Private equity firm Riverstone Holdings said it will invest $500m in Three Rivers Natural Resources III, an Austin-based oil and gas company.Riverstone Global Energy and Power Fund VI will invest $333m and Riverstone Energy will contribute with the remaining $167m.The company said it will focus on oil and gas acquisition opportunities in the Permian Basin, similarly with the two previous partnerships with Three Rivers.Pierre Lapeyre and David Leuschen, co-founders of Riverstone, said: "This investment exemplifies Riverstone's strategy of re-partnering with proven management teams with deep, basin-focused operating expertise."We look forward to repeating the success we had with both Three Rivers I and Three Rivers II."