There are also deposits of copper, lead, manganese, and mercury. Colombia has the largest coal deposits in South America. Colombia's oilfields produced 17.4 millio tons of oil in 1988 (an average of 451,000 barrels per day in 1989). Scattered over wide areas, the main oilfields are near the Venezuelan frontier in the northeast and in the central and lower Magdalena Valley. Natural gas fields in the Guajira peninsula produced 5,280 million cubic meters of gas. Hydroelectric power stations produce 70 per cent of the country's electricity.