Alentejo Basin, Portugal; Unexplored Plays—from Carbonate Build-Ups to Deepwater Systems

Frode Sandnes¹ and Reidun Myklebust^2
1 TGS, N-3478 Naersnes, Norway
² TGS,

The Alentejo Basin is located south on the Atlantic Margin of Portugal, south of Lisbon. It extends over more than 30 000 km² and grades into the Tagus Abyssal Plain.

Only two wells have been drilled in the Alentejo Basin; one on the eastern flank of the basin (penetrating a thick Mesozoic carbonate platform) and the other one within a submarine channel system that allowed clastic sediments (transported by the palaeo-Tejo River system that drained a large portion of Iberia) to bypass the carbonate platform into deepwater depositional systems.

Interpretation of modern, high quality, 2D regional seismic data has revealed a multitude of different untested play types in the basin. Total sedimentary thickness exceeds 10 km. Basin development is similar to that of the Jeanne d'Arc Basin, with the exception of a Tertiary compressive overprint. Structural styles range from extensional (tilted fault blocks) to trans-tensional (gentle rollovers, inverted sub-basins). Triassic salt domes also exist.

Potential source rocks have been projected from nearby explored basins; Late Jurassic and Early Cretaceous shale units are considered most important. Modeling indicates the source rocks to be mature for hydrocarbon generation. Oil stained dredge samples, seismic hydrocarbon indicators and satellite detected oil seeps confirm a live petroleum system. Potential reservoir units include carbonate build-ups in the shallow portion of the basin, shallow water sandstones and deepwater clastic depositional units.

Several structural and stratigraphic leads have giant potential, many of these in water depths shallower than 2000 m.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005