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Uruguay is underlain by a Precambrian shield which is part of the Río de la Plata craton; an extensive area of stable crystalline basement that spreads across Uruguay and large areas of eastern Argentina and southern Brazil. In the north the shield is covered by younger volcanic rocks and sediments deposited during the initial opening of the South Atlantic up to Cretaceous time.
Two sedimentary basins are recognised onshore; the Norte Basin and the Santa Lucia basin. The Norte Basin is a sub-basin of the Parana Basin, a large intra-cratonic area which extends over the north of Uruguay, the south of Brazil, the northeast of Argentina and the south of Paraguay.
It has an early Devonian to Late Cretaceous fill of shales, siltstones, sandstones and volcanics. A number of wells were drilled in the basin in the mid-1950s and mid-1980s but there appears to be no conventional petroleum potential. However, some shale oil potential may be present in the Permian Mungrullo Formation especially near the Brazilian border as determined by several hundred shallow wells drilled in the 1970s and 1980s.
The Santa Lucia Basin is an intra-cratonic basin, pull-apart basin which developed during the Jurassic and Cretaceous in the south of the country. It primarily contains sandstones and volcanics but Globalshift believes it is unlikely to contain viable source rocks.
In the offshore waters of Uruguay sedimentation within the Atlantic rift system has continued through to the present day. Three basins are recognized; Punta del Este, Pelotas (southern part) and the Oriental del Plata.
These basins were created during the break-up of Gondwana and opening of the Atlantic Ocean. The Punta del Este Basin in the west is an aborted rift, perpendicular to the continental margin. The Pelotas Basin in the east is the flexural border of the south Atlantic rift. Since the Palaeocene both basins have developed as one. The Oriental del Plata Basin in the south underlies ultra-deep waters over a basement of transitional and oceanic crust.
The stratigraphy of these basins includes a syn-rift sequence (Jurassic-Neocomian) of aluvio-fluvial and lacustrine deposits, interbedded with volcanics; a post-rift sequence of transitional transgressive marine sediments (Barremian-Aptian) and regressive marine deposits (Late Cretaceous).
During the Cenozoic sedimentation continued to fluctuate in type as the sea level rose and fell. Source rocks may have matured in depocentres of the Cretaceous post-rift in the Punta del Este Basin and the Cenozoic post-rift in the Pelotas Basin.
Only 2 shallow wells had been drilled up to 2016, both in the Punta del Este Basin. Very little is known about the Oriental del Plata Basin but a well is due to be drilled soon.
URUGUAY: SEDIMENTARY BASINS
Globalshift.co.uk (source: GEO ExPro)