Sedimentary rock

Oil shale

Organic-rich, fine-grained rock containing kerogen that yields oil only when heated (pyrolysis). It is not the same as tight-oil shales like the Bakken. Organic matter comes mainly from algae, spores, pollen and plant debris, mostly in marine or lake settings; the Green River Formation (Colorado, Utah, Wyoming) is the world's largest known deposit. Kerogen types: Type 1 (algal, oil-prone), Type 2 (mixed, may produce oil), Type 3 (woody, gas-prone).