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.