Oil production in South Sudan has fallen by a quarter since independence four months ago, Unity State's environment minister has told the BBC.Since the split from the north there has been a shortage of skilled workers, the minister, William Garjang, said.The rainy season was also making it difficult to repair faulty machinery and several roads have been mined by rebels, he said.Revenue from oil provides 98% of the new nation's budget.South Sudan seceded from Sudan in July - the culmination of a peace deal signed in 2005 that ended decades of civil war.The two countries have yet to agree on sharing their oil wealth.The BBC's James Copnall in Bentiu, capital of Unity State, says there is growing concern about South Sudan's strong dependency on oil, particularly as the supply may run out in the next two or three decades.A rebellion waged by a new rebel group, the South Sudan Liberation Army (SSLA), has further complicated matters.Its fighters are concentrated in Unity State, near many of South Sudan's lucrative oil fields.The SSLA took up arms earlier this year in protest against corruption, mismanagement of oil revenues and what it believes is the domination of the Dinka ethnic group in the government.But Mr Garjang told the BBC the rebellion was not disrupting oil production.Part of the problem was the rainy season, which made it difficult to get machinery to faulty oil facilities to repair them, he said.But the other major problem was the absence of qualified technical personnel, as many northerners left the oilfields once South Sudan became independent, Mr Garjang said.