Exploring the shelf margin plays of NW Africa and the Southern Atlantic

Abstract

The shelf margin plays of passive margins of North West Africa and the Southern Atlantic are linked by the common characteristic of being little explored oil plays in shallow to moderate water depths, yet their geologies are strikingly different. An attempt to fragment Gondwana occurred through the Permian and Triassic, developing a number of rift systems across Africa and South America, it was not until the Jurassic that North America began to drift north west signaling the breakup of the super continent. Clastic syn-rift sections, assumed to be oriented sub parallel to the Atlantic rift comprise clastic and salt sequences down the margin from Morocco to Guinea Bissau. In the post drift of the Early Cretaceous, up to the end Aptian, carbonate platforms developed along the margin, building out and over the syn-rift fabric. Loading the salt halofer created intra-platformal salt diapirism, potentially connecting intra platformal Jurassic to overlying sequences. In the Albian clastic deltas systems prograded west up to and over the carbonate platform margin in the Albian. Two events that may be related have occurred together to create a prospective shelf margin play. Firstly the edge of the carbonate platform margin has been dramatically uplifted creating counter regional dip for the western edge of the Aptian carbonate platform, and likewise the overlying Albian Deltaics. Secondly, the Pre-Miocene unconformity (the Senonian unconformity) or set of unconformities has eroded the platform margin and sediments above this and a series of highly mud prone but unstable sequences were deposited on top of this unconformity creating top seal. This system is charged by a Turonian and perhaps Aptian source deposited beyond the shelf edge to create the trap and play that was successfully tested by the SNE discovery and is applicable down the unexplored margin from Senegal to Guinea Bissau. The Atlantic fully opened south Gondwana in the earliest Cretaceous. In the north opening along the Jurassic extension to the initial break-up transform fault, yet south of the equatorial transform margin the Atlantic rift has a north-south orientation almost orthogonal to the Jurassic (and Karoo) aged rift axis. Whilst in the north (Northern Margins and Sergipe Basins of Brazil) the shelf edge plays and oil discoveries are dominated by Early Cretaceous syn-rift systems. Further south in Argentina the Cretaceous shelf margin is dominated by transport systems taking huge quantities of clastics across the shelf along the paleo Jurassic grabens before pouring them down the slope into deep water play systems. In some of these Jurassic grabens, source rocks of Permian, Jurassic and even Aptian are buried by 10 km of sediment. Therefore the shelf edge play in Argentina comprises Jurassic clastic plays in syn-rift traps of Jurassic and/or Early Cretaceous age, charged from the Jurassic grabens or from the Atlantic basin to the east. This talk will use newly reprocessed (2017) deghosted seismic data from Senegal and Guinea Bissau to examine the development and characteristics of the play elements at the Cretaceous shelf margin of North West Africa, and compare with newly acquired (2017/18) seismic data from the shelf margin plays of the Colorado Basin and Ewing Terrace of Argentina. Differences in structural development of each margin will be reviewed in context of the subsequent sedimentary systems, depositional facies, source rocks and play systems. These passive margins of the Atlantic have different histories, yet the hydrocarbon systems may have strong linkage in ancestral fabric and process. Both represent opportunities to explore for Oil in shallow to moderate water depths. In both cases the experience of success at the shelf margin will lead to greater success on the slope and basin floor.

AAPG Datapages/Search and Discovery Article #90325 © 2018 AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin, Lisbon, Portugal, May 2-3, 2018