Kuwaiti oil wells have an installed capacity estimated at 2.9 mbd, with a maximum sustainable capacity of 2.5 mbd. In 1972 an upper ceiling of 2 mbd was imposed to prevent too rapid a depletion of Kuwait's most valuable resource. The ceiling was revised to 1.5 mbd in April 19 80, and subsequently to 1.25 mbd. At current production levels (1.08 mbd in January 1984, slightly in excess of its OPEC quota of 1.05 mbd), proven reserves (estimated at 69 billion barrels, making Kuwait the third largest in the world in oil reserves) are expected to last for as long as 180 years. Oil income for 1984 is projected at ,10.4 billion, slightly higher than the 1983 level of ,10.3 billion. Kuwaiti oil is heavier and has a higher sulphur content than Saudi oil, but Kuwaiti oil is produced under very favorable conditions, as evidenced by its low production cost, ,0.15 per barrel. In 1983, domestic consumption of oil was estimated at 0.169 mbd.