Evidence for Jurassic Deposition Over the Highly Extended Continental Crust of the Alentejo Basin (Offshore Portugal)

Abstract

The Alentejo basin in the deepwater offshore Portugal is a frontier area where new 3D seismic data has been acquired, enhancing the understanding of the tectono-sedimentary evolution. The basin evolved as a magma-poor rift basin during the opening of the North Atlantic and was later inverted from Late Cretaceous onwards. Using both seismic and well data, it was possible to characterize the entire Jurassic section in the Alentejo basin, and individualize each epoch.

This new seismic data improved the imaging of the deeper sedimentary strata in the basin, particularly in the ultra deepwater region. Facies distribution and thickness of sediments for both source rocks and reservoirs differ greatly, depending on the area considered. The reconnaissance of a thick seismic sequence of a low-moderate amplitude and sub-parallel reflectors, with a distinctive ductile rheological behavior, allowed us to assume the presence of a decoupling layer between the rift faults at basement level and the overlain strata. This same structural style is also seen in the early-middle Jurassic of the Lusitanian basin, where it is indicative of an organic-rich shaly carbonate, known as the best source rock facies in the onshore.

The Early Jurassic was recognized from the wells in the edge of the basin to the domain of highly extended crust on seismic. In this distal domain, as well as in the more proximal areas of the basin, this unit is still characterized by a thick, ductile deformed sediment package which is indicative of a high shale content. However, Triassic Dagorda salt is not directly observed in the Alentejo basin, but rather inferred on the more distal part due to this distinctive tectonic style.

The Middle Jurassic is characterized by the presence of reef complexes whereas the Upper Jurassic clearly denotes a differential subsidence, which will condition the carbonate growth, with shallow carbonate platforms forming in isolated horsts and basinal carbonates developing in local depocentres. Often the basal Upper Jurassic is onlapping on older strata, independently of the depositional environment where it was deposited.

All in all, in the proximal area the entire Jurassic is well developed and allows a better constraint of the stratigraphy, whereas in the distal area the Jurassic section is thinner and less developed, with less facies heterogeneity. The stratigraphy ranges from shallow to deep water environments, displaying the high variability of this basin.

AAPG Datapages/Search and Discovery Article #90226 © 2015 European Regional Conference and Exhibition, Lisbon, Portugal, May 18-19, 2015