Oil shale is a strategic energy resource that constitutes about 4% of Estonia's gross domestic product. The oil shale industry in Estonia is one of the most developed in the world. In 2012, the country's oil shale industry employed 6,500 people – about 1% of the national workforce. Of all the oil shale fired power stations in the world, the two largest are in this country. In 2012, 70% of mined oil shale was used for electricity generation, accounting for about 85% of Estonia's total electricity production. A smaller proportion of the mined oil shale is used to produce shale oil, a type of synthetic oil extracted from shale by pyrolysis, which is sufficient to keep Estonia as the second largest shale oil producer in the world after China. In addition, oil shale and its products are used in Estonia for district heating and as a feedstock material for the cement industry.