African oil explorers Petrel Resources and Clontarf Energy are taking legal action to stop rivals potentially drilling for oil in a block in Ghana which they own.Petrel and Clontarf have told their lawyers to seek an injunction to prevent the award of any part of their Tano 2A block, in which they have stakes of 30% and 60% respectively, to a third party.The groups acted after media reports and speculation suggested the Ghanaian parliament had ratified an agreement allowing exploration in "a Tano Basin shallow water block".Petrel and Clontarf, which are awaiting government approval of their own deal to explore for oil in Tano 2A, said they had reason to believe the ratification supposedly already granted may involve part of Tano 2A."Petrel is urgently seeking clarification from the Ghanaian authorities that the ratification does not relate to an area covered by Tano 2A," Petrel said in a statement that was also issued by Clontarf. "The company will seek to enforce its rights. Further announcements will be made in due course."Shares in Petrel fell 1.5p or 11.8% to 11.25p at 15:16 in London, while Clontarf dropped 0.2p or 20.9% to 0.85p.PW