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Identification and Modelling of Hydrocarbon Leakage Indicators: Implications on the Nature of a Large Seafloor-Mounded Belt in the Lower Congo Basin

Zahie Anka¹, Robert Ondrak¹, Astrid Kowitz², and Niels Schødt³
¹GFZ German Research Centre for Geosciences. Potsdam, Germany
²Museum für Naturkunde der Humboldt-Universität, Berlin, Germany
³Maersk Olie og Gas AS, Copenhagen, Denmark

We present a combined approach of interpretation of a large 2D seismic-reflection dataset, covering almost 1400 km2, and numerical modelling of hydrocarbon generation and migration across the slope of the Lower Congo Basin, in order to investigate the factors controlling timing and distribution of hydrocarbon leakage indicators in this area.

We identified a variety of past and present hydrocarbon leakage features comprising three main families: (1) Mid-Upper Miocene seismic chimneys concentrated basinwards and eventually ending up on buried pockmarks, and (2) Plio-Pleistocene chimneys which are rather clustered to the east of a large central graben and often ending up in seafloor pockmarks with diameters between 200m and 500 m. A third family is represented by minor amounts of scattered seismic chimneys identified within the Miocene sequences and eventually ending up in shallow enhanced reflectors or "Flat spots". The distribution of these features can be associated either to stratigraphic or structural elements of the basin.

At least two major events of leakage were identified during the Middle-Late Miocene and the Pliocene-Present. External factors as increased sediment supply seems the main controlling factor on the Miocene leakage event, while internal structural elements would have driven Pliocene to present-day leakage. A major positive morphological feature, represented by a margin-paralleled belt of 1-km-wide seafloor mounds, was identified above major growth faults in the eastern zone of the study area.

Based on these observations, we setup a data constrained 2D HC generation and migration model for two sections across the northern and southern slopes in order to reproduce the migration features observed on the seismic data. We defined several post-rift source rocks within the Upper Cretaceous to Eocene interval and tested different heat flow scenarios. The model was then calibrated with available Temperature and maturity (Vitrinite reflectance) borehole data. Our results point to the feasibility of a genetic link between the seafloor mounded structures and vertical migration of thermogenic fluids from either overmature deep source rocks or shallower leaking reservoirs, with the mounds possibly representing methane-derived carbonate build-ups.

In summary, despite the inherent limitations of 2D migration models, when combined and calibrated with observations from seismic data they can be used as a valid tool to assess petroleum migration and predict migration routes and seal failure in sedimentary basins.

AAPG Search and Discovery Article #120098©2013 AAPG Hedberg Conference Petroleum Systems: Modeling the Past, Planning the Future, Nice, France, October 1-5, 2012