Russian paleobotanist Mikhail Zalessky in 1916. The name reflects the German name for Kukruse Manor where oil shale samples were obtained. Kukersite deposits in Estonia are the world's second highest-grade oil shale deposits after the Australian torbanite. Its organic content varies from 15% to 55%, averaging over 40%. Correspondingly, its mean calorific value is 15 MJ/kg (3,600 kcal/kg). The conversion ratio of its organic content into usable energy (shale oil and oil shale gas) is between 65 and 67%, and its Fischer Assay oil yield is 30 to 47%. The principal organic component of kukersite is telalginite, which originated from the fossil green alga Gloeocapsomorpha prisca, deposited in a shallow marine basin. Kukersite lies at depths of . The most significant kukersite deposits in Estonia – the Estonian and the Tapa – cover about , and together with the Leningrad deposit (an extension of the Estonian) form the Baltic Oil Shale Basin. The Estonian deposit, which covers about , is used industrially. It consists of 23 exploration and mining fields. The Tapa deposit is not accounted as a reserve due to its lower calorific value, which makes its extraction economically inexpedient. In northern Estonia there are 50 layers of kukersite; the six lowest of these form a thick mineable bed. In this area kukersite lies near the surface. To the south and west it lies deeper and its thickness and quality decrease. According to the International Energy Agency, Estonia's kukersite represents about 1.1% of global and 17% of European oil shale resources. The total kukersite resources in Estonia are estimated to be about 4.8 billion tonnes, including 1.3 billion tonnes of economically proven and probable reserves. Economically proven and probable reserves consist of mineable deposits with energy ratings of at least 35 gigajoules per square metre and calorific values of at least 8 MJ/kg, located in areas without environmental restrictions. Up to 650 million tonnes of economically proven and probable reserves are designated as recoverable. History Early history It is often reported that 18th-century naturalist and explorer Johann Anton Güldenstädt had mentioned a discovery of a "burning rock" near Jõhvi in 1725, but his published travel notes mention neither oil shale nor Estonia. It is also often reported that the earliest documented record of oil shale in Estonia, authored by the Baltic German publicist and linguist August Wilhelm Hupel, dates to 1777. However, this is based on a misinterpretation of