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By 09:17 BST its shares were down by over 12 per cent, accounting for nearly half of the FTSE 100's total dip, Reuters reports.
The index was 1.6 per cent - or 82.39 points - lower on 5,106.04 points, with other energy firms also retreating.
Tullow Oil and Cairn Energy shed 0.8 per cent and 1.2 per cent respectively, while Royal Dutch Shell dipped 2.4 per cent.
Elsewhere, domestic banks suffered on continued concerns regarding eurozone debt, while weaker metal prices caused BHP Billiton, Lonmin, Kazakhmys and Anglo American to all begin the day lower.
Similar falls were seen across Europe, with the FTSEurofirst 300 dropping 1.4 per cent to 986.86 points at 09:24 BST.
Last month, the pan-European index lost 5.8 per cent of its value.
Posted by Clive Arneil
