AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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The African Conjugate Basins of the Equatorial Atlantic Margins – what are we missing?


Exploration along the Equatorial Atlantic Margins, outside of the Niger Delta, has promised much but too frequently failed to deliver. Modest sized fields on the shelf and onshore tar belts have proven working petroleum systems, which can be demonstrated to be present in deeper water. However, deep-water exploration has too often resulted in sub-commercial discoveries. The Benin (Dahomey) Embayment is a typical example, but it must be noted that the number of deep-water wells is extremely limited, and there have been a few successful commercial fields (Aje, Ogo). The main exception along the Equatorial Margin is the Tano Basin, where early deepwater exploration success spurred a recent concerted exploration drilling campaign by both majors and large independents, that is now paying off with substantial production levels. A further exploration phase in acreage surrounding the main commercial fields is about to get under way in the Tano Basin. Future exploration in the area should seek to apply learnings from the Tano Basin into neighboring basins, on both margins, where success has been much more limited. Conversely, our understanding of the conjugate and neighboring basins can provide new insights into the geology and plays of the Tano basin as exploration moves into this new phase. In this paper we will review the petroleum systems of two sets of conjugates: the Benin Embayment and the Ceara Basin, and the Tano Basin and the Barreirinhas Basin. We will highlight aspects that appear to have positive impact in the Tano basin, and review which of the conjugates might have similar play characteristics. We will note certain peculiarities in the current understanding of the petroleum systems of these basins, often the result of viewing each basin in isolation without realizing that key data may already be evident in a conjugate basin. In particular we will review the detailed stratigraphy of each basin to address some key questions: - The differing timing of opening in the basins straddling the Romanche Fracture Zone. - There is salt in the Ceara Basin, proven in the shelf and potentially thickening into deeper water. Why would it not be present in the conjugates? Could the presalt plays extend into the Equatorial margins? How would salt potentially influence the tectonics of these basins? - The Aptian is present and a key source rock in the Ceara Basin. Why is it not recognized in the Benin Embayment? - Carbonates within the ‘transition’ provide a play, albeit not yet very successful, in Brazil: is this play being overlooked on the African margin? - What precisely is the trapping mechanism in the Jubilee field, and where might it be replicated? While it will be difficult to provide definitive answers to these questions, we hope that the presentation of the limited available data will stimulate a more critical phase of exploration thinking and play analysis. Explorationists are all too aware that it may take years and multiple exploration campaigns before a basin may realise its true potential.