Iran has oil reserves to last for the next 150 years but faces declining output as fields mature, the head of its state oil company said on Saturday.

Iran has played down the impact of sanctions -- which have discouraged many western companies from investing in its oil and gas sector -- and Ahmad Qalebani, managing director of the National Iranian Oil Company, told a news conference an output decline of 25,000 barrels per day over the last year was a natural phenomenon, given the age of the oil fields.

'Our reservoirs are in the second half of their lives,' Qalebani told reporters at an oil and gas fair in Tehran. 'This drop is natural for any reservoir.'

He said the cost of maintaining current output levels, which he did not specify, was $8 billion per year. Iranian media reported in October that Iran was pumping 3.66 million barrels per day.

Qalebani said Iran has raised the estimate of its oil reserves to 155 billion barrels. In October, one week after Iraq upped its reserves estimate to 143 billion barrels, surpassing Iran, Tehran increased its own estimate to 150.31 billion barrels, up from a previous 138 billion barrels.

'Iran will have oil, producing and exporting, for at least the next 150 years,' he told the Mehr news agency on the sidelines of the conference.

The world's fifth-biggest oil exporter has said it is happy with oil prices, which hit a two-a-half year high above $127 this week. Iran's national budget is based on a price of $80 per barrel and a price above that could create a healthy surplus. – Reuters