Oil Shale in Estonia

There are two kinds of oil shale in Estonia, both of which are sedimentary rocks laid down during the Ordovician geologic period. Graptolitic argillite is the larger oil shale resource, but, because its organic mattercontent is relatively low, it is not used industrially. The other is kukersite, which has been mined for more than a hundred years. Kukersite deposits in Estonia account for 1% of global oil shale deposits.