Discovery rates are still smaller than depletion rates, and have been since the 1960's.

Its reserves, however, are declining steadily, and the company now has only an eight-and-a-half-year supply of oil left at current depletion rates, according to Mr. Jacoboski.

The professor said that the depletion rate was currently believed to be 13 parts per million per year, but that accurate measurement was urgently needed.

The depletion rate over the US has been measured at 4 to 5 per cent since 1978.

The oil field is best known for the controversy surrounding its depletion rate.

This is used to understand depletion rates of materials consumed as a by-product to extraction.

Around 200,000 hectares a year are being lost, equivalent to an annual depletion rate of 0.5 per cent- one of the highest in the world.

That "accelerated the depletion rate and the time to uncontrolled decline," or the point where the field's production drops dramatically, he said.

And, the February 6, 2013, imposition of sanctions on Iran's ability to repatriate hard currency could cause the depletion rate to increase.

But investors shouldn't be entirely dazzled by the trusts' current payouts, he said, adding that it was important to study the depletion rates of their energy reserves.