The fossil fuels include coal, petroleum and natural gas. Bulgaria possesses significant reserves of coal estimated at 4,8 billion tons. More than 92% of them, or 4,5 billion tons, is lignite, which is the lowest rank of coal due to its relatively low heat content but is widely used for electricity generation. With reserves of 2,856 billion tons Maritsa Iztok, situated in the Upper Thracian Plain, is by far the largest coal basin in the country which powers Maritsa Iztok Complex, the largest energy complex in South-Eastern Europe. Other lignite basins include Sofia valley (reserves of 870 million tons), Elhovo (656 million tons), Lom (277 million tons), Maritsa Zapad (170 million tons). The reserves of sub-bituminous coal are 300 million tons, situated mainly near Bobov Dol, Pernik and Burgas. The recoverable reserves of bituminous coal and anthracite are insignificant – only 10 and 2,5 million tons respectively. However, there is a huge basin of bituminous coal in Southern Dobruja with estimated reserves of over 1 billion tons but its large depth (1370–1950 m) is an obstacle for its commercial exploitation.