Oil shale is a thinly layered, blocky to fissile (breaking into thin layers), sedimentary rock rich in preserved organic material that yields significant amounts of oil and gas when heated sufficiently. The global resource of oil potentially available from oil shale is substantially larger than the amount of oil produced historically. Potential reserves have been estimated at 2.8 trillion barrels of oil. However, recent revision of Chinese resource estimates may potentially add approximately 300 billion barrels to the global resource. All estimates of recoverable shale oil are uncertain at this time. Oil shale was commonly deposited in large, fresh to highly saline lakes or in shallow marine environments. The organic material consists of kerogen, a complex mix of fossilized remains consisting mainly of algae and cyanobacteria.