Seismic and acoustic imaging of fluid seepage structures in different sedimentological and tectonic settings in the Lower Congo Basin

Using various geophysical methods including multichannel seismics, bathymetric mapping and sediment echosounding, several active gas seepage sites on the seafloor in the deep Lower Congo Basin were investigated. Gas is formed within Oligocene to Miocene fan deposits and relies on migration pathways to reach the seafloor. To the South of the Congo Canyon, continued seaward moving deformation induced by Aptian salt movement is shown to promote active gas seepage at the deformation front while further landward sites remain inactive. In the vicinity of the post-Miocene Congo Fan to the North of the Congo Canyon, active seepage can be shown to depend on salt tectonic faulting that connects gas-charged Pliocene fan deposits to the seafloor in the absence of widespread polygonal faulting in hemipelagic sediments. These investigated seepage sites act as an example of possible gas seepage configurations at the front of compressional regimes that can be applied to other similar areas

Seismische und akustische Untersuchungen von Fluidaustrittstellen unter verschiedenen sedimentologischen und tektonischen Vorraussetzungen im Lower Congo Basin

Using various geophysical methods including multichannel seismics, bathymetric mapping and sediment echosounding, several active gas seepage sites on the seafloor in the deep Lower Congo Basin were investigated. Gas is formed within Oligocene to Miocene fan deposits and relies on migration pathways to reach the seafloor. To the South of the Congo Canyon, continued seaward moving deformation induced by Aptian salt movement is shown to promote active gas seepage at the deformation front while further landward sites remain inactive. In the vicinity of the post-Miocene Congo Fan to the North of the Congo Canyon, active seepage can be shown to depend on salt tectonic faulting that connects gas-charged Pliocene fan deposits to the seafloor in the absence of widespread polygonal faulting in hemipelagic sediments. These investigated seepage sites act as an example of possible gas seepage configurations at the front of compressional regimes that can be applied to other similar areas

Bathymetric and multichannel seismic data from the Congo deep sea fan

Seafloor seepage is a widespread phenomenon within salt‐influenced basins as the deformation provides pathways for hydrocarbons to reach the seafloor. However, only minor attention has been given to the distal parts of such systems where the impact of salt‐tectonic deformation is relatively unpronounced. The stress put on the sedimentary column by moving salt on a continental margin may influence fluid flow systems even outside of the salt province. This stress may lead to overpressure formation within reservoirs and determine the orientation of overpressure‐induced fractures. Seepage in the Congo Fan has been discovered in such a distal position at the Regab pockmark, about 35 km west of the salt front and its geology and biology have been studied extensively in recent years. We present high‐resolution multichannel seismic data from the Regab pockmark that reveal the underlying migration pathways from a buried channel flank 300 mbsf to the seafloor via hydraulic fractures in the sealing overburden. Local doming of the reservoir and the remobilization and uplift of sedimentary strata along the migration pathways are interpreted as the result of overpressure within the reservoir. The orientation of the hydraulic fractures is WSW‐ENE and the fracture outline corresponds to the area of most intense seepage activity within the seafloor pockmark. Along with a similar orientation of other fractures in the vicinity, we propose that this alignment is due to the stress imposed on the sedimentary column in the fan by the seaward moving salt and rafting sedimentary packages of the salt province further east

Geological development of the Limpopo Shelf (southern Mozambique) during the last sealevel cycle

Paleo-shorelines on continental shelves give insights into the complex development of coastlines during sealevel cycles. This study investigates the geologic development of the Limpopo Shelf during the last sealevel cycle using multichannel seismic and acoustic datasets acquired on the shelf in front of the Limpopo River mouth. A detailed investigation of seismic facies, shelf bathymetry, and a correlation to sea level revealed the presence of numerous submerged shorelines on the shelf. These shorelines are characterized by distinct topographic ridges and are interpreted as coastal dune ridges that formed in periods of intermittent sealevel still-/slowstand during transgression. The shorelines are preserved due to periods of rapid sealevel rise (melt water pulses) that led to the overstepping of the dune ridges as well as due to early cementation of accumulated sediments that increased the erosive resistance of the ridges. The high along-shelf variability of the submerged dune ridges is interpreted as a result of pre-existing topography affecting shoreline positions during transgression. The pre-existing topography is controlled by the underlying sedimentary deposits that are linked to varying fluvial sediment input at different points on the shelf. The numerous prominent submerged dune ridges form barriers for the modern fluvial sediment from the Limpopo River and dam sediment on the inner shelf. They may also facilitate along-shelf current-induced sediment transport.Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/50110000165