Vertical evolution of fluid venting structures in relation to gas flux, in the Neogene-Quaternary of the Lower Congo Basin, Offshore Angola

The processes preceding the genesis of pockmarks and their subsequent evolution has been rarely studied since pockmarks were discovered in 1970. A high-resolution 3-D seismic survey from the continental slope of the Lower Congo basin allows us to study in detail the structures that developed before the formation of pockmarks, as well as their subsequent geometric evolution.This paper aims at reporting new type of fluid-related structures that relate to pockmarks. The objective of this research is to understand: 1) the evolution of these structures and their genetic relationship with pockmarks, and 2) how their morphologies may be influenced by changes in venting intensity over time.Pockmark and chimney structures in this survey are hosted within Late Miocene to Quaternary hemipelagites, above clastic reservoirs in a petroliferous basin. They are all associated with positive high amplitude anomalies (PHAAs) on seismic data. PHAAs are interpreted from their dramatically higher acoustic impedance relative to the background sedimentary values of the host units, and by their association with seismic pull-up effects. From their planform and cross-sectional geometry, PHAAs are primarily interpreted as being due to the development of methane-related carbonates but they may be associated with gas hydrates in one or two examples.Methanogenic carbonates form either by authigenic accumulation or by anaerobic methane oxidation (AMO) once upwardly migrating methane enters the sulfate-methane transition zone (SMTZ), typically within the first few tens of meters below the seabed. Therefore, vertical accumulations of PHAAS are interpreted to reflect the temporal evolution of methane flux migrating upward to reach the paleo-seabed or SMTZ. Timing of local scale fluid leakage can therefore be diagnosed by analyzing PHAAs at seep locations that occur in association with pockmark/chimney structures.A conceptual model for the evolution of different fluid venting structures is proposed here, and is based on the analysis of venting structures that we expressed by 3-D seismic reflections. In this model the main features are linked into a process-based sequence, culminating in the infill and termination of the pockmarks as fluid venting sites. Our model summarizes different rhythms of venting which are expressed by different types of PHAAs along with venting structures. Linear PHAAs and conduits inferred slow seeps. They can be succeeded by sub-circular PHAAs and shallow depressions which inferred slow to moderate seeps. Then the seep can become either a fast vent and terminated into a pockmark crater; or it can become less active and be sealed afterwards. Subsurface sediment fluidizations and remobilizations are likely to happen in the last case and are probably caused by hydrate dissociations.

► Positive high amplitude anomalies (PHAAs) are interpreted as methanogenic carbonates.
► Pockmarks and PHAAs geometries reflected the temporal evolution of methane flux.
► Pockmarks and PHAAs recorded the timing and location of fluid leakages.
► Their chronology in a vertical succession depended on the flux of episodic vents.