A South African court has sentenced Nigerian oil militant Henry Okah to an effective 24 years in jail.He was found guilty of 13 terrorism-related charges over twin car bombings during Nigeria's independence day celebrations in 2010.At least 12 people were killed and 36 others injured.Okah led a group which said it was fighting to help Niger Delta residents gain a greater share of the oil wealth from their part of southern Nigeria.The court established that Okah was the former leader of the Movement for the Emancipation of the Niger Delta, Mend.He also received a 13-year jail term for threats made to the South African government after his arrest in October 2010 but this runs concurrently with his 24-year sentence.The judge found that the state had proven Okah's guilt beyond a reasonable doubt because he didn't testify in his own defence during the trial.He had repeatedly denied any involvement in the bombings.Prosecutors have argued that although Okah is not a South Africa citizen, the country had the jurisdiction to try him under the International Co-operation in Criminal Matters Act.Analysts believe it would have been too dangerous for him to be tried in Nigeria because of the presence of his militant supporters.The BBC's Will Ross in Lagos says Okah may no longer be a headache for the Nigerian authorities but the threat of instability in the Niger Delta remains strong, despite a fall in the levels of violence since a 2009 amnesty.While Mend and other militant groups say they were fighting for a political cause, criminal gangs have taken advantage of the region's instability to make money from ransoms paid by oil companies, and by stealing oil.At its peak, the instability in the Niger Delta cost Nigeria about $1bn (£630m) in lost revenue, Reuters news agency quotes the central bank as saying.Nigeria is Africa's biggest oil producer, but most of its people live in poverty.