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Alentejo Basin, Portugal; Unexplored Plays - from Carbonate Build-ups to Deepwater Systems*
By
Frode Sandnes1 and Reidun Myklebust1
Search and Discovery Article #10099 (2006)
Posted February 19, 2006
*Modified from extended abstract prepared for presentation at AAPG Annual Convention, Calgary, Alberta, June 19-22, 2005
1TGS, Naersnes, Norway ([email protected])
Abstract
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 paleo-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
transtensional (gentle rollovers, inverted sub-basins). Triassic salt domes also
exist.
Potential source rocks have been projected from
nearby explored basins; Upper Jurassic and Lower 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..
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SummaryThe Alentejo Basin is located south on the Atlantic Margin of Portugal, south of Lisbon (Figure 1). It extends over more than 30 000 km2 and grades into the Tagus Abyssal Plain.
Interpretation of modern, high quality, 2D
regional
Total sedimentary thickness appears to exceed
10 km based on Only two wells have been drilled in the Alentejo Basin (Figure 3); Pescada-1 on the eastern flank of the basin penetrateed a thick Mesozoic carbonate platform and Golfinho-1 is on the flank of a submarine channel system that allowed clastic sediments (transported by the paleo-Tejo River system that drained a large portion of Iberia) to bypass the carbonate platform into deepwater depositional systems.
Potential source rocks have been projected
from nearby explored basins; Upper Jurassic and Lower Cretaceous shale
units are considered most important.
Potential reservoir units include carbonate
build-ups in the shallow portion of the basin, shallow water sandstones,
and deepwater clastic depositional units. Examples of the Cretaceous and Tertiary deepwater depositional systems are recognized. Figure 5 is an isochron map (TWT) of such a unit. The map indicates a fill and spill depositional system in the southern portion. Several mapped structural and stratigraphic leads have giant potential. Many of these are located in water depths shallower than 2000 m. Once discoveries are made, fields can be put on stream; infrastructure is in place. |