Opec and Russia are set to boost oil supply in an effort to cool prices, but questions remain over how the increase will work, writes David Sheppard.

Under a deal between Opec and Russia that started last year, big producers aimed to reduce supplies by about 1.8m barrels a day, but the cuts have gone further than intended, helping push prices above $80 a barrel this month.

There are three reasons why the cut in oil supply went too far. First, Venezuela’s output collapsed amid a social and economic crisis. Other members such as Angola are also battling drops in output, with natural decline rates at fields accelerating due to under-investment when prices were low. Finally, some countries, such as Saudi Arabia, have cut more than required under the agreement.