lower Congo Basin for some years. Several research cruises from Bremen (e.g. the RV Meteor cruise M76/3 in 2008) were carried out to investigate the setting and formation of the associated seepage features together with gas hydrate occurrences and seafloor features in detail.
The ongoing PhD work of Stefan Wenau focuses on the investigation of these seepage features in terms of their mode of formation and their relation to the geological setting of the Angolan continental margin. The Lower Congo Basin shows quite dynamic processes affecting the margin. Large-scale sediment input by the Congo River and subsequent distribution by its submarine canyon and a deep-sea fan have built up thick sedimentary sequences. Active salt tectonics induced by Aptian evaporites moving seawards in the sub-seafloor deform overlying sediments and decisively affect seepage in the area by creating pathways for fluid and gas migration towards the seafloor. Hydrocarbons are provided by source rocks at various stratigraphic levels, many also forming economically valuable reservoirs that are currently being explored and exploited.
The seismo-acoustic data set to be used includes swath bathymetry multibeam echosounder data, which allow for the mapping of topographic features such as diapiric ridges, or small-scale seep-related structures such as pockmarks. Echosounder profiles show the shallow sub-seafloor in very high resolution as well as gas bubbles from active seep sites rising through the water column. Multichannel seismic data image sedimentary structures up to several hundred meters below the seafloor at a high resolution to allow mapping gas advection pathways and to investigate the geological environment of the observed seepage at the seafloor.
Understanding the processes leading to methane seepage in the observed geological setting, facilitates predicting sites of gas seepage in other settings around the world. Furthermore, it is crucial to know the temporal variation of seep activity to help better estimate the effect of methane release from the seafloor on the global system. The amount of methane reaching the atmosphere from marine seeps at various depths is still only poorly constrained, but given the increasingly widespread recognition of seepage features on the seafloor, it may be considerable.
Deep seep structure offshore Pakistan
Meteor cruises M74/2 and M74/3 focused primarily on identification and investigation of ongoing seepage on the Makran Accretionary System. Active seepage was observed, among other locations, at the Nascent Ridge Vent Site, which is situated near the deformation front in a water depth of 2.8 km.
Current analysis of seismo-acoustic data aims at understanding the interaction between compression and uplift, shallow sedimentation, dewatering of subducted material, and gas hydrate stability. Key factors are the physical properties such as permeability, the pressure and temperature regimes, and their temporal variation. A conceptual model is being developed to explain the fate of fluid and gas from depth through the hydrate stability zone to the seafloor, undergoing advection along pathways, lateral migration along strata, redirection by gas hydrates, and trapping at distinct subsurface depth levels.
Seismic image across Nascent Ridge