Saudi Arabia and its Gulf Opec allies have lined up to try and quell mounting fears of a prolonged supply glut in the oil market, warning of future shortages in the sector if investments fall further. A year after the de facto leaders of Opec opened the taps to squeeze high-cost rivals, they are struggling to reassure cash-strapped members of the cartel that the strategy is working. Prices are languishing near $40 a barrel and showing little sign of recovering, despite major oil companies slashing billions of dollars in investment already this year. At a conference in Bahrain on Thursday, Ali Al Naimi, Saudi Arabia’s oil minister, told oil executives and financiers that the world must continue to plough money into projects in order to guarantee the “stability” of the market in the long run. “Demand for oil is continuing to rise,” Mr Naimi said, predicting growth would rise at 1m barrels a day, a faster pace than earlier this decade. He said $700bn in investment would be needed over the next decade to “provide a surplus to guarantee the security of the market”.