Guaraní Aquifer System sedimentary petrography of outcropping formations and hydrogeologic implications

Petrographic analyses are presented for the Rivera, Tacuarembó and Cuchilla Ombú formations, which constitute the outcropping Guaraní Aquifer System in the Rivera and Tacuarembó departments, Uruguay. Sedimentary petrography studies were carried out with the objective of qualitatively and quantitatively evaluate the porosity of sedimentary rocks. To determine the porosity in sandstones, the method of impregnation of petrographic thin sections followed by point-counting of total pore area and interconnected pore area was used. Clast and matrix composition was determined using both optical microscope and scanning electron microscope (SEM), with an energy dispersive X-ray analyzer (EDS) The study shows average values of total porosity of 17% and effective porosity of 14% for the Rivera and Cuchilla Ombú aquifer units and average value of total porosity of 15% and effective porosity of 9% for the Tacuarembó unit. There are compositional differences between sandstones of the Rivera and Tacuarembó formations, the former being essentially quartz-arenites and the latter predominantly subarkoses and sublithic arenites.