Offshore Algarve (SW Iberia): Nature of the lithosphere and its impact on the exploration potential of the area.
The study area is located in the Algarve basin, offshore the southern coast of Portugal that was the focus of a study performed by Repsol and Partex in 2013 to evaluate its hydrocarbon exploration potential.
The geological evolution of the basin started with Permian-Triassic to earliest Jurassic rifting, followed by a passive margin stage during the Jurassic-Cretaceous. From the latest Cretaceous onward a convergent margin stage developed in which the Betic-Rif orogeny and the emplacement of the associated olistostrome took place.
A regional view of the basin structures can be appreciated on a gravity map. The most striking feature is a prominent ENE-WSW-oriented anomaly that extends about 200 km and marks the southern border of the basin. Gravimetric and magnetic modeling performed along three dip lines shows that the anomaly can be reproduced only by significantly raising the interpretation of the mantle, hence expressing an important structural feature that marks the transition between different lithospheric domains. North of the anomaly, the model indicates the presence of continental crust with normal thickness, while south of the anomaly, the crustal thinning suggests a transition towards oceanic crust.
On 2D seismic, the area of the gravity anomaly is characterized by prominent inverted structures that are associated with pronounced halokinetic features. These inverted structures might be the expression of a zone of transpression resulting from the interaction between the African and Iberian plates.
Various academic publications support the presence of a major plate boundary in the offshore Algarve and interpret the existence of a remnant of the ocean that connected the Central Atlantic and the Tethys in the Late Jurassic.
These works conclude that a slab of oceanic crust is present in the center of the Gulf of Cádiz, enclosed between the African and Iberian continental crust, and dipping eastward beneath the Rifean-Betic arc (Spakman and Wortel, 2004; Gutscher et al., 2009; Sallarès et al., 2011; Ranero et al., 2013).
The fact that the southern edge of the Algarve basin is probably bordered by an oceanic domain has an impact on the evaluation of its exploration potential. In terms of source rock, it lies close to what used to be a narrow and segmented ocean in Jurassic time that presumably favoured the accumulation of organic matter. It appears affected by prominent inverted structures and associated salt diapirism, presumably the result of wrench tectonics, which have developed a large variety of traps. It is also characterised by the development of a main depocenter which has provided both depositional environments for potential reservoirs and overburden for potential source rock maturation.
AAPG Datapages/Search and Discovery Article #90226 © 2015 European Regional Conference and Exhibition, Lisbon, Portugal, May 18-19, 2015