When Opec ministers meet today in Vienna, they will be looking in dismay at tumbling oil prices. But some might be even more concerned about activity in an obscure corner of the energy market – options trading, where investors have amassed a record position betting that prices could fall even further.

The strong buying of put options – an insurance against falling prices as the contracts give holders the right to sell oil at a predetermined price and date – comes as spot oil prices fell this week to a 16-month low of $66.2 a barrel.