AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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Modelling Petroleum Systems of Hyperextended Margins: the Angola Case Study

Abstract

The Kwanza Basin of Angola is a South Atlantic basin which evolution was controlled by the Early Cretaceous rifting and later transition to a passive margin basin. It is interpreted by several authors as lying over a Hyperextended Margin (e.g. Unternehr, 2010). In such a setting thinning of the lithosphere is not linear from undeformed continental to oceanic domain like in the pure shear model (Mc Kenzie, 1978) but present “hyperextended” portions where crust or mantle lithosphere has been preferentially removed (Huismans & Beaumont, 2011). The Kwanza Basin infill is characterized by a thick Aptian salt layer that separates different petroleum plays. The post-salt plays were the first explored in the offshore, targeting Albian carbonates and Late Cretaceous - Tertiary clastics. The pre-salt play that was already tested in the onshore and shallow offshore generated great interest in deep and ultra-deep offshore during the last decade in search of analogs to the prolific mirror Campos and Santos basins of Brazil. Massive inversion and an intense drilling campaign of 23 exploration wells (2011 to 2016) were realized but gave contrasting results from the pre-salt play including giant oil and condensate discoveries, dry gas and CO2 rich accumulations, but also a number of dry wells. This situation reveal a complex petroleum system that might only be understood at a regional scale, taking into account deep geodynamics and related thermal and geochemical processes. That was the objective of a regional petroleum system analysis that will be presented here. The applied workflow included basin scale structural and facies mapping, well database construction, review and interpretation of organic geochemical data, integration of petrographic and non-organic geochemical data, and finally building and simulation of a 3D basin model to be used as an integration and visualization tool testing different hypotheses. 3D basin modelling of such a hyperextended margin was a challenging novelty and required the implementation of a specific workflow to face an unusual crustal structure and thermal history, both of them further complicated by a thick salt layer. After a detail try and essay process a good calibration of the model was achieved. Modelling results show that the thermal impact is higher than in a classical pure shear model and the difficulty consists in estimating the limits of the different crustal domains, the timing of rifting and the extent of the rise of the asthenosphere and its consequences on thermicity. This case study also highlighted new processes to decipher and take into account, and software limitations inherent to this specific margin type when assessing petroleum systems.