Print this pageSeismic Interpretation and Numerical Modelling of Hydrocarbon Leakage Indicators in the Post-Rift Sequences from the Slope of the Angola Basin, West African Margin
The analysis of hundreds of km of 2D seismic-reflection profiles on the deep-offshore of the Angola basin, to the south of the Congo submarine canyon, allowed us to identify a variety of features probably related to both past or active leakage of both liquid and gas hydrocarbons. In addition, an interval of highly-dense vertical faulting within the Pliocene to Present seem to act as a conduit for hydrocarbon seepage up to the sea-floor. No evidence of gas sequestration or gas hydrates deposits as BSR was identified. The leakage features, which range from gas chimneys, buried and present-day pockmarks, to seabed-mounded structures, seem to be linked to either stratigraphic (Miocene paleo-channels and prograding clastic wedges) or structural (growth faults, salt diapirs) elements.
Based on these observations, we setup a conceptual model that was used for a 2D basin model of two sections on the northern and southern slopes in order to reproduce the hydrocarbon migration features observed on the seismic data. We defined several post-rift source rocks within the Upper Cretaceous to Eocene interval. Sedimentation increases rapidly since the Oligocene due to the deposition of the Tertiary Congo fan. The evolution of the salt diapirs is treated in a schematic approach to consider the thermal effect of these structures. The model allowed us to (1) compare the burial history and the thermal evolution of both sections, (2) simulate the hydrocarbon generation history, and (3) reconstruct the possible hydrocarbon migration pathways.
We tested different heat flow scenarios to model HC maturation and migration. We calibrated the model with available Temp and Ro borehole data. Assuming an average heat flow of 55 mW/m2 the results predict the onset of hydrocarbon generation during the lower Miocene, and continuing at Present. Migration started on Middle Miocene onwards. Further different scenarios of maturation and migration will be modelled in order to calibrate the results with the identified leakage indicators.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil