The world’s largest oil and gas groups shed more than a billion barrels of reserves in 2014, the sharpest decline in at least six years, according to figures that show their exploration record has worsened as big discoveries dwindle.

The latest annual reports for the "Big Five" energy majors – BP, Chevron, ExxonMobil, Royal Dutch Shell and Total – show that proved reserves for the group as a whole shrank to 78.6 billion barrels of oil equivalent (BOE) last year, from a little over 80billion the previous year, the steepest drop since at least 2008.

Behind the fall is a substantial decline in the number of barrels added as a result of recent discoveries and extensions to existing oil and gas fields, according to Morgan Stanley analysis of the data. That figure fell 24 per cent last year to 2.3 billion BOE and has nearly halved from 4.4 billion BOE in 2011.

Though these figures can be volatile, and depend in part on companies’ approach to booking barrels as “proved” reserves, the deteriorating exploration performance – if sustained – will raise questions over their ability to grow in the long run without making acquisitions.

Martijn Rats, analyst at Morgan Stanley, said last year was "really quite disappointing" for discoveries made through exploratory drilling. Big finds, such as Statoil's Johan Sverdrup field in the Norwegian North Sea in 2010, are increasingly rare.

“Discoveries are drying up,” he said.

“It’s becoming harder and harder to find oil outside the US. There are great success stories in the US with shale gas and ‘tight’ oil, but, outside that, conventional drilling is becoming less and less successful.” – (Copyright The Financial Times Ltd)