The Tres Islas Fm. (Early Permian) is formed by fine to medium sandstones with some levels of clays and sometimes conglomerates very common in marine – deltaic environmental. The unit is located in Norte Basin (see map) and it is correlated with the Rio Bonito Fm. of the Brazilian Paraná Basin. The Rio Bonito Fm. contains the majority of the coal levels that are exploited in South Brazil including the Candiota mine located very close to the Uruguayan border.
In Uruguay, the coal exploration background is very low, unreliable and in many cases has uncertain location. Since 2009 ANCAP has made an effort to establish a new geological model that includes the deltaic –marine cycle of Permo- carboniferous mega-sequence of the Norte Basin, particularly in the third top of the Tres Islas Fm. In the same way, ANCAP has made prospection activities between 2010 and 2012, in addition to make a geological investigation in cooperation with Science Collage of the República University.
As results of different studies it is concluded that the possibility to have coal is in the third top of the Tres Islas Fm. related to the barrier – lagoon environmental. The coal seams range between few centimeters to meter of thickness. The region with the best exploration opportunity, with structural control of the Tres Islas Fm. and preservation of the third top of the unit is toward Northeast region of the Norte Basin close to Brazilian border.
Distribution of Tres Islas Fm. in Norte Basin. Tres Islas Fm. outcrops in Norte Basin