A high content of organic matter consisting of clay minerals and silt-sized quartz grains from which oil may be extracted by distillation: "The spaces between the grains of oil shales are microscopic, making the rock highly impermeable and trapped within these spaces is a solid, combustible hydrocarbon, substance called kerogen which is a fossilized material in shale and other sedimentary rocks."

"When heated, the kerogen in oil shales breaks down and produces oil."