AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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Hydrocarbon Systems Analysis of the Portuguese Atlantic Margin: Projecting a Working Hydrocarbon System from the Lusitania Basin to the Deep-Water Offshore Basins


The Lusitania Basin, Portugal has a proven, working hydrocarbon system. Most of the 162 wells drilled to date in the basin encountered hydrocarbon shows whilst drilling and oil seeps are observed onshore. Oil samples, both from wells and from seeps, have been typed to two source intervals: the Upper Jurassic, Oxfordian, Cabacos Formation and the Lower Jurassic, Toarcian to Sinemurian, Brenha and Coimbra formations. Projection of the Lusitania Basin hydrocarbon system into the deep-water basins of the Portuguese Atlantic Margin provides significant challenges. The Iberian continental margin is a hyper-extended, passive margin, with basins such as Alentejo, Peniche and Galicia overlying continental crust that has been significantly thinned. To date no exploration wells have been drilled in these basins and only limited stratigraphic information is available from ODP and DSDP boreholes. Mapping of “tectonic domains”, using both seismic and gravity data, has helped to build confidence in specific age models but there is much uncertainty and alternative models must always be considered. Despite the uncertainty, it is highly likely that Jurassic sections are preserved in these deep-water basins and that intervals with organic-enrichment are present. A series of 1D basin models conducted along the Portuguese Atlantic Margin indicates that in many basinal locations potential Jurassic source intervals are mature for hydrocarbon generation. Sensitivity analyses conducted on the 1D models indicate that in many scenarios the timing of hydrocarbon expulsion (primarily oil), is favourable to charge traps which formed during late Jurassic to early Cretaceous extension. However, some of the mapped traps are late inversion structures and for these to contain commercial volumes of hydrocarbons different sources or different charge mechanisms are required. These include invoking alternative, but as yet unproven, source intervals within the passive fill of the basins, invoking asymmetric crustal extension, or relying-on alternative charge mechanisms, such as “hotelling”. In conclusion, it is predicted that mature source intervals are present in the deep-water basins of the Portuguese Atlantic Margin and that in many, but not all, cases the timing of hydrocarbon generation was favourable to charge the mapped traps.