When the oil industry descends on Vienna this week it will survey a market transformed by Opec’s historic decision six months ago not to lower output to prop up falling prices. But the main event may not be Friday’s ministerial meeting, at which the 12-member cartel is widely expected to stick with the policy it launched in November. Instead, many will be focused on a summit that will bring the world’s most powerful big oil executives face to face with the architects of Opec’s strategy that has forced them to slash costs and cut investment in new projects.