The oldest deposits of the most important Estonian fossil – oil shale – have formed during the Ordovician age, about 400-450 million years ago. The remains of the sea flora and fauna have settled in coastal waters, and after compression have formed a gray mass a sapropel, the main constituent of oil shale. As a result of bacterial vital activity, the initial material have changed beyond recognition and a new substance appeared. That was kerogen (a solid mixture of organic chemical compounds), the organic part of oil shale, consisting of carbon, hydrogen and oxygen.
The interchange of oil shale and limestone layers can be explained by climatic changes in reservoirs. Due to the scientists’ opinion, limestone is a consequence of bacterial vital activity as well. Limestone bacteria have successfully developed only in warm water. As the temperature has decreased, aquatic plants have started to predominate; their remains were a basis for the formation of the oil shale. This sort of periodical climate change explains the arrangement of oil shale and limestone layers.
As well as living organisms in structure of oil shale there are several mineral sediments, that’s why during the process of burning, oil shale educes less heat and more ashes than coal. It is more expedient to use the oil shale as a raw material for the chemical industry, and less, as a fuel. The oil shale as a sedimentary rock is widespread all over the world. It can be found on the territory of 80 countries, the largest deposits are located in, Russia, the USA, Brazil, China and Canada. The color of the oil shale can vary from russet to black.
Oil shale deposits in Estonia are a part of the Baltic Oil Shale Basin, which is divided by convention to the Estonian and the Leningrad deposits, boundary between them is the Narva River. An outcrop along the Gulf of Finland coastline marks the northern boundary of the Estonian deposit. The line of a layer augmentation is bending along the Tallinn-Narva railway. The layers are almost horizontal. Oil shale layers descend to the South approximately to 3,5 meters for every kilometer. Two deposits are located in the north-eastern part of Estonia, namely Estonian and Tapa deposit, with geological reserve is more than 6 milliard tons of oil shale.