The OPEC has failed to approve issues related to oil-production during the last meeting and will try to reach an agreement next week. The forthcoming summit in Vienna next week is going to be quite complex amid instable situation on the oil markets due to protests over nuclear program in Iran, Libyan restart of oil production after war and spreading euro-zone crisis. The main issue to be covered is impact of global downturn on the oil market, said Anas Alhajji, chief economist at NGP Energy Capital Management.