Geophysical characterisation of active thermogenic oil seeps in the salt province of the lower Congo basin. Part II: A regional validation
Résumé
The Lower Congo Basin is known to discharge a substantial volume of oil towards the sea surface, from more than one hundred seafloor seep sites distributed throughout the deep province of the Lower Congo Basin. A large geochemical coring survey confirmed the presence of oil on the seabed. The combination with the seismic data considerably improved the identification of the origin of the oil slicks on the seabed. Multiple specific geophysical characteristics of thermogenic hydrocarbon seep sites were highlighted in a previous detailed analysis of seismic datasets. This study aims to test the characteristics previously identified at regional-scale. The active discharge zone is limited to the distal province of the basin. It is characterised by strong compression/shortening due to the sliding of the post-salt super-sequence, resulting in numerous salt diapirs that control the location of seafloor oil seep sites. The paper describes the bathymetric, reflectivity, amplitude and sub-bottom profiler characteristics of a group of thermogenic seep sites. They correspond essentially to submarine mounds or pockmarks of complex and irregular shape surrounded by hummocky mounds. Active oil pockmarks are systematically associated with positive amplitude anomalies on the seabed and are linked to vertical high-amplitude columns rooted in the seismic reflector and associated with the base of the gas hydrate stability zone. The sub-bottom profiler data shows that the hummocky mounds are connected by a network of faults to high-amplitude bodies buried under a consistent sediment thickness. Based on the definition of specific geophysical features (seafloor mounds, complex shape and irregular pockmarks, positive anomalies of seafloor amplitude, high-amplitude vertical pipes), we identified a series of potential oil seep sites at basin scale. The mounds are particularly recognizable using the seismic curvature attribute; we have identified 2946 individual hummocks that are grouped in 50 zones with a density of 35–240 per km2. They are believed to be associated with asphalt storage on the seafloor and related to the biodegradation of heavy oils during hydrocarbon dysmigration through the sedimentary pile. Unusual double BSRs occur over the study area; these are also a specific feature of thermogenic seep sites. Barely half of the potential sites identified on the seismic datasets are associated with recurrent oil slicks at the sea surface. The proportion of remaining anomalies may be associated with inactive seep sites over the period of satellite-based monitoring or gas-dominated seep sites.
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