Modelling Hydrocarbon Generation, Migration and Present-Day Hydrocarbon Seepage from the Deep-Offshore Slope of Angola

Anka, Zahie ¹; Ondrak, Robert V.¹; Clausen, Lene F.^2
1 Organic Geochemistry, GFZ German Research Centre for Geosciences, Potsdam, Germany.
² Mærsk Olie og Gas AS, Kopenhagen, Denmark.

We analysed several 2D seismic-reflection profiles on the deep-offshore of the Angola basin, to the south of the Congo submarine canyon. Although some extensive structures can be identified, most of the observed salt structures are mainly contractional, switching from vertical diapirs in the east to massive salt build-up to the west.

The seismic data reveal the existence of abundant features probably related to both liquid and gas hydrocarbons accumulation and leakage. These features range from gas chimneys, located on the crestal faulting above salt diapirs, to bright and flat spots within Miocene channels flanking the salt structures. In addition, an interval of highly-dense vertical faulting within the Pliocene to Present, seem to act as a conduit for hydrocarbon seepage to the sea-floor.

With the units identified in the seismic, we setup a conceptual model that was used for a 2D basin model in order to (1) elucidate the burial history and the thermal evolution, (2) simulate the hydrocarbon generation history, and (3) reconstruct the possible hydrocarbon migration pathways. The model focuses on the sedimentary evolution of the post-rift sequences above the Aptian salt. We defined several source rocks within the condensed Upper Cretaceous to Eocene interval. Sedimentation increases rapidly from Oligocene on with the deposition of Tertiary Congo fan. The evolution of the salt diapirs and walls is treated in a schematic approach to consider the thermal effect of these structures.

We tested different heat flow scenarios to model HC maturation and migration and calibrated the model with available borehole data. Assuming an average heat flow of 70 mW/m² the model predicts the onset of hydrocarbon generation during the lower Miocene, continuing at present. Migration started in Middle Miocene onwards. Furthermore, different scenarios of maturation and migration will be modelled in order test the sensitivity of the model and to calibrate the results with leakage features observed on the seismic lines.

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009