marine depositional environment, very different from the Green River Basins. These deposits have also undergone commercialization attempts; they are also resources for natural gas and have been mined for low-grade oil shale. These oil shales were formed during the Late Devonian and Early Mississippian periods. During this time, much of the eastern United States was covered by a large shallow sea. The oil shale is thought to have been the result of slow deposition of planktonic algae, under anoxic conditions. In parts of the basin close to the shoreline, the organic mixture that helped form the oil shale contains organic-rich sediment from the rising Appalachian mountains. Formations in Brazil Brazil has nine significant locations of oil shale deposits. The size, location and quality of oil shale deposits in the Paraíba Valley and the Irati Formation have attracted the most attention. These two contain an estimated 1.4 billion barrels of in-situ shale oil with total resources as much as more than three billion barrels. While the "Irati formation" deposit is the smaller of the two, containing an estimated 600 million barrels in-situ compared to 840 million in the Paraíba Valley formation, the former is more economically viable. The Irati Formation consists of two oil shale beds separated by of limestone and shale. The upper layer is thicker () but the thinner lower bed () is of greater value; the weight percent of shale oil yield is around 12% for the lower layer as compared to 7% for the upper one. The oil shale yield varies laterally, and may be as little as 7% for the lower layer and 4% for the upper layer. The formation is a very fine grained and laminated deposit ranging in color from dark gray to brown to black. While 60–70% of the shale consists of clay minerals, the balance is made up of organic matter. No consensus has been reached on the exact depositional nature of the Irati oil shale. One theory suggests that the organic material in the Irati oil shale originated from algae deposited in a lacustrine environment with salinity varying from that of freshwater to brackish water. Other theory suggests that the organic sediment may have been deposited in a shallow, partially restricted marine environment. Hutton's classification describes it as a marine source oil shale. Formation in Estonia The kukersite oil shale of Ordovician age in Estonia is part of the Baltic oil