AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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Alentejo Basin, offshore Portugal – Deep-water Interpretation Issues

Abstract

Alentejo Basin is located on the southwest coast off Portugal, southwards of Lusitanian and Peniche basins. It runs roughly parallel to the coastline straddling from Estremadura Spur, in the north, towards Sagres Spur, to the south. The basin is rather un-explored although its hydrocarbon potential has attracted attention of the petroleum industry with its very first deep-water well scheduled for drilling in 2018. Its stratigraphy is marked by basal Triassic red-beds partly filled by postulated mudflats layered evaporite sequences and dolomites. Massive lava flows (200 MA) above the basal rift are characteristic of this basin, with outcrop thickness between 20 to 100 m. Currently, the basin is considered without salt, although volcanics may have blocked salt migration upwards into younger beds, unlike what happens in the neighbour salt basins towards north without these volcanic rocks. In Algarve Basin, in a context of oblique margin with intensive deformation and embracing volcanic rocks as well, salt diapirs migrates upwards. To note that in the Alentejo Basin the lava layers precludes the seismic imaging of the strata beneath basalt. The well imaged overlaying succession above basalt consists of a sequence of Sinemurian dolomites and thick carbonate and siliciclastic deposits of Jurassic and Cretaceous age. The basin geology may be reconstructed based on existing legacy seismic, by tying the key geologic markers to interpreted seismic reflectors. The deep-water basin is also delimited by ridge type rift shoulders that compartmentalize it into several mini-basins. As observed onshore in Lusitanian Basin, the major rift faults probably work as inversion sliding fault planes, during the period from Late Cretaceous to Miocene. Prospect plays in the Alentejo Basin are represented mostly by anticlines with four-way dip closures and stratigraphic pinch-out up-dip terminations. The stratigraphy of the basin will be better understood after drilling the first well.