The outcrop of the Irati Formation has the form of a great "S" that begins in the state of Sao Paulo and extends nearly continuously for 1,700 km to the frontier of Brazil and Uruguay. In the southern part of the states of Parana and Rio Grande do Sul, two distinct beds of oil shale are separated by nonbituminous shale and limestone. In the northern part of Parana, in Santa Catarina and Sao Paulo, thin layers of oil shale are intercalated with nonbituminous shale and dolomite throughout the section. The oil shale beds are thickest in the vicinity of Sao Gabriel, in Rio Grande do Sul, where the upper bed averages about 9 m, the lower bed about 4.5 m, and the intervening shale-limestone sequence about 12 m. Basalt and diabase intrusive rocks are present in the Irati Formati n at many localities in its outcrop.

The oil shale is dark brown to black, finely laminated, fissile, and hard, but weathers to a reddish-brown regolith that in places is 10 m thick. It contains 2-12 percent oil recoverable by destructive distillation; the density ranges from about 1.80 to 2.45, generally decreasing with increasing content of organic matter.

Three areas have been sampled in detail by drilling; reserves of 600 million bbl have been proved in the two beds (which average 10.7 m in combined thickness and 7.4 percent in oil yield) in the Sao Mateus do Sul area in Parana. Reserves of similar magnitude are present in the lower bed (which averages nearly 7 percent in oil yield) in the Sao Gabriel and Dom Pedrito areas of the Rio Grande do Sul; the upper bed in those areas yields only 2-3 percent oil and is not considered suitable for exploitation. A semi-industrial scale prototype plant is planned for construction at Sao Mateus do Sul.