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Surface oil seeps and asphalt occurrences were identified in central Portugal within the Lusitanian basin in the 19th century and asphalt and bitumen have been mined at several locations.
The basin was the site of periodic oil and gas exploration activity since the late 1930s. Early shallow drilling focused on salt-related surface structures associated with oil seeps. Oil shows and minor oil recovery was reported but no commercial production could be established. Onshore drilling ceased in 1963 and did not resume until 1981.
In 1969 international companies began regional seismic, gravity, and magnetic surveys offshore and concessions were awarded in the early 1970s, leading to the drilling of 19 wells through to 1982 but only 3 wells have been drilled since then. Oil and gas shows were recorded and live oil was recovered from 2 wells but no commercial oil or gas fields were identified.
Mohave Oil and Gas commenced a study of the oil and gas potential of onshore Portugal in early 1992 and acquired 3 onshore licenses in the northern part of the basin in 1993 and 2 additional licenses in 1997. Four wells were drilled but no discoveries were made.
In 2002 the government offered 14 deepwater blocks for exploration and production contracts. After the licensing round expired only Repsol-YPF, in partnership with RWE, made bids for Blocks 13 and 14. In 2007 Petrogal (Galp) licensed 3 deep water blocks in the Alentejo basin with Petrobras. Petrobras relinquished its rights in 2014 which were taken by ENI. A regional reprocessing of seismic data was undertaken in 2015 but as at 2018 no offshore wells had been drilled since 1995 and Globalshift does not forecast any future production of oil or gas from the country.
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Portugal lies on the western side of the Iberian Peninsula, a Hercynian cratonic block known as the Iberian Massif. The Massif is bounded by Alpine fold belts of the Pyrenees in the north and the Betics in the south, both located in Spain. To the west off Portugal the peninsula is delimited by a continental boundary formed by the opening of the Atlantic Ocean.
In the north of Portugal are outcropping and thrusted Palaeozoic metamorphic rocks. In the south the South Portuguese Zone (SPZ) is an exotic terrane that came from Laurasia and was attached to what later became the Grand Banks in Canada. This continent was to the north of Iberia but now forms a thin triangle of Upper Devonian to Carboniferous metamorphic rocks on the south end of Portugal.
In central Portugal and offshore the Iberian Massif is buried by Mesozoic and Cenozoic sediments. The Lusitanian Basin covers much of central Portugal partly onshore and partly offshore, limited on the east by the Iberian Massif. It is a Mesozoic rift basin that opened in the Late Triassic and developed into an open marine carbonate shelf in the Jurassic. It was covered by westward-prograding siliciclastics during the latest Jurassic and Cretaceous as the North Atlantic opened. It was then subjected to transpression during Tertiary Alpine events. Although many wells have been drilled in the Lusitanian basin only oil and gas shows have been encountered and structures are generally breached.
Other basins off Portugal are less well explored. Off the northern coast of Portugal is the Porto Basin, elongated in a north-south direction which formed at the same time as the Lusitanian basin. West of a ridge of shallow basement (to the west of the Lusitania basin) is the Peniche basin containing deep water turbidite deposits. It has similarities to the Jeanne d'Arc basin in the Grand Banks of Canada but with a thin Tertiary overburden and much greater water depth. No wells have been drilled.
Off the south of the country are the Alentajo, Sagres and Algarve basins. These have thick sections of Triassic (including salt), Jurassic, and Lower Cretaceous sediments also similar to the Jeanne d'Arc basin. Only two wells have been drilled in the Alentejo Basin, one on the eastern flank penetrating a thick Mesozoic carbonate platform and the other within a submarine channel system that bypassed the platform to deposit deepwater turbidites.
The Sagres basin has a thick, untested section of siliciclastic and carbonates whilst the Algarve basin is a continuation of the Guadalquivir basin in Spain where biogenic gas fields are producing from Upper Miocene and Pliocene turbidites. Unlike in Jeanne d’Arc, all these basins have a Tertiary compressive overprint that may have disrupted potential traps.
Along the Atlantic continental margin, between the continental and oceanic crust, there is a 100 km wide zone of exhumed continental mantle which was uplifted during rifting of Newfoundland from Iberia and then further offshore is the Tagus Abyssal Plain of oceanic crust.
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