I worked in the oil industry (in IT) for a bit more than 10 years, in the 90s. I worked for a drilling contractor named R&B Falcon. Transocean bought R&B in 2001. I just happened to have installed some of the warehouse management software I designed and managed development of on the Deepwater Horizon when it was brand new (for those who have been following the Gulf oil spill)
But that's another story.
Brasil is probably your best bet in South America. Although there are still foreign companies involved directly in exploration and production, Brasil is trying to get as much as possible done "in-house." I just don't know how far being a foreigner will get you these days - probably pretty far, but I'm not sure. I haven't spent much time in Brasil since 2000 (we had operations for Petrobras going out of Macae, Rio and a couple of places to the north) and I don't know if the government has put emphasis on training petro geologists in the universities, which might make it harder to get in.
I have done some work for Chevron (Houston office) over the last couple of years. Discussion has come up at times about their offices here in Buenos Aires. No one I knew there in Houston knew anyone working for Chevron here. Almost like it was a punishment to be sent here. There just is not much of a foreign presence related to oilfield markets.
Part of that (or an example of why) is because of the hell Shell got here when Nestor Kirchner decided he needed a whipping boy for the cost of refined products here and tried to force them to lower their prices.
My uncle runs a small outfit in Venezuela (he used to be president of Tenneco and then Penzoil, decided to do his own thing when he retired), and is constantly worried about Chavez nationalizing their equipment. I think many companies think of the same things when thinking of Argentina - it's difficult to business here anyway, even if not as bad as Venezuela.