Discoveries in the Orange Basin, Namibia, have renewed exploration interest in Uruguay's offshore basins which is the conjugate margin analog on the South America side of the Atlantic margin. 

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The Namibia discoveries are located within Cretaceous age formations which are present in Uruguay but have not been tested. Seismic data indicate similar features between Uruguay and the Namibian discoveries suggesting that an analogous petroleum system is possible.

The Uruguayan offshore basins were formed during the break-up of Gondwana and the later opening of the Atlantic Ocean during the Late Jurassic to Early Cretaceous period. The breakup between Africa and South America commenced with the formation of a rift system in the Jurassic but progressively evolved into a passive continental margin. The opening of the Atlantic Ocean contains four major tectonic segments each of which is bounded by major fracture zones that are perpendicular to the axis of rifting. Offshore Uruguay is located (see figure below) within the southern Austral segment bounded to the south by the Falkland fracture zone (FAFZ) and to the north by the Rio Grande fracture zone (RGFZ). Spreading associated with the Atlantic opening initiated from south to north within the Austral segment meaning the rifting phases get progressively younger northwards.

Free-Air Gravity (Sandwell & Smith 2014) map over the South Atlantic Ocean, highlighting oceanic tectonic features, Namibian deepwater discoveries (green stars) and the deepwater area in Uruguay (blue star).

The rifting generated a series of failed rift basins which formed perpendicular to the continental margin. In the Uruguayan offshore, the Punta del Este basin represents the most northern of these failed rift basins within the Austral segment. Punta del Este is separated from the Argentinian Salado basin to the south by the Plata High whilst its northern boundary is defined by the Polonia High which separates it from the Pelotas Basin. Unlike Punta del Este, the Pelotas basin is a typical passive margin basin. The northern limit is defined by the Florianopolis fracture zone beyond which lies the Santos Basin in the Brazilian offshore.

Deposition into the Uruguay basins is aligned with rift initiation moving progressively northwards. In the Punta del Este basin, a depocenter formed during the Cretaceous whilst depocenter formation began in the Cenozoic for the Pelotas basin. The Polonia structural high which separates the basins in the shallow water rapidly deepens seaward meaning that post-rifting in deep water, there depositional is consistency between the two basins.

Punta del Este and Pelotas Basin Stratigraphy

The stratigraphy of both basins is broadly similar and can be classified as

  • Pre-rift Paleozoic sediments comprising interbedded standstones and shales. Pre-rift sediments are most developed in the Punta del Este basin
  • Syn-rift Jurassic to early Cretaceous are best developed in Punta del Este where volcanic-sedimentary fill can exceed 3 000 m within connected graben and half grabens. In the Pelotas basin, the syn-rift volcanic-sediment fill is found in isolated half grabens typically not exceeding 1 000 m in thickness
  • Transition phase with geometries similar to a sag phase (likely early Cretaceous in date) is only present in the Punta del Este basin. Mixed clastic sediments over-top the graben shoulders indicating major fault movement ceases at this time.
  • Post-rift sediments from early to mid-Cretaceous to Quaternary in age can reach up to 6 km. There is a well-defined unconformity at the Cretaceous-Paleogene boundary which also marks a migration of the sedimentary depocenter northwards into the Pelotas basin.

The basement in both basins comprises Pre-Cambrian granite and gneisses which have been dated both by onshore outcrops and drilling in onshore Argentina. Typically, seaward dipping reflectors (SDR’s) are visible which transition into horizontal oceanic crust seaward.

A complete description of the development of the South America and West Africa rift system can be found in the ANCAP technical library (ANCAP Geology & Prospectivity).

The corresponding African continental margin to Uruguay is south of the Walvis Ridge such as the Orange Basin located in southern Namibia and South Africa.

Play Type and Analogies

Uruguay’s offshore is relative under-explored with only three wells drilled to date

  • Shallow water Lobo well (1976) completed in a Mesozoic syn-rift section in Punta del Este basin
  • Shallow water Gaviotin well (1976) completed in a Paleozoic pre-rift section in Punta del Este basin
  • Deepwater Raya-1 well (2016) completed in Oligocene post-rift sediments in the Pelotas basin

None of the wells encountered hydrocarbons despite finding good quality and substantial sands that confirm potential reservoir presence.

The shallow water wells that were drilled in Punta del Este basin were drilled to depths of 2 700 m and 3 600 m and targeted the shoulders of the inboard half grabens. This shallow water play concept relied on significant migration pathways from the thicker seaward sediment accumulations and associated seal presence above the early Cretaceous pre-rift and syn-rift sections.

The Raya-1 well in the Pelotas Basin drilled into a turbiditic sand complex which has been dated as Oligocene in age. The Oligocene section is approximately mid-way through the post-rift sedimentary sequence and like the 1976 targets in Punta del Este, requires a migration pathway from deeper source rocks.

Considering that there are only been three exploration wells across the entirety of Uruguay’s offshore, the Punta del Este and Pelotas basins remain prospective. The prospectivity potential is further reinforced by recent discoveries from the Graff-1 and Venus-1 wells in Namibia. The Namibian discoveries are deepwater turbidite fan systems directly overlaying Aptian age source rock. The Venus and Graff discoveries proved the Cretaceous level reservoir concept was viable on the African conjugate margin which further increases prospectivity potential for similar play types on the South American margin, specifically offshore Uruguay. None of the current exploration wells in Uruguay tested the analogous concept to the Namibia discoveries.

PGS has extensive data coverage across Uruguay’s offshore acreage with high-quality reprocessed 2D Seismic (2022) and 3D Seismic (2015) available. Data coverage across the basin has been re-evaluated to assess the petroleum system elements using insights from the Venus and Graff discoveries. The newly reprocessed data (see below) provides much-improved data quality than was previously available, especially for the deep Cretaceous section overlying the syn-rift architecture. There is evidence of Deepwater fan systems overlying sediment-filled grabens and half grabens, possibly of Aptian age. In combination, these elements suggest an analogous petroleum system to that which has been successfully proven in Namibia.

Illustrating potential play types offshore Uruguay using 2022 reprocessed data. 

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Please contact a member of our South America team for more information.