Everything you ever wanted to know about the Sudan and the genocide going on in the south against the non-Arab population can be found in this report from the Human Rights Watch. Probably far more than you ever wanted to know, actually.
Far too much to summarize, except to note that over the past 20 years, the government of Sudan, with the collusion of oil companies from Sweden, France and China, have been systematically ridding the southern oil fields of the indiginous population, by any means necessary.
No US firms operate in the Sudan.