| 11. | Between the World Wars oil shale projects were begun in several countries.
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| 12. | From 1938, the town became the centre of an oil shale industry.
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| 13. | The oil shale industry was established in China already in the 1920s.
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| 14. | During World War II the value of the Estonian oil shale deposit grew.
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| 15. | The all existing oil shale industry was merged into that subsidiary.
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| 16. | Oil shale has also been burnt directly as a low-grade fuel.
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| 17. | The village originated in the oil shale mining boom of the 19th century.
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| 18. | Instead domestic oil shales were heated to produce the needed petrol.
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| 19. | There are other sources of oil, such as tar sands and oil shale.
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| 20. | Examples include extra heavy oil, oil sand, and oil shale deposits.
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