A New Frontier – Blocks Available

The Uruguayan offshore sedimentary basins are virtually unexplored. The time is now right to test the potential of rift basins on trend with the productive San Jorge Basin in Argentina and Campos Basin in Brazil.
This article appeared in Vol. 5, No. 6 - 2008


The basement arch Alto del Polonio that separates Punta del Este from Pelotas basin is associated with potential stratigraphic traps that can be identified with the new seismic. Courtesy of Wavefield Inseis The offshore Uruguayan sedimentary basins, comprising more than 80,000 km2, are largely unexplored. With only two wells drilled in an area equivalent to some 13 North Sea quadrants, there is certainly a need for more exploratory work to be done to test the true hydrocarbon potential. Consequently, ANCAP, the National Oil Company, has announced a licensing round with a deadline in June 2009.

A minimum of exploration

The onshore and offshore sedimentary basins of Uruguay are largely unexplored. The offshore basins, encompassing some 80,000 km2, are now being promoted by ANCAP. The first offshore licensing round ever opened in December and will close in June 2009. Illustration: ANCAP Offshore exploration was initiated in the mid 1970's. Two exploration wells, Lobo and Gaviotín, were drilled on structural highs in the Punta del Este Basin, but none of them encountered hydrocarbons.  

- Additional seismic suggests that these wells were both drilled on prospects with no closure, says Hector de Santa Ana, E&P Manager of ANCAP.  

In the 1980's, there was virtually no exploration activity in these basins. Only one seismic survey in shallow and transitional waters was shot.  

A regional seismic survey preformed in 2002 confirmed the occurrence of thick sedimentary basins in the deep and ultra deep waters.  

- After almost 30 years of limited exploration, the national government, through ANCAP, joined forces with Wavefield Inseis in order to conduct a comprehensive geological study of all the offshore basins, the deep water included. For that purpose a regional 2D seismic survey of 7125 km was carried out in 2007. An infill of 2909 km was completed this year in the Punta del Este and Oriental del Plata basins, says de Santa Ana.  

The data is now available and makes up the foundation for the current licensing round.

The Petroleum System

Wavefield Inseis has, in co-operation with ANCAP of Uruguay, completed the acquisition of 7,125 km of Regional and 2,909 km of semi-detailed Long Offset 2D seismic data covering the entire Uruguayan offshore sector. Gravity and magnetic data have also been recorded. Illustration: Wavefield Inseis The rift basins of the Uruguayan offshore were generated following the breakup of the supercontinent Gondwana in Permian-Triassic times and contain up to 7000 meters of Palaeozoic, Mesozoic and Cenozoic rocks.  

The four classical stages in the development of a passive margin sedimentary basin are clearly identified in the evolution of the offshore basins: pre-rift, rift, sag and passive margin.  

Potential source rocks are associated with the prerift, synrift and sag sequences.  

Sediments associated with the prerift sequence, only present in Punta del Este Basin, do have equivalents in the onshore Norte basin (TOC between 1.0 and 12%), and may be preserved in Palaeozoic half-grabens.  

Lacustrine shales, associated with the synrift phase, have a good source potential in the Santa Lucia basin (TOC between 1.0 and 1.5%), another Uruguayan Mesozoic onshore basin. From seismic data, its presence can be inferred in the deepest part of the grabens of Punta del Este Basin.  

Marine sequences associated with the sag phase (Aptian), located from intermediate to deep waters of the three basins, are also considered potential source rocks.  

Reservoir rocks of proven quality are present in all offshore basins. Sandy sequences of deltaic systems in the sag sequence and fluvial systems in the synrift sequence have the best reservoir potential. Deep channel and turbiditic successions, located at different stratigraphic levels of the sag and passive margin sequences, should also be considered.  

The new seismic data has demonstrated many trapping mechanisms typical of an extensional tectonic setting with transcurrent movements.

Upstream Uruguay Round 2009

Outcrop of the Cambrian Sierra de Animas complex of volcanic-intrusive rocks along the Uruguayan Atlantic coast. Photo: ANCAP - Offshore Uruguayan basins are today considered frontier areas with high exploration risk. This is in part due to the insufficient and outdated information and in part to lack of exploration, says de Santa Ana.  

On the other hand, there appear to be numerous hydrocarbon indications.  

- Direct hydrocarbon indications include oil of 32 °API as detected in fluid inclusions analysis performed on cuttings from offshore wells. Furthermore, other forms of hydrocarbon indications seen on the seismic data and the well logs are quite promising.  

In May 2008, the Executive Power of República Oriental del Uruguay entitled ANCAP, the National Oil Company of Uruguay, and the entity in charge of upstream activities in the country, implemented the ‘Uruguay Round 2009'.  

According to the plan, offers from companies will be received by ANCAP from June 15th 2009 to July 1st. Blocks will then be awarded at the end of 2009.  

- Uruguay hopes this round can be an example of transparency in a framework of equal opportunities for all the oil companies. Possibly the companies will focus the analysis in the exploratory potential of Uruguay, rather than legal, economical, social, environmental and political aspects, which count with the best trust indexes and then, minimal risk, says Hector de Santa Ana.


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