How does Opec know how closely its members are complying with production quotas? They use spies, of course – companies known as ‘tanker trackers’.

The most reliable data, used even by Opec countries themselves, come not from the cartel member’s energy ministries, but from so-called secondary sources – a network of spies watching, binoculars in hand, the movement of tankers in and out of the world’s biggest export terminals.

There are three main tanker trackers are Petro-Logistics, Oil Movements and Lloyd’s Intelligence Marine Unit.

Conrad Geber, head of Petro-Logistics… relies on multiple sources – from “spies” at oil ports to “friendly” officials at oil companies leaking data. But even so, he concedes the information is never 100 per cent accurate.

“There are black holes such as Nigeria and Venezuela where all you can come by is a reasonable estimate,” he says.

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The confusion and distrust about production is so deep that Opec members regularly request data about fellow members’ production from the International Energy Agency. This is ironic because the IEA, created after the 1970s oil shocks as the western countries’ oil watchdog, is basically to Opec what Nato was to the Warsaw Pact.